Assessment of surface water storage options and reticulation infrastructure in the Roper catchment A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid Australia’s National Science Agency Cuan Petheram1, Ang Yang1, Lynn Seo1, Lee Rogers1, Fred Baynes2, Kevin Devlin2, Steve Marvanek1, Justin Hughes1, Rocio Ponce Reyes1, Peter Wilson1, Danial Stratford1, Seonaid Philip1 1 CSIRO, 2 Independent consultant ISBN 978-1-4863-1925-1 (print) ISBN 978-1-4863-1926-8 (online) Citation Petheram C, Yang A, Seo L, Rogers L, Baynes F, Devlin K, Marvanek S, Hughes J, Ponce Reyes R, Wilson P, Stratford D, Philip S (2022) Assessment of surface water storage options and reticulation infrastructure in the Roper catchment. A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Copyright © Commonwealth Scientific and Industrial Research Organisation 2022. To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO. Important disclaimer CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. CSIRO is committed to providing web accessible content wherever possible. If you are having difficulties with accessing this document please contact Email CSIRO Enquiries . CSIRO Roper River Water Resource Assessment acknowledgements This report was funded through the National Water Grid’s Science Program, which sits within the Australian Government’s Department of Climate Change, Energy, the Environment and Water. Aspects of the Assessment have been undertaken in conjunction with the Northern Territory Government. The Assessment was guided by two committees: i.The Assessment’s Governance Committee: CRC for Northern Australia/James Cook University; CSIRO; National Water Grid (Department of Climate Change, Energy, the Environment and Water); NT Department of Environment, Parks and Water Security; NT Department of Industry, Tourism and Trade; Office of Northern Australia; Qld Department of Agriculture and Fisheries; Qld Department of Regional Development, Manufacturing and Water ii.The Assessment’s joint Roper and Victoria River catchments Steering Committee: Amateur Fishermen’s Association of the NT; Austrade; Centrefarm; CSIRO, National Water Grid (Department of Climate Change, Energy, the Environment and Water); Northern Land Council; NT Cattlemen’s Association; NT Department of Environment, Parks Australia; Parks and Water Security; NT Department of Industry, Tourism and Trade; Regional Development Australia; NT Farmers; NT Seafood Council; Office of Northern Australia; Roper Gulf Regional Council Shire Responsibility for the Assessment’s content lies with CSIRO. The Assessment’s committees did not have an opportunity to review the Assessment results or outputs prior to its release. This report was reviewed by Fazlul Karim (CSIRO) Acknowledgement of Country CSIRO acknowledges the Traditional Owners of the lands, seas and waters, of the area that we live and work on across Australia. We acknowledge their continuing connection to their culture and pay our respects to their Elders past and present. Photo Farm-scale gully dam in Roper catchment. Source: CSIRO Director’s foreword Sustainable regional development is a priority for the Australian and Northern Territory governments. Across northern Australia, however, there is a scarcity of scientific information on land and water resources to complement local information held by Indigenous owners and landholders. Sustainable regional development requires knowledge of the scale, nature, location and distribution of the likely environmental, social and economic opportunities and the risks of any proposed development. Especially where resource use is contested, this knowledge informs the consultation and planning that underpins the resource security required to unlock investment. In 2019 the Australian Government commissioned CSIRO to complete the Roper River Water Resource Assessment. In response, CSIRO accessed expertise and collaborations from across Australia to provide data and insight to support consideration of the use of land and water resources for development in the Roper catchment. While the Assessment focuses mainly on the potential for agriculture, the detailed information provided on land and water resources, their potential uses and the impacts of those uses are relevant to a wider range of regional-scale planning considerations by Indigenous owners, landholders, citizens, investors, local government, the Northern Territory and federal governments. Importantly the Assessment will not recommend one development over another, nor assume any particular development pathway. It provides a range of possibilities and the information required to interpret them - including risks that may attend any opportunities - consistent with regional values and aspirations. All data and reports produced by the Assessment will be publicly available. Chris Chilcott Project Director C:\Users\bru119\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.Word\C_Chilcott_high.jpg The Roper River Water Resource Assessment Team Project Director Chris Chilcott Project Leaders Cuan Petheram, Ian Watson Project Support Caroline Bruce Communications Chanel Koeleman/Kate Cranney, Siobhan Duffy, Amy Edwards Activities Agriculture and socio- economics Chris Stokes, Caroline Bruce, Shokhrukh Jalilov, Diane Jarvis1, Adam Liedloff, Yvette Oliver, Alex Peachey2, Allan Peake, Maxine Piggott, Perry Poulton, Di Prestwidge, Thomas Vanderbyl7, Tony Webster, Steve Yeates Climate David McJannet, Lynn Seo Ecology Groundwater hydrology Indigenous water values, rights, interests and development goals Danial Stratford, Laura Blamey, Rik Buckworth, Pascal Castellazzi, Bayley Costin, Roy Aijun Deng, Ruan Gannon, Sophie Gilbey, Rob Kenyon, Darran King, Keller Kopf3, Stacey Kopf3, Simon Linke, Heather McGinness, Linda Merrin, Colton Perna3, Eva Plaganyi, Rocio Ponce Reyes, Jodie Pritchard, Nathan Waltham9 Andrew R. Taylor, Karen Barry, Russell Crosbie, Phil Davies, Alec Deslandes, Katelyn Dooley, Clement Duvert8, Geoff Hodgson, Lindsay Hutley8, Anthony Knapton4, Sebastien Lamontagne, Steven Tickell5, Sarah Marshall, Axel Suckow, Chris Turnadge Pethie Lyons, Marcus Barber, Peta Braedon, Kristina Fisher, Petina Pert Land suitability Ian Watson, Jenet Austin, Elisabeth Bui, Bart Edmeades5, John Gallant, Linda Gregory, Jason Hill5, Seonaid Philip, Ross Searle, Uta Stockmann, Mark Thomas, Francis Wait5, Peter L. Wilson, Peter R. Wilson Surface water hydrology Justin Hughes, Shaun Kim, Steve Marvanek, Catherine Ticehurst, Biao Wang Surface water storage Cuan Petheram, Fred Baynes6, Kevin Devlin7, Arthur Read, Lee Rogers, Ang Yang Note: Assessment team as at June 15, 2023. All contributors are affiliated with CSIRO unless indicated otherwise. Activity Leaders are underlined. 1James Cook University; 2NT Department of Industry, Tourism and Trade; 3 Research Institute for the Environment and Livelihoods. College of Engineering, IT & Environment. Charles Darwin University; 4CloudGMS; 5NT Department of Environment, Parks and Water Security; 6Baynes Geologic; 7independent consultant; 8Charles Darwin University; 9Centre for Tropical Water and Aquatic Ecosystem Research. James Cook University. ii | Surface water storage and reticulation Shortened forms SHORT FORM FULL FORM AEP annual exceedance probability AHD Australian Height Datum ALOS Advanced Land Observing Satellite AMTD adopted middle thread distances ANCOLD Australian National Committee on Large Dams APE areal potential evaporation APSIM Agricultural Production Systems sIMulator AWRA-L Australian Water Resources Assessment landscape model AWRA-R Australian Water Resources Assessment river system model AWRC Australian Water Resources Council BHA behaviour analysis CC conventional concrete DEM digital elevation model DEM-H national 1 second hydrological digital elevation model DIWA Directory of Important Wetlands in Australia DOI Digital Object Identifier EPBC Environmental Protection and Biodiversity Conservation act FSL full supply level GDG Gould–Dincer Gamma algorithm (or method) GRP glass reinforced plastic EGM96 Earth Gravitational Model 1996 geoid, which is the datum upon which SRTM and DEM-H are based PMF probable maximum flood RCC roller compacted concrete SGG Soil generic group SRTM Shuttle Radar Topographic Mission TDC Total direct costs TOC Total out turn costs Units UNIT DESCRIPTION m3 cubic metre GL gigalitre ha hectare km kilometre mEGM96 EGM96 geoid height in metres ML megalitre ML/year megalitres per year (ML/y) mm millimetre Mt million tonnes m2 square metre y year t tonne Preface Sustainable regional development is a priority for the Australian and Northern Territory governments. For example, in 2023 the Northern Territory Government committed to the implementation of a new Territory Water Plan. One of the priority actions announced by the government was the acceleration of the existing water science program ‘to support best practice water resource management and sustainable development’. The efficient use of Australia’s natural resources by food producers and processors requires a good understanding of soil, water and energy resources so they can be managed sustainably. Finely tuned strategic planning will be required to ensure that investment and government expenditure on development are soundly targeted and designed. Northern Australia presents a globally unique opportunity (a greenfield development opportunity in a first-world country) to strategically consider and plan development. Northern Australia also contains ecological and cultural assets of high value and decisions about development will need to be made within that context. Good information is critical to these decisions. Most of northern Australia’s land and water resources, however, have not been mapped in sufficient detail to provide for reliable resource allocation, mitigate investment or environmental risks, or build policy settings that can support decisions. Better data are required to inform decisions on private investment and government expenditure, to account for intersections between existing and potential resource users, and to ensure that net development benefits are maximised. In consultation with the Northern Territory Government, the Australian Government prioritised the catchment of the Roper River for investigation (Preface Figure 1-1) and establishment of baseline information on soil, water and the environment. Northern Australia is defined as the part of Australia north of the Tropic of Capricorn. The Murray– Darling Basin and major irrigation areas and major dams (greater than 500 GL capacity) in Australia are shown for context. The Roper River Water Resource Assessment (the Assessment) provides a comprehensive and integrated evaluation of the feasibility, economic viability and sustainability of water and agricultural development. While agricultural developments are the primary focus of the Assessment, it also considers opportunities for and intersections between other types of water-dependent development. For example, the Assessment explores the nature, scale, location and impacts of developments relating to industrial and urban development and aquaculture, in relevant locations. The Assessment was designed to inform consideration of development, not to enable any particular development to occur. As such, the Assessment informs – but does not seek to replace – existing planning, regulatory or approval processes. Importantly, the Assessment does not assume a given policy or regulatory environment. As policy and regulations can change, this enables the results to be applied to the widest range of uses for the longest possible time frame. Preface Figure 1-1 Map of Australia showing Assessment area It was not the intention – and nor was it possible – for the Assessment to generate new information on all topics related to water and irrigation development in northern Australia. Topics not directly examined in the Assessment are discussed with reference to and in the context of the existing literature. Functionally, the Assessment adopted an activities-based approach (reflected in the content and structure of the outputs and products), comprising eight activity groups; each contributes its part to create a cohesive picture of regional development opportunities, costs and benefits. Preface Figure 1-2 illustrates the high-level links between the eight activities and the general flow of information in the Assessment. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Preface Figure 1-2 Schematic diagram of the high-level linkages between the eight activities and the general flow of information in the Assessment. Assessment reporting structure Development opportunities and their impacts are frequently highly interdependent and consequently, so is the research undertaken through this Assessment. While each report may be read as a stand-alone document, the suite of reports most reliably informs discussion and decisions concerning regional development when read as a whole. The Assessment has produced a series of cascading reports and information products: • Technical reports; that present scientific work at a level of detail sufficient for technical and scientific experts to reproduce the work. Each of the eight activities has one or more corresponding technical report. • A Catchment report; that for the Roper catchment synthesises key material from the technical reports, providing well-informed (but not necessarily-scientifically trained) readers with the information required to make decisions about the opportunities, costs and benefits associated with irrigated agriculture and other development options. • A Summary report; that for the Roper catchment provides a summary and narrative for a general public audience in plain English. • A Summary factsheet; that for the Roper catchment provides key findings for a general public audience in the shortest possible format. The Assessment has also developed online information products to enable the reader to better access information that is not readily available in a static form. All of these reports, information tools and data products are available online at https://www.csiro.au/roperriver. The website provides readers with a communications suite including factsheets, multimedia content, FAQs, reports and links to other related sites, particularly about other research in northern Australia. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Executive summary Current allocations of surface water in the Roper catchment are low, relative to the catchment’s median annual streamflow (<0.1%). The development of the surface water resources of this highly seasonal catchment to enable regional economic development, as has occurred in the south of Australia, would in many instances require rivers to be regulated and water stored. This report presents information on the broad-scale opportunities for and risks of storing surface water in the Roper catchment, including large engineered dams and large farm-scale offstream storages (i.e. ringtanks) and large farm-scale gully dams. The information is provided to support informed deliberations and discussions around the construction of dams in the Roper catchment. Large instream dams No previous studies of large dams in the Roper catchment have been undertaken. In order to undertake an objective analysis the DamSite model, a series of algorithms that automatically determines favourable locations in the landscape as sites for large instream and offstream dams was used to assess over 50 million potential dam sites in the Roper catchment. The more favourable sites in terms of cost per ML released from the dam wall in distinct geographic parts of the Roper catchment were evaluated together with broad-scale geological data and land suitability data to select dam sites for a desktop pre-feasibility analysis. Potentially feasible damsites in the Roper catchment occur where resistant ridges of Proterozoic sandstone beds have been incised by the river systems and outcrop on both sides of river valleys. The sandstones are generally weathered to varying degrees and the depth of weathering and the amount of sandstone outcrop on the valley slopes is a fundamental control on the suitability of the potential damsites. Where the sandstones are relatively unweathered and outcrop on the abutments of the potential damsite less stripping will be required to achieve a satisfactory founding level for the dam. The other fundamental control on the suitability of potential dams in the Roper catchment is the extent and depth of the Quaternary alluvial sands and gravels in the floor of the valley as these materials will have to be removed to achieve a satisfactory founding level for any dam. In general, where stripping removes the more weathered rock, it is anticipated that the Proterozoic sandstones will form a reasonably watertight dam foundation requiring conventional grout curtains and foundation preparation. In those parts of the catchment where potentially soluble dolomites occur within the Proterozoic sequences (soluble over a geological time scale) then it is possible that potentially leaky dam abutments and reservoir rims may be present requiring specialised and costly foundation treatment such as extensive grouting. In the lower reaches of the Roper River where the river is tidal, the presence of soft estuarine sediments has the potential to make dam design more challenging and construction more expensive, which may compromise the feasibility of a dam. Four potential dam sites were selected based on their potential to supply water for irrigation and one potential dam site (on the Wilton River) was selected based in its potential to supply water for hydro-electric power generation. These sites are summarised in Preface Table 1. Two potential sites were short-listed to develop conceptual arrangements and preliminary manually derived cost estimates. The remaining costs were modelled using the dam cost algorithm used in the DamSite model. Preface Table 1 Potential dam sites in the Roper catchment examined as part of this Assessment NAME DAM TYPE* SPILLWAY HEIGHT ABOVE BED ** (M) CAPACITY AT FSL (GL) CATCHMENT AREA (KM2) ANNUAL WATER YIELD *** (GL) CAPITAL COST# ($ MILLION) UNIT COST## ($/ML) EQUIVALENT ANNUAL UNIT COST & O&M### ($/Y PER ML/Y) Waterhouse River West Branch dam site RCC 23 128 795 48 415 8646 640 Waterhouse River RCC 21 219 1,477 89 253 2,843 211 Flying Fox Creek RCC 22 133 1,173 68 318 4,676 346 Jalboi River RCC 25 174 828 40 463 11,575 857 Wilton River RCC 39 4,541 12,073 926 849& 916 68 FSL = full supply level * Roller compacted concrete dam (RCC). ** The height of the dam abutments and saddle dams will be higher than the spillway height. *** Water yield is based on 85% annual time-based reliability using a perennial demand pattern for the baseline river model under Scenario A. This is yield at the dam wall (i.e. does not take into account distribution losses or downstream transmission losses). These yield values do not take into account downstream existing entitlement holders or environmental considerations. #  Indicates manually derived preliminary cost estimate, which is likely to be –10% to +50% of ‘true cost’.  Indicates modelled preliminary cost estimate, which is likely to be –25% to +75% of ‘true’ cost. Should site geotechnical investigations reveal unknown unfavourable geological conditions, costs could be substantially higher. ## This is the unit cost of annual water supply and is calculated as the capital cost of the dam divided by the water yield at 85% annual time reliability. ### Assuming a 7% real discount rate and a dam service life of 100 years. Includes operation and maintenance costs, assuming operation and maintenance costs are 0.4% of the total capital cost. §§Note there is limited soil suitable for irrigated agriculture downstream of this potential dam site Although there are potential dam sites in the lower Roper River, Hodgson River and Wilton River that could potentially supply large quantities of water at a relatively low levelized cost (i.e. ~$60/y per ML/y), there is limited land downstream of these sites suitable for irrigated agriculture. The site with the lowest levelized cost upstream of moderately large contiguous areas of soils suitable for irrigated agriculture is on the Waterhouse River. This site could potentially supply a modest amount of water at a levelized cost of $211/y per ML/y and the site is situated on Aboriginal Land scheduled under ALRA near the community of Beswick and is classified under ‘inalienable freehold title’, which means that it cannot be bought, acquired or mortgaged. Other sites with relatively low levelized cost on lease hold land, more favourable geology for roller compacted concrete dams and small to moderate contiguous areas of suitable soil downstream are on the upper Flying Fox Creek and Jalboi River. Situated on relatively small catchments these are relatively low yielding sites (68 and 40 GL in 85% of years respectively) and have levelized costs of about 346 and 857 per $/y per ML/y respectively. Although the potential dam site on the Wilton River has capacity to generate large quantities of hydro-electric power there is no transmission line infrastructure in the Roper catchment to support the development of hydro-electric power. No information relating to sacred sites or cultural heritage values of the potential dam sites was made available to the Assessment. The Roper catchment is very likely to contain a large number of Indigenous cultural sites, including archaeological pre-contact sites some of which are likely to be of national scientific significance. Previous studies in northern and southern Australia clearly show that Indigenous people lived along major watercourses and drainage lines. The cultural heritage value of these landforms and their immediate surrounds is therefore assumed to be moderate to very high. Potential dams in the Roper catchment, which were examined as part of the Assessment, were estimated to have less than 1% sediment infilling after 30 years and less than 3% sediment infilling after 100 years. If any of the potential dams examined in the Assessment were to be constructed, sediment yields would need to be recomputed by undertaking a detailed field measurement and modelling program of downstream impacts on river channels and an assessment of estuarine and coastal geomorphology. A desktop assessment of potential environmental issues associated with large potential dam sites in the Roper catchment was undertaken. Assessment of potential impacts was based on fish distribution and passage, for which reasonable information exists, and consideration of general environmental issues that commonly arise in dam developments in similar habitats elsewhere, particularly the Burdekin Falls Dam (Lake Dalrymple) and the Ord River Dam (Lake Argyle). A large dam constructed on the lower Wilton River may limit the migration, movement or colonisation of habitat by fish species. Potential dam sites in the headwaters of the Roper catchment (i.e. Waterhouse River, Flying Fox Creek and Jalboi River) will have less impact because the restriction on species movement is small relative to the downstream areas and the number of fish species typically decreases with distance from the coast. It should be noted that the investigation of a potential large dam site generally involves an iterative process of increasingly detailed studies over a period of years, occasionally over as few as 2 or 3 years but often over 10 or more years. For any of the options listed in this report to advance to construction, far more comprehensive studies would be needed. Studies at that detail are beyond the scope of this regional-scale resource Assessment. Farm-scale gully and hillside dams and offstream storages in the Roper catchment This report provides a broad-scale assessment of the suitability of farm-scale gully and hillside dams and offstream water storage locations in the Roper catchments. It does not attempt to produce individual engineering farm-dam or water-harvesting infrastructure designs for individual producers. A desktop assessment of the suitability of farm-scale offstream storages in the Roper catchment was undertaken based on soil parameter grids developed by the Assessment. These data were sourced from the companion technical report on digital soil mapping (Thomas et al., 2022). Because the Assessment only sampled soil to a depth of 1.5 m, this suitability assessment does not give consideration to the nature of subsurface material below 1.5 m depth. The largest areas suitable for farm-scale offstream storages in the Roper catchment are along the recent alluvial soils adjacent to the Roper River. This area is, however, susceptible to flooding. Elsewhere in the Roper catchment the soils are too sandy or landscape too steep and rocky for farm-scale offstream storages. Farm-scale gully and hillside dams were modelled using the DamSite model. Those areas that are more topographically favourable for gully dams have the smallest areas of soil suitable for irrigated agriculture. The cumulative effect of water extraction for farm-scale ringtanks is examined in the companion technical report on river system simulation (Hughes et al., 2023) and the companion technical report on ecology (Stratford et al., 2023). Assuming an average seepage loss of 2 mm/day a 4.25 m high ringtank in the Roper catchment was calculated to have a levelized cost of $135, 175 and $239/y per ML/y for irrigating crops with short, medium and long/perennial growing seasons. For crops with short and medium length growing seasons the levelized cost is considerably less than that for large engineered dams. For crops with a long growing season (i.e. double cropping system or perennial) the levelized cost of a 4.25 m high ringtank is slightly higher than the most cost-effective large engineered dams in the Roper catchment, however, the levelized cost of a 6.75 m ringtank is slightly less than the most cost-effective large engineered dam. With a levelized cost typically between $50 and $75/y per ML/y gully dams were found to be considerably more cost effective than ringtanks and large engineered dams. However, this is under idealised conditions where suitable soils for construction are readily available, and the site is upstream of soils suitable for irrigated agriculture. The combination of suitable topography for gully dams, soils for construction and irrigated cropping in the Roper catchment are rare. Contents Director’s foreword .......................................................................................................................... i The Roper River Water Resource Assessment Team ...................................................................... ii Shortened forms .............................................................................................................................iii Units ............................................................................................................................... iv Prefac e ............................................................................................................................. v Executive summary ....................................................................................................................... viii Part I Introduction 1 1 Introduction ........................................................................................................................ 2 1.1 Types of water storages ........................................................................................ 4 1.2 Stages of investigation in design, costing and construction of dams ................. 10 1.3 Dam safety and models of dam labour construction .......................................... 11 1.4 Key terminology and concepts ............................................................................ 13 2 Methods ............................................................................................................................ 16 2.1 Large instream and offstream dams ................................................................... 16 2.2 Farm-scale storages ............................................................................................. 21 Part II Large instream and offstream dams 25 3 Opportunity analysis of potential dam sites .................................................................... 26 3.1 DamSite modelling .............................................................................................. 26 3.2 Long-list of potential dam sites ........................................................................... 30 4 Sites short-listed ............................................................................................................... 35 4.1 Waterhouse River dam site on the Waterhouse River; ATMD 70.5 km ............. 35 4.2 Flying Fox Creek dam site on Flying Fox Creek; ATMD 105 km ........................... 45 5 Re-regulating structures ................................................................................................... 55 5.1 Sheet piling weirs ................................................................................................ 55 5.2 Concrete gravity type weirs ................................................................................. 56 5.3 Sand dams ........................................................................................................... 57 6 Farm-scale storages .......................................................................................................... 58 6.1 Offstream farm-scale storages (ringtanks) .......................................................... 58 6.2 Farm-scale gully and hillside dams ...................................................................... 66 Part III Scheme-scale reticulation infrastructure 75 7 Potential reticulated infrastructure and irrigation development .................................... 76 7.1 Learnings from other northern Australian Irrigation Developments relevant to potential Gulf developments ............................................................................................ 76 7.2 Areas serviced by the potential dam site on the Waterhouse River ATMD 70.5 km ............................................................................................................................. 77 7.3 Area serviced by the potential dam site on upper Flying Fox Creek ATMD 105 km ............................................................................................................................. 83 Part IV Summary comments 91 8 Summary comments ......................................................................................................... 92 References ............................................................................................................................. 94 Part V Appendices 97 Detailed costings for short-listed dam sites in the Roper catchment ................. 98 Potential dam site summary tables ................................................................... 112 Figures Preface Figure 1-1 Map of Australia showing Assessment area ..................................................... vi Preface Figure 1-2 Schematic diagram of the high-level linkages between the 8 activities and the general flow of information in the Assessment. ............................................................................ vii Figure 1-1 Schematic cross-section diagram of a rockfill embankment dam with a clay core ...... 6 Figure 1-2 Schematic cross-section of a concrete-faced rockfill dam ............................................ 7 Figure 1-3 Rectangular ringtank in the Flinders catchment ........................................................... 9 Figure 2-1 Constant monthly demand pattern used in the BHA modelling for the Roper catchment ..................................................................................................................................... 21 Figure 3-1 Potential storage sites in the Roper catchment based on minimum cost per ML storage capacity ............................................................................................................................ 27 Figure 3-2 Potential storage sites in the Roper catchment based on minimum cost per ML yield at the dam wall ............................................................................................................................. 28 Figure 3-3 Roper catchment hydro-electric power generation opportunity map ....................... 29 Figure 3-4 Long-listed potential dam sites and the main geological units of the Roper catchment ....................................................................................................................................................... 32 Figure 4-1 Location map of potential Waterhouse River dam site, reservoir extent and catchment area ............................................................................................................................. 38 Figure 4-2 Potential Waterhouse River dam reservoir and property boundaries ........................ 39 Figure 4-3 Known water-dependent ecological assets in the vicinity of the potential Waterhouse River dam site and reservoir ......................................................................................................... 40 Figure 4-4 Geology underlying the potential Waterhouse River dam site and reservoir ............ 41 Figure 4-5 Waterhouse River potential dam site topographic dimensions and inflow hydrology ....................................................................................................................................................... 42 Figure 4-6 Waterhouse River potential dam site cost, water yield at the dam wall and evaporation ................................................................................................................................... 43 Figure 4-7 Waterhouse River potential dam site and storage levels and water yield ................. 44 Figure 4-8 Flying Fox Creek potential dam site AMTD 105 km looking upstream ....................... 48 Figure 4-9 Location map of potential Flying Fox Creek dam site, reservoir extent and catchment area ............................................................................................................................................... 48 Figure 4-10 Potential Flying Fox Creek dam reservoir and property boundaries ......................... 49 Figure 4-11 Known water-dependent ecological assets in the vicinity of the potential Flying Fox Creek dam site and potential reservoir extent ............................................................................. 50 Figure 4-12 Geology underlying the potential Flying Fox Creek dam site and reservoir ............. 51 Figure 4-13 Flying Fox Creek potential dam site topographic dimensions and inflow hydrology 52 Figure 4-14 Flying Fox Creek potential dam site cost, water yield at the dam wall and evaporation ................................................................................................................................... 53 Figure 4-15 Flying Fox Creek potential dam site and storage levels and water yield .................. 54 Figure 5-1 Schematic cross-section diagram of sheet piling weir ................................................ 55 Figure 5-2 Donkey Camp Weir on the Katherine River (Daly River catchment, Northern Territory) ....................................................................................................................................... 57 Figure 6-1 Suitability of farm-scale offstream water storage (ringtanks) in the Roper catchment ....................................................................................................................................................... 60 Figure 6-2 Turkey nest offstream storage in the Roper catchment ............................................. 61 Figure 6-3 80% exceedance of annual streamflow in the Roper catchment under Scenario A ... 62 Figure 6-4 Most economically suitable locations for large farm-scale gully dams in the Roper catchment overlaid on map of versatile agricultural land ............................................................ 68 Figure 6-5 Suitability of soils for construction of gully dams ........................................................ 70 Figure 7-1 Potential piped reticulated layout ............................................................................... 80 Figure 7-2 Nominal conceptual layout of potential irrigation area on Flying Fox Creek .............. 86 Figure 7-3 Elevation profile of channel ......................................................................................... 88 Tables Table 1-1 Types of farm-scale water storages ................................................................................ 9 Table 2-1 Criteria used to assess potential dam sites ................................................................... 18 Table 2-2 AWRA-R nodes from which simulated streamflow data were extracted for use in the behaviour analysis model at the five potential dam sites selected for pre-feasibility analysis ... 20 Table 2-3 Suitability scores for the construction of farm-scale earth embankment structures .. 22 Table 2-4 Subset of rules used to assess suitability of land for construction of farm-scale earth embankment structures................................................................................................................ 22 Table 3-1 Rapid desktop evaluation of long-listed potential dam sites in Roper catchment ...... 33 Table 5-1 Estimated generic construction cost of 3-m high sheet piling weir ............................. 56 Table 6-1 Effective volume after net evaporation and seepage for ringtanks of three average water depths and under three seepage rates near junction of Jalboi River and Roper River in the Roper catchment ........................................................................................................................... 64 Table 6-2 Indicative costs for a 4000-ML ringtank ........................................................................ 65 Table 6-3 Annualised cost for the construction and operation of three ringtank configurations 65 Table 6-4 Equivalent annual cost per ML for two different capacity ringtanks under three seepage rates near Jalboi River in the Roper catchment ............................................................. 66 Table 6-5 Cost of three hypothetical farm-scale gully dams of 4 GL capacity .............................. 71 Table 6-6 High-level breakdown of capital costs for three hypothetical farm-scale gully dams of 4 GL capacity ................................................................................................................................. 72 Table 6-7 Effective volumes and cost per ML for three 4 GL storages with different mean depths and seepage loss rates near the Waterhouse River in the Roper catchment .............................. 72 Table 6-8 Cost of construction and operation of three hypothetical farm-scale gully dams of 4 GL capacity .................................................................................................................................... 73 Table 6-9 Equivalent annualised cost and effective volume for three hypothetical farm-scale gully dams of 4 GL capacity near the Waterhouse River in the Roper catchment ....................... 73 Table 7-1 Advantages and disadvantages of the three development options ............................. 78 Table 7-2 Reach parameters for nominal conceptual layout of reticulation scheme .................. 81 Table 7-3 Preliminary costs for nominal conceptual layout ......................................................... 82 Table 7-4 Reach parameters for nominal conceptual layout of boosted scheme ....................... 82 Table 7-5 Preliminary costs for nominal conceptual layout ......................................................... 83 Table 7-6 Pipe parameters for nominal conceptual layout of scheme servicing soils downstream of potential dam on upper Flying Fox Creek ................................................................................. 87 Table 7-7 Channel parameters for nominal conceptual layout of scheme servicing soils downstream of potential dam on upper Flying Fox Creek ........................................................... 87 Table 7-8 Details of contributing catchments adjacent to main channel ..................................... 88 Table 7-9 Cost summary ............................................................................................................... 89 Table 7-10 Costs for the pump station and rising main ................................................................ 90 Table 8-1 Summary comments for potential dams in the Roper catchment ............................... 92 Part I Introduction 1 Introduction Current allocations of surface water in the Roper catchment are low relative to the catchment’s median annual discharge (i.e. <1%). Large-scale development of the surface water resources of this highly seasonal catchment to enable regional economic development would generally require rivers to be regulated and water stored. In southern Australia, constructing large reservoirs (Preface Figure 1) has effectively delivered reliable water supplies in a dry and variable temperate climate. The elaborate series of dams and tunnels constructed as part of the Snowy Mountain Hydro-Electric Scheme has enabled watering of much of the irrigated land in the Murray–Darling Basin. A number of commentators have observed that no country or region in a tropical or sub- tropical climate has made significant economic progress without harnessing adequately its water resources (Bisawas, 2012). Large instream dams are not the only methods of storing water. Although Petheram et al. (2014) found that large dams presented the greatest opportunity for enabling broad-scale irrigated agriculture across northern Australia, they also stated that other methods, while capable of supplying far smaller volumes of water than instream dams, may play a role in maximising the cost-effectiveness of water supply. Furthermore, the large, often public, capital expenditure requirements and often unpredictable environmental and social changes associated with large instream dams have led some sectors of the public to question whether they are an appropriate pathway for development (O’Donnell and Hart, 2016; International Rivers, 2014; WCD, 2000). Thus, decisions around river regulation and water storage are complex, and the consequences of decisions are inter-generational – even small inappropriate releases of water may preclude possibly larger and more appropriate developments in the future. The primary purpose of this report is to provide a comprehensive overview of the different surface water storage options available in the Roper catchment to help decision makers take a long-term view of water resource development (i.e. >20 years), which will also inform shorter-term allocation decisions. Having a wide range of reliable information prior to making decisions, including on all the different ways water can be stored, can have long-lasting benefits to government and communities and facilitate an open and transparent debate. The broad types of dams and water storage options likely to be used in northern Australia are described in Section 1.1. The first step in assessing potential water storage options in a catchment is to examine existing water storages. Any available unused water in these storages is likely to be the most cost-effective source of additional water. No large engineered dams exist in the Roper catchment. It is important to note that this surface water storage analysis was of a pre-feasibility nature. The broad steps involved in investigating a large dam are described in Section 1.2. Report objectives The objectives of this report were to: • review all previous studies (published and unpublished) on large dams in the Roper catchment • identify and assess every location in the study area for its potential for the construction of large instream and offstream dams, including an estimate of water yield and modelled cost • undertake a manual cost estimate (for 2 short-listed sites) at a nominated full supply level (FSL) height • undertake a pre-feasibility assessment of the best opportunities for farm-scale instream (i.e. gully and hillside dams) and offstream (i.e. ringtank) storages • identify the more promising surface water storage option in the study area in terms of yield per unit cost and proximity to soil suitable for irrigated agriculture. • present the results in a consistent tabulated format that facilitates site comparisons • provide a conceptual layout and indicative costs for two scheme-scale reticulations systems in more-promising parts of the catchment. Site-specific field investigations of individual farm-scale storage sites were beyond the scope of this pre-feasibility assessment. However, the performance of hypothetical farm-scale storage is discussed, generalised cost estimates are provided and farm-scale storages were modelled using the best available information. Report outline This report is divided into four parts and two appendices. Part I Introduction contains two chapters. Chapter 1 provides introductory material to aspects of large instream and offstream dams and farm-scale dams, key terminology and concepts, and Chapter 2 details the methods undertaken to assess dams in the Assessment area, including the DamSite modelling process. Part II Large instream and offstream dams contains four chapters. Chapter 3 analyses a long-list of dams used to identify potential sites for pre-feasibility analysis. Chapter 4 presents detailed information on two short-listed dam sites in the Roper catchments. Chapter 5 contains general information on regulating structures, such as weirs and sand dams and Chapter 6 examines farm- scale storages in the Roper catchment. Part III Scheme-scale reticulation infrastructure contains Chapter 7 which examines the scope for broad-scale irrigation developments serviced by the two short-listed dam sites. Part IV Summary comments contains the summary comments in Chapter 8 and references in Chapter 9. Appendix A provides detailed costings for the two short-listed potential dam sites in the Roper catchment. Appendix B provides summary tables for the three potential dam sites selected for pre-feasibility analysis but were not short-listed for a more detailed costing. Section outline The remainder of this introductory chapter is structured so as to give well-informed but non- technical readers some of the background information on surface water storage infrastructure needed to understand subsequent technical sections of the report. Large parts of this section are reproduced from Petheram et al. (2017a) as the material is generic and provides good contextual information for the specific analysis of the Roper catchment presented in Part 2. Section 1.1, provides an overview of the different types of large dams and farm-scale water storage infrastructure in the Roper catchment. Section 1.2 outlines the broad steps in the investigation of a large dam site, which provides the context for the additional work needed in order to for a site to be considered ‘shovel ready’. In Section 1.3 a brief overview of dam safety, provides the context for a discussion on who might build different types of dams, which influences their cost. Section 1.4 defines key terminology and concepts used in the report. Introductory information on environmental and cultural heritage considerations and deriving dam axis elevation profiles and reservoir volumes using Shuttle Radar Topographic Mission (SRTM) data is provided in Petheram et al. (2013). Key linkages with other activities of the Roper River Water Resource Assessment This report draws heavily on information and models generated by other activities in the Assessment, in particularly the rivers system model calibration report (Hughes et al., 2023), ecology asset description report (Stratford et al., 2022), digital soil mapping and land suitability report (Thomas et al., 2022), agriculture and socio-economic viability report (Stokes et al., 2023) and the flood mapping and modelling report (Kim et al., 2023). 1.1 Types of water storages The Assessment undertook a pre-feasibility level assessment of four types of surface water storage options. These were (i) large dams that supply water to multiple properties, (ii) farm-scale or on- farm dams that supply water to a single property, (iii) re-regulating structures such as weirs, and (iv) natural water bodies. Although the last does not require construction, their capacity may be enhanced with strategically constructed embankments. Both large dams and farm-scale dams can be further classified as either instream or offstream water storages. In this Assessment, instream water storages are defined as structures that intercept a drainage line (creek or river) and are not supplemented with water from another drainage lines. Offstream water storages are defined as structures that (i) do not intercept a drainage line, or (ii) intercept a drainage line and are supplemented with water extracted from another larger drainage line. Ringtanks and turkey nest tanks are examples of offstream storages with a continuous earth embankment. Large dams, farm-scale dams, offstream storages, re- regulating structures and natural water bodies are briefly discussed below. Large instream dams are usually constructed from earth, rock or concrete materials as a barrier across a river to store water in the reservoir created. They need to be able to safely discharge the largest flood flows likely to enter the reservoir, and the structure must be designed so the dam meets its purpose, generally for at least 100 years. Note, however, that some dams have been in continuous operation for over 1000 years. For example, the Kofini Dam in Greece and the Anfengtang Dam in China are still in operation 3300 and 2600 years, respectively, after their construction (Schnitter, 1994). Schnitter (1994) consequently described dams as ‘the useful pyramids’. The requirements of a good dam have long been established. For example, inscriptions near the Anantharaja Dam in southern India, completed in 1369, detail 12 requirements of a good reservoir (taken from Vadera (1965) in Schnitter (1994)), that could still be claimed as valid today: 1. A king (i.e. owner or client) endowed with righteousness, rich, happy and desirous of acquiring fame 2. A person well versed in hydrology 3. A reservoir bed of hard soil 4. A river conveying sweet water from a distance of about 40 km 5. Two projecting portions of hills in contact with the river 6. Between these projecting portions of hills a dam built of compact stone, not too long but firm 7. The two extremities of the hills to be devoid of fruit-bearing land (i.e. humus) 8. The bed of the reservoir to be extensive and deep 9. A quarry containing straight and long stones 10. Fertile low and level (i.e. irrigable) area in the neighbourhood 11. A watercourse having strong eddies in the mountain region 12. A group of men skilled in the art of dam construction. The inscription also lists six faults that should be avoided: 1. Oozing of water from the dam 2. Saline soil 3. Site at the boundary of two kingdoms 4. High ground in the middle of the reservoir 5. Scanty water supply and an extensive area to be irrigated 6. Too little land to be irrigated and excessive supply of water. An attraction of large dams is that if the reservoir is large enough relative to the demands on the dam (i.e. water supplied for consumptive use, evaporation and seepage), when the reservoir is full water can last two or more years. This has the advantage of providing water during dry seasons and mitigating against years with low inflows to the reservoir. For this reason large dams are sometimes referred to as carry-over storages. An advantage of large instream dams is that they provide a very efficient way of intercepting the flow in a river, effectively trapping all flow until the FSL is reached. For this reason, however, they also provide a very effective barrier to the movement of fish and other species within a river system and can also inundate large areas of land. Two types of dams are particularly suited to northern Australia: embankment dams and concrete gravity dams. In the hazard framework outlined in Section 1.3, large dams fall within Category 3, and it is recommended that their construction be undertaken by an engineering consultant specialising in large dams. Embankment dams Embankment dams are usually the most economical, provided that suitable construction materials can be found locally, and are best suited to smaller catchment areas where the spillway capacity requirement is small. There are two major types of embankment dams: earthfill embankment dams and rockfill embankment dams. Earthfill embankment dams are the most popular dams worldwide (61%) because they can be built on a wide range of foundation conditions. In Australia, however, only 33% of large dams are classified as earthfill, while 40% are rockfill. Like earthfill dams, rockfill dams can be built on a wide range of foundation conditions, but they require less material as they can be built with much steeper sides. They can also be constructed during rain and remain stable even under high seepage conditions. Rockfill embankment dams also have an advantage over earthfill embankment dams in that methods have been devised for reinforcing the downstream rockfill slope to protect it from erosion. Indeed several rockfill dams in Australia have survived overtopping during flood events with minimal damage. Two common types of rockfill dams for which there are examples in northern Australia are shown in Figure 1-1 and Figure 1-2. In the first case the dam has a central earth core within the embankment that provides a watertight barrier to prevent water percolating through the rockfill (e.g. Belmore Creek Dam (officially known as Lake Belmore), Norman catchment). In the second case the seepage barrier is a thin reinforced concrete slab placed on the upstream face of the rockfill (e.g. Corella Dam, Flinders catchment). Figure 1-1 Schematic cross-section diagram of a rockfill embankment dam with a clay core Source: Petheram et al. (2013) For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 1-2 Schematic cross-section of a concrete-faced rockfill dam Source: Petheram et al. (2013) Where sound foundation rock is not available at reasonable depth, an embankment type dam can be founded on a ‘soft’ foundation provided that any permeable layers in the foundation can be cut off effectively and water pressures within the foundation limited, for example by pressure relief wells. Many offstream storage embankment dams are founded on soil foundations where spillway requirements are minimal. Concrete gravity dams In contrast to embankment dams, concrete gravity dams require sound foundation rock because the leakage path through the foundation under the water barrier is short (Kinstler, 2000a). Where a large capacity spillway is needed to discharge flood inflows, a concrete gravity dam with a central overflow spillway is generally the most suitable type. Traditionally, concrete gravity dams were constructed by placing conventional concrete (CC) in formed ‘lifts’. However, Kidston Dam (officially known as the Copperfield River Gorge Dam) in the Gilbert catchment was the first dam in Australia where roller compacted concrete (RCC) was used, with low-cement concrete placed in continuous thin layers from bank to bank and compacted with vibrating rollers. This approach allows large dams to be constructed in a far shorter time frame than is required for CC construction. The use of RCC over CC was estimated to reduce the cost of a concrete gravity dam at Copperfield Gorge by about 40% (Doherty, 1999), and the introduction of this technique resulted in an increase in the proportion of concrete gravity type dams built in Australia. RCC is best used for high dams where a larger-scale plant can provide significant economies of scale. This is now the favoured type of construction in Australia whenever foundation rock is available within reasonable depth, and where a larger capacity spillway is required. Other types of large dams Two other major types of dams are concrete buttress and arch dams. Each can be favourable when concrete is expensive and labour is cheap (i.e. they require more formwork and are of greater complexity), such as occurred during the Great Depression and after World War II. However, in recent decades the high cost of labour has made these types of dams less economical than concrete gravity or embankment dams. Furthermore, arch dams are generally not very For a description of this image please contact Enquires@csiro.au. suitable in Australia due to a lack of suitable topography; they have the greatest benefits over concrete gravity dams where the valley width is narrow and the rock is structurally sound. A note on offstream storages Offstream water storages are not a new concept; they were among the first constructed water storages because people initially lacked the capacity to build structures that could block rivers and withstand large flood events. For example, in the 12th Dynasty of Ancient Egypt, water was diverted from the Nile River into the El Fayyum Depression (Nace, 1972), while one of the largest Mayan cities was constructed around offstream water storages (Scarborough and Gallopin, 1991). In Australia there is evidence that Indigenous people, prior to European settlement, engineered structures in the Roper catchment to divert dry-season baseflow into adjacent wetlands (Barber and Jackson, 2011). Offstream water storages can take the form of farm-scale ringtanks (e.g. 100 to 10,000 ML storage capacity) or large dam structures (>10,000 ML). Figure 1-3 shows an example of an approximately 4000 ML ringtank. The most suitable type of offstream water storage depends on a number of factors, including topography, availability of suitable soils, excavation costs and source of water (e.g. groundwater or surface water pumping, flood harvesting). One of the advantages of offstream storages is that, if properly designed, they can cause less disruption of the natural flow regime than do large instream dams, provided that water is extracted from the river using pumps or there is a diversion structure with raiseable gates that allow water and aquatic species to pass when not in use. However, raiseable gates are typically expensive to operate and maintain, particularly in remote areas, and the structures supporting the gates need to be designed to withstand large flood events, which increases the cost of the diversion structure considerably. Weirs can also be used in conjunction with offstream water storages, whereby the weir is used to raise the upstream water level to allow diversion into an offstream storage or the creation of a pumping pool. However, an often-overlooked aspect of offstream storages is that the amount of water that can be diverted into an offstream storage using a diversion structure in a river depends on the relative difference of the height of water in the river and the height of water in the storage. Water must run downhill from the point of diversion to the storage location. To achieve adequate flow rates in the diversion channel, the diversion structure has to be sufficiently high to generate the required head of water. This is particularly the case in northern Australia where river water levels rise and fall very rapidly (Petheram et al., 2008) and there is little time for extraction or diversion. Kim et al. (2013) provide an example of a ‘hydraulic’ analysis for an offstream storage and diversion structure in the Flinders catchment. Because the risk of failure of offstream storages due to overtopping is typically lower than for an instream dam, it is more feasible that a regionally based contractor overseen by a regionally based engineer (i.e. suitable to build a Category 2 dam) could construct a large offstream storage than a large instream dam. Figure 1-3 Rectangular ringtank in the Flinders catchment Photo: CSIRO 1.1.1 Farm-scale dams and water storages Farm-scale dams, also referred to as on-farm dams, are typically used to supply water for stock and domestic purposes or for mosaics of small-scale irrigation supplying the one property. They can take the form of gully and hillside dams, ringtanks, turkey nest tanks and excavated tanks. A summary of the different types of on-farm dams and indicative storage-to-excavation ratios is provided in Table 1-1. These structures are evaluated in Chapter 6. Table 1-1 Types of farm-scale water storages TYPE OF ON-FARM DAM DESCRIPTION STORAGE-TO-EXCAVATION RATIO Gully dam Earth embankment built across a drainage line. Dams are normally built from material located in the storage area upstream of dam site. Gully dams can also be used in conjunction with offstream water storages, where the weir is used to raise the upstream water level to allow diversion into offstream storage or the creation of a pumping pool. 10:1 (favourable conditions) Hillside dam An earth dam located on a hillside or slope and not in a defined depression or drainage line. 5:1 (on flatter terrain) 1:1 (on steeper slopes) Ringtank A storage confined entirely within a continuous embankment built from material obtained within the storage basin. 1.5:1 (small tank) 4.5:1 (large tank) 10:1 (very large tank) Turkey nest tanks A storage confined entirely within a continuous embankment but built from material borrowed from outside the storage area. All water is therefore held above ground level. Usually smaller than ringtanks and lower storage-to-excavation ratio Excavated tanks Restricted to flat sites and comprise excavations below the natural surface. Excavated material is wasted. Generally limited to stock and domestic use and irrigation of high-value crops. Low storage-to-excavation ratio Source: Adapted from Lewis (2002). For more information on this figure please contact CSIRO on enquiries@csiro.au 1.1.2 Re-regulating structures Re-regulating structures such as weirs differ from dams in that they are lower barriers located entirely within stream banks and are totally overtopped during flood events. Weirs are typically used as re-regulating structures downstream of large dams to allow for more efficient releases from the storages and for some additional water yield from the weir storage itself, thereby reducing the transmission losses normally involved in supplemented river systems. Re-regulating structures can range from concrete gravity weirs to sheet piling weirs to simple sand dams – mounds of river sand within the bed of a river to create a pumping pool. These types of structures are discussed in more detail in Chapter 7. 1.2 Stages of investigation in design, costing and construction of dams The investigation of a potential dam site involves an iterative process of increasingly detailed studies, sometimes occurring over as few as 2 or 3 years but often over 10 or more years. It is not unusual for the cost of the geotechnical investigations for a dam site alone to exceed several million dollars. Given the high costs and time involved and the likelihood of many potential dam sites in a catchment, an important stage of developing the surface water resources of a catchment is a pre-feasibility assessment. The pre-feasibility assessment is the first of five stages of a dam project. Fell et al. (2015) outlined these five stages as pre-feasibility, feasibility and site selection, design and specification, construction, and operation. The pre-feasibility stage, including this Assessment, typically involves a detailed desktop investigation and site visit to acquire significant information for numerous dam sites in an area, including determining whether the: • topography favours the creation of a large storage volume by a dam of a height and length likely to be economically viable • regional and local geology are likely to impose constraints or additional cost to construction • streamflow characteristics are appropriate for a storage to meet the forecast demand • dam site location is in the vicinity of the forecast demand for water and soils suitable for irrigation • storage would affect existing land uses, existing infrastructure or environmental, social or cultural values • impacts are likely to be acceptable to investors and other stakeholders. This is particularly relevant to the Darwin catchments which have a large amount of infrastructure development relative to other parts of northern Australia. The geological assessment should include a visit to each site by an experienced infrastructure geologist. The likelihood of dam sites being suitable for future detailed evaluation can often be determined from a preliminary assessment of available information including maps, geology and streamflow data, and particularly from site inspections. An initial desktop assessment of the impacts of a storage development on existing land uses, existing infrastructure and environmental values may indicate at an early stage whether the impacts are likely to be acceptable to investors or other stakeholders. More-promising potential dam sites may have been the subject of earlier investigations, in which case the available study reports can be particularly useful in any reassessment. A pre-feasibility analysis commonly short-lists the better sites for a more detailed desktop analysis, including more time-demanding analyses such as preliminary flood design assessment (e.g. to assess the additional height above the FSL (or freeboard), which can significantly affect dam cost). One such preliminary assessment was recently undertaken in northern Australia in the Flinders and Gilbert catchments by Petheram et al. (2013). This process makes it possible to confidently select the most appropriate dam sites on which to undertake more detailed and costly ground- based investigations. To progress a dam proposal from a desktop assessment to the commencement of construction requires a series of comprehensive and often iterative studies. These include: • detailed topographic surveys • detailed hydrological studies calculating the reservoir yield and reliability and the magnitude of flood inflows that could be experienced during the period of construction and operation of the dam • geotechnical studies, including geological mapping of the site and inundated area, seismic surveys, trenching and drilling to assess foundation conditions for each of the proposed structural elements and to assess potential sources of construction materials. Geotechnical assessments are required at all five stages of a dam project (Fell et al., 2015) • engineering studies of dam type and layout, including for the main cross-river wall, any necessary saddle dams, spillways and outlet works as well as provisions required to address impacts, particularly in the storage area • impact assessment studies including environmental, social and cultural heritage impacts and the development of strategies to avoid or manage impacts • consideration of needs and costs for processing, transport and marketing of the products of irrigated agriculture • economic and financial studies that compare estimated costs and benefits and which develop proposals for funding the construction and operation of the works including the water supply charges proposed. Ultimately the studies need to acquire the necessary level of detail and certainty to obtain the required approvals. The final step should consider how implementation of the project should proceed, including institutional arrangements for construction and ongoing operation and maintenance of the scheme, for the entire operational life of the dam. 1.3 Dam safety and models of dam labour construction The Australian National Committee on Large Dams (ANCOLD) is an incorporated voluntary association of organisations and individual professionals with an interest in dams in Australia. It organises technical working groups and issues guidelines on topics related to dams. The ANCOLD dam consequence categories (ANCOLD, 2012) define seven hazard groups (Very low, Low, Significant, High C, High B, High A and Extreme), where the higher the hazard category assigned to a dam the more work is required to ensure the risk to downstream communities is mitigated to an acceptable level. These seven hazard categories can be grouped into three categories that broadly reflect the amount of work required for the operation of a safe dam: • Category 1. Dams in the ‘Very low’ or ‘Low’ hazard category. Depending upon the jurisdiction and the dimensions of the structure and reservoir, these dams may require a permit to undertake dam works, but in general, these require the least amount of detail to satisfy statutory planning, construction, maintenance and reporting requirements. • Category 2. Dams that fall into a hazard category of ‘Significant’ and ‘High C’, or all dams that are over 10 m but less than 25 m in height and not in Category 3. These require considerably more detail to satisfy planning, construction, maintenance and reporting requirements than do dams in Category 1. • Category 3. Dams that are at the higher end of the hazard category scale. These dams include ‘High B’, ‘High A’ and ‘Extreme’ hazard dams and dams that are over 25 m in height. These generally require the services of an engineering consultancy service specialising in the construction of large dams, require considerable planning and approval, and usually require an environmental impact statement and approval under the Commonwealth’s Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). In the context of the Assessment, these three categories are useful for helping to broadly categorise dam labour construction models, the capital costs and ongoing operation and maintenance costs. Dam labour construction models and implications to cost Most on-farm dams fall into Category 1, but larger farm-scale dams may be Category 2. It is technically feasible for farm dams that fall into Category 1 to be constructed by the landholder where they own their own plant or can purchase new or second-hand plant and compaction equipment, and the structure satisfies jurisdictional requirements. There are no jurisdictional regulations covering construction of farm dams in the NT. Construction by landholders using their own plant and equipment may often be a cheaper form of surface water storage construction because: • landholders often employ lower design standards, preferring intermittent annual costs associated with maintaining a structure rather than high upfront costs (e.g. preferring to repair batter slopes with their own equipment as needed rather than use rock protection) • contractor and other project overheads (e.g. ensuring access and operation of the structure comply with health and safety regulations) are substantially lower than if a regionally based contractor and engineer were used • the cost of maintenance and repair of machinery and the opportunity cost of the landholder undertaking their own design, survey and project management of the structure are rarely considered by landholders. Although farm dams constructed by a landholder may in some circumstances be cheaper than those using a regionally based contractor and engineer, the dam’s service life is typically lower and the ongoing maintenance costs and risk of failure are typically higher. For example, ANCOLD (1992) report that a study in NSW found a 23% failure rate for farm dams in that state. Dams that fall within Category 2 could feasibly be constructed by a regionally based contractor, with investigation and design being undertaken by a regionally based engineering consultant. Under this model of construction, the upfront cost of constructing the dam would be higher than for a Category 1 dam but probably less than for a Category 3 dam. Category 2 dams are generally cheaper than Category 3 dams because they require less technical design and investigation and typically have lower contractor overheads, such as project risk and site accommodation, because of the smaller scale and complexity of the operation. Larger dams such as Darwin River Dam and Manton Dam in the Darwin catchments are Category 3 dams. These dams are the domain of professional engineering companies that specialise in the construction of large dams. Costs are usually high because investigation costs are expensive and the structures are generally designed and constructed to a very high standard, usually with at least a 100-year service life. Contingency is typically high because these structures carry the highest risk as considerable subsurface works are required. 1.4 Key terminology and concepts 1.4.1 Dam terminology In this report the word ‘dam’ refers to the structure including the dam wall, primary and secondary spillways and outlet structures. ‘Reservoir’ is reserved for the water body upstream of the dam wall. Dam volume is defined here as the volume of material in the dam wall, and the reservoir capacity is the volume of the reservoir at FSL. The total freeboard is the sum of the wet freeboard and the dry freeboard, where the wet freeboard is the height above the FSL below which a design flood event may pass (also referred to as flood surge) and the dry freeboard is the height above the wet freeboard to account for wind-generated waves overtopping the structure. 1.4.2 Water yield Yield is the amount of water that can be released in a controlled manner from a reservoir system. Yield values are accompanied by a reliability value where, for all other factors held constant, increasing the reliability decreases the yield. Other terms used synonymously with yield are release, draft and regulation. In this report all yield and reliability values are expressed in terms of annual time reliability, which is calculated as per Equation 1. 𝑅𝑅𝑡𝑡=𝑁𝑁𝑆𝑆 𝑁𝑁 (1) Where 𝑅𝑅𝑡𝑡 is the time-based reliability, 𝑁𝑁𝑆𝑆 is the total number of intervals during which the demand was met; and 𝑁𝑁 is the total number of time intervals in the simulation. For annual time reliability 𝑁𝑁𝑆𝑆 becomes the number of successful years and 𝑁𝑁 becomes the number of years in the simulation period. 1.4.3 Water year and wet and dry seasons Northern Australia has a highly seasonal climate, with most rain falling from December to March. Unless specified otherwise, this Assessment defines the wet season as the 6-month period from 1 November to 30 April and the dry season as the 6-month period from 1 May to 31 October. All results in the Assessment are reported over the water year, defined as the period 1 September to 31 August, unless specified otherwise. This allows each individual wet season to be counted in a single 12-month period, rather than being split over two calendar years (i.e. counted as two separate seasons). This is more realistic for reporting climate statistics from a hydrological and agricultural assessment viewpoint. 1.4.4 Scenario definitions Four scenarios are presented in this report: • Scenario A – historical climate and current development • Scenario B – historical climate and hypothetical future water resource development • Scenario C – future climate and current development • Scenario D – future climate and hypothetical future water resource development. Scenario A Scenario A is historical climate and current development. The historical climate series is defined as the observed climate (rainfall, temperature and potential evaporation for water years from 1 September 1910 to 31 August 2019). All results presented in this report are calculated over this period unless specified otherwise. The current level of surface water, groundwater and economic development were assumed (as at 2021). Scenario A was used as the baseline against which assessments of relative change were made. Historical tidal data were used to specify downstream boundary conditions for the flood modelling. Historical climate data were sourced from the scientific information for landowners (SILO) data drill database, http://www.longpaddock.qld.gov.au/silo/ (Jeffrey et al., 2001). SILO provides surfaces of daily climate data interpolated and infilled from point measurements made by the observation network developed and maintained by the Bureau of Meteorology. Scenario B Scenario B is historical climate and future development, as generated in the Assessment. Scenario B used the same historical climate series as Scenario A. River inflow, groundwater recharge and flow, and agricultural productivity were modified to reflect potential future development. All price and cost information was indexed to mid-2021. The impacts of changes in flow due to this future development were assessed, including impacts on: • instream, floodplain and near-shore ecology • Indigenous water values • economic costs and benefits • opportunity costs of expanding irrigation • institutional, economic and social considerations that may impede or enable adoption of irrigated agriculture. Scenario C Scenario C is future climate and current levels of surface water and ground development assessed at ~2060. It will be based on the 109-year climate series (as in Scenario A) derived from GCM projections for an approximate 1.6°C global temperature rise (~2060) relative to the 1990 scenario, representing Shared Socioeconomic Pathway, SSP2-4.5. The GCM projections will be used to modify the observed historical daily climate sequences. Scenario D Scenario D is future climate and future development. It used the same future climate series as Scenario C. River inflow, groundwater recharge and flow, and agricultural productivity were modified to reflect potential future development, as in Scenario B. Therefore, in this report, the climate data for Scenarios A and B are the same (historical observations from 1 September 1890 to 31 August 2019) and the climate data for Scenarios C and D are the same (the above historical data scaled to reflect a plausible range of future climates). 1.4.5 Reporting DEM-H elevation and height data Elevation data are fundamental to assessing dam design and evaluating water storage capacities. For the Roper catchment, the national 1 second hydrological digital elevation model (DEM-H) (~30 m horizontal grid), derived from the Shuttle Radar Topographic Mission (SRTM) data (Gallant et al., 2011), is the finest resolution digital elevation dataset available. This dataset covers the entire continent and constitutes the best available data over most of Australia, particularly northern Australia. The SRTM is based on the Earth Gravitational Model 1996 (EGM96) geoid, for which there is a vertical datum difference with the Australian Height Datum (AHD). The difference between the datum of the two elevation datasets is poorly defined due to the lack of a well-defined AHD surface across the Australian continent; however, it is generally less than 1 m. For this reason, all heights and elevations derived using the DEM-H are reported here as EGM96 geoid height in metres (mEGM96). It is important to understand the strengths and weaknesses of any elevation dataset used, and the reader is referred to Petheram et al. (2013) for a brief discussion of the DEM-H. 2 Methods 2.1 Large instream and offstream dams The first phase of the investigation into large dams involved (i) reviewing reports describing all large dam proposals that had been the subject of earlier or current investigations, and (ii) running the DamSite model to ensure no potential dam options had been overlooked. These two activities were undertaken concurrently and are described in Section 2.1.1 and Section 2.1.2, respectively. Based on the review of existing literature and the DamSite model results, Section 2.1.3 lists the sites selected for pre-feasibility analysis and Section 2.1.4 summarises the methods for which each potential dam site was assessed and outlines the method by which potential dam sites were selected for pre-feasibility analysis. 2.1.1 Review of past literature No previous studies examining dam sites in the Roper catchment have been identified. 2.1.2 DamSite modelling To ensure that potential dam sites across the Roper catchment were objectively and consistently assessed, the DamSite model (Read et al., 2012; Petheram et al., 2017b) was applied across the entire study area. This model is a series of algorithms that automatically determines favourable locations in the landscape as sites for intermediate-to-large water storages and has been previously applied successfully to the Flinders and Gilbert catchments (Petheram et al., 2013). Broadly, the approach involved calculating the potential dam and reservoir dimensions of every location in the Roper catchment at 1 m height increments, constructing saddle dams as required, and using the DEM-H, the best freely available DEM across northern Australia. The DamSite model then calculated a ‘preliminary yield’ (85% annual time reliability) at the dam wall using the computationally efficient Gould–Dincer Gamma (GDG) method (McMahon and Adeloye, 2005; Petheram et al., 2008) for calculating the water yield of carry-over storages (i.e. large dams where water can be carried over from one year to the next) and a within-year storage yield method (Petheram et al., 2017b). This was done for each 1 m increment dam height at each site, and for over 100 heights at some sites. For each height increment, the ‘preliminary yield’ was selected from the larger of the GDG yield and within-year yield estimates. At each site and for each 1 m increment dam height, the model calculated an approximate unit cost for a dam structure based on the vegetation-corrected Advanced Land Observing Satellite (ALOS) DEM elevation profile along the dam and saddle dam axis and type, and the quantity of material required and their unit cost rates. The cost algorithm effectively applies a penalty to higher and longer dam wall structures. More detail on the DamSite model is provided by Read et al. (2012), Petheram et al. (2013) and Petheram et al. (2017b). Since these publications, however, the DamSite model dam cost algorithm has been substantially revised. The new cost algorithm is described in Petheram et al. (2017a). DamSite model parameters as applied to the Roper catchment The DamSite model as applied to the Roper catchment for assessing large dams was parameterised as follows: • The minimum catchment area assessed was 2 km2. • The minimum wall height was 6 m. • Gridded runoff data were sourced from the Australian Water Resources Assessment landscape (AWRA-L) model runs for the catchment (see companion technical report on river model calibration (Hughes et al., 2023)). The results of the DamSite model were summarised to identify the most cost-effective potential dam sites. This was done by presenting the results in terms of: • the ratio of reservoir capacity (i.e. reservoir volume at FSL) to cost of dam construction, also referred to as maximum volume per unit cost (VpUCmax). This measure is useful for identifying parts of the landscape that are particularly topographically suitable for large offstream storages • the ratio of reservoir yield to cost of dam construction, also referred to as maximum yield per unit cost (YpUCmax). This measure is particularly useful for identifying sites that are topographically and hydrologically suitable for the construction of large instream dams. The results of the DamSite analysis in the Roper catchment is presented in Section 3.1. Note that the DamSite model was also used to assess on-farm dams using a smaller catchment area and wall height. The method and parameters used to assess on-farm dams are detailed in Section 2.2.2. 2.1.3 Potential dam site selection Based on the DamSite modelling, a ‘long list’ of approximately 20 potential dam sites geographically spread across the Roper catchment were selected for an initial cursory desktop geological evaluation. The sites were selected on the basis of having a relatively favourable ratio of reservoir yield to unit cost as calculated by the DamSite model. The geological desktop evaluation was undertaken using satellite imagery and 1:250,000 geological mapping data. Based on the initial desktop geological evaluation, a smaller number of dam sites were selected for an evaluation of water yield based on inflows from the Roper River system model (Hughes et al., 2023) and a revised modelled cost using the ALOS DEM to extract the dam site axis elevation profile (as opposed to the DamSite model costing, which was based on the SRTM-H). Based on these data five sites were selected for pre-feasibility analysis. For any of these options to advance to construction, far more comprehensive studies would be required, as outlined in Section 1.2. Studies of that level of detail were beyond the scope of this pre-feasibility level Assessment. 2.1.4 Summary of criteria used to assess large dams as part of pre-feasibility analysis and methods used to select sites Five of the more favourable potential dam sites (in terms of topography of the dam axis, geological conditions, proximity to suitable soils, general geographic location and water yield) in different geographic regions of the catchment were selected for pre-feasibility analysis. To facilitate comparison of different sites, each site was assessed and reported against a standard set of 19 criteria listed in Table 2-1. This table summarises the methods by which the criteria were investigated and reported. Two of the more favourable sites selected for pre-feasibility analysis were short-listed (in terms of topography of the dam axis, geological conditions, proximity to suitable soils, general geographic location and water yield) for a manual detailed dam cost estimate for a nominal FSL. The tables summarising the characteristics of the two short-listed sites in Chapter 4 present the assessment criteria in the same structure as shown here. While these sites represent some of the more promising large instream and offstream dams in the Roper catchment, other sites may be more favourable depending upon the location and nature of the demand. Table 2-1 Criteria used to assess potential dam sites PARAMETER DESCRIPTION Previous investigations Based on web-based searches and searches of NT Government databases, no literature on past dam studies in the Roper catchment were identified. Description of potential dam configuration An overview of a dam configuration based on recent data, methods and contemporary thinking. Regional geology The regional geology for each dam site was assessed using satellite imagery and the NT 1:250,000 geology series. Site geology The site geology for each dam site was assessed using the NT 1:250,000 geology series, and some sites where permission was granted were visited by the Assessment geologist. Reservoir rim stability and leakage potential These parameters were assessed by overlaying inundated area at the selected FSL and 1% AEP flood event on satellite imagery and 1:250,000 geology data, and some sites were inspected from helicopter by the Assessment geologist. Potential structural arrangement Potential conceptual arrangements were developed by the Assessment’s water infrastructure planner, based on contemporary dam design concepts and thinking. Availability of construction materials Based on 1:250,000 geology data and proximity to known quarry locations. Catchment area Catchment areas were derived from DEM-H. Flow data Simulated streamflow metrics were calculated using output from the AWRA-R (river system) models produced by the Assessment (see river modelling companion technical report (Hughes et al., 2023)). Storage capacity Storage capacity was derived from the DEM-H, unless stated otherwise. For potential dams the dead storage volume was assumed to occur at 5 m above the river bed (typically 1 to 2% of the reservoir capacity at FSL). PARAMETER DESCRIPTION Reservoir yield assessment at dam wall A behaviour analysis (BHA) model (McMahon and Adeloye, 2005) was used to assess the relationship between yield, reliability and storage volume under Scenario B (historical daily climate data, potential dam development) for a range of dam wall heights (i.e. yield was assessed at 1 m height increments from 5 m above river bed to a maximum height beyond which a dam would not be feasible) and a perennial crop demand pattern (Figure 2-1) using the baseline river model (Hughes et al., 2023). Inflows to the BHA model were generated by the locally calibrated AWRA-R (river system) model and the AWRA-L (landscape) model (see companion technical report on river model calibration (Hughes et al., 2023)). Table 2-2 lists the AWRA-R nodes from which simulated streamflow data were extracted for input into the behaviour analysis model. The perennial crop demand pattern was based on the Agricultural Production Systems sIMulator (APSIM) (Keating et al., 2003) simulations using the sugarcane module (see companion technical report on agriculture (Stokes et al., 2023)). The performance of each reservoir was reported in terms of the annual time reliability and the volumetric reliability (McMahon and Adeloye, 2005). These performance criteria are sensitive to particular aspects of unsatisfactory operation during periods of low reservoir inflows. The inability of a reservoir or system of reservoirs to provide the target demand during a given period is commonly described as a supply failure. Potential use of supply Based on soil and land suitability information compiled by the Assessment (see companion technical report on digital soil mapping and land suitability for the Roper catchment, Thomas et al. (2022)). Estimated rates of reservoir sedimentation Sedimentation rates were calculated using estimated sediment yields and the FSL dam capacity for each site. Sediment yields were calculated using an empirical relationship derived from ten sediment yield studies across northern Australia (Tomkins, 2013). Rates of reservoir sedimentation are presented for 30 and 100 years and the number of years taken to 100% infill. Minimum (best-case), expected and maximum (worst-case) estimates are provided. Storage impacts Based on review of past studies, satellite imagery, GIS overlays and site visit. Environmental considerations Mapped data on the ecological assets and the fish species distribution for the Roper catchment were sourced from the companion technical report on aquatic ecology (Stratford et al., 2022) and complemented with datasets of endangered vertebrate and plant species at a national (Species and Ecological Communities of National Significance) and territory level. It should be noted that due to a range of factors, records of species are sparse across this catchment. Key datasets for the assessments were: • Atlas of Living Australia (Atlas of Living Australia, 2022) • Fish Atlas Database • CDU University- Fish dataset • NT Government Fauna Atlas (2019) • Species of National Environmental Significance Database (2019) • Ecological Communities of National Environmental Significance (public grids) Database 10 km Grids (Australian Government Department of the Environment, 2016) • Wildnet wildlife records (2021) • Department of Environment Parks and Water Security (2019) • OBIS (2022) • Directory of Important Wetlands in Australia (DIWA) Spatial Database (Public) (2018) • Important Areas for Birds (Birdlife International, year) • Ramsar Wetlands of Australia (2015) • Threatened animals and plants (Northern Territory Government, 2021). Barrier to movement of aquatic species The likely impacts of a dam wall in terms of creating a barrier to the movement of aquatic species was assessed by examining the distribution of species within the catchments and the relative position of the dam within the catchment. Ecological implications of inundation PARAMETER DESCRIPTION The ecological implications of inundation were assessed in terms of potential habitat loss for the local biodiversity. This was done intersecting the species datasets against the catchment boundary and the inundation region. Of special interest were species listed as ‘of concern’, ‘vulnerable’, ‘endangered’, ‘critically endangered’ or ‘migratory’ in national (EPBC), state or territory (NT, WA and Queensland) and/or international level (in particular, migratory birds). Water quality and stratification considerations No specific assessment of water quality and stratification was made as part of the Assessment. It has been assumed that selective withdrawal baulks will be provided in the intake works so that best quality water can be drawn from the storage when releases are made. Indigenous land tenure, native title and cultural heritage considerations No site-specific evaluation of cultural heritage considerations was possible as pre-existing Indigenous cultural heritage site records were not made available to the Assessment from the NT Government. Land tenure and native title information were derived from regional land councils and the National Native Title Tribunal. Estimated cost For the two short-listed potential dam sites, cost estimates were calculated manually by the Assessment’s water infrastructure planner. This was done by developing conceptual arrangements for each of the storages. Dam and saddle dam profile axes were calculated using the ALOS DEM. Unit cost rates applied for each item of work were originally derived from earlier estimates for studies the proposed Green Hills and Connors River dams and the existing Wyaralong Dam and then using more recent estimates from Hells Gates and Palmer River dam studies. The uncertainty in cost associated with the quantity of material for short- listed sites was estimated to be between −20% and +50%. However, if non-trivial geological issues were identified as part of a feasibility analysis or during dam construction then the final cost of construction could be increased by considerably more than 30%. For those three pre-feasibility dams that were not short-listed, modelled dam costs were obtained using the cost algorithm in the DamSite model (see Section 2.1.2) using a dam axis elevation profile derived from the ALOS DEM. The uncertainty in cost associated with the quantity of material estimated by the DamSite model is estimated to be between −25% and +75%. However, if non-trivial geological issues were identified as part of a feasibility analysis or during dam construction then the final cost of construction could be increased by considerably more than 50%. Estimated cost / ML of supply Estimated capital cost divided by the water yield at 85% reliability as computed by the Assessment under the nominated structural arrangement. Summary comment As provided by Assessment personnel. Table 2-2 AWRA-R nodes from which simulated streamflow data were extracted for use in the behaviour analysis model at the five potential dam sites selected for pre-feasibility analysis ROPER POTENTIAL DAM SITES NODE IDENTIFIER Wilton River at ATMD 33 km 90301460 Waterhouse River at ATMD 70.5 km 90300890 Flying Fox Creek at ATMD 105 km 90301080 Waterhouse River West Branch 90300890 Jalboi River at ATMD 53 km 90302505 Figure 2-1 Constant monthly demand pattern used in the BHA modelling for the Roper catchment 2.2 Farm-scale storages Because large farm-scale water storages are typically no more than several gigalitres in capacity and are constructed to serve one farm or paddock/location, they could feasibly occur at multiple locations within a landscape. For a catchment-scale investigation of farm-scale water storages, it was not feasible to visit and assess the many hundreds of possible locations. Rather, the Assessment used a broad-scale analysis to identify areas with the greatest (and least) potential for farm-scale storages, to help focus on-ground assessments and guide policy and planning decisions related to farm-scale storages. The high-level catchment-scale investigations analysed: • earth embankment offstream storage suitability • locations most suitable for gravity drainage • topographic and hydrological characteristics of suitable locations for gully dams. The methods employed in these investigations are briefly discussed in turn below. 2.2.1 Identification of areas suitable for farm-scale offstream storages The suitability of landscapes and soil for the construction of earth embankment farm dams (both ringtanks and gully or hillside dams) was assessed using 30 m by 30 m gridded data of selected soil attributes generated for the Roper catchment (see companion technical report on digital soil mapping attributes (Thomas et al., 2022)). These gridded datasets were generated using a relatively new approach called digital soil mapping, which makes use of advances in computing and statistics. Digital soil mapping allows soil properties (variables), such as clay content, sampled at specific locations, to be related to an expanding Australian database of national covariates. Covariates, which are GIS-format datasets, are selected because they directly correlate to landscape and soil properties. Examples of covariates are slope, correlating to soil depth, and rainfall deficit, correlating to leaching intensity and pH. Digital soil mapping (i) enables discovery of relationships at the geographic intersection of the sampled variable (e.g. pH) and multiple ‘stacked’ covariate datasets, (ii) builds statistical models from these relationships, and then (iii) applies the models to predict (map) the variable values at all other unsampled locations in the Assessment area from the covariates (McBratney et al., 2003). Unlike traditional soil mapping used to map soil types, digital For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 0.000.040.080.120.16JFMAMJJASONDFraction of annual demandMonth soil mapping produces maps of individual soil properties (e.g. pH or permeability). As a result, the approach is especially suited to land suitability assessment. A particular strength of digital soil mapping methods over the traditional mapping methods is that the former produces spatial statistical measures of the quality of the mapped parameter that can be readily displayed. The assessment of the suitability of earth embankment structures across the study area was undertaken on a grid cell by grid cell basis and by examining all possible combinations of four gridded soil parameters. The four parameters and the categories, listed from least to most favourable, used for each parameter are: • clay content – 0 to 10%, 10 to 25%, 25 to 35%, 35 to 50% and greater than 50% • permeability – rapid, moderate, slow and very slow NCST (2009) • soil depth – <1 m, 1 to 1.5 m and >1.5 m • slope – >5%, 2 to 5%, 1 to 2% and <1%. Those grid cells characterised as being most suitable for the construction of farm-scale earth embankment structures were assigned a suitability score of 1, and those least suitable were assigned a 4 (Table 2-3). A subset of rules to illustrate the concept is provided in Table 2-4. Table 2-3 Suitability scores for the construction of farm-scale earth embankment structures SUITABILITY SCORE DESCRIPTION 1 Likely to be suitable 2 Possibly suitable 3 Unlikely to be suitable 4 Not suitable Table 2-4 Subset of rules used to assess suitability of land for construction of farm-scale earth embankment structures CLAY CONTENT PERMEABILITY SOIL DEPTH SLOPE SUITABILITY SCORE 25 to 35% Slow 1 to 1.5 m <1% 2 25 to 35% Slow 1 to 1.5 m 1 to 2% 2 25 to 35% Slow 1 to 1.5 m 2 to 5% 3 25 to 35% Slow 1 to 1.5 m >5% 4 25 to 35% Moderate 1 to 1.5 m <1% 3 25 to 35% Moderate 1 to 1.5 m 1 to 2% 3 25 to 35% Moderate 1 to 1.5 m 2 to 5% 3 25 to 35% Moderate 1 to 1.5 m >5% 4 Irrespective of the values of the other parameters, a grid cell was assigned a suitability score of 4 if at that location the: • soil depth was less than 1 m • slope was greater than 5% • permeability was rapidly draining, or • clay content was between 0 and 10%. In total there were 240 possible permutations of the clay content, permeability, soil depth and slope classes. Eight of these permutations resulted in a suitability score of 1, 19 permutations resulted in a suitability score of 2, 41 permutations resulted in a suitability score of 3, and 172 permutations resulted in a suitability score of 4. 2.2.2 Topographic and hydrological analysis of suitable locations for gully and hillside dams The topographic and hydrological potential for gully and hillside dams across the study area was assessed using the DamSite model. As outlined in Section 2.1.2 for large instream and offstream dams, the DamSite model requires hydro-climate data (i.e. runoff, rainfall and evaporation), a DEM and an algorithm for costing the structure. The DamSite model was run using 85% gridded annual exceedance runoff datasets generated by the Assessment for the study area (see companion technical report on river model calibration (Hughes et al., 2023)), and net evaporative losses were calculated by multiplying the reservoir surface area at 0.7 capacity by the median net evaporation between March and August (inclusive) for the Roper catchment. Seepage losses were estimated to be 2 mm/day over the reservoir surface area at 0.7 capacity. Every SRTM grid cell location with a catchment area greater than 1 km2 and less than 40 km2 was assessed for its potential as a farm-scale dam by constructing earth embankment structures at 1 m height intervals between 5 and 20 m in height, including freeboard. Dam wall heights of less than 5 m were not examined in this analysis because the uncertainty in the DEM-H elevations were deemed to be too large relative to the height of the dam and capacity of the reservoir. Dam walls were constructed assuming a 3:1 (horizontal to vertical) ratio on the upstream face and a 2.5:1 ratio on the downstream face with a crest width of the square root of the height +1. The results are reported in terms of GL per 1000 m3 of earth moved. The dry freeboard was a function of the reservoir surface area plus 0.5 m wet freeboard. These values are broadly in line with the recommendations in the farm water supplies design manual (QWRC, 1984). Part II Large instream and offstream dams 3 Opportunity analysis of potential dam sites The opportunity analysis of potential dam sites in the Roper catchment involved a three-tier process to select the short-listed sites. 3.1 DamSite modelling To ensure that no options had been overlooked, the DamSite model (see Section 2.1.2) was used to undertake a preliminary assessment of over 50 million potential dam sites in the Roper catchment. A desktop geological suitability assessment of the results of the DamSite model was undertaken by overlaying the dam locations on 1:250,000 geology data (see Section 2.1.3). The DamSite model results were then ranked using different criteria, and the locations compared likely arable land. 3.1.1 Large dams for irrigation and water supply in the Roper catchment Large offstream storages in the Roper catchment Figure 3-1 displays the most promising sites across the Roper catchment in terms of storage volume (GL) per million dollars of construction cost. Only locations with a ratio of storage to cost less than $5000/ML are shown. This provides a simple way of displaying locations in the Roper catchment with the most favourable topography for a large reservoir relative to the size (i.e. cost) of the dam wall necessary to create the reservoir. This figure is particularly useful for identifying more-promising sites for ‘offstream’ storage (i.e. where some or all of the water is pumped into the reservoir from an adjacent drainage line). The threshold value of $5000/ML is nominal and is used to minimise the amount of data displayed. Note that this analysis does not consider evaporation or hydrology. Figure 3-1 shows that the parts of the Roper catchment with the most favourable topography for storing water are along the Wilton and lower Roper rivers and major tributaries either side of the Roper River and Waterhouse River in the headwaters of the Roper catchment. Figure 3-1 Potential storage sites in the Roper catchment based on minimum cost per ML storage capacity This figure can be used to identify locations where topography is suitable for large offstream storages. At each location the minimum cost per ML storage capacity is displayed. The smaller the minimum cost per ML storage capacity ($/ML) the more suitable the site for a large offstream storage. Analysis does not take into consideration geological considerations, hydrology or proximity to water. Only sites with a minimum cost to storage volume ratio of less than $5000/ML are shown. $1000/ML is equivalent to 1 GL per million dollars. Costs are based on unit rates and quantity of material and site establishment for a roller compacted concrete (RCC) dam. Data are underlain by a shaded relief map. Inset displays height of full supply level (FSL) at the minimum cost per ML storage capacity. Large instream storages in the Roper catchment In addition to suitable topography (and geology), instream dams require sufficient inflows to meet a potential demand. Potential dams that command smaller catchments with lower runoff have smaller yields. Results relating to this criterion can be summarised and conveniently presented in terms of cost of constructing the dam per ML of yield. This is very similar to the cost of constructing a dam per ML of storage volume described above. The DamSite model was initially run using a preliminary storage-yield-reliability calculation method, the GDG method (see Section 2.1.2), which is very rapid to apply. Only the top 10,000 sites for the Roper catchment ranked in terms of the cost per ML GDG yield was the yield recalculated using the more numerically For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au intensive behaviour analysis. Figure 3-2 only shows those sites with a cost of $5000/ML. The DamSite modelling indicates that the most cost-effective potential dam sites are on the Wilton River and lower Roper and Hodgson rivers. Figure 3-2 Potential storage sites in the Roper catchment based on minimum cost per ML yield at the dam wall This figure indicates those sites more suitable for major dams in terms of cost per ML yield at the dam wall in 85% of years overlain on versatile land surface (see companion technical report on land suitability, Thomas et al., 2022). At each location the minimum cost per ML storage capacity is displayed. The smaller the cost per ML yield ($/ML) the more favourable the site for a large instream dam. Only sites with a minimum cost to yield ratio less than $10,000/ML are shown. Costs are based on unit rates and quantity of material required for a roller compacted concrete (RCC) dam with a flood design of 1 in 10,000. Right inset displays height of full supply level (FSL) at the minimum cost per ML yield and left inset displays width of FSL at the minimum cost per ML yield. Letters indicate location of potential dams A – Waterhouse River west branch; B – Waterhouse River; C – upper Flying Fox Creek; D – Jalboi River; E – Wilton River. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 3.1.2 Large dams for hydro-electric power generation potential The potential for major instream dams to generate hydro-electric power is presented in Figure 3-3, following a reconnaissance assessment of more than 50 million sites in the Roper catchment. This figure provides indicative estimates of hydro-electric power generation potential but does not consider the existence of supporting infrastructure. Figure 3-3 Roper catchment hydro-electric power generation opportunity map Costs are based on unit rates and quantity of material required for a RCC dam with a flood design of 1 year in 10,000. Cost includes site establishment, fish lifts/traps (high dams) or ladders (low dams) and land resumption for the area of land impounded by a flood event of AEP 1%. Data are underlain by a shaded relief map. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 3.2 Long-list of potential dam sites Potential dam sites with the higher DamSite modelled water yield per unit cost in different geographic parts of the Roper catchment were selected for a rapid desktop geological evaluation. The long-listed sites are shown in Figure 3-4 overlaid on a geological map. Geology of the Roper catchment with relevance to surface water storage The landscape of the Roper catchment is relatively featureless with a topography that is generally flat to undulating. The landscape drains into the Gulf of Carpentaria and is characterised by broad alluvial plains associated with the Roper River and its tributaries, and low rubbly hills or strike ridges of various resistant strata. Local relief ranges between 20 and 120 m. Vegetation cover is dominated by open woodland with denser stands of trees along major watercourses and soil plains supporting grasses and low shrubs. The oldest rocks in the area are of Proterozoic age (2500 to 540 million years old) and consist of repeated thick sequences of sediments and volcanics that include numerous prominent beds of sandstone. They were deposited in a series of basins extending across the area and then folded, faulted and intruded by igneous rocks to form mountain chains. Towards the end of the Proterozoic the mountain chains had been eroded down to a level not far above that of the current topography. During the Cambrian period (540 to 485 million years ago) there was widespread extrusion of basalt lava, which was followed by deposition of limestones and dolomites. The Cambrian strata only occur south-west of the Roper River where the limestones and dolomites are affected by karst (solution effects producing underground cave systems) and provide an important regional groundwater source. Erosion recommenced after the Cambrian and continued to the mid-Cretaceous period (about 100 million years ago) when subsidence and high global sea levels resulted in deposition of a thin succession of Cretaceous shallow marine sandstone, conglomerate and mudstone across the Roper catchment. The present landscape has been produced by warping and dissection of a series of erosion surfaces formed during several cycles of erosion that started in the Late Cretaceous about 70 million years ago and ended in the mid-Cenozoic era about 25 million years ago. During this time, stable crustal conditions and subaerial exposure led to patchy erosion of the Cretaceous rocks and prolonged subaerial weathering of the remaining Cretaceous and Proterozoic rocks and resulted in the formation of deep weathering profiles and associated iron-cemented capping. Between the mid-Cenozoic and the present day, there has been gentle uplift and warping of the various surfaces and their weathered capping. Continued erosion led to the emergence of the present-day landscape, which involved the removal of Cretaceous strata from most of the region and the etching out of structures in the underlying Proterozoic rocks (mainly Roper Group). Extensive floodplains and coastal deposits were built up on the margins of modern drainage systems and the coastline, respectively, in the region. Potentially feasible dam sites occur where resistant ridges of Proterozoic sandstone beds that have been incised by the river systems outcrop on both sides of river valleys. The sandstones are generally weathered to varying degrees, and the depth of weathering and the amount of sandstone outcrop on the valley slopes is a fundamental control on the suitability of the potential dam sites. Where the sandstones are relatively unweathered and outcrop on the abutments of the dam site less, stripping will be required to achieve a satisfactory founding level for the dam. The other fundamental control on the suitability of the dam site is the extent and depth of Quaternary alluvial sands and gravels in the floor of the valley, as these materials will have to be removed to achieve a satisfactory founding level for the dam. In general, where stripping removes the more weathered rock, it is anticipated that the Proterozoic sandstones will form a reasonably watertight dam foundation requiring conventional grout curtains and foundation preparation. Where potentially soluble dolomites occur within the Proterozoic sequences (soluble over a geological timescale) then it is possible that potentially leaky dam abutments and reservoir rims may be present, which will require specialised and costly foundation treatment such as extensive grouting. Where this condition is possible based on review of the 250,000 geological map sheets, it has been noted. Figure 3-4 Long-listed potential dam sites and the main geological units of the Roper catchment For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Table 3-1 Rapid desktop evaluation of long-listed potential dam sites in Roper catchment Note: Grade 1 is best grade 5 is worst – holistic assessment based on whether bedrock is exposed at site, likely depth of weathering/stripping on abutments, likely depth of cut-off and presence of deep alluvium and overall height to width assessment. DAM ID GEOLOGY LITHOLOGY ALLUVIAL TRACT GEOMORPHOLOGY COMMENT GRADE 3 Wide Qa/Prr Sandstone and mudstone Wide Qa Incised bedrock surface, rock exposed, deep channel Good dam site 1 6 Prh, Prom Thickly bedded sandstone Incised bedrock surface, rock exposed, deep channel Good dam site 1 25 Ridge Prh, regional fault Thickly bedded sandstone, regional fault Nick point, Incised bedrock surface, no rock exposed Good dam site 1 30 Wide Qa/Poo some Pri, Cz Dolomite, siltstone, sandstone some sandstone, older alluvium, some calcrete Wide Qa Incised palaeosurface, deep river channel, downstream fan 4 52 Wide Qa/Poo Dolomite, siltstone, sandstone Wide Qa Incised palaeosurface, braided alluvium 4 55 Phs Coarse sandstone Incised bedrock ridge, rock exposed, Good dam site 1 79 Pooj Sandstone, breccia, chert, dolostone Older palaeosurface removed, incised channel 3 103 Wide Qa, Prh Thickly bedded sandstone Wide Qa Incised palaeosurface, braided alluvium 4 106 Wide Qa Elb Cross-bedded sandstone with minor shale Wide Qa Incised palaeosurface, braided alluvium, calcrete 4 137 Phw Pon Sandstones minor volcanics, sandstones Older palaeosurface removed, incised channel 3 145 Prom over Prh Over thickly bedded sandstone Incised palaeosurface, alluvial infill 3 158 Prr Sandstone, siltstone, greywacke Deeply incised palaeosurface, mainly bedrock ?nick point?, 3 160 Prv over Pdd Siltstone and mudstone over dolerite Incised palaeosurface, alluvial infill? 3 165 Wide Qa, Pri Sandstone and siltstone Wide Qa Older palaeosurface removed, incised channel, some bedrock shapes 4 185 Prx Cross-bedded sandstone Older palaeosurface, incised, calcrete?, some bedrock structure 3 200 Wide Qa, Prm Sandstone and siltstone Wide Qa Incised palaeosurface, braided alluvium, calcrete 4 233 Poo Dolomite, siltstone, sandstone Older palaeosurface removed, incised channel 3 281 Pom Conglomerates and sandstone Incised palaeosurface, some rock exposed Fair dam site 2 DAM ID GEOLOGY LITHOLOGY ALLUVIAL TRACT GEOMORPHOLOGY COMMENT GRADE 291 Wide Qa, Prl, Kl Sandstone, conglomerate; sandstone claystone, conglomerate Wide Qa Palaeosurface, broad meandering alluvial tract Poor dam site 5 325 Prc Flaggy sandstone and sitststone Incised bedrock surface, rock exposed? Fair dam site 2 332 Wide Qa/Poo Dolomite, siltstone, sandstone Wide Qa Incised palaeosurface, very broad alluvial tract Poor dam site 5 415 Phg Cross-bedded sandstone, minor volcanics Incised palaeosurface, calcrete? 3 4 Sites short-listed Five potential dam sites listed in Table 3-1 were selected for pre-feasibility analysis and two short- listed for a more detailed costing. Details of the two short-listed sites are documented in sections 4.1 and 4.2. The three remaining sites selected for pre-feasibility analysis are documented in Appendix B. 4.1 Waterhouse River dam site on the Waterhouse River; ATMD 70.5 km See Table 3-1 and Figure 3-4 PARAMETER DESCRIPTION Previous investigations No previous investigations of this site have been located. The site was identified from CSIRO DamSite modelling. References No references to investigation of this site have been located. Description of potential dam This description is for a potential dam with FSL 200 mEGM96 (i.e. 21 m above (ALOS) bed level) and storage capacity of 219 GL. A storage at this site could provide supply water for irrigation to moderately suitable soils downstream of the storage. Under this hypothetical conceptual arrangement releases would be made from the storage to the stream for diversion by irrigators. The location of the potential site is shown in Figure 4-1 and Figure 4-2. Figure 4-3 shows the known water-dependent ecological assets in the vicinity of the potential dam site. Regional geology The Roper catchment landscape is flat to undulating and consists of a series of erosion surfaces developed over the last 70 million years which are characterised by deep weathering profiles and associated iron-cemented capping. The continued erosion has led to the emergence of the present-day landscape and involved the removal of Cretaceous strata from most of the region and the etching out of structures in the underlying Proterozoic rocks. The resulting landscape is characterised by broad alluvial plains associated with the Roper River and its tributaries, and low rubbly hills or strike ridges formed by folded and faulted resistant thick Proterozoic sandstone formations. Site geology Based on geological mapping (high-level fly-over only) The dam site is located on Proterozoic rocks of the Katherine River Group (Phs), which consist of medium to coarse and pebbly, trough cross-bedded quartz sandstone and dip to the north-west at 5°. The deeply weathered erosion surface characteristic of the region appeared to have been locally removed by erosion at the dam site. On the dam abutments, blocky weathered sandstone bedrock is exposed and open joints were observed, suggesting erosion of weathered material from between the blocks; assume a mean depth of stripping of 4 m. The river channel has rock bars and a mobile unvegetated tract of alluvium (sand and gravel). The depth of alluvium is anticipated to be 2 to 3 m locally. The geology of the site is shown in Figure 4-4. Reservoir rim stability and leakage potential There is no evidence of instability or leakage potential around the reservoir rim. Potential structural arrangement The site appears to be suitable for a roller compacted concrete type dam with a central uncontrolled spillway with crest length of 75 m. Outlet works and a fish lift facility would be located on the left bank of the dam. PARAMETER DESCRIPTION Access to the left bank of the site could be via 15 km of new road branching from the Central Arnhem Highway a short distance east of the Beswick settlement. The total distance from Katherine via the Stuart and Central Arnhem highways would be some 127 km. Availability of construction materials A quarry in the Proterozoic sandstone with suitable aggregate for a RCC dam might be found within 5 km of the dam site. For estimating purposes a ratio of useful rock excavated to total volume excavated of 0.5 could be assumed. Hard rock quarries for higher quality aggregate to construct an outer layer of RCC for the dam could probably be sourced from Katherine. Catchment area 1477 km2 Modelled annual inflow data Parameter Scenario A GL/yr Scenario Cdry GL/yr Scenario Cmid GL/yr Scenario Cwet GL/yr Max 764 579 681 986 Mean 192 148 170 264 Median 158 118 132 219 Min 3 3 3 3 Reservoir characteristics Reservoir characteristics are shown in Figure 4-5. Reservoirs with FSLs of different heights are tabulated below. FSL (mEGM96) Surface area (ha) Capacity (GL) 198 2838 155 200 3540 219 202 4368 298 Reservoir yield assessment at dam wall FSL 198 mEGM96 Estimated yield at 85% annual time reliability 80 GL FSL 200 mEGM96 Estimated yield at 85% annual time reliability 89 GL FSL 202 mEGM96 Estimated yield at 85% annual time reliability 92 GL Reservoir yields under projected future climates shown in Figure 4-7. Estimated rates of reservoir sedimentation at FSL 200 (mEGM96) Best case Expected Worst case 30 years (%) 0.8 1.1 1.3 100 years (%) 2.5 3.8 4.2 Years to fill 3946 2631 2368 Potential use of supply The area below the junction of the upper Waterhouse River and Waterhouse River West Branch is dominated by dissected tablelands of the Sturt Plateau in the upper catchment, alluvial plains adjacent to the river, and sandplains in the lower sections. The tablelands and associated scarps have well-drained, predominantly moderately deep to shallow sandy-surfaced red Kandosols (SGG 4.1) with abundant ironstone gravels throughout the soil profile, and abundant rock on and adjacent to the scarps (SGG 7). The deeper soils with less gravels are suitable for a diverse range of spray- and trickle-irrigated crops, predominantly horticultural crops. However, these areas are heavily fragmented by shallow or rocky areas resulting in small usable areas, mainly for trickle-irrigated horticulture. PARAMETER DESCRIPTION The alluvial plains in the upper catchment are dominated by very deep sandy- surfaced brown Dermosols (SGG 2) with moderately permeable, moderately well- drained to imperfectly drained, mottled structured clay subsoils. Sandy-surfaced Kandosols with brown and yellow massive loamy subsoils (SGG 4.2) and red subsoils (SGG 4.1) are intermixed. Small areas of gilgaied grey cracking clays (SGG 9) occur on the alluvial back-plains in the Beswick area. The brown and grey soils are subject to seasonal wetness (wet season) and may be subject to flooding. Soils are suited to a diverse range of dry-season spray-irrigated grain, pulse and forage crops; horticultural small crops; wetness-tolerant horticultural tree crops, sugarcane and cotton. Wet-season crops are restricted to crops with a moderate tolerance to wetness such as spray-irrigated forage crops, sunflower, sesame and rice. Fragmentation of the area by creeks and soil distribution may limit the usable areas for various uses. The red, brown and yellow massive loamy soils with sandy surfaces (Kandosols SGG 4.1) occur on the gently undulating elevated sandplains and level plains on recent alluvium adjacent to the lower reaches of the Waterhouse River. Moderately deep to very deep (0.5 to >1 m), well-drained to imperfectly drained, red (SGG 4.1) and mottled brown or yellow (SGG 4.2), sandy-surfaced massive soils occur as a mosaic over the landscape, probably reflecting the depth to the underlying rock with red soils on the deeper areas. These moderately permeable soils have moderate to high (100 to 140 mm to 1 m) soil water storage and are highly suited to a broad range of spray- or trickle-irrigated crops. However, fragmentation of the area by creeks and soil distribution may limit the usable areas for various uses. Storage impacts As well as the dam storage area at FSL, an additional flood surcharge area above FSL would need to be acquired. This area would affect the Beswick Aboriginal Land Trust area. Environmental considerations Barrier to movement of aquatic species Records of fish at this site were limited, resulting in no records of fish at this site. Fish whose movement may be impeded by a dam include the mouth almighty (Glossamia aprion), giant gudgeon (Oxyeleotris selheimi), spangled grunter (Leiopotherapon unicolor), barramundi (Lates calcarifer), Hyrtl's catfish (Neosilurus hyrtlii), as well as the northern snapping turtle (Elseya dentata) as they all occur in the neighbouring streams. Ecological implications of inundation The vegetation at this potential dam site is ‘Arnhem Plateau Sandstone Shrubland Complex’ listed as Endangered Ecological Community (EPBC Act). The potential inundated area at FSL for this site (200 mEMG96) may impact this ecological community. Freshwater mangrove (Barringtonia acutangular) has been recorded downstream of the potential site. The potential for ecological change as a result of changes to the downstream flow regime is examined in the companion technical report on ecology (Stratford et al., 2023). Indigenous land tenure, native title and cultural heritage considerations There is a high likelihood of unrecorded sites in the inundation area. Estimated cost A manual cost estimate undertaken as part of the Assessment for a RCC dam on the Waterhouse River Creek site at FSL 200 mEGM96 found the dam would cost approximately $253 million. Details of this cost estimate are provided in Appendix A. To enable a like-for-like comparison with the non-short-listed sites, dam costs were calculated using CSIRO’s generalised dam costing algorithm, which takes into account major cost elements for RCC type dams with central overflow spillways. These are reported for a selection of FSL below. FSL 198 mEGM96 $230 million FSL 200 mEGM96 $259 million FSL 202 mEGM96 $303 million PARAMETER DESCRIPTION Estimated cost / ML of supply Based on the yields estimated by CSIRO BHA modelling and the costs derived from the CSIRO generalised costing algorithm, the estimated cost/ML of supply at the following storage levels are as follows: FSL 198 mEGM96 $2875/ML FSL 200 mEGM96 $2910/ML FSL 202 mEGM96 $3293/ML Based on the manual cost estimate, the cost/ML of supply at FSL of 200 mEGM96 is $2842/ML. Summary comment The Waterhouse River potential dam site has one of the highest yield-to-cost ratios in the upper parts of the Roper catchment. It is also upstream of large areas of sandy loam soils moderately suitable for irrigated agriculture. The site is situated on Aboriginal land scheduled under the Commonwealth Aboriginal Land Rights (Northern Territory) Act 1976 and is near the community of Beswick and is classified under ‘inalienable freehold title’, which means that it cannot be bought, acquired or mortgaged. Although data from the NT cultural heritage sites register were not made available to the Assessment, it is likely that the site and parts of the potential inundation area would contain cultural heritage sites of significance. Figure 4-1 Location map of potential Waterhouse River dam site, reservoir extent and catchment area For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 4-2 Potential Waterhouse River dam reservoir and property boundaries For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 4-3 Known water-dependent ecological assets in the vicinity of the potential Waterhouse River dam site and reservoir For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 4-4 Geology underlying the potential Waterhouse River dam site and reservoir For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 4-5 Waterhouse River potential dam site topographic dimensions and inflow hydrology (a) Elevation profile along dam axis; (b) reservoir volume, surface area and height relationship; (c) dam wall height versus dam width and flood rise for 1:10,000 and 1:50,000 AEP and probable maximum flood (PMF) events plotted against full supply level (FSL); (d) annual streamflow; (e) annual flow exceedance. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure4-6Waterhouse River potential dam site cost,wateryield at the dam wall and evaporation (a) Dam length and dam costversus full supply level (FSL); (b) dam yield at75% and 85%annual time reliability andyield per million dollars at75% and 85% annual time reliability; (c) annual time reliability plotted against yield fordifferent FSL; (d) volumetric reliability plotted against yieldfor different FSL; (e) yield at75% and 85% annual time reliability and degree of regulation (ratio of total controlled releases to total reservoir inflows) plotted against FSL; (f) yield and net evaporation (evaporation minus rainfall) divided by yield plotted against annual time reliability. Chapter4 Sites short-listed|43 Figure 4-7 Waterhouse River potential dam site and storage levels and water yield (a) Maximum and minimum annual storage trace at the selected FSL (200 mEGM96) and annual spilled volume (i.e. uncontrolled releases); (b) annual exceedance of ratio of annual quantity of water released to annual demand (i.e. yield) under conditions where the reservoir was operated to supply the full demand (yield) in 55% to 95% of years at the selected FSL; (c) annual exceedance plot of released volume under conditions where the reservoir was operated to supply the full demand (yield) in 55% to 95% of years at the selected FSL; (d) annual yield at 85% annual time reliability plotted against FSL under Scenario A (baseline) and Scenario D; (e) annual time reliability vs yield for FSL 200 mEMG96 under Scenario A (baseline) and Scenario D. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 4.2 Flying Fox Creek dam site on Flying Fox Creek; ATMD 105 km See Table 3-1 and Figure 3-4 PARAMETER DESCRIPTION Previous investigations No previous investigations of this site have been located. The site was identified from CSIRO DamSite modelling. References No references to investigations of this site have been located. Description of potential dam This is a potential dam with FSL 173 mEGM96 (22 m above (ALOS) bed level) and capacity of 133 GL. A storage at this site could release 99 GL of water in 85% of years for irrigation of moderately suitable soils downstream of the storage. Under this hypothetical conceptual arrangement, releases would be made from the storage downstream to a regulating weir for diversion by irrigators. Figure 4-8 shows the potential dam site looking upstream. The location of the potential site is shown in Figure 4-9 and Figure 4-10. Figure 4-11 shows the known water-dependent ecological assets in the vicinity of the potential dam site. Regional geology The Roper catchment landscape is flat to undulating and consists of a series of erosion surfaces developed over the last 70 million years, which are characterised by deep weathering profiles and associated iron-cemented capping. The continued erosion has led to the emergence of the present-day landscape and involved the removal of Cretaceous strata from most of the region and the etching out of structures in the underlying Proterozoic rocks. The resulting landscape is characterised by broad alluvial plains associated with the Roper River and its tributaries, and low rubbly hills or strike ridges formed by folded and faulted resistant thick Proterozoic sandstone formations. Site geology Site visit The dam site is located on Proterozoic rocks of the Mount Rigg Group (Pooj), which consist of medium- to very thick-bedded quartz sandstone; chert and sandstone clast pebble to cobble conglomerate; minor dolomitic siltstone with a sub-horizontal dip. The deeply weathered erosion surface characteristic of the region appeared to have been partially removed by erosion at the dam site. On the dam abutments, weathered sandstone bedrock is partially exposed with some talus (blocky slope deposits) on the surface. Possibly 1 to 3 m stripping would be required so assume a mean depth of stripping of 2 m. The river channel consists of a tract of vegetated alluvium (sand and gravel) and the depth of alluvium is anticipated to be 3 to 5 m locally, with some additional alluvial terraces 2 m deep. The geology of the site is shown in Figure 4-12. Reservoir rim stability and leakage potential There is no evidence of instability or leakage potential around the reservoir rim. Potential structural arrangement The site appears to be suitable for a roller compacted concrete type dam with a central uncontrolled spillway 150 m wide. Outlet works and a fish lift facility would be located on the right bank. No saddle dams are required at this level of development. Access to the right bank of the site would be via a 10 km new road from the Central Arnhem Highway branching before the creek crossing. The total distance via the Stuart and Central Arnhem highways from Katherine would be some 207 km. Availability of construction materials A quarry in the Proterozoic sandstone with suitable aggregate for a RCC dam might be found within 10 km of the dam site. For estimating purposes a ratio of useful rock excavated to total volume excavated of 0.5 could be assumed. Hard rock quarries for higher quality aggregate to construct an outer layer of RCC for the dam could probably be sourced from Katherine. Catchment area 1173 km2 PARAMETER DESCRIPTION Modelled annual inflow data Parameter Scenario A GL/yr Scenario Cdry GL/yr Scenario Cmid GL/yr Scenario Cwet GL/yr Max 656 500 599 834 Mean 151 118 140 211 Median 131 102 124 178 Min 2 2 2 2 Reservoir characteristics Reservoir characteristics are shown in Figure 4-13. Reservoirs with FSL of different heights are tabulated below. FSL (mEGM96) Surface area (ha) Capacity (GL) 171 1603 97 173 1924 133 175 2260 174 Reservoir yield assessment at dam wall FSL 171 mEGM96 Estimated yield at 85% annual time reliability 59 GL FSL 173 mEGM96 Estimated yield at 85% annual time reliability 68 GL FSL 175 mEGM96 Estimated yield at 85% annual time reliability 71 GL Reservoir yields under projected future climates shown in Figure 4-15. Estimated rates of reservoir sedimentation at FSL 173 mEGM96 Best case Expected Worst case 30 years (%) 1.0 1.5 1.7 100 years (%) 3.3 5.0 5.6 Years to fill 2990 1993 1794 Potential use of supply The area below the potential dam site on the upper Flying Fox Creek is dominated by hills and undulating rises dissected by a narrow alluvial plain along the creek, then a relatively large area of alluvial plains of Flying Fox Creek around the Central Arnhem Road, and broad alluvial plains associated with the lower Flying Fox Creek approximately 45 km downstream of the Central Arnhem Road. The hills and rises have shallow and/or rocky soils (SGG 7) that are unsuitable for irrigation development. The alluvial plains in the upper catchment are dominated by very deep sandy- surfaced brown Dermosols (SGG 2) with moderately permeable, moderately well- drained to imperfectly drained, mottled structured clay subsoils. These soils are subject to seasonal wetness (wet season) and may be subject to flooding. Soils are suited to a diverse range of dry-season spray-irrigated grain, pulse and forage crops; spray- or trickle-irrigated horticultural small crops; wetness-tolerant horticultural tree crops; and spray-irrigated sugarcane and cotton. Wet-season crops are restricted to crops with a moderate tolerance to wetness such as spray- irrigated forage crops, sunflower, sesame and rice. Fragmentation of the area by creeks and soil distribution may limit the usable areas for various uses, particularly on the alluvial plains with numerous braided channels approximately 10 km downstream of the Central Arnhem Road. The broad alluvial plains approximately 45 km downstream of the Central Arnhem Road are dominated by very deep, moderately well-drained to imperfectly drained, slowly permeable brown to grey cracking clay soils (SGG 9) with strongly sodic subsoils, and soft self-mulching or hard-setting structured clay surfaces. Soils have high to very high water-holding capacity (>140 mm to 1 m) but may have a restricted rooting depth due to very high salt levels in the subsoil. The self- mulching and structured brown and grey cracking clay soils are suited to a variety of dry-season flood- or spray-irrigated grain, forage and pulse crops, sugarcane and cotton. Much of the clay plains are subject to regular flooding and frequently have small (<0.3 m) gilgai depressions and numerous flood channels. However, the plains with braided channels are frequently in narrow, ribbon form, which limit opportunities for agricultural development. PARAMETER DESCRIPTION Impacts In addition to the storage area, a flood margin area would also need to be acquired. Environmental impacts Barrier to movement of aquatic species At this site there were no records of any species. Ecological implications of inundation The ‘Arnhem Plateau Sandstone Shrubland Complex’ listed as Endangered Ecological Community (EPBC Act) surrounds the potential inundated area at FSL for this site (173 mEMG96). The potential for ecological change as a result of changes to the downstream flow regime is examined in the companion technical report on ecology (Stratford et al., 2022). Indigenous land tenure, native title and cultural heritage considerations There is a high likelihood of unrecorded sites in the inundation area. Estimated cost A manual cost estimate undertaken as part of the Assessment for a RCC dam on the upper Flying Fox Creek potential dam site at FSL 173 mEGM96 found the dam would cost approximately $318 million. Details of this cost estimate are provided in Appendix A. To enable a like-for-like comparison with the non-short-listed sites, dam costs were calculated using CSIRO’s generalised dam costing algorithm, which takes into account major cost elements for RCC type dams with central overflow spillways. These are reported for a selection of FSL below. FSL 171 mEGM96 $288 million FSL 173 mEGM96 $312 million FSL 175 mEGM96 $339 million Estimated cost / ML of supply Based on the yields estimated by CSIRO BHA modelling and the costs derived from the CSIRO generalised costing algorithm, the estimated cost/ML of supply at the following storage levels are as follows: FSL 171 mEGM96 $4881/ML FSL 173 mEGM96 $4588/ML FSL 175 mEGM96 $4775/ML Based on the manual cost estimate, the cost/ML of supply at FSL of 235 mEGM96 is $4676/ML. Summary comment The upper Flying Fox Creek potential dam site is one of the sites with the highest yield-to-cost ratio on non-Indigenous land, with potentially favourable geology and upstream of large areas of sandy loam soils moderately suitable for irrigated agriculture. Figure 4-8 Flying Fox Creek potential dam site AMTD 105 km looking upstream Figure 4-9 Location map of potential Flying Fox Creek dam site, reservoir extent and catchment area For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 4-10 Potential Flying Fox Creek dam reservoir and property boundaries For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 4-11 Known water-dependent ecological assets in the vicinity of the potential Flying Fox Creek dam site and potential reservoir extent For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 4-12 Geology underlying the potential Flying Fox Creek dam site and reservoir For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 4-13 Flying Fox Creek potential dam site topographic dimensions and inflow hydrology (a) Elevation profile along dam axis; (b) reservoir volume, surface area and height relationship; (c) dam wall height versus dam width and flood rise for 1:10,000 and 1:50,000 AEP and probable maximum flood (PMF) events plotted against full supply level (FSL); (d) annual streamflow; (e) annual flow exceedance For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 4-14 Flying Fox Creek potential dam site cost, water yield at the dam wall and evaporation (a) Dam length and dam cost versus full supply level (FSL); (b) dam yield at 75% and 85% annual time reliability and yield per million dollars at 75% and 85% annual time reliability; (c) annual time reliability plotted against yield for different FSL; (d) volumetric reliability plotted against yield for different FSL; (e) yield at 75% and 85% annual time reliability and degree of regulation (ratio of total controlled releases to total reservoir inflows) plotted against FSL; (f) yield and net evaporation (evaporation minus rainfall) divided by yield plotted against annual time reliability. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 4-15 Flying Fox Creek potential dam site and storage levels and water yield (a) Maximum and minimum annual storage trace at the selected FSL (172 mEGM96) and annual spilled volume (i.e. uncontrolled releases); (b) annual exceedance of ratio of annual quantity of water released to annual demand (i.e. yield) under conditions where the reservoir was operated to supply the full demand (yield) in 55% to 95% of years at the selected FSL; (c) annual exceedance plot of released volume under conditions where the reservoir was operated to supply the full demand (yield) in 55% to 95% of years at the selected FSL; (d) annual yield at 85% annual time reliability plotted against FSL under Scenario A (baseline) and Scenario D; (e) annual time reliability vs yield for FSL 172 mEMG96 under Scenario A (baseline) and Scenario D. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 5 Re-regulating structures Re-regulating structures, such as weirs, are typically located downstream of large dams. They allow for more efficient releases from the storages and for some additional water yield from the weir storage itself, thereby reducing the transmission losses normally involved in supplemented river systems. As a rule of thumb, however, weirs are constructed to one-half to two-thirds the river bank height. This height lets the weirs achieve maximum capacity while ensuring that the change in downstream hydraulic conditions does not result in excessive erosion of the toe of the structure and also that large flow events can still be passed without causing excessive flooding upstream. Broadly speaking, there are two types of weir structure: concrete gravity weirs and sheet piling weirs. These are discussed below. Both weir types often use rock-filled mattresses on the stream banks extending downstream of the weir to protect erodible areas from flood erosion. Sand dams are also briefly discussed. Note that weirs, sand dams and diversion structures obstruct the movement of fish in a similar way to dams during the dry season. 5.1 Sheet piling weirs Where rock foundations are not available, stepped steel sheet piling weirs have been successfully used in many locations across Queensland. These weirs consist of parallel rows of steel sheet piling, generally about 6 m apart with a step of about 1.5 to 1.8 m high between each row. Reinforced concrete slabs placed between each row of piling absorb much of the energy as flood flows cascade over each step. The upstream row of piling is the longest, driven to a sufficient depth to cut off the flow of water through the most permeable material (Figure 5-1). Indicative costs are provided in Table 5-1. Figure 5-1 Schematic cross-section diagram of sheet piling weir Source: Petheram et al. (2013a) For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Table 5-1 Estimated generic construction cost of 3-m high sheet piling weir WEIR CREST LENGTH (M) ESTIMATED CAPITAL COST ($ MILLION) 100 28 150 36 200 43 5.1.1 Stepped steel sheet piling weir on Flying Fox Creek AMTD 36 km A potential weir site was investigated on Flying Fox Creek AMTD 36 km, which could be used to regulate releases from an upstream dam on Flying Fox Creek AMTD 105 km reducing transmission losses. The weir storage would also increase system yield based on storing inflows (mean annual inflow from residual catchment was modelled to be 101.5 GL) from that part of the catchment downstream of the potential dam site. The potential weir site is on a broad vegetated wide alluvial tract with patchy unvegetated areas suggesting alluvial sands and gravels locally mobilised under current flow regime. The bedload is likely to be mobile during large flood events. The alluvial tract is estimated to be about 400 m wide and 5 to 10 m deep. No rock exposed on either abutment suggesting deeply weathered siltstones and mudstones over dolerite. Although unsuitable for a concrete gravity storage structure the ground conditions are well suited to a steel sheet piling weir. A nominal conceptual layout would entail a stepped steel sheet piling weir approximately 500 m in length with a central sloping low flow section and reinforced concrete slab protection between the piling rows. Four rows of piling would be required. Outlet works and a vertical slot type fish ladder would be located on the right abutment. It has been assumed that dry season flows would be diverted through a 1,800 mm diameter RC pipe which would also provide for environmental releases from the weir storage to the stream. Access to the right bank of the site would be via a 38 km new road from the Central Arnhem Highway branching before the creek crossing. The total distance via the Stuart and Central Arnhem highways from Katherine would be some 229 km. The access road would also provide access to a pump station servicing a potential irrigation area. A weir with a FSL 4 m above the bed level would have a capacity of about 10 GL and with no environmental flow releases the system yield of the weir and a potential dam on Flying Fox Creek AMTD 105 km FSL 173 mEMG96 would be 87 GL in 85% of years (compared to 68 GL in 85% of years released from a with FLS 173 mEMG96 at Flying Fox Creek ATMD 105 km). It is estimated that the weir would cost $89 million to construct. 5.2 Concrete gravity type weirs Where rock bars are exposed at bed level across the stream, concrete gravity type weirs have been built on the rock at numerous locations across Queensland. This type of construction is less vulnerable to flood erosion damage both during construction and in service. Figure 5-2 shows an example of a concrete gravity weir on the Katherine River (NT). Figure 5-2 Donkey Camp Weir on the Katherine River (Daly River catchment, Northern Territory) An example of a reinforced concrete weir structure in the Northern Territory. Water from Donkey Camp Weir is treated and supplied to Katherine. Photo: CSIRO 5.3 Sand dams As many of the large rivers in northern Australia are very wide (e.g. >300 m), weirs are likely to be impractical and expensive at many locations. An alternative structure is sand dams, which are low embankments built of sand constructed at the start of each dry season during periods of low or no flow when heavy earth-moving machinery can access the bed of the river. They are constructed to form a pool of sufficient depth to enable pumping (i.e. typically greater than 4 m depth) and are widely used in the Burdekin River near Ayr, where the river is too wide to construct a weir. Typically, sand dams take three to four large excavators about 2 to 3 weeks to construct, and no further maintenance is required until they need to be reconstructed again after the wet season. Bulldozers can construct a sand dam more quickly than can a team of excavators, but they have greater access difficulties. Because sand dams only need to form a pool of sufficient size and depth from which to pump water, they usually only partially span a river and are typically constructed immediately downstream of large, naturally formed waterholes. The cost of 12 weeks of hire for a 20 t excavator and float (i.e. for transportation) is approximately $85,000. Although sand dams are cheap to construct relative to a concrete or sheet piling weir, they require annual rebuilding and have much larger seepage losses beneath and through the dam wall. No studies are known to have quantified losses from sand dams. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 6 Farm-scale storages The primary aim of this section is to provide a broad-scale assessment of the suitability of farm- scale water storage locations in the Roper catchment. It also provides a summary of farm-scale dam construction and operation and maintenance costs detailed in the Northern Australia Water Resource Assessment report on farm-scale dam costs (Benjamin, 2018) indexed to 2021. Note, however, in assessing regional-scale economics of water harvesting schemes, local variations in scale and site-specific nuances (e.g. length of supply channel, amount of diesel required for pumping, removal of sediment deposited in diversion channels, replacement of worn and damaged equipment, availability of materials, remoteness) result in considerably different construction and operational costs from one site to the next. Hence, operationally, each site would require its own specifically tailored engineering design. Many landholders will have observed the way water moves across their land and will have given considerable thought to their most suitable water harvesting configurations. This report does not attempt to produce engineering water harvesting infrastructure designs for individual producers. Nor does this report seek to provide instruction on the design and construction of offstream water storages. Numerous other texts and online tools provide detailed information on nearly all facets of offstream water storage. For instructional information the reader is directed in the first instance to QWRC (1984), Lewis (2002) and IAA (2007). This section describes a desktop analysis of two types of farm-scale dams. Section 6.1 examines offstream storages, such as ringtanks, into which water is pumped from an adjacent drainage line, and Section 6.2 examines gully and hillside dams, which intercept and store runoff generated directly from the dam’s catchment. 6.1 Offstream farm-scale storages (ringtanks) This section presents the results of a desktop land suitability assessment for farm-scale offstream storages in the Roper catchment. This assessment is based on the soils data in the top 1.5 m of the soil profile generated as part of the Assessment (see companion technical report on digital soil mapping (Thomas et al., 2022)). Because of a lack of data on soils below a depth of 1.5 m, this analysis does not consider the suitability of subsurface material below this depth. Farm-scale offstream water storages require consideration at a scale finer than is possible to assess in a regional-scale resource assessment. Hence, the results presented here are only indicative of potential suitable locations. Design and construction of offstream water storages should only be undertaken following a site investigation by a suitability qualified professional. Land suitability requirements for ringtanks include impermeable soils, slopes less than 5%, clay content greater than 20%, no rock and deep soils. Suitability criteria include: • slowly permeable to very slowly permeable soils (<50 mm/day) to reduce unnecessary water losses from deep drainage and avoid rising watertables or potential secondary salinisation in the vicinity of the tanks • level to gentle slopes to enable construction of a ‘large storage’ and reduce unnecessary excavation into hillslopes • soils texture greater than 20% clay to the depth of excavation to allow machinery to compact the tank floor and walls to reduce deep drainage. Clay textures (>35% clay) are preferred • non-rocky soils to enable ease of construction and uniform compaction • deep soils, preferably greater than 1 m deep, to allow excavation of tanks with sufficient storage depth and wall height. Most of the catchment is unsuitable for offstream storages (Figure 6-1), predominantly due to excessive slope, shallow soils, rockiness and permeable soils. Several land types are likely to be suitable for ringtanks. These include the poorly drained coastal marine clay plains, the cracking clay soils on the alluvial plains of the Roper River and major tributaries, and the Cenozoic clay plains on the Sturt Plateau. The low-lying, very deep (>1.5 m), very poorly drained, strongly mottled grey saline clay soils with potential acid sulfate deposits in the profile on the coastal marine plains are likely to be suitable for ringtanks but are subject to tidal inundation and storm surge from cyclones. Other areas likely to be suitable are the slowly permeable cracking clay soils on the alluvial plains of the Roper River and other major rivers, which have very deep (>1.5 m), moderately well to imperfectly drained, slowly permeable brown to grey cracking clays that are usually strongly sodic at depth. The clay plains of the Roper River are subject to regular flooding and frequently have small (<0.3 m) gilgai depressions and numerous flood channels. These soils on the alluvial plains grade to seasonally wet soils in the catchment below Ngukurr. The Cenozoic clay plains of the Sturt Plateau with very deep (>1.5 m), impermeable, imperfectly drained grey cracking clay soils often have large deep gilgai (>0.3 to 0.8 m). This relict alluvium occurs in drainage depressions, enabling collection and storage of overland flows. The non-cracking clay soils associated with the Cenozoic clay plains on the Sturt Plateau may be suitable for ringtanks. These very deep (>1.5 m), gilgaied non-cracking soils with moderately permeable clay loam to clay surfaces up to 1 m deep overly impermeable, mottled, structured, brown vertic (relating to shrink–swell properties) clay subsoils. Other soils that may be suitable are the gently undulating plains with deep (1 to 1.5 m), non-rocky, non-cracking clay soils developed on mudstones in the upper Hodgson River catchment. These soils frequently occur in association with shallow or rocky soils on slopes that are dissected by numerous creeks. Figure 6-1 Suitability of farm-scale offstream water storage (ringtanks) in the Roper catchment Soil and subsurface data were only available to a depth of 1.5 m, so this Assessment does not consider the suitability of subsurface material below this depth. This figure does not take into consideration flood risk or the availability of water. Data are underlain by a shaded relief map. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 6.1.1 River flow exceeded in 80% of years To enable a first-pass assessment of the potential for water harvesting in different parts of the Roper catchments, information on annual streamflow exceeded in 80% of years is presented in Figure 6-3. However, note that physical pumping constraints, environmental flow considerations and existing downstream usage mean the actual amount of water available for extraction may be considerably less than shown. Figure 6-3 displays the 80% exceedance of annual streamflow in the Roper catchment under the historical climate (Scenario A). It shows that, with the exception of the Wilton and Hodgson rivers, the tributaries joining the Roper River have relatively low 80% exceedance of annual streamflow. This is significant in terms of both offstream storage and gully dams, as it indicates that most of the tributaries of the Roper River will only be able to support limited farm-scale water storage developments. Although the 80% exceedance of annual streamflow in the Roper River downstream of the junction with the Wilton River is large, there are limited areas adjacent to the river that are suitable for cropping. Figure 6-2 Turkey nest offstream storage in the Roper catchment Photo: CSIRO For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 6-3 80% exceedance of annual streamflow in the Roper catchment under Scenario A 6.1.2 Evaporative and seepage losses Losses from the reservoir of a farm-scale dam occur through evaporation and seepage. When calculating evaporative losses from a storage, it is important to calculate net evaporation (i.e. evaporation minus rainfall) rather than just evaporation. Strategies to minimise evaporation include liquid and solid barriers, but these are typically expensive per unit of inundated area (e.g. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au $12 to $40 per m2. In non-laboratory settings, liquid barriers such as oils are susceptible to being dispersed by wind and have not been shown to reduce evaporation from a water body (Barnes, 2008). Solid barriers can be effective in reducing evaporation but are expensive. For example, covering the reservoir surface (110 ha) of the 4000 ML hypothetical ringtank detailed below (Table 6-1 to Table 6-4) with an impermeable barrier to prevent evaporation at a cost of $25/m2 would increase the capital cost of the storage from $2 million to $30 million, more than a factor of ten. Evaporation losses from a ringtank can also be reduced slightly by subdividing the storage into multiple cells and extracting water from each cell in turn to minimise the total surface water area. However, constructing a ringtank with multiple cells requires more earthworks and incurs higher construction costs than outlined in this section. A study of 138 farm dams ranging in capacity from 75 ML to 14,000 ML from southern NSW to central Queensland by the Cotton Catchment Communities CRC (2011) found mean seepage and evaporation rates of 2.3 and 4.2 mm/day, respectively. Of the 138 dams examined, 88% had seepage values of less than 4 mm/day and 64% had seepage values less than 2 mm/day. These results largely concur with IAA (2007), which states that reservoirs constructed on suitable soils will have seepage losses equal to or less than 1 to 2 mm/day and seepage losses will be greater than 5 mm/day if sited on less suitable (i.e. permeable) soils. Ringtanks with greater mean water depth lose a lower percentage of their total storage capacity to evaporation and seepage losses; however, they have a smaller storage capacity-to-excavation ratio. In Table 6-1 effective volume refers to the actual volume of water that could be used for consumptive purposes after losses due to evaporation and seepage. For example, if water is stored in a ringtank in the Roper catchment (near the junction of the Roper and Jalboi rivers) with mean water depth of 3.5 m until October and the mean seepage loss is 2 mm/day, about 45% of the stored volume would be lost to evaporation and seepage. The examples provided in Table 6-1 are for 4000 ML ringtanks in the Roper catchment, but the effective volume expressed as a percentage of the ringtank capacity is applicable to any storage (e.g. ringtanks or gully dams) of any capacity for mean water depths of 3.5, 6 and 8.5 m. In the Roper catchment, most moderate to large streamflow events occur before the end of March. Assuming the storage is full at this time, one strategy is to sow suitable crops during the early dry season to minimise evaporative and seepage losses and enable crops to use existing soil water. In the Roper catchment, however, the alluvial clay soils are not likely to be trafficable before May in many years. Hence the configurations in the following tables provide general information on construction costs and effective volumes in the Roper catchment for three seepage rates (1 mm/day, 2 mm/day and 5 mm/day) and for three storage durations (5 months, 7 months and 10 months), assuming the ringtank is full at the end of March. Sorghum planted for hay is an example of a crop where water may be required for irrigation for a 4-month period (i.e. May to August), sorghum planted for grazing is an example of a crop where water may be required for irrigation for a 6-month period (May to October) and Rhodes grass is an example of a perennial crop or a crop for which water is needed throughout the dry season (i.e. water may be required over a 10-month period). Table 6-1 Effective volume after net evaporation and seepage for ringtanks of three average water depths and under three seepage rates near junction of Jalboi River and Roper River in the Roper catchment Effective volume refers to the actual volume of water that could be used for consumptive purposes as a result of losses due to net evaporation and seepage, assuming a storage capacity is 4000 ML. For storages of 4000-ML capacity and average water depths of 3.5, 6 and 8.5 m, reservoir surface areas are 110, 65 and 45 ha, respectively. S:E ratio is the storage capacity to excavation ratio. Effective volumes calculated based on the 20% exceedance net evaporation. AVERAGE WATER DEPTH† (M) S:E RATIO SEEPAGE LOSS (MM/DAY) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENTAGE OF CAPACITY (%) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENTAGE OF CAPACITY (%) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENTAGE OF CAPACITY (%) 5 months (April to August) 7 months (April to October) 10 months (April to January) 3.5 14:1 1 2970 74 2447 61 2002 50 14:1 2 2803 70 2213 55 1666 42 14:1 5 2301 58 1510 38 660 16 6 7.5:1 1 3388 85 3077 77 2811 70 7.5:1 2 3289 82 2938 73 2613 65 7.5:1 5 2993 75 2523 63 2018 50 8.5 5:1 1 3574 89 3358 84 3173 79 5:1 2 3506 88 3262 82 3036 76 5:1 5 3301 83 2975 74 2624 66 6.1.3 Indicative capital, operation and maintenance costs of offstream storages In this analysis the cost of a farm-scale offstream storage scheme includes the cost of the water storage, pumping infrastructure, limited length of supply channel/piping, levee banks, and operation and maintenance of the scheme. For a given storage capacity, the construction costs (and opportunity cost of land used in the construction) vary considerably, depending on the way the storage is built. For example, circular storages have a better storage volume to cost ratio than rectangular or square storages. It is also considerably more expensive to double the height of an embankment wall than double its length. Table 6-2 provides a high-level breakdown of the capital and operation and maintenance (O&M) costs of a large farm-scale ringtank, including the cost of the water storage, pumping infrastructure and up to 100 m of pipes, and operation and maintenance of the scheme. The costs and analyses presented in Table 6-3 and Table 6-4 are based on costs of $5/m3 for earthfill and topsoil and $6.50/ m3 for compacted clay (Benjamin 2018), indexed to 2021; it was assumed this includes the cost of compaction and that all earth can be obtained within close vicinity of the site. In this example it is assumed that the ringtank is within 100 m of the river and pumping infrastructure. It should be noted that the cost of pumping infrastructure and conveying water from the river to the storage is particularly site-specific. For more detailed breakdown of ringtank costs see the Northern Australia Water Resource Assessment technical report on large farm-scale dams (Benjamin, 2018). In flood-prone areas where flood waters move at moderate to high velocities, riprap protection may be required, and this may increase the construction costs presented in Table 6 4 and Table 6 5 by 10 to 20% depending upon volume of rock required and proximity to a quarry with suitable rock. Table 6-2 Indicative costs for a 4000-ML ringtank Assumes a 4.25-m wall height, 0.75-m freeboard, 3:1 ratio on upstream slope, 3:1 ratio on downstream slope and crest width of 3.1 m, approximately 60% of material can be excavated from within storage, and cost of earthfill and compacted clay is $5.4/m3 and $7/m3, respectively. Earthwork costs include vegetation clearing, mobilisation/demobilisation of machinery and contractor accommodation. For more detail on costs see Northern Australia Water Resource Assessment technical report on large farm-scale dams (Benjamin, 2018). Pump station costs from companion technical report on pump stations in northern Australia (Devlin, 2023). Costs indexed to 2021. Pump station O&M costs assume cost of diesel of $1.49/L. SITE DESCRIPTION/ CONFIGURATION EARTHWORKS ($) GOVERNMENT PERMITS AND FEES ($) INVESTIGATION AND DESIGN FEES ($) PUMP STATION ($) TOTAL CAPITAL COST ($) O&M OF RINGTANK ($/Y) O&M OF PUMP STATION ($/Y) TOTAL O&M ($/Y) 4000-ML ringtank 1,725,000 38,200 81,800 1,100,000 2,945,000 18,300 107,000 125,300 The capital costs can be expressed over the service life of the infrastructure (assuming a 7% discount rate, see Chapter 6) and combined with O&M costs to give an equivalent annual cost for construction and operation. This enables infrastructure with differing capital and O&M costs and service lives to be compared. The total equivalent annual costs for the construction and operation of a 1000-ML ringtank with 4.25-m high embankments and 55 ML/day pumping infrastructure is about $149,000 (Table 6-3). For a 4000-ML ringtank with 4.25-m high embankments and 160 ML/day pumping infrastructure, the total equivalent annual cost is about $374,400. For a 4000-ML ringtank with 6.75-m high embankments and 160 ML/day pumping infrastructure, the total equivalent annual cost is about $510,900. Table 6-3 Annualised cost for the construction and operation of three ringtank configurations Assumes freeboard of 0.75 m, pumping infrastructure can fill ringtank in 25 days and assumes a 7% discount rate. Costs based on those provided for 4000 ML provided in companion technical report on large farm-scale dams (Benjamin, 2018). Costs indexed to 2021. Pump station O&M costs assume cost of diesel of $1.49/L. CAPACITY AND EMBANKMENT HEIGHT ITEM CAPITAL COST ($) LIFESPAN (Y) ANNUALISED CAPITAL COST ($) ANNUAL O&M COST ($) 1000 ML and 4.25 m Ringtank 925,000 40 69,400 9,250 Pumping infrastructure† 380,000 15 41,700 7,600 Pumping cost (diesel) na na na 21,000 4000 ML and 4.25 m Ringtank 1,725,000 40 129,400 17,250 Pumping infrastructure† 1,100,000 15 120,800 22,000 Pumping cost (diesel) na na ns 85,000 4000 ML and 6.75 m Ringtank 3,330,000 40 249,800 33,300 Pumping infrastructure† 1,100,000 15 120,800 22,000 Pumping cost (diesel) ns ns ns 85,000 na = not applicable. †Costs include rising-main, large-diameter concrete or multiple strings of high density polypipe, control valves and fittings, concrete thrust-blocks and head-walls, dissipater, civil works and installation. ‡Value assumes water is piped between river pumping infrastructure and ringtank. Although ringtanks with an average water depth of 3.5 m (embankment height of 4.25 m) lose a higher percentage of their capacity to evaporative and seepage losses than ringtanks of equivalent capacity with average water depth of 6 m (embankment height of 6.75 m) (Table 6-1), their levelized cost (i.e. annualised cost divided by water supplied) are lower (Table 6-4) due to the considerably lower cost of constructing embankments with lower walls. In Table 6-4 the equivalent annual cost of the water supplied from the ringtank takes into consideration net evaporation and seepage from the storage, which increase with the length of time water is stored (i.e. crops with longer growing seasons will require water to be stored longer). In these tables, the results are presented for the equivalent annual cost of water yield from a ringtank of different seepage rates and lengths of time for storing water. Table 6-4 Equivalent annual cost per ML for two different capacity ringtanks under three seepage rates near Jalboi River in the Roper catchment Assumes a 0.75-m freeboard, 3:1 ratio on upstream slope, 3:1 ratio on downstream slope. Crest widths are 3.1 m and 3.6 m for embankments with heights of 4.25 m and 6.75 m respectively and assumes earthfill and compacted clay costs $5/m3 and $6.50/m3 respectively. Earthwork costs include vegetation clearing, mobilisation/demobilisation of machinery and contractor accommodation. 1000-ML ringtank reservoir has surface area of 28 ha and storage volume to excavation ratio of about 7:1. 4000-ML ringtank and 4.25 m embankment height reservoir has surface area of 114 ha and storage volume to excavation ratio of about 14:1. 4000-ML ringtank with 6.75 m embankment height reservoir has surface area of 64 ha and storage volume to excavation ratio of about 7.5:1. CAPACITY AND EMBANKMENT HEIGHT ANNUAL- ISED COST* ($) SEEPAGE LOSS (MM/DAY) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/Y PER ML/Y) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/Y PER ML/Y) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/Y PER ML/Y) 5 months (April to August) 7 months (April to October) 10 months (April to January) 1000 ML and 4.25 m 148,955 1 1769 202 2158 246 2656 303 148,955 2 1877 214 2394 273 3214 367 148,955 5 2299 262 3564 407 8712 994 4000 ML and 4.25 m 374,415 1 965 128 1187 157 1475 196 374,415 2 1026 136 1321 175 1803 239 374,415 5 1265 168 2004 266 5391 714 4000 ML and 6.75 m 510,855 1 1312 151 1448 167 1590 183 510,855 2 1352 156 1518 175 1713 198 510,855 5 1489 172 1775 205 2235 258 Taking into consideration the cost of constructing ringtanks and net evaporation and seepage losses, the optimal embankment height will vary depending upon the capacity of the storage and the time required to store water. 6.2 Farm-scale gully and hillside dams Large farm-scale gully dams are generally constructed of earth or earth and rockfill embankments with compacted clay cores, and usually to a maximum height of about 20 m. Dams with a crest height of over 10 or 12 m typically require some form of downstream batter drainage incorporated in embankments. Large farm-scale gully dams typically have a maximum catchment area of about 30 km2 due to the challenges in passing peak floods from large catchments (large farm-scale gully dams are generally designed to pass an event with an AEP of 1%), unless a site has an exceptionally good spillway option. Like ringtanks, large farm-scale gully dams are a compromise between best-practice engineering and affordability. Designers need to follow accepted engineering principles relating to important aspects of materials classification, compaction of the clay core and selection of appropriate embankment cross-section. However, costs are often minimised where possible, for example, by employing earth bywashes and grass protection for erosion control rather than the more expensive concrete spillways and rock protection found on major dams. This can compromise the integrity of the structure during extreme events and its longevity as well as increase the ongoing maintenance costs but can considerably reduce the upfront capital costs. In this section the following assessments are reported: • suitability of the landscape for large farm-scale gully dams • indicative capital, operating and maintenance costs of large farm-scale gully dams. Net evaporation and seepage losses also occur from large farm-scale gully dams. 6.2.1 DamSite model results The DamSite model (Petheram et al., 2017b) was used to assess every location in the Roper catchment for their potential as a farm-scale earth embankment gully or hillside dam. As discussed in Section 2.1.2, the model was used to assess dams of between 5 m and 20 m in height. Figure 6-4 shows locations where it may be relatively economical to construct large farm-scale gully dams in the Roper catchment and the likely density of options. This analysis considers sites likely to have relatively favourable topography. It does not explicitly consider whether sites are underlain by soil that is suitable for the construction of the embankment and that will minimise seepage from the reservoir base. Soil suitability is shown in Figure 6-5. Dams can be constructed on eroded or skeletal soils provided there is access to a clay borrow pit nearby for the cut-off trench and core zone. However, those sites are likely to be less economically viable. Figure 6-4 Most economically suitable locations for large farm-scale gully dams in the Roper catchment overlaid on map of versatile agricultural land Gully dam data overlaid on agricultural versatility data. Versatile data sourced from companion technical report on digital soil mapping (Thomas et al., 2022). Agricultural versatility data show the parts of the catchment that are more or less versatile for irrigated agriculture. This Assessment does not consider the suitability of subsurface material. Sites with catchment areas greater than 40 km2 or yield-to-excavation ratios less than 20:1 are not displayed. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au In the Roper catchment, 1542 locations were modelled as having a maximum water yield of 20 ML per 1000 m3 of excavation or greater. However, note that in many of these locations the soils are unsuitable for constructing embankment dams because the soil is too shallow to provide sufficient material for construction (Figure 6-5). This means dam walls would have to be constructed using rockfill, cement and imported clay soils. The maximum yield per 1000 m3 of excavation was observed to be independent of catchment area. Data on farm-scale gully and hillside dams showing those locations with the highest yield-to-cost ratios are available through the Northern Australia Water Resource Assessment Explorer (https://nawra-explorer.csiro.au/). It should be noted that the results presented in Figure 6-4 are modelled and consequently only indicative of the general locations where siting a gully dam may be most economically suitable. This analysis may be subject to errors in the underlying digital elevation model, such as effects due to the vegetation removal process. An important factor not considered in this analysis was the availability of a natural spillway. Site-specific investigations by a suitably qualified professional should always be undertaken prior to the construction of a gully dam. Figure 6-5 Suitability of soils for construction of gully dams For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 6.2.2 Indicative capital, operation and maintenance costs of farm-scale gully and hillside dams The cost of a large farm-scale gully dam will vary depending upon a range of factors including the suitability of the topography of the site, the size of the catchment area, quantity of runoff, proximity of site to good-quality clay, availability of durable rock in the upper bank for a spillway and the size of the embankment. The height of the embankment, in particular, has a strong influence on cost. An earth dam to a height of 8 m is about 3.3 times more expensive to construct than a 4 m high dam. A dam to a height of 16 m will require 3.6 times more material than the 8 m high version, but the cost may be more than five times greater, depending on design and construction complexity (Benjamin, 2018). Performance and cost of three hypothetical farm-scale gully dams in northern Australia Table 6-5 summarises the key parameters for three hypothetical farm-scale gully dam configurations of with a capacity of 4 GL, and Table 6-6 provides a high-level breakdown of the major components of the capital costs for each configuration. Detailed costs for the three sites are provided in the companion technical report on large farm-scale dams (Benjamin, 2018). Table 6-5 Cost of three hypothetical farm-scale gully dams of 4 GL capacity Costs include government permits and fees, investigation and design, and fish passage. For a complete list of costs and assumptions, see the companion technical report on farm-scale dams (Benjamin, 2018). Costs are indexed to 2021. SITE DESCRIPTION/ CONFIGURATION CATCHMENT AREA (KM2) EMBANK-MENT HEIGHT (M) EMBANK-MENT LENGTH (M) S:E RATIO MEAN DEPTH (M) RESERVOIR SURFACE AREA (HA) TOTAL CAPITAL COST ($) O&M COST ($) Favourable site with large catchment, suitable topography and simple spillway (e.g. natural saddle) 30 9.5 1100 29:1 5.0 80 1,380,000 60,000 Unfavourable site with small catchment, challenging topography and limited spillway options (e.g. steep gully banks, no natural saddle) 15 14 750 21:1 6.3 63 1,590,000 38,000 Unfavourable site with moderate catchment, challenging topography and limited spillway options (e.g. steep gully banks, no natural saddle) 20 14 750 21:1 6.3 63 1,670,000 43,000 Table 6-6 High-level breakdown of capital costs for three hypothetical farm-scale gully dams of 4 GL capacity Earthworks include vegetation clearing, mobilisation and demobilisation of equipment, and contractor accommodation. Investigation and design fees include design and investigation of fish passage device and failure impact assessment (i.e. investigation of possible existence of fish population at risk downstream of site). Costs are based on experience in north Queensland – costs associated with government permits and fees in NT may differ. For a complete list of costs and assumptions, see the companion technical report on farm-scale dams (Benjamin, 2018). Costs are indexed to 2021. SITE DESCRIPTION/CONFIGURATION EARTHWORKS ($) GOVERNMENT PERMITS AND FEES ($) INVESTIGATION AND DESIGN FEES ($) TOTAL CAPITAL COST ($) Favourable site with large catchment, suitable topography and simple spillway (e.g. natural saddle) 1,247,000 40,000 93,000 1,380,000 Unfavourable site with small catchment, challenging topography and limited spillway options (e.g. steep gully banks, no natural saddle) 1,446,000 43,000 101,000 1,590,000 Unfavourable site with moderate catchment, challenging topography and limited spillway options (e.g. steep gully banks, no natural saddle) 1,526,000 43,000 101,000 1,670,000 Table 6-7 presents calculations of the effective volume for three configurations of 4-GL capacity gully dams (varying mean water depth/embankment height) for combinations of three seepage losses and water storage over three time periods in the Roper catchment. Table 6-7 Effective volumes and cost per ML for three 4 GL storages with different mean depths and seepage loss rates near the Waterhouse River in the Roper catchment Evaporation and seepage losses assumed to occur over 70% of the reservoir surface area. AVERAGE DEPTH AND RESERVOIR SURFACE AREA CON- STRUCTION COST ($) COST ($/ML) SEEPAGE LOSS (MM/D) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENT- AGE OF CAPACITY (%) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENT- AGE OF CAPACITY (%) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENT- AGE OF CAPACITY (%) 5 months (April to August) 7 months (April to October) 10 months (April to January) 3 m and 133 ha 1,067,000 250 1 3131 78 2693 67 2394 60 1,067,000 250 2 2990 75 2495 62 2111 53 1,067,000 250 5 2566 64 1901 48 1260 31 6 m and 66 ha 1,614,000 375 1 3567 89 3348 84 3198 80 1,614,000 375 2 3497 87 3250 81 3058 76 1,614,000 375 5 3287 82 2956 74 2637 66 9 m and 44 ha 2,152,000 500 1 3707 93 3559 89 3457 86 2,152,000 500 2 3660 91 3493 87 3362 84 2,152,000 500 5 3518 88 3295 82 3078 77 Table 6-8 presents cost information for three hypothetical farm-scale gully dams and Table 6-9 explores the sensitivity of equivalent annual unit cost of three hypothetical farm-scale gully dams to changes in seepage rate and time of water storage. Table 6-8 Cost of construction and operation of three hypothetical farm-scale gully dams of 4 GL capacity Assumes operation and maintenance (O&M) cost of 3% of capital cost and a 7% discount rate. Figures have been rounded. AVERAGE DEPTH AND RESERVOIR SURFACE AREA ITEM CAPITAL COST ($) ANNUALISED CAPITAL COST ($) ANNUAL O&M COST ($) EQUIVALENT ANNUAL COST ($/Y) 3 m and 133 ha Low embankment wide gully dam 1,076,000 92,300 30,000 124,600 6 m and 66 ha Moderate embankment gully dam 1,614,000 138,500 45,000 186,900 9 m and 44 ha High embankment narrow gully dam 2,152,000 184,700 60,000 249,200 Table 6-9 Equivalent annualised cost and effective volume for three hypothetical farm-scale gully dams of 4 GL capacity near the Waterhouse River in the Roper catchment Dam details are in Table 6-8. Annual cost assumes a 7% discount rate. AVERAGE DEPTH AND RESERVOIR SURFACE AREA EQUIVALENT ANNUAL COST ($/Y) SEEPAGE LOSS (MM/D) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/Y PER ML/Y) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/Y PER ML/Y) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/Y PER ML/Y) 5 months (April to August) 7 months (April to October) 10 months (April to January) 3 m and 133 ha 124,600 1 344 40 400 46 449 52 124,600 2 360 42 431 50 510 59 124,600 5 419 49 566 66 854 99 6 m and 66 ha 186,900 1 453 52 482 56 505 58 186,900 2 462 53 497 58 528 61 186,900 5 491 57 546 63 612 71 9 m and 44 ha 249,200 1 581 67 605 70 623 72 249,200 2 588 68 616 71 640 74 249,200 5 612 71 653 76 699 81 Where the topography is suitable for large farm-scale gully dams and a natural spillway is present, large farm-scale gully dams are typically cheaper to construct than ringtanks. . Part III Scheme-scale reticulation infrastructure 7 Potential reticulated infrastructure and irrigation development This chapter examines the scope for broad-scale irrigation developments serviced by the two short-listed dam sites described in sections 4.1 and 4.2. The digital soil modelling and land suitability analysis (Thomas et al., 2022) indicates that relatively limited soils are serviced directly by either storage site, so selection of the area served has been undertaken using the following principles: • Aggregation of suitable soils. The focus has been on areas with aggregations of suitable soils rather than isolated patches of suitable soils. The main target of potential development will be the alluvium adjacent to the streams being impounded, downstream of the storage site. • Proximity to source. The two short-listed storages are relatively modest in size. Proximity is important for two main reasons: (i) it limits the capital cost of transfer infrastructure to get the water from the source impoundment, whether by connector pipeline or channel, or downstream regulating structure and re-lift, and (ii) it limits losses in transferring the water from source to point of use in all cases other than the fully piped option. • Compatibility to topography. Both short-listed storages are in sections of the river where the stream is relatively incised, and hence distribution of water by realising it downstream or by channel conveyance will only potentially serve areas further down the catchment. Distribution of water from the storages by pipeline has the potential to reach adjacent catchments, but at the expense of additional re-lift pumping. • Compatibility with a range of crop types. Preference will be given to soils suitable for a range of crop types rather than soils suitable for a limited suite of crops. The inescapable conclusion for both short-listed dam sites is that the potential for irrigation development in the immediate proximity to the dam sites is limited. Therefore, the means of conveyance of water from the storage to the development site will be the most crucial consideration in both cases. 7.1 Learnings from other northern Australian Irrigation Developments relevant to potential Gulf developments A number of larger-scale irrigation developments in northern Australia in recent decades hold potential lessons for any potential irrigation development in the Roper catchment or elsewhere across the north. The following discussion draws on such lessons from four schemes: • Emerald Irrigation Scheme, Central Queensland – both in-situ derived basaltic soils and associated alluvial deposits along the Nogoa River • Burdekin Irrigation Scheme, north Queensland – a range of soil types on the Burdekin and Haughton River floodplain, and associated upslope areas • Ord Stage 2, Kimberley region of WA – mostly clay alluvium deposits on the Weaber Plain • Cane supplementation schemes, in particular Pioneer Valley Water Board and Proserpine Water Board – pumping from rivers and piped reticulation. Pumping pools are important for any river re-lift pumps to ensure adequate submergence and avoid complications from flood siltation. Since the total river flow for a good proportion of the year will only comprise the irrigation releases – and is likely to only average some 900 ML/day at the dam and be decreasing downstream – providing adequate submergence will normally mean either a flow constriction or a constructed re-regulating weir. As some options could potentially be served by each parcel having its own pump site, this submergence requirement will be a major limitation. Infrastructure must be aligned to cater for flood flows in internal and adjacent catchments. This is more of an issue for schemes involving open flow reticulation, rather than piped and pumped schemes, but will apply to some degree to all schemes. Farm units are best shaped by existing topography and soils distribution. This is especially the case for spray systems, as spray system design can cater for reasonably irregular layouts. Hydrogeology is critical to long-term sustainability. That is, any irrigation system must have a mechanism to cater for the increased accessions to groundwater that are an unavoidable part of irrigation. This is mainly because accessions from rainfall are greater in the areas under irrigation than in dryland, due to the higher mean antecedent moisture profile in the soil. In this situation, riparian lands, above but adjacent to a river system are normally better for irrigated agriculture than isolated lands without drainage incisions. Natural country slope also plays a part in this requirement. Water use efficiency needs to be designed at the start. For example open reticulation system should be designed with control structures, overflows and be implemented with Total Channel Control technology etc. Long systems involving substantial travel time can be inefficient and waste valuable water in operational overflows if the above components are not included. 7.2 Areas serviced by the potential dam site on the Waterhouse River ATMD 70.5 km This site has the potential to serve up to 10,150 ha, based on the following assumptions: • dam yield at 85% annual reliability: ~90 GL/year • crop demand assuming dry-season field crops or perennial trees under spray: 8 ML/ha • irrigation efficiency for spray application: 85% • irrigation efficiency for trickle application: 90% • distribution efficiency for open-channel distribution: 80% • distribution efficiency for piped reticulation: 98% • river reticulation efficiency: 80% • percentage of gross area irrigated: 95%. Targeted gross areas are therefore between 7,800 ha and 10,141 ha, depending on crop type, irrigation method and reticulation arrangement. The soils downstream of the potential Waterhouse River dam site can be characterised as follows: • Soils suitable for a broad range of cropping options are limited in the immediate vicinity of the dam site. The closest large contiguous areas of suitable soils are immediately to the north and east of Mataranka, some 55 km from the potential dam site. • Areas closer to the dam site exist in two main configurations: some areas adjacent to the river are in reasonably contiguous parcels, and a significant block of land along the Waterhouse River West Branch is geographically close to the dam but in an adjacent catchment. • Soils targeted for development are mapped by the digital soils mapping as predominantly SGG4.1 red loamy soils and SGG 2 friable non-cracking clay or clay loam soils. These are rated as suitability class 2 or 3 for a broad range of dry-season crops under spray, with less suitability to wet-season cropping or furrow application. The soils are particularly suited to perennial tree crops, dry-season cultivation of intensive horticulture and root crops under trickle and to a lesser extent spray, grain and fibre crops under spray, small-seeded crops, pulse crops, and forage crops under spray. Three development themes are possible for use of the water from this potential dam site, namely: • riparian development of suitable soils, extending as far as required to achieve the targeted gross areas described above • development of the area along Waterhouse River West Branch as well as sufficient lands adjacent to the main Waterhouse River downstream of the dam to achieve the targeted gross areas described above • release of water into the Waterhouse River with a re-regulating structure at a major re-lift point near Mataranka to allow development of the major contiguous areas of suitable soils. Each option has some advantages and disadvantages, as summarised in Table 7-1. Table 7-1 Advantages and disadvantages of the three development options DEVELOPMENT THEMES ADVANTAGES DISADVANTAGES Theme 1: Riparian development on Waterhouse River Some of the target areas are relatively close to the dam. Majority of target areas are a long way downstream, meaning expensive reticulation, or significant losses if reticulation is not piped. Soil types are a mixture of red loamy soils and friable non- cracking clays or clay loamy soils, so will require more management than the more uniform area downstream. Theme 2: Waterhouse River West Branch lands plus Theme 1 areas Gives access to large contiguous areas in relative proximity to the dam. Has the same soil complexity as the Theme 1 areas. The larger area to the west of the Waterhouse West Branch will require some form of re-lift pumping, and a very extensive pipe network. Theme 3: Re-regulating structure near Mataranka Has access to more uniform areas of red loamy soils. This option will have better existing road connections than the other two themes do. Will require a significant structure in the river, and multiple pump stations. The first suitable areas close to the river are upstream of the Waterhouse River – Roper River junction east of Mataranka town. Below this area, while significant areas of suitable soils exist, they are not near the river. Another factor to consider in evaluating the development themes will be the mechanism for transferal of water from the potential Waterhouse River dam site to the demand areas. Theme 1 would allow either a piped reticulation, or a combined channel and pipe system, where pipes are used to cross the river from a channel system on the west bank of the Waterhouse River. The overall river gradient from the dam site to the head of the last block served is approximately 1 m/km, making a piped system potentially economically viable compared to an equivalent capacity open channel system, which would have to be membrane lined in these soil types. Other factors against an open channel solution are the number of cross-drainage facilities required and the large number of control and drop structures required to cater for that slope. Theme 2 would potentially have to be mostly fully piped, and the section on the Waterhouse West Branch would require a lateral pipeline of about 13 km overall length, with in-line boosting to reach the elevations required. However the main reticulation on the east bank would have to extend to at least point L in Figure 7-1 to serve an area equivalent to that of Theme 1. Theme 3 would require a re-regulating weir at the head of the best aggregation of suitable soils. There are two main options for this. The first is a site some 43.5 km downstream of the potential dam, where there is a single section of stream after a braided reach and suitable soils on either bank. However, the area serviced from this re-regulation point would be segmented, and strung out along the river, in much the same manner as Theme 1 above. There are no major inflows to this reach, so all river releases would be subject to instream losses, with no opportunity for water yield increases due to storages operating in conjunction. The second option is much further downstream, just after the junction of the Roper River and Elsey Creek. This option opens up the opportunity of serving the largest area of contiguous Class 2 soils, but all will require re-lifting from the Roper River. Depending on the area served, this re-lift would have to be to 10 km long and have a static lift of up to 30 m. While this pump site is 88 km from the dam, and subject to significant losses in transmission, the system does have the advantage that system yield could be increased due to the inflows from the Elsey Creek and Roper River systems. This option is not covered by the present analysis. Given the above, the development theme chosen for further investigation is Theme 1, Riparian development on Waterhouse River via pipe reticulation. 7.2.1 Piped reticulation layout Design of the pipe reticulation will be based on the following assumptions. Gross area targeted will be based on: • fully piped reticulation, based on 98% efficiency, to allow for initial filling and some minor system losses • spray irrigation, with assumed efficiency of 85% • net land usage of 95%, to allow for on farm infrastructure etc. It has been assumed that more detailed soils surveys may change the configuration of the suitable soils, but not the overall availability •annual crop demand of8ML/ha allowing for a range of cropping options as outlined above.Forthe dam yield of~90GL/year, a net area of 9,580hawill be targeted.This corresponds to agross area of 10, 100ha,made up ofareas 1to 11in Figure7-1. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure7-1Potential piped reticulated layout 80|Surface water storage and reticulation Additional assumptions are as follows: • Daily crop demand is based on Et=p*f1*0.8*E0, where p is climate factor, assumed as 0.7, f1 is crop factor assumed as 1.0, and E0 is assumed at 11.9 mm/day, based on SILO values for Mataranka. Et is therefore 6.66 mm/day. • Irrigation demand is 7.8 mm/day/ha, assuming spray allocation. • No diversity factor is applied, as the total area is small and soil types reasonably uniform. • Flowrate at the head of the system is therefore 8.71 m3/s, with flow decreasing progressively downstream as areas are serviced. The adopted design involves selecting pipe sizes to meet the following criteria: • Indicative alignment as shown in Figure 7-1. This allows for command of the high point in each parcel of served land and avoids intermediate high points. Some of the parcels served are large and will presumably require further pipelines within the parcel. However, they will also require re-pressurisation at some point, which would make most sense at the top of the area. The subsequent pipelines can therefore be considered as part of the on-farm works rather than the backbone reticulation infrastructure. The exception to this is the service to parcels 7 and 10 on the west bank, where the parcels are large, and re-pressurisation would realistically be a little further down towards the centroid of the area served. In these cases, the main reticulation lines have been extended further down-slope. • Offtake from the dam at a maximum drawdown of 5 m above bed level. • A minimum residual head of 2 m at each of the offtake points for the 11 served areas. • The use of glass reinforced plastic (GRP) pipe throughout, mainly driven by the large diameters required. An effective roughness of k=0.06 mm is assumed, representing an achievable long- term value. Head loss is calculated using the Colebrook–White equation. • The design for the gravity option is summarised in Table 7-2 . Table 7-2 Reach parameters for nominal conceptual layout of reticulation scheme REACH LENGTH (M) FLOW RATE (L/SEC) PIPE REQUIREMENTS Dam-A 5,272 7,640 DN3000 PN10 GRP A-B 3,102 7,340 DN2000 PN10 GRP B-C 2,261 7,250 DN2000 PN10 GRP C-D 6,421 6,750 DN2000 PN10 GRP D-E 5,716 6,030 DN1900 PN10 GRP E-F 10,076 4,000 DN2000 PN10 GRP F-L 1,899 3,690 DN1600 PN10 GRP L-G 7,012 2,930 DN1600 PN10 GRP Lat A-H 952 230 DN373 DICL Lat E-K 2,916 2,030 DN1200 PN10 GRP Lat G-I 2,841 1,620 DN1200 PN10 GRP Pipe reticulation costing Preliminarycosts forthe above design(Table7-3) cometo$15,025.27/ha forthe 9,560haofirrigated area. Table7-3Preliminary costs for nominal conceptual layout ITEMCOST ($) Pipes supply andinstallation105,958,000 Structures2,259,500 Contractors overheads5,333,000 Design and constructionoverheads11,355,000 Contingency18,736,000 Total$143,641,500 Boostedscheme The above schemerepresents a conservativedesign, sincethe pipelinesystem will have excesscapacityat all timeswhen the storage is abovethe minimum 5mabove bed level.Another approach that makes a less conservative assumption is to assumethat the storageoperates bygravity at higher storagelevels but is boosted bypumping at the dam outlet for lower levels.There is a practical limit totheextent this approach can beused, asthe pipelinevelocities get higher forhigherlevels of pressure boost,and consequent waterhammer issueswill become difficult toresolve.A 10mboost was found to result in pipeline velocitiesbelow 2.5m/sec, where the surge issues will be controllable. The arrangement adopted isfora pipeline alignment identical to that adopted above,but with aboosterpump arrangement at thedam outlet.Thiswould operateso that it is onlyactivated when pipelinepressures at the outlets fell below their prescribed value of 2mresidual head.A variablefrequency drive(VFD)would allow tuning of thepump boost to the minimum required, subject tolimits of the VFD’s operation.The pump would feature anon-returnvalve inthe bypass, allowinggravity operation atall other times. Parametersfor the reaches of theboosted scheme aredetailed inTable7-4. That scheme allowsthe use of smaller pipediametersthan areused inthe gravity caseabove. Table7-4Reach parameters for nominal conceptual layout of boosted scheme REACHLENGTH (M)FLOW RATE (L/SEC)PIPE REQUIREMENTS Dam-A5,272 7,640 DN2000 PN10 GRPA-B3,102 7,340 DN2000 PN10 GRPB-C2,261 7,250 DN2000 PN10 GRPC-D6,421 6,750 DN2000 PN10 GRPD-E5,716 6,030 DN2000 PN10 GRPE-F10,076 4,000 DN2000 PN10 GRPF-L1,899 3,690 DN1500 PN10 GRPL-G7,012 2,930 DN1500 PN10 GRP 82|Surface water storage and reticulation REACH LENGTH (M) FLOW RATE (L/SEC) PIPE REQUIREMENTS Lat A-H 952 230 DN373 DICL Lat E-K 2,916 2,030 DN1100 PN10 GRP Lat G-I 2,841 1,620 DN1100 PN10 GRP Boosted pipe reticulation costing Preliminary costs for the above design (Table 7-5) come to $13,223.13 /ha for the 9,560 ha of irrigated area. Table 7-5 Preliminary costs for nominal conceptual layout ITEM COST ($) Pipes supply and installation 91,195,500 Structures 3,705,500 Contractors overheads 5,030,000 Design and construction overheads 9,993,000 Contingency 16,488,500 Total 126,412,500 Option evaluation The only difference in the ongoing cost of the two reticulated options described above is the operation of the pump station. There is about $17 million difference in the capital cost of the two schemes. Since the proportion of the dam releases that need boosting cannot easily be predicted at this time, an upper limit of these costs can be calculated by assuming that all flow has to be boosted. This indicates an NPV of below $7, meaning the boosted scheme is likely to be the more cost effective option. 7.3 Area serviced by the potential dam site on upper Flying Fox Creek ATMD 105 km This potential dam site, on Flying Fox Creek ATMD 10 km or about 11 km north of the Central Arnhem Highway, is smaller than the potential dam site on the Waterhouse River and has a smaller water yield. Using similar criteria to that developed for the site on the Waterhouse River, it has the potential to serve up to 6,800 ha, depending on the transmission and irrigation method chosen. The major difference is that the target areas are likely to feature clay soils, and hence the soils are less versatile in terms of the range of irrigated crops compared to the sandy loamy soils downstream of the potential Waterhouse River dam site. Calculations are based on the following assumptions: • dam yield at 85% annual reliability: 68 GL/year • crop demand assuming dry-season field crops: 8 ML/ha • irrigation efficiency for spray application: 85% • irrigation efficiency for trickle application: 90% • irrigation efficiency for furrow irrigation: 80% • distribution efficiency for open-channel distribution: 90% • distribution efficiency for piped reticulation: 98% • river reticulation efficiency: 80% • percentage of gross area irrigated: 95% Targeted gross areas are therefore between 5,448 ha and 6,811 ha, depending on crop type, irrigation method and reticulation arrangement. The soils downstream of the potential upper Flying Fox Creek dam site can be characterised as follows: • A limited area of suitable soils exists some 10 km south of the Central Arnhem Highway. However, this is a section where the river is braided into at least six channels, and most of the suitable area is within the braided sections, limiting the usefulness due to flooding risk. • The largest aggregation of suitable soils is much further downstream, some 42 km south of the Arnhem Highway, where the creek runs to the northern side of a large area of clay soils. • Soils targeted for development are mapped by the digital soil modelling (Thomas et al., 2022) as predominantly SGG 2 friable non-cracking clay or clay loam soils and SGG 9 cracking clay soils. These are rated as suitability Class 3 for a broad range of dry-season crops under spray and furrow, with less suitability to wet-season cropping. They are particularly suited to dry-season cultivation of intensive horticulture under trickle and to a lesser extent spray, grain and fibre crops under spray and furrow, small-seeded crops, pulse crops, and forage crops under spray. Some of the soils are also rated as suitable for wet-season cultivation of rice and industrial crops. The major challenge in serving this area of soils is the fact that the suitable soils are some 53 km below the potential dam site. Distribution of water this far, without substantial demand on the way, will not be economical. This only leaves river distribution with a re-regulating structure as the likely mode of development. While this will result in significant losses in transmission, there is the potential to pick up additional yield from inflow from the intervening catchment, in particular that form Derim Derim Creek and Maori Creek. The potential re-regulating structure is assumed to be located on Flying Fox Creek ATMD 36 km and is descripted in Section 5.1.1. Other elements of the potential scheme for service of this area are: • a pump station at the re-regulating structure able to meet the full demand of the channel distribution network • an associated rising main, of sufficient length to (i) reach an elevation where the required area can be served by a gravity channel system, and (ii) extend far enough that the cross slope, which is very steep near the river, is flat enough to allow practical construction of an open-channel system. In practical terms, this indicates a cross slope below about one in six • an open-channel distribution system along the western edge of the serviced area. A significant feature of this channel system will be allowance for cross drainage from the upslope catchments. This will be by way of both cross-drainage culverts and drainage overpasses featuring inverted siphons. The former will allow the gradeline to be maintained, whereas the latter will involve a reduction in gradeline due to the head loss associated with the piped section of the inverted siphon under the overpass. Since the main limitation to this area will be wetness related to river flooding, there will be incentive to maintain the gradeline as high as possible. Areas were chosen for development based on the following: • suitability for a broad range of crop types; however, the base case chosen for the selection was Crop Type 7 under dry-season spray • allowance for the drainage network that will be required to get flows from above the channel through the developed area • a preference for regular-shaped areas that may be suited to both spray and furrow irrigation. Note that this implies the inclusion of some minor areas of Class 4 soils. The area able to be irrigated is calculated as 5,200 ha, based on the following assumptions: • Available water yield is assumed as 80% of 68 GL. Note that this assumes the contribution from the intervening catchments is minor, with the majority of flows released for environmental flow purposes. • Crop demand is assumed as 8 ML/ha. • Irrigation efficiency is assumed as 85% for spray. • Channel distribution efficiency is assumed as 90%. Note that this reflects the relatively short system and assumes some supervisory control system, such as Total Channel Control, is implemented. This corresponds to a gross area of some 5,485 ha, using the same 95% land usage factor as above. The location of the serviced areas and the alignment of the channel serving these areas are shown in Figure 7-2. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 7-2 Nominal conceptual layout of potential irrigation area on Flying Fox Creek The irrigation demand is assessed as 7.8 mm/ha/day. The capacities of the main reaches are shown in Table 7-6. Table 7-6 Pipe parameters for nominal conceptual layout of scheme servicing soils downstream of potential dam on upper Flying Fox Creek REACH CHAINAGE (KM) LENGTH (M) AREA COMMANDED (HA) FLOWRATE (CUMEC) From To Rising Main 0 1.15 1,150 5.13 A-B 0 1.625 1,625 550 5.13 B-C 1.625 12.481 10,856 997 4.63 C-D 12.481 18.915 6,434 2,511 3.73 D-E 18.915 21.157 2,242 1,597 1.45 Total 5,655 The total area shown in Table 7-6 is slightly above the gross area calculated above, but will be used for this preliminary design, to ensure the design is conservative. Adopted channel parameters are provided in Table 7-7 based on: • all channels being earth lined from material borrowed from adjacent sections. Some sections are in shallow, lighter-textured soils, and are assumed to be over-excavated . Given the soils distribution, this is a realistic assumption • Manning’s equation, using n = 0.04 • a bank width of 4 m, one side gravelled • a 1:10,000 slope to maintain command over the suitable soil areas • channel commencement at an elevation of 80 mEMG96, where the majority of the suitable soils identified can be served, and where cross falls are below one in six. Table 7-7 Channel parameters for nominal conceptual layout of scheme servicing soils downstream of potential dam on upper Flying Fox Creek REACH DESIGN FLOWRATE (CUMEC) BED WIDTH (M) WATER DEPTH (M) FREEBOARD (M) SLOPE (M/KM) VELOCITY (M/SEC) A-B 5.13 4 2.15 0.5 0.1 0.3 B-C 4.63 4 2.0 0.5 0.1 0.29 C-D 3.73 4 1.8 0.5 0.1 0.27 D-E 1.45 3 1.5 0.4 0.1 0.24 Cross drainage will be a major consideration for this channel system, since it is essentially aligned to accrue water draining into the river system. The channel will also require a number of control points to allow the water level to be maintained at a minimum level in the channel at all times of operation, to minimise both erosion potential during increases in flow and weed growth. Contributing catchments that need to be safely passed across the channel alignment and align with downstream drainage lines are shown in Figure 7-2. Details are provided in Table 7-8. Table7-8Details of contributing catchments adjacent tomain channel CATCHMENTAREA(SQ KM) Q50(M3/S) ADOPTED DETAILS A1.925 43 Cross drain6*DN1500 B10.692 116 Siphon2*DN1800C31.6 218 Siphon2*DN1800D26.8 190 Siphon2*DN1800 Flowsfor the contributing catchments arederived via the Regional FloodFrequency EstimationModel and arefor a 1 in50 AEP. Eachsiphon identified aboveis also used asaregulation point to maintainminimum flowdepthsin the channel andallow foradequate flow control.Rubicon Flow Gateswould beused forregulation andwouldberemotely controlledusing the Total Channel Control technology. The longitudinal profile adopted and the resulting hydraulic gradeline is shownin Figure7-3.Thechainages in thisinstance commence at there-regulating weir. 707274767880828401000020000ElevationChainage from Re-Reg WeirSite 79 ChannelProfilen/sDFLPoolBedTop ofBank Figure7-3Elevation profile ofchannel 7.3.1Costestimate Costing of the channel and associated structuresis based on the followingassumptions for different soil generic groups: •Channels in SGG 7 areassumed to be fully over-excavated and lined, with rockexcavation below 1mdepth. •Channels in SGG 4 areassumed to be fully over-excavated and lined. 88|Surface water storage and reticulation •Channels in SGG2 are assumed to notrequire lining. •Channels in SGG9 are assumed to notrequire lining. •All channels aredesigned for roughly balanced cut and fill, taking intoaccount the use ofexcavated material in the various zonesinthe channel.This shouldbe attainable by lateral movement of the channel alignment. Costs are summarisedinTable7-9. Table7-9Cost summary COMPONENTCOST ($) Channel earthworks8,903,671 Structures2,355,637 Contractors overheads$,814,826 Design and construction overheads1,407,413 Contingency2,322,232 Total17,803,780 7.3.2Pump station andrisingmain Risingmain As outlined above, the rising mainwould run from the re-regulatingweirto a section on the ridgeadjacenttothe rightbank where slopes meet thecriteria above.Thisresults in a length of risingmainof 2.67km.Therising main has been sized using anet present valueapproach,wherethecapital cost of the risingmain and associated pumps is added tothe amortised value of thepowercost.The result is that aDN1800 PN10 GRP pipe is selected asthe optimum choice.Note that duetothe relatively short rising main length, the costcurves are flat andthe final choice is likelytobedriven bythe relativetax treatment of capital and recurrent expensesby the constructingauthority. Site 79 RisingMain 8.7 8.6 8.5 8.4 8.3 8.1 Rising Main Dia (mm) NPV Cost (M) 1,5001,6001,7001,8001,9002,000 Chapter7 Potential reticulatedinfrastructure and irrigation development|89 Pump station The above analysis indicates that the pump station will require an installed capacity with: • a flow rate of 5.13 m3/s • installed power of 650 kW. The arrangement would feature two submersible units in a pump well capable of closing the inlet during major flooding events. As little information is available on the site, the costing (Table 7-10) has been based on cost functions derived from similar installations in north Queensland. Table 7-10 Costs for the pump station and rising main COMPONENT COST ($) Pipeline 5,034,206 Structures incl pump station 2,640,542 Contractors overheads 886,675 Design and construction overheads 856,142 Contingency 2,696,848 Total 12,114,413 The combined cost of both the earthworks components and the rising main and pump station is therefore some $29.9 million, equivalent to some $5,700/ha of the 5,200 ha of serviced land. Part IV Summary comments 8 Summary comments The relatively undeveloped state of the water resources across northern Australia represents a globally unique opportunity for governments and communities to take a long-term view to water resource development and undertake a considered evaluation of different potential development pathways, including ‘do nothing’. This report documents the results of a catchment-scale pre- feasibility assessment of surface water storage options in the Roper catchment. Larger sites were a major focus of this study as the design and construction of smaller farm-scale dams is highly site specific. Overall the landscape of the Roper catchment is relatively subdued and as a result sites topographically suitable for dam construction are relatively uncommon. The best potential dam sites in terms of yield per unit cost are along the lower Roper River, lower Hodgson River, Wilton River and Waterhouse River, the latter two areas scheduled as Aboriginal Land under ALRA. Sites along the lower Roper River and Wilton River, while relatively high yielding by virtue of having large catchment areas, there are limited areas of contiguous soil suitable for irrigated agriculture downstream of these locations. Potential dam sites in the Roper catchment occur where resistant ridges of Proterozoic sandstone beds have been incised by the river systems and outcrop on both sides of river valleys. To the north of the Roper River the majority of the potential dam sites are located in the headwater of tributaries of the Roper River. Many of these sites and their potential irrigated areas would be difficult to access during the wet season without the construction of substantial bridge and road infrastructure. To the south of the Roper River there are few locations suitable for large dams as the topography is particularly subdued, and the better locations tend to be closer to the Roper River. The Sturt Plateau has no sites suitable for large dams. A major limitation to the use of large dams for irrigated agriculture in the Roper catchment is the disaggregated nature of soils suitable for irrigated agriculture. Few locations have moderately large contiguous areas of soils suitable for irrigation. Five potential dam sites were examined as part of a pre-feasibility analysis, four for the purpose of supplying water for irrigation and the fifth (on the Wilton River) for the purpose of hydro-electric power generation. These sites are summarised in Table 8-1. Table 8-1 Summary comments for potential dams in the Roper catchment NAME SUMMARY COMMENT Waterhouse River West Branch ATMD 70.5 km A low yielding site on the Waterhouse River West Branch. Upstream of large areas of sandy loam soils moderately suitable for irrigated agriculture. The site is situated also on Aboriginal land scheduled under ALRA and is classified under ‘inalienable freehold title’, which means that it cannot be bought, acquired or mortgaged. Although data from the NT cultural heritage sites register were not made available to the Assessment, it is likely that the site and parts of the potential inundation area would contain cultural heritage sites of significance. Waterhouse River ATMD 70.5 km The Waterhouse River potential dam site has one of the highest yield to cost ratios in the upper parts of the Roper catchment. The site appears to be suitable for a roller compacted concrete type dam with a central uncontrolled spillway The potential dam site is upstream of large areas of sandy loam soils moderately suitable for irrigated agriculture. The site is situated also on Aboriginal land scheduled under ALRA and is classified under ‘inalienable freehold title’, which means that it cannot be bought, acquired or mortgaged. Although data from the NT cultural heritage sites register were not made available to the Assessment, it is likely that the site and parts of the potential inundation area would contain cultural heritage sites of significance. NAME SUMMARY COMMENT Upper Flying Fox Creek ATMD 105 km A moderately high yielding site relative to other sites in the Roper catchment that appears to be suitable for a roller compacted concrete type dam with a central uncontrolled spillway 150 m wide. There is potential for a regulating weir downstream from the storage that would enable additional inflows to be captured and allow for the more efficient use of water released from the storage. conceptual arrangement releases would be made from the storage downstream to a regulating weir for diversion by irrigators. There is a high likelihood of unrecorded cultural heritage sites in the inundation area. Jalboi River ATMD 53 km A low yielding site on the Jalboi River that appears to be suitable for a roller compacted concrete type dam with a central uncontrolled spillway. The potential dam site a lower yield to cost ratio that other sites selected for pre-feasibility analysis; however, it is relatively close to large areas of alluvial soils moderately suitable for irrigated agriculture. Being north of the Roper River accessing the dam site and potential irrigation areas during the wet season would be challenging without major road and bridge infrastructure. The affected area would primarily affect the Lonesome Dove pastoral lease area. There is a high likelihood of unrecorded cultural heritage sites in the inundation area. Wilton River ATMD 33 km A very high yield potential dam site. However, there are limited areas of land that would be suitable for irrigated agriculture downstream of the site. The site has the most suitable characteristics for a dam supplying water for hydro-electric power in the Roper catchment, however, the site is remote and there is no electricity transmission infrastructure. The site is situated also on Aboriginal land scheduled under ALRA and is classified under ‘inalienable freehold title’, which means that it cannot be bought, acquired or mortgaged. Although data from the NT cultural heritage sites register were not made available to the Assessment, it is highly likely that the site and parts of the potential inundation area would contain cultural heritage sites of significance. In the Roper catchment land suitable for siting farm-scale offstream storages is limited in extent and geographically constrained to the alluvial soils adjacent to the Roper River and its major tributaries. In many of the better locations broad-scale flooding may be a concern, which may make access and wet season cropping challenging. In the Roper catchment the largest contiguous areas likely to be suitable for siting offstream storages were along the recent alluvium of the Roper River. References ANCOLD (1992) Status of dam safety in Australia. ANCOLD Bulletin 91, 9–29. Australian National Committee on Large Dams, Hobart. Barber M and Jackson S (2011) Indigenous water values and water planning in the upper Roper River, Northern Territory. CSIRO Water for a Healthy Country National Research Flagship. Benjamin J (2018) Farm-scale dam design and costs. A technical report to the Australian Government from the CSIRO Northern Australia Water Resource Assessment, part of the National Water Infrastructure Development Fund: Water Resource Assessments. CSIRO, Australia. Bisawas AK (2012) Preface. In: Tortajada C, Altinbilek D and Bisawas AK (eds) Impacts of large dams: A global assessment. Water Resources Development and Management. ISSN: 1614- 810X. © Springer-Verlag, Berlin, Heidelberg. DOI: 10.1007/978-3-642-23571-9. Cotton Catchment Communities CRC (2011) Storage seepage & evaporation. Final summary of results. Cotton storages project 2011. Cotton Catchment Communities CRC. Doherty J (1999) Chapter 4. Gravity dams. In: Cole B (ed.) A history of dam technology in Australia 1850–1999. Institution of Engineers, Australia and Australian National Committee on Large Dams, Penrith. Fell R, MacGregor P, Stapledon D, Bell G and Foste M (2015) Geotechnical engineering of dams. Second edition. Taylor and Francis, London. Gallant JC, Dowling TI, Read AM, Wilson N, Tickle P, Inskeep C (2011) 1 second SRTM Derived Digital Elevation Models User Guide. Geoscience Australia www.ga.gov.au/topographic- mapping/digital-elevation-data.html. Hughes J, Yang A, Marvanek S, Wang B, Petheram C and Philip S (2023) River model calibration and scenario analysis for the Roper catchment. A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. IAA (2007) Guidelines for ring tank storages. Ed. Hugh Barrett. Irrigation Association of Australia Ltd. International Rivers (2017) True cost of hydropower in China. Viewed 5 May 2017, https://www.internationalrivers.org/sites/default/files/attached- files/truecostofhydro_en_small.pdf. Jeffrey SJ, Carter JO, Moodie KM and Beswick AR (2001). Using spatial interpolation to construct a comprehensive archive of Australian climate data, Environ. Modell. Softw., 16(4), 309-330. Keating BA, Carberry PS, Hammer GL, Probert ME, Robertson MJ, Holzworth D, Huth NI, Hargreaves JNG, Meinke H and Hochman Z (2003) An overview of APSIM, a model designed for farming systems simulation. European Journal of Agronomy 18, 267–288. Kim S, Hughes J, Ticehurst C, Stratford D, Merrin L, Marvanek S and Petheram C (2023) Floodplain inundation mapping and modelling for the Roper catchment. A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Kim S, Petheram C, Hornbuckle J, Holz L, Kehoe M, Podger S, Aramini D, Wilson P, McJannet D, Read A, Munday T, Davis A, Poulton P, Rassam D, Waltham N, Burrows D, Hughes J, Brennan McKellar L, Podger G, Philip S, Barber M and Schmidt RK (2013). Chapter 9: O’Connell Creek off-stream storage and irrigated rice. In: Petheram C, Watson I and Stone P (eds) Agricultural resource assessment for the Flinders catchment. A report to the Australian Government from the CSIRO Flinders and Gilbert. Kinstler F (2000). Buttress dams, Chapter 7 in ‘Dam Technology in Australia 1850–1999’, Editor Bruce Cole, Australian National Committee on Large Dams Inc. Lewis B (2002) Farm dams: planning, construction and maintenance. Landlinks Press, Collingwood. McBratney AB, Mendonca ML and Minasny B (2003) On digital soil mapping. Geoderma 117, 3–52. McMahon TA and Adeloye AJ (2005) Water resources yield. Water Resources Publications, LLC, Highlands Ranch, Colorado. Nace RL (1972). Water problems and developments of the past. Journal of the American Water Resources. NCST (2009) Australian soil and land survey field handbook. National Committee on Soil and Terrain. CSIRO Publishing 2009. O’Donnell E and Hart B (2016) Damming northern Australia: we need to learn hard lessons from the south. The Conversation. Viewed 5 May 2017, https://theconversation.com/damming- northern-australia-we-need-to-learn-hard-lessons-from-the-south-53885. Petheram C, Gallant J, Read, A. (2017b). DamSite: An automated and rapid method for identifying dam wall locations. Environmental modelling and software, 92, 189-201. Petheram C, Gallant J, Wilson P, Stone P, Eades G, Roger L, Read A, Tickell S, Commander P, Moon A, McFarlane D, Marvanek S (2014) Northern rivers and dams: a preliminary assessment of surface water storage potential for northern Australia. CSIRO Land and Water Flagship Technical Report. CSIRO, Australia. 74 | Northern rivers and dams. Petheram C, McMahon TA, Peel MC (2008) Flow characteristics of rivers in northern Australia: implications for development, Journal of Hydrology 58(1–2), 93–111. Petheram C, Rogers L, Eades G, Marvanek S, Gallant J, Read A, Sherman B, Yang A, Waltham N, McIntyre-Tamwoy S, Burrows D, Kim S, Podger S, Tomkins K, Poulton, P, Holz L, Bird M, Atkinson F, Gallant S and Kehoe, M (2013) Assessment of surface water storage options in the Flinders and Gilbert catchments, A technical report to the Australian Government from the CSIRO Flinders and Gilbert Agricultural Resource Assessment, part of the North Queensland Irrigated Agriculture Strategy, CSIRO Water for a Healthy Country and Sustainable Agriculture flagships, Australia. Petheram C, Rogers L, Read A, Gallant J, Moon A, Yang A, Gonzalez D, Seo L, Marvanek S, Hughes J, Ponce Reyes R, Wilson P, Wang B, Ticehurst C and Barber M (2017a) Assessment of surface water storage options in the Fitzroy, Darwin and Mitchell catchments. A technical report to the Australian Government from the CSIRO Northern Australia Water Resource Assessment, part of the National Water Infrastructure Development Fund: Water Resource Assessments. CSIRO, Australia. QWRC (1984) Farm water supplies design manual. Volume I Farm storages. Second Edition. Edited by AJ Horton and GA Jobling. Farm Water Supplies Section, Irrigation Branch, Queensland Water Resources Commission. Read, A., Gallant, J., and Petheram, C. (2012). DamSite: An automated method for the regional scale identification of dam wall locations. 34th Hydrology & Water Resources Symposium, Sydney, Australia, 19-22 November 2012. Engineers Australia. Scarborough and Gallopin (1991) A water storage adaptation in the Maya lowlands. Science 251(4994), 658–662. DOI: 10.1126/science.251.4994.658. Schnitter NJ (1994) A history of dams: the useful pyramids. AA Balkema, Rotterdam. Stokes C, Jarvis D, Webster A, Watson I, Jalilov S, Oliver Y, Peake A, Peachey A, Yeates S, Bruce C, Philip S, Prestwidge D, Liedloff A, Poulton P, Price B and McFallan S (2023) Financial and socio-economic viability of irrigated agricultural development in the Roper catchment. A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Stratford D, Kenyon R, Pritchard J, Merrin L, Linke S, Ponce Reyes R, Blamey L, Buckworth R, Castellazzi P, Costin B, Deng R, Gannon R, Gilbey S, King D, Kopf K, Kopf S, McGinness H, McInerney P, Perna C, Plaganyi E and Waltham N (2022) Ecological assets of northern Australia to inform water resource assessments. A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Stratford D, Merrin L, Linke S, Kenyon R, Ponce Reyes R, Buckworth R, Deng R, McGinness H, Pritchard J, Seo L and Waltham N (2024) Assessment of the potential ecological outcomes of water resource development in the Roper catchment. A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Thomas M, Philip S, Stockman U, Wilson PR, Searle, R, Hill J, Bui E, Gregory, L, Watson, I, Wilson PL and Gallant G (2022) Soils and land suitability for the Roper catchment, Northern Territory. A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Tomkins K (2013) Estimated infilling rates for dams in northern Australia based on a review of previous literature. A technical report to the Australian Government from the CSIRO Flinders and Gilbert Agricultural Resource Assessment, part of the North Queensland Irrigated Agriculture Strategy. CSIRO Water for a Healthy Country and Sustainable Agriculture flagships, Australia. WCD (2000) Dams and development, a new framework for decision-making. World Commission on Dams. Earthscan Publications Ltd, London and Sterling, VA. Part V Appendices Detailed costings for short-listed dam sites in the Roper catchment A.1 Potential dam site on the Waterhouse River ATMD 70.5 km Apx Table A-1 Potential dam site on the Waterhouse River ATMD 70.5 km - direct construction costs DIRECT CONSTRUCTION COSTS UNIT QUANTITY RATE ($) AMOUNT ($) General Environmental management Lump sum 2,000,000 Cultural heritage management Lump sum 600,000 Community consultation Lump sum 200,000 Mobilisation and demobilisation Establishment of workforce accommodation Lump sum 6.000,000 Establishment of survey control Lump sum 500,000 Establish construction power supply (temporary) Lump sum 2,500,000 Establish communications Lump sum 250,000 Mobilisation of major plant Lump sum 3,500,000 Demobilisation of major plant Lump sum 875,000 Demobilisation of workforce accommodation Lump sum 1,800,000 Clear site and 50% of storage area ha 1,720 2,800 4,816,000 Mobilize/demobilise site laboratory Lump sum 400,000 Access Access road to site from Central Arnhem Highway km 15 1,150,000 17,250,000 Establish site access roads Lump sum 2,000,000 Rehabilitation of roads on construction completion Lump sum 1,500,000 Construction Material sources Remove quarry overburden Lump sum 300,000 Develop quarry Lump sum 450,000 Access road to quarry km 2 700,000 1,400,000 Access road to sand gravel sources km 5 700,000 3,500,000 DIRECT CONSTRUCTION COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Diversion and care of river Excavate LB diversion channel (rock) cu m 17,500 60 1,050,000 Excavation for coffer dams cu m 4,760 20 95,000 Place material for coffer dams cu m 17,750 30 532,000 Dewatering Lump sum 350,000 Divert river Lump sum 400,000 Removal of coffer dams cu m 17,750 25 444,000 Foundations cross river section Excavate sand from river bed cu m 8,890 15 133,000 Excavate rock from abutments and bed cu m 57,645 50 2,882,000 Detailed excavation cu m 1,000 250 250,000 Detailed clean up sq m 11,020 130 1,433,000 Dental concrete cu m 1,000 800 800,000 Foundation grouting cross river section Concrete to grouting plinth cu m 1,072 800 858,000 Reinforcement to grout plinth tonne 43 5,500 235,000 Drill grout holes m 5,220 100 522,000 Supply and install standpipes no 480 200 96,000 Hook ups and pressure tests no 564 200 113,000 Pressure grouting bags 10,440 50 522,000 RCC dam river section Mobilise RCC placement plant Lump sum 3,500,000 De mobilise RCC placement plant Lump sum 1,050,000 Trial mixes Lump sum 400,000 Diversion channel plug cu m 2,640 700 1,848,000 Conventional concrete to faces cu m 9,680 800 7,744,000 RCC concrete to dam wall cu m 68,775 535 36,795,000 Gallery floor units and precast slabs m 146 4,000 584,000 Conventional concrete to spillway crest cu m 900 1,800 1,620,000 Reinforcement to spillway crest tonne 36 7,000 252,000 Conventional concrete to spillway crest cu m 1,200 800 960,000 Reinforcement to spillway apron tonne 48 6,000 288,000 Conventional concrete to end sill cu m 253 1,500 380,000 Reinforcement to end sill tonne 15 7,000 105,000 DIRECT CONSTRUCTION COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Drill holes for apron anchors m 1,680 150 252,000 Supply and install apron anchors tonnes 11 7,000 77,000 Conventional concrete to training walls cu m 1,234 1,700 2,098,000 Reinforcement to training walls m 74 7,500 555,000 Drill drainage holes m 384 150 58,000 Water stops - supply and install m 370 200 740,000 Backfill on abutments cu m 3,580 50 179,000 Instrumentation HW/TW recorders etc Lump sum 200,000 Miscellaneous metalwork Lump sum 100,000 Outlet works Intake tower concrete cu m 262 2,000 524,000 Intake tower reinforcement tonne 15 8,000 120,000 Intake tower guides and seals Lump sum 300,000 Trashracks item 12 40,000 480,000 Selective withdrawal baulks item 12 30,000 360,000 Bulkhead gate Lump sum 160,000 Hoist crane, install and commission Lump sum 200,000 Ladders and platforms Lump sum 200,000 Supply and install 1,200 mm dia outlet conduits tonne 70 14,000 980,000 Outlet conduits concrete encasement cu m 168 2,000 336,000 Outlet conduits concrete reinforcement tonne 7 8,000 56,000 Valve house concrete cu m 140 2,000 280,000 Valve house reinforcement tonne 5.6 8,000 45,000 Outlet works pipework Lump sum 350,000 Butterfly valves and actuators no 2 200,000 400,000 Fixed cone regulating valves no 2 250,000 500,000 Outlet works hoist crane Lump sum 150,000 Electrical hydraulic installations Lump sum 300,000 Fish transfer facility Concrete to intake channels, hopper chamber and valve pit cu m 500 2,000 1,000,000 Reinforcement to intake channels, hopper chamber and valve pit tonne 25 8,000 200,000 Fish attraction pipework, valves and diffusers Lump sum 500,000 Fish traps Lump sum 250,000 Fish lift hopper no 250,000 DIRECT CONSTRUCTION COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Hopper tracks Lump sum 300,000 Overhead crane at crest Lump sum 350,000 Monitoring equipment Lump sum 300,000 Electrical and mechanical installations Lump sum 300,000 Fish lift commissioning Lump sum 400,000 Permanent downstream access crossing Lump sum 2,500,000 Total direct construction costs (TDC) 126,432,000 Apx Table A-2 Potential dam site on the Waterhouse River ATMD 70.5 km – on site costs ON SITE COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Project and field staff Lump sum 3% of TDC 3,792,960 Staff recruitment and training Lump sum 0.6% of TDC 758,592 Camp operations Lump sum 3.5% of TDC 4,425,120 Site office expenses Lump sum 0.6% of TDC 758,592 Site water services Lump sum 0.06% of TDC 75,859 Operating cost temporary power supply Lump sum 2,250,000 Site communication, IT expenses Lump sum 0.45% of TDC 568,944 Site cleaning, rubbish removal Lump sum 0.04% of TDC 50,573 Project control testing Lump sum 650,000 Misc. travel expenses Lump sum 0.2% of TDC 252,864 Insurances, public liability Lump sum 3.4% OF TDC 4,298,688 Total on site overheads (OSO) 17,882,192 Total direct and on site overhead costs 144,314,192 Profit and off site overheads 10% of TDC and OSO 14,431,419 Total Out Turn Costs (TOC) 158,745,611 Apx Table A-3 Potential dam site on the Waterhouse River ATMD 70.5 km – owner costs OWNERS COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Investigation and design Preliminary design Lump sum 0.5% of TDC 632,160 Geotechnical and materials Lump sum 2.0% of TDC 2,528,640 Hydraulic model study Lump sum 750,000 Detailed design and documentation Lump sum 2.5%of TDC 3,160,800 Acquisition and Approvals Environmental assessment and approvals Lump sum 4,000,000 Cultural heritage Lump sum 3,000,000 Native title Lump sum 500,000 Storage area acquisition ha 5,400 750 4,050,000 Storage area access relocations Lump sum 500,000 Surveys and legals Lump sum 500,000 Permanent onsite buildings and services Lump sum 500,000 Principal's insurances (1.1% of TOC) Lump sum 1,746,202 Owners management and supervision (0.15% of TOC) Lump sum 238,118 Total owners costs 22,105,920 A.2 Potential dam site on the Flying Fox Creek ATMD 105 km Apx Table A-4 Potential dam site on the Flying Fox Creek ATMD 105 km – direct construction costs DIRECT CONSTRUCTION COSTS UNIT QUANTITY RATE ($) AMOUNT ($) General Environmental management Lump sum 2,000,000 Cultural heritage management Lump sum 400,000 Community consultation Lump sum 200,000 Mobilisation and demobilisation Establishment of workforce accommodation Lump sum 6,500,000 Establishment of survey control Lump sum 300,000 Establish construction power supply (temporary) Lump sum 2,500,000 Establish communications Lump sum 500,000 Mobilisation of major plant Lump sum 4,000,000 Demobilisation of major plant Lump sum 875,000 Demobilisation of workforce accommodation Lump sum 1,800,000 Clear site and 50% of storage area ha 885 3,000 2,655,000 Mobilise/demobilise site laboratory Lump sum 400,000 Access Access to site from Central Arnhem Highway km 10 1,150,000 11,500,000 Establish site access roads Lump sum 2.000.000 Rehabilitation of roads on construction completion Lump sum 1.500.000 Construction Material sources Remove quarry overburden Lump sum 350,000 Develop quarry Lump sum 500,000 Access road to quarry km 2 700,000 1,400,000 Access road to sand gravel sources km 5 700,000 3,500,000 Diversion and care of river Excavate LB diversion channel (rock) cu m 18,240 60 1.094.000 Excavation for coffer dams cu m 15,770 17.5 276,000 Place material for coffer dams cu m 49,700 30 1,491,000 Dewatering Lump sum 450,000 Divert river Lump sum 400,000 DIRECT CONSTRUCTION COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Removal of coffer dams cu m 49,700 20 994,000 Foundation excavation and treatment Excavate sand from river bed cu m 18,000 15 270,000 Excavate rock from bed and abutments cu m 55,460 50 2,773,000 Detailed excavation cu m 1,000 250 250,000 Detailed clean up sq m 9,320 130 1,212,000 Dental concrete cu m 1,000 800 800,000 Foundation grouting cross river section Concrete to grouting plinth cu m 788 800 630,000 Reinforcement to grout plinth tonne 32 6,000 192,000 Drill grout holes m 5,290 100 529,000 Supply and install standpipes no 350 200 70,000 Hook ups and pressure tests no 690 200 138,000 Pressure grouting bags 10,580 50 529,000 RCC dam river section Mobilise RCC placement plant Lump sum 4,000,000 De mobilise RCC placement plant Lump sum 1,200,000 Trial mixes Lump sum 400,000 Diversion channel plug cu m 2,640 700 1,848,000 Conventional concrete to faces cu m 19,465 775 15,085,000 RCC concrete to dam wall cu m 130,275 500 65,138,000 Gallery floor units and precast slabs m 350 3,500 1,225,000 Conventional concrete to spillway crest cu m 1,800 1,750 3,150,000 Reinforcement to spillway crest tonne 72 7,000 504,000 Conventional concrete to spillway apron cu m 2,400 800 1,920,000 Reinforcement to spillway apron tonne 96 6,000 576,000 Conventional concrete to end sill cu m 506 1,400 708,000 Reinforcement to end sill tonne 30 7,000 210,000 Drill holes for apron anchors m 2,880 150 432,000 Supply and install apron anchors tonnes 21 7,000 147,000 Conventional concrete to training walls cu m 1,234 1,700 2,098,000 Reinforcement to training walls tonne 74 7,500 555,000 Drill drainage holes m 975 150 145,000 Water stops - supply and install m 645 200 129,000 Backfill on abutments cu m 3,220 50 161,000 DIRECT CONSTRUCTION COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Instrumentation HW/TW recorders etc Lump sum 250,000 Miscellaneous metalwork Lump sum 150,000 Outlet works Intake tower concrete cu m 262 2,000 524,000 Intake tower reinforcement tonne 15 8,000 120,000 Intake tower guides and seals Lump sum 300,000 Trashracks item 12 40,000 480,000 Selective withdrawal baulks item 12 30,000 360,000 Bulkhead gate Lump sum 160,000 Hoist crane, supply, install and commission Lump sum 200,000 Ladders and platforms Lump sum 200,000 Supply and install 1200 mm dia outlet conduits tonnes 70 14,000 980,000 Place concrete surround cu m 168 2,000 336,000 Reinforcement to concrete surround tonnes 7 8,000 56,000 Supply and install 1200 mm dia butterfly guard valves Item 2 200,000 400,000 Supply and install 900 mm dia regulating valves Item 2 250,000 500,000 Valve house pipework Lump sum 350,000 Valve house concrete cu m 140 2,000 280,000 Reinforcement to valve house tonne 5.6 8,000 45,000 Electrical mechanical installations Lump sum 100,000 Hoist crane, supply, install and commission Lump sum 150,000 Fish transfer facility Concrete to intake channels, hopper chamber and valve pit cu m 500 2,000 1,000,000 Reinforcement to intake channels, hopper chamber and valve pit tonne 25 8,000 200,000 Fish attraction pipework, valves and diffusers Lump sum 500,000 Fish traps Lump sum 250,000 Fish lift hopper Lump sum 250,000 Hopper tracks Lump sum 300,000 Overhead crane at crest Lump sum 350,000 Monitoring equipment Lump sum 300,000 Electrical and mechanical installations Lump sum 300,000 Fish lift commissioning Lump sum 400,000 Permanent downstream crossing Lump sum 4,000,000 DIRECT CONSTRUCTION COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Total direct construction costs (TDC) 163,806,000 Apx Table A-5 Potential dam site on the Flying Fox Creek ATMD 105 km – on site overheads ON SITE OVERHEAD UNIT QUANTITY RATE ($) AMOUNT ($) ON SITE OVERHEADS Project and field staff Lump sum 3% of TDC 4,914,180 Staff recruitment and training Lump sum 0.6% of TDC 982,836 Camp operations Lump sum 3.5% of TDC 5,733,210 Site office expenses Lump sum 0.6% of TDC 982,836 Site water services Lump sum 0.06% of TDC 98,284 Operating cost temporary power supply Lump sum 2,500,000 Site communication, IT expenses Lump sum 0.45% of TDC 737,127 Site cleaning, rubbish removal Lump sum 0.04% of TDC 65,522 Project control testing Lump sum 750,000 Misc. travel expenses Lump sum 0.2% of TDC 327,612 Insurances, public liability Lump sum 3.4% OF TDC 5,569,404 Total on site overheads (OSO) 22,661,011 Total direct and on site overhead costs 186,467,011 Profit and off site overheads 10% of TDC and OSO 18,646,701 Total Out Turn Costs (TOC) 205,113,712 Apx Table A-6 Potential dam site on the Flying Fox Creek ATMD 105 km – owner costs OWNERS COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Investigation and design Preliminary design Lump sum 0.5% of TDC 819,030 Geotechnical and materials Lump sum 2.0% of TDC 3,276,120 Hydraulic model study Lump sum 750,000 Detailed design and documentation Lump sum 2.5% of TDC 4,095,150 Acquisition and Approvals OWNERS COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Environmental assessment and approvals Lump sum 4,000,000 Cultural heritage Lump sum 3,000,000 Native title Lump sum 500,000 Storage area acquisition ha 2600 700 1,820,000 Storage area access relocations Lump sum 500,000 Surveys and legals Lump sum 500,000 Permanent onsite buildings and services 500,000 Principal's insurances Lump sum1.1% of TOC 2256251 Owners management and supervision Lump sum 0.015% of TOC 307,671 Total owners costs 22,324,221 TOTAL PROJECT COSTS (TPC) 227,437,934 Risk adjustment 90,975,173 TOTAL CAPITAL COST 318,413,107 A.3 Potential weir site on the Flying Fox Creek ATMD 36 km Apx Table A-7 Potential weir site on the Flying Fox Creek ATMD 36 km – direct construction costs DIRECT CONSTRUCTION COSTS UNIT QUANTITY RATE ($) AMOUNT ($) General Environmental management Lump sum 150,000 Cultural heritage management Lump sum 150,000 Community consultation Lump sum 50,000 Mobilisation and demobilisation Establishment of workforce accommodation Lump sum 3,000,000 Establishment of survey control Lump sum 100,000 Establish construction power supply (temporary) Lump sum 750,000 Establish communications Lump sum 200,000 Mobilisation of major plant Lump sum 1,500,000 Demobilisation of major plant Lump sum 500,000 Demobilisation of workforce accommodation Lump sum 1,000,000 Clear site and 50% of storage area ha 265 5,000 1,325,000 Site laboratory Lump sum 200,000 Access Access road to site km 38 500,000 19,000,000 Establish site access roads Lump sum 500,000 Rehabilitation of roads on construction completion Lump sum 200,000 Supply sheet piling to site tonnes 1,892 2,500 4,730,000 CONSTRUCTION Material sources Develop sources for clay and sand materials Lump sum 100,000 Establish source of rock spalls Lump sum 150,000 Diversion and care of river Excavate diversion channel cu m 13,440 20 269,000 Excavation for coffer dams cu m 3,840 15 58,000 Place material for coffer dams cu m 22,100 30 663,000 Dewatering Lump sum 400,000 Divert river Lump sum 200,000 Removal of coffer dams cu m 22,100 15 332,000 DIRECT CONSTRUCTION COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Foundations Excavate OTR from river bed and abutments cu m 26,760 15 401,000 Over flow section Drive row 4 piling sq m 3,120 420 1,310,000 Drive row 3 piling sq m 3,360 430 1,445,000 Drive row 2 piling sq m 4,080 440 1,795,000 Drive row 1 piling sq m 4,560 450 2,052,000 Downstream protective apron slabs cu m 3,270 600 1,962,000 Reinforcement to slabs tonne 131 6,000 786,000 Upstream concrete slab cu m 654 600 392,000 Reinforcement to upstream slab tonne 26 6,000 156,000 Concrete low flow channel cu m 456 650 296,000 Reinforcement to low flow channel tonne 18 6,000 108,000 Waterstops m 960 200 192,000 Drains through piling rows 2,3 and 4 no 140 300 42,000 Filter fabric under mattresses sq m 6,540 5 33,000 Rockfilled mattresses sq m 6,540 150 981,000 Compacted sand cu m 10,000 25 250,000 Compacted clay cu m 7,350 40 294,000 Left and right abutment sections Drive rows 1-4 sq m 972 420 408,000 Concrete protection slabs cu m 216 800 173,000 Reinforcement to slabs tonne 9 6,000 54,000 Filter fabric under matresses sq m 1,566 10 16,000 Rockfilled mattresses sq m 1,566 160 251,000 Outlet works Inlet box concrete cu m 40 2,000 80,000 Inlet box reinforcement tonne 2 8,000 16,000 Trashscreens Lump sum 10,000 Outlet pipe supply and install m 100 2,000 200,000 Backfill around outlet pipe cu m 8,950 50 447,000 Outlet box Lump sum 27 2,000 54,000 Outlet box reinforcement tonne 1 8,000 88,000 Outlet box pipework Lump sum 25,000 DIRECT CONSTRUCTION COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Regulating valve Lump sum 50,000 Guard valve Lump sum 35,000 Miscellaneous items Lump sum 50,000 Vertical slot fish ladder Concrete to floor and walls of ladder cu m 60 2,000 120,000 Reinforcement tonne 2.4 8,000 19,000 Install baffles for slots no 50 750 37,000 Cover gratings Lump sum 15,000 Total Direct Construction Costs (TDC) 50,170,000 Apx Table A-8 Potential weir site on the Flying Fox Creek ATMD 36 km – on site overheads ON SITE OVERHEADS UNIT QUANTITY RATE ($) AMOUNT ($) Project and field staff Lump sum 3% of TDC 1505100 Staff recruitment and training Lump sum 0.6% of TDC 301020 Camp operations Lump sum 3.5% of TDC 1755950 Site office expenses Lump sum 0.6% of TDC 301020 Site water and power expenses Lump sum 0.1% of TDC 50170 Site communication, IT expenses Lump sum 0.45% of TDC 225765 Site cleaning, rubbish removal Lump sum 0.04% of TDC 20068 Project control testing Lump sum Misc. travel expenses Lump sum 0.2% of TDC 100340 Insurances, public liability Lump sum 3.4% OF TDC 1705780 Total On Site Overheads 5965213 TDC and OSO costs 56,135,213 Profit and off site overheads (10% of TDC and OSO) 5,613,521 Total Out Turn Costs (TOC) 61,748,734 Apx Table A-9 Potential weir site on the Flying Fox Creek ATMD 36 km – owner costs OWNER COSTS UNIT QUANTITY RATE ($) AMOUNT ($) Investigation and design Preliminary design Lump sum 0.5% of TDC 250850 Geotechnical and materials Lump sum 2.0% of TDC 1003400 Detailed design and documentation Lump sum 2.5%of TDC 1254250 Acquisition and Approvals Environmental assessment and approvals Lump sum 400,000 Cultural heritage Lump sum 200,000 Native title Lump sum 75,000 Site acquisitions Lump sum 200,000 Surveys and legals Lump sum 150,000 Principal's insurances Lump sum 1.1% of TDC 551,870 Owners management and supervision Lump sum 0.4% of TDC 200,680 Total owners costs 4,286,050 TOTAL PROJECT COSTS 66,034,784 Risk adjustment 23,112,175 TOTAL CAPITAL COST 89,146,959 Potential dam site summary tables B.1 Waterhouse River West Branch dam site on Waterhouse River West Branch; ATMD 70.5 km PARAMETER DESCRIPTION Previous investigations No previous investigations of this site have been located. The site was identified from CSIRO DamSite modelling. References No references to any investigation of this site have been located. Description of potential dam This is a potential dam with FSL 23 m (elevation 188 mEMG96) above bed level and a storage capacity of 128 GL. A storage at this site could provide supply for irrigation of suitable lands downstream of the storage. Releases would be made from the storage to the stream for diversion by irrigators. The location of the potential site is shown in Apx Figure B-1 and Apx Figure B-2. Apx Figure B-3 shows the known water-dependent ecological assets in the vicinity of the potential dam site. Regional geology The Roper catchment landscape is flat to undulating and consists of a series of erosion surfaces developed over the last 70 million years which are characterised by deep weathering profiles and associated iron-cemented capping. The continued erosion has led to the emergence of the present-day landscape and involved the removal of Cretaceous strata from most of the region and the etching out of structures in the underlying Proterozoic rocks. The resulting landscape is characterised by broad alluvial plains associated with the Roper River and its tributaries, and low rubbly hills or strike ridges formed by folded and faulted resistant thick Proterozoic sandstone formations. Site geology Based on geological mapping (high-level fly-over only) The potential dam site is located on Proterozoic rocks of the Mount Rigg Group (Pon) which consist of quartz sandstone, commonly cross-bedded and which overlie rocks of the Katherine River Group (Phw) which consist of basalt: massive and vesicular and dolerite and dip to the north at about 10°. The deeply weathered erosion surface characteristic of the region appeared to have been removed by erosion at the dam site. On the dam abutments weathered sandstone bedrock outcrops as cliff lines with some talus (blocky slope deposits) on the surface; possibly 2 to 5 m stripping would be required. The river channel consists of a tract of mobile unvegetated alluvium (sand and gravel) and the depth of alluvium is anticipated to be 3 to 5 m locally. The geology of the site is shown in Apx Figure B-4. Reservoir rim stability and leakage potential There is no evidence of instability or leakage potential around the reservoir rim. Potential structural arrangement The site appears suitable for a roller compacted concrete type dam with a central uncontrolled spillway with crest length of 75 m. Outlet works and a fish lift facility would be located on the right abutment. A saddle dam some 2 km long will be required on the western side of the storage to contain the storage and flood rises. Access to the right bank of the site would be via 19 km of new road branching from the Central Arnhem Highway. The total distance from Katherine would be 128 km. A crossing over a tributary of Beswick Creek would be required to reach the site. PARAMETER DESCRIPTION Availability of construction materials A quarry in the Proterozoic sandstone with suitable aggregate for an RCC dam might be found within 5 km of the dam site. Fine aggregate might be won from the river bed. For estimating purposes a ratio of useful rock excavated to total volume excavated of 0.5 could be assumed. Hard rock quarries for higher quality aggregate to construct an outer layer of RCC for the dam could probably be sourced from the Katherine area. Catchment area 795 km2 Modelled annual inflow data Parameter Scenario A (GL/yr) Scenario Cdry (GL/yr) Scenario Cmid (GL/yr) Scenario Cwet (GL/yr) Max 412 312 367 531 Mean 103 80 92 142 Median 85 63 71 118 Min 2 1 2 2 Reservoir characteristics Reservoir characteristics are shown in Apx Figure B-5. Reservoirs with FSLs of different heights are tabulated below. FSL (mEGM96) Surface area (ha) Capacity (GL) 186 1628 91 188 2067 128 190 2564 174 Reservoir yield assessment at dam wall FSL 186 mEGM96 Estimated yield at 85% annual time reliability 45 GL FSL 188 mEGM96 Estimated yield at 85% annual time reliability 48 GL FSL 190 mEGM96 Estimated yield at 85% annual time reliability 52 GL Reservoir yields under projected future climates shown in Apx Figure B-7. Estimated rates of reservoir sedimentation at FSL 188 mEGM96 Best case Expected Worst case 30 years (%) 0.7 1.1 1.2 100 years (%) 2.4 3.6 4.0 Years to fill 4201 2801 2521 Potential use of supply The area below the junction of the upper Waterhouse River and Waterhouse River West Branch is dominated by dissected tablelands of the Sturt Plateau in the upper catchment, alluvial plains adjacent to the river, and sandplains in the lower sections. The tablelands and associated scarps have well-drained, predominantly moderately deep to shallow sandy-surfaced red Kandosols (SGG 4.1) with abundant ironstone gravels throughout the soil profile, and abundant rock on and adjacent to the scarps (SGG 7). The deeper soils with less gravels are suitable for a diverse range of spray- and trickle-irrigated crops, predominantly horticultural crops. However, these areas are heavily fragmented by shallow or rocky areas resulting in small usable areas, mainly for trickle-irrigated horticulture. The alluvial plains in the upper catchment are dominated by very deep sandy-surfaced brown Dermosols (SGG 2) with moderately permeable, moderately well-drained to imperfectly drained, mottled structured clay subsoils. Sandy-surfaced Kandosols with brown and yellow massive loamy subsoils (SGG 4.2) and red subsoils (SGG 4.1) are intermixed. Small areas of gilgaied grey cracking clays (SGG 9) occur on the alluvial back-plains in the Beswick area. The brown and grey soils are subject to seasonal wetness (wet season) and may be subject to flooding. Soils are suited to a diverse range of dry-season spray-irrigated grain, pulse and forage crops; horticultural small crops; wetness-tolerant horticultural tree crops, sugarcane and cotton. Wet-season crops are restricted to crops with a moderate tolerance to wetness such as spray- irrigated forage crops, sunflower, sesame and rice. Fragmentation of the area by creeks and soil distribution may limit the usable areas for various uses. PARAMETER DESCRIPTION The red, brown and yellow massive loamy soils with sandy surfaces (Kandosols SGG 4.1) occur on the gently undulating elevated sandplains and level plains on recent alluvium adjacent to the lower reaches of the Waterhouse River. Moderately deep to very deep (0.5 to >1 m), well-drained to imperfectly drained, red (SGG 4.1) and mottled brown or yellow (SGG 4.2), sandy-surfaced massive soils occur as a mosaic over the landscape, probably reflecting the depth to the underlying rock with red soils on the deeper areas. These moderately permeable soils have moderate to high (100 to 140 mm to 1 m) soil water storage and are highly suited to a broad range of spray- or trickle-irrigated crops. However, fragmentation of the area by creeks and soil distribution may limit the usable areas for various uses. Impacts In addition to an area of 2067 ha at FSL 188 mEMG96, a flood margin area would also need to be acquired. This area would primarily affect the Beswick Aboriginal Land Trust area. Environmental impacts Barrier to movement of aquatic species At this site there were no records of any species. However, in nearby streams there are records of catfish (Porochilus rendahli) and barramundi (Lates calcarifer) and their movement may be impeded by a dam. Ecological implications of inundation The ‘Arnhem Plateau Sandstone Shrubland Complex’ listed as Endangered Ecological Community (EPBC Act) surrounds the potential inundated area at FSL for this site (188 mEMG96). The potential for ecological change as a result of changes to the downstream flow regime is examined in the companion technical report on ecology (Stratford et al., 2023). Indigenous land tenure, native title and cultural heritage considerations There is a high likelihood of unrecorded sites in the inundation area. Estimated cost To enable a like-for-like comparison with the non-short-listed sites, dam costs were calculated using CSIRO’s generalised dam costing algorithm, which takes into account major cost elements for RCC type dams with central overflow spillways. These are reported for a selection of FSL below. FSL 186 mEGM96 $361 million FSL 188 mEGM96 $415 million FSL 190 mEGM96 $478 million. Estimated cost / ML of supply Based on the yields estimated by CSIRO BHA modelling and the costs derived from the CSIRO generalised costing algorithm, the estimated cost/ML of supply at the following storage levels are as follows: FSL 186 mEGM96 $8022/ML FSL 188 mEGM96 $8646/ML FSL 190 mEGM96 $9192/ML Summary comment A low yielding site on the Waterhouse River West Branch. Upstream of large areas of sandy loam soils moderately suitable for irrigated agriculture. The site is situated also on Aboriginal land scheduled under ALRA and is classified under ‘inalienable freehold title’, which means that it cannot be bought, acquired or mortgaged. Although data from the NT cultural heritage sites register were not made available to the Assessment, it is likely that the site and parts of the potential inundation area would contain cultural heritage sites of significance. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-1 Location map of potential Waterhouse River West Branch dam site, reservoir extent and catchment area For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-2 Potential Waterhouse River West Branch dam reservoir and property boundaries For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-3 Known water-dependent ecological assets in the vicinity of the potential Waterhouse River West Branch dam site and depth of reservoir as full capacity For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-4 Geology underlying the potential Waterhouse River West Branch dam site and reservoir For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-5 Waterhouse River West Branch potential dam site topographic dimensions and inflow hydrology (a) Elevation profile along dam axis; (b) reservoir volume, surface area and height relationship; (c) dam wall height versus dam width and flood rise for 1:10,000 and 1:50,000 AEP and probable maximum flood (PMF) events plotted against full supply level (FSL); (d) annual streamflow; (e) annual flow exceedance For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-6 Waterhouse River West Branch potential dam site cost, yield at the dam wall and evaporation (a) Dam length and dam cost versus full supply level (FSL); (b) dam yield at 75% and 85% annual time reliability and yield per million dollars at 75% and 85% annual time reliability; (c) annual time reliability plotted against yield for different FSL; (d) volumetric reliability plotted against yield for different FSL; (e) yield at 75% and 85% annual time reliability and degree of regulation (ratio of total controlled releases to total reservoir inflows) plotted against FSL; (f) yield and net evaporation (evaporation minus rainfall) divided by yield plotted against annual time reliability. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-7 Waterhouse River West Branch potential dam site and storage levels and water yield (a) Maximum and minimum annual storage trace at the selected FSL (188 mEGM96) and annual spilled volume (i.e. uncontrolled releases); (b) annual exceedance of ratio of annual quantity of water released to annual demand (i.e. yield) under conditions where the reservoir was operated to supply the full demand (yield) in 55% to 95% of years at the selected FSL; (c) annual exceedance plot of released volume under conditions where the reservoir was operated to supply the full demand (yield) in 55% to 95% of years at the selected FSL; (d) annual yield at 85% annual time reliability plotted against FSL under Scenario A (baseline) and Scenario D; (e) annual time reliability vs yield for FSL 188 mEMG96 under Scenario A (baseline) and Scenario D. B.2 Jalboi River dam site on the Jalboi River; ATMD 53 km PARAMETER DESCRIPTION Previous investigations No previous investigations of this proposal have been located. The site has been identified from CSIRO DamSite modelling. References No references to investigation of this site have been located. Description of potential dam This table details a potential dam with FSL 25 m (elevation 73 mEMG96) above bed level and a storage capacity of 189 GL. A storage at this site could provide supply for irrigation of suitable lands downstream of the storage. Releases would be made from the storage to the stream for diversion by irrigators. The location of the potential site is shown in Apx Figure B-9 and Apx Figure B-10. Apx Figure B-11 shows the known water-dependent ecological assets in the vicinity of the potential dam site. Regional geology The Roper catchment landscape is flat to undulating and consists of a series of erosion surfaces developed over the last 70 million years which are characterised by deep weathering profiles and associated iron-cemented capping. The continued erosion has led to the emergence of the present-day landscape and involved the removal of Cretaceous strata from most of the region and the etching out of structures in the underlying Proterozoic rocks. The resulting landscape is characterised by broad alluvial plains associated with the Roper River and its tributaries, and low rubbly hills or strike ridges formed by folded and faulted resistant thick Proterozoic sandstone formations. Site geology Site visit The dam site is located on Proterozoic rocks of the Roper Group (Prom, Prh & Prj), which consist of blocky and flaggy quartz sandstone and ferruginous medium to very coarse sandstone and dip to the south-west at 10°. The deeply weathered erosion surface characteristic of the region appeared to have been partially removed by erosion at the dam site. On the dam abutments weathered sandstone bedrock is partially exposed with some talus (blocky slope deposits) on the surface; possibly 1 to 3 m stripping would be required. The river channel consists of a tract of vegetated alluvium (sand and gravel) and the depth of alluvium is anticipated to be 4 to 8 m locally. The geology of the site is shown in Apx Figure B-12. Reservoir rim stability and leakage potential There is no evidence of instability or leakage potential around the reservoir rim. Potential structural arrangement The site appears to be suitable for a roller compacted concrete type dam with a central uncontrolled spillway with crest length of 75 m. Outlet works and a fish lock facility would be located on the right abutment. Access to the right bank of the site would be via 38 km of new road branching from the Central Arnhem Highway. The total distance from Katherine via the Stuart and Central Arnhem highways would be some 255 km. Availability of construction materials A quarry in the Proterozoic sandstone with suitable aggregate for an RCC dam might be found within 10 km of the dam site. For estimating purposes a ratio of useful rock excavated to total volume excavated of 0.5 could be assumed. Hard rock quarries for higher quality aggregate to construct an outer layer of RCC for the dam could probably be sourced from the Katherine area. Catchment area 828 km2 Modelled annual inflow data Parameter Scenario A (GL/yr) Scenario Cdry (GL/yr Scenario Cmid (GL/yr) Scenario Cwet (GL/yr Max 300 237 285 454 Mean 84 63 78 126 Median 66 47 56 99 PARAMETER DESCRIPTION Min 1 1 1 1 Reservoir characteristics Storage characteristics are shown in Apx Figure B-13. Storages with FSLs of different heights are tabulated below. FSL Surface area (ha) Capacity (GL) 71 1742 137 73 1960 174 75 2285 216 Reservoir yield assessment at dam wall FSL 71 mEGM96 Estimated yield at 85% annual time reliability 36 GL FSL 73 mEGM96 Estimated yield at 85% annual time reliability 40 GL FSL 75 mEGM96 Estimated yield at 85% annual time reliability 42 GL Reservoir yields under projected future climates shown in Apx Figure B-15. Estimated rates of reservoir sedimentation at FSL 73 mEGM96 Best case Expected Worst case 30 years (%) 0.5 0.8 0.9 100 years (%) 1.8 2.7 3.0 Years to fill 5502 3668 3301 Potential use of supply The area below the potential Jalboi River dam is dominated by a broad alluvial plain with abundant braided channels. Broad level plains with no channels also exist. The areas adjacent to the plains are dominated by undulating to steep hills and rises on Proterozoic sediments and dolerites. The broad alluvial plains are dominated by very deep, moderately well to imperfectly drained, slowly permeable brown to grey cracking clay soils (SGG 9) with strongly sodic subsoils, and soft self-mulching or hard-setting structured clay surfaces. Soils have high to very high water-holding capacity (>140 mm to 1 m) but may have a restricted rooting depth due to very high salt levels in the subsoil. These cracking clay soils are suited to a variety of dry-season flood or spray-irrigated grain, forage and pulse crops, sugarcane and cotton. Much of the clay plains are subject to regular flooding and frequently have small (<0.3 m) gilgai depressions and numerous flood channels. However, the plains with braided channels are frequently in narrow, ribbon form which limit opportunities for agricultural development. Intermixed with the clay soils on the braided plains are very deep, well-drained, sandy-surfaced red Kandosols (SGG 4.1), adding to the soil complexity and further limiting agricultural opportunities. Minor areas of moderately deep to deep (0.5 to 1.5 m), well-drained, red friable clay soils (Red Ferrosols SGG 2) with few stones or boulders occur on gentle plains. Soils are developed on dolerite. These soils are suitable for horticultural tree crops, but the small areas limit agricultural opportunities. Impacts In addition to the storage area of 1,958 ha, a flood margin area would also need to be acquired. The affected area would primarily affect the Lonesome Dove pastoral lease area. Environmental impacts Barrier to movement of aquatic species Fish whose movement may be impeded by a dam at this site include the sooty grunter (Hephaestus fuliginosus), mouth almighty (Glossamia aprion) and sleepy cod (Oxyeleotris lineolata). Ecological implications of inundation The potential inundated area at FSL for this site (73 mEMG96). Part of this potential site is within the Wongalara Private Nature Reserve. The potential for ecological change as a result of changes to the downstream flow regime is examined in the companion technical report on ecology (Stratford et al., 2023). PARAMETER DESCRIPTION Indigenous land tenure, native title and cultural heritage considerations There is a high likelihood of unrecorded sites in the inundation area. Estimated cost To enable a like-for-like comparison with the non-short-listed sites, dam costs were calculated using CSIRO’s generalised dam costing algorithm, which takes into account major cost elements for RCC type dams with central overflow spillways. These are reported for a selection of FSL below. FSL 71 mEGM96 $437 million FSL 73 mEGM96 $463 million FSL 75 mEGM96 $491 million. Estimated cost / ML of supply Based on the yields estimated by CSIRO BHA modelling and the costs derived from the CSIRO generalised costing algorithm, the estimated cost/ML of supply at the following storage levels are as follows: FSL 171 mEGM96 $12,139/ML FSL 173 mEGM96 $11,575/ML FSL 175 mEGM96 $11,690/ML Summary comment A low yielding site on the Jalboi River that appears to be suitable for a roller compacted concrete type dam with a central uncontrolled spillway. The potential dam site a lower yield to cost ratio that other sites selected for pre-feasibility analysis; however, it is relatively close to large areas of alluvial soils moderately suitable for irrigated agriculture. Being north of the Roper River accessing the dam site and potential irrigation areas during the wet season would be challenging without major road and bridge infrastructure. The affected area would primarily affect the Lonesome Dove pastoral lease area. There is a high likelihood of unrecorded sites in the inundation area. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-8 Jalboi River potential dam site AMTD 53 km looking upstream For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-9 Location map of potential Jalboi River dam site, reservoir extent and catchment area For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-10 Potential Jalboi River dam reservoir and property boundaries For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-11 Known water-dependent ecological assets in the vicinity of the potential Jalboi River dam site and depth of reservoir as full capacity For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-12 Geology underlying the potential Jalboi River dam site and reservoir For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-13 Jalboi River potential dam site topographic dimensions and inflow hydrology (a) Elevation profile along dam axis; (b) reservoir volume, surface area and height relationship; (c) dam wall height versus dam width and flood rise for 1:10,000 and 1:50,000 AEP and probable maximum flood (PMF) events plotted against full supply level (FSL); (d) annual streamflow; (e) annual flow exceedance For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-14 Jalboi River potential dam site cost, water yield at the dam wall and evaporation (a) Dam length and dam cost versus full supply level (FSL); (b) dam yield at 75% and 85% annual time reliability and yield per million dollars at 75% and 85% annual time reliability; (c) annual time reliability plotted against yield for different FSL; (d) volumetric reliability plotted against yield for different FSL; (e) yield at 75% and 85% annual time reliability and degree of regulation (ratio of total controlled releases to total reservoir inflows) plotted against FSL; (f) yield and net evaporation (evaporation minus rainfall) divided by yield plotted against annual time reliability. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-15 Jalboi River potential dam site and storage levels and water yield (a) Maximum and minimum annual storage trace at the selected FSL (73 mEGM96) and annual spilled volume (i.e. uncontrolled releases); (b) annual exceedance of ratio of annual quantity of water released to annual demand (i.e. yield) under conditions where the reservoir was operated to supply the full demand (yield) in 55% to 95% of years at the selected FSL; (c) annual exceedance plot of released volume under conditions where the reservoir was operated to supply the full demand (yield) in 55% to 95% of years at the selected FSL; (d) annual yield at 85% annual time reliability plotted against FSL under Scenario A (baseline) and Scenario D; (e) annual time reliability vs yield for FSL 73 mEMG96 under Scenario A (baseline) and Scenario D. B.3 Wilton River dam site on the Wilton River; ATMD 33 km PARAMETER DESCRIPTION Previous investigations No previous investigations of this site have been located. The site was identified from CSIRO DamSite modelling. References No references to investigation of this site have been located. Description of potential dam This table details a potential dam with FSL 39 m (elevation 62 mEMG96) above bed level and a storage capacity of 2253 GL. A storage at this site would have the largest hydro-electric power generation capacity in the Roper catchment. Water released from the storage could also provide for irrigation of limited areas of suitable lands downstream. The location of the potential site is shown in Apx Figure B-16 and Apx Figure B-17. Apx Figure B-18 shows the known water-dependent ecological assets in the vicinity of the potential dam site. Regional geology The Roper catchment landscape is flat to undulating and consists of a series of erosion surfaces developed over the last 70 million years which are characterised by deep weathering profiles and associated iron-cemented capping. The continued erosion has led to the emergence of the present-day landscape and involved the removal of Cretaceous strata from most of the region and the etching out of structures in the underlying Proterozoic rocks. The resulting landscape is characterised by broad alluvial plains associated with the Roper River and its tributaries, and low rubbly hills or strike ridges formed by folded and faulted resistant thick Proterozoic sandstone formations. Site geology Geological mapping (no inspection) The dam site is located on Proterozoic rocks of the Roper Group (Prr) which consist of flaggy micaceous and feldspathic sandstone with glauconite and dip gently to the north-west at about 7°. The deeply weathered erosion surface characteristic of the region appears to be present at the dam site and there is no rock exposed on the abutments so assume a mean depth of stripping of 10 m. The Wilton River channel is about 70 to 100 m wide at the dam site and appears to be still water. However, there appear to be rock bars downstream of the dam site, so the channel is unlikely to be over-deepened. Assume founding of the dam structure level 8 m below river level. The geology of the site is shown in Apx Figure B-19. Reservoir rim stability and leakage potential There is no evidence of instability or leakage potential around the reservoir rim. Potential structural arrangement The site may be suitable for a roller compacted concrete type dam with a central uncontrolled spillway 300 m wide. Outlet works and a fish lift facility would be located on the right abutment side. Access to the right abutment at the site would be by 30 km of new road branching from the Roper Highway a short distance downstream of the Roper Bar crossing. The total distance from Katherine to the site would be 310 km. Availability of construction materials A quarry in the Proterozoic sandstone with suitable aggregate for an RCC dam could be found within 20 kms of the potential damsite. For estimating purposes the ratio of useful rock excavated to total volume excavated of 0.5 could be assumed. Hard rock quarries for higher quality aggregate to construct an outer layer of RCC for the dam could probably be sourced from Katherine. Catchment area 12,073 km2 Modelled annual inflow data Parameter Scenario A (GL/yr) Scenario Cdry (GL/yr) Scenario Cmid (GL/yr Scenario Cwet (GL/yr) Max 5965 4030 5573 7745 Mean 1495 1113 1412 1990 PARAMETER DESCRIPTION Median 1227 930 1120 1651 Min 20 20 21 23 Reservoir characteristics Reservoir characteristics are shown in Apx Figure B-20. Reservoirs with FSLs of different heights are tabulated below. FSL (mEGM96) Surface area (ha) Capacity (GL) 60 34,372 3,809 62 39,055 4,541 64 47,703 5,405 Reservoir yield assessment at dam wall FSL 60 mEGM96 Estimated yield at 85% annual time reliability 899 GL FSL 62 mEGM96 Estimated yield at 85% annual time reliability 926 GL FSL 64 mEGM96 Estimated yield at 85% annual time reliability 929 GL Reservoir yields under projected future climates shown in Apx Figure B-22. Estimated rates of reservoir sedimentation at FSL 62 mEGM96 Best case Expected Worst case 30 years (%) 0.3 0.4 0.5 100 years (%) 0.9 1.4 1.6 Years to fill 10,687 7,125 6,412 Potential use of supply There are limited areas of land suitable for irrigated agriculture downstream of this site on the Wilton River and the confluence of the Wilton River and the Roper River. This site was examined for its hydro-electric power generating potential, however, there is no transmission line infrastructure that would support this development. Impacts In addition to the inundated area of 22,290 ha at FSL, a flood margin area would also need to be acquired. The area required would primarily affect the Urapunga pastoral lease area. Environmental impacts Barrier to movement of aquatic species Fish whose movement may be impeded by a dam at this site include the banded grunter (Amniataba percoides), spangled grunter (Leiopotherapon unicolor), sooty grunter (Hephaestus fulginosus), silver cobbler (Neoarius midgleyi), mouth almighty (Glossamia aprion), giant gudgeon (Oxyeleotris selheimi), shovelnose catfish (Sciades paucus), blue catfish (Neoarius graeffei), black catfish (Neosilurus ater), sleepy cod (Oxyeleotris lineolata), common archerfish (Toxotes chatareus), barramundi (Lates calcarifer), freshwater anchovy (Thryssa scratchleyi), bony bream (Nematalosa erebi), Rendahl's Tandan (Porochilus rendahli) and the freshwater sawfish (Pristis pristis), listed as vulnerable in the EPBC Act. Similarly, the movement of freshwater turtles, such as, the gulf snapping turtle (Elseya lavarackorum), northern snapping turtle (Elseya dentata), northern snake-necked turtle, oblong turtle (Chelodina oblonga oblonga), Cann's snake-necked turtle (Chelodina canni) and Worrell's turtle (Emydura subglobosa worrelli) could be affected. Ecological implications of inundation A dam development at this site would inundate a large area of the Wilton River floodplain, which contain important habitat for small river-floodplain specialist fish, including: Anodontoglanis dahli, flyspecked hardyhead (Craterocephalus stercusmuscarum), Gulf grunter (Scortum ogilbyi) and for waterbirds, such as, straw- necked ibis (Threskiornis spinicollis), white-faced heron (Egretta novaehollandiae), and Nankeen Night-Heron (Nycticorax caledonicus). Freshwater mangrove (Barringtonia acutangular) has also been recorded in this potential site. The potential inundated area at FSL for this site (62 mEMG96) may affect these species by changing their habitat. Most of the inundated area is within the South-East Arnhem Land Indigenous Protected Area or the Wongalara Private Nature Reserve. PARAMETER DESCRIPTION The potential for ecological change as a result of changes to the downstream flow regime is examined in the companion technical report on ecology (Stratford et al., 2023). Indigenous land tenure, native title and cultural heritage considerations There is a high likelihood of unrecorded sites in the inundation area. Estimated cost To enable a like-for-like comparison with the non-short-listed sites, dam costs were calculated using CSIRO’s generalised dam costing algorithm, which takes into account major cost elements for RCC type dams with central overflow spillways. These are reported for a selection of FSL below. FSL 60 mEGM96 $814 million FSL 62 mEGM96 $849 million FSL 64 mEGM96 $889 million Estimated cost / ML of supply Based on the yields estimated by CSIRO BHA modelling and the costs derived from the CSIRO generalised costing algorithm, the estimated cost/ML of supply at the following storage levels are as follows: FSL 171 mEGM96 $905/ML FSL 173 mEGM96 $916/ML FSL 175 mEGM96 $957/ML Summary comment A very high yield potential dam site. However, there are limited areas of land that would be suitable for irrigated agriculture downstream of the site. The site has the most suitable characteristics for a dam supplying water for hydro-electric power in the Roper catchment, however, the site is remote and there is no electricity transmission infrastructure. The site is situated also on Aboriginal land scheduled under ALRA and is classified under ‘inalienable freehold title’, which means that it cannot be bought, acquired or mortgaged. Although data from the NT cultural heritage sites register were not made available to the Assessment, it is highly likely that the site and parts of the potential inundation area would contain cultural heritage sites of significance. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-16 Location map of potential Wilton River dam site, reservoir extent and catchment area For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-17 Potential Wilton River dam reservoir and property boundaries For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-18 Known water-dependent ecological assets in the vicinity of the potential Wilton River dam site and depth of reservoir as full capacity For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-19 Geology underlying the potential Wilton River dam site and reservoir For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-20 Wilton River potential dam site topographic dimensions and inflow hydrology (a) Elevation profile along dam axis; (b) reservoir volume, surface area and height relationship; (c) dam wall height versus dam width and flood rise for 1:10,000 and 1:50,000 AEP and probable maximum flood (PMF) events plotted against full supply level (FSL); (d) annual streamflow; (e) annual flow exceedance For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-21 Wilton River potential dam site cost, water yield at the dam wall and evaporation (a) Dam length and dam cost versus full supply level (FSL); (b) dam yield at 75% and 85% annual time reliability and yield per million dollars at 75% and 85% annual time reliability; (c) annual time reliability plotted against yield for different FSL; (d) volumetric reliability plotted against yield for different FSL; (e) yield at 75% and 85% annual time reliability and degree of regulation (ratio of total controlled releases to total reservoir inflows) plotted against FSL; (f) yield and net evaporation (evaporation minus rainfall) divided by yield plotted against annual time reliability. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Apx Figure B-22 Wilton River potential dam site and storage levels and water yield (a) Maximum and minimum annual storage trace at the selected FSL (62 mEGM96) and annual spilled volume (i.e. uncontrolled releases); (b) annual exceedance of ratio of annual quantity of water released to annual demand (i.e. yield) under conditions where the reservoir was operated to supply the full demand (yield) in 55% to 95% of years at the selected FSL; (c) annual exceedance plot of released volume under conditions where the reservoir was operated to supply the full demand (yield) in 55% to 95% of years at the selected FSL; (d) annual yield at 85% annual time reliability plotted against FSL under Scenario A (baseline) and Scenario D; (e) annual time reliability vs yield for FSL 62 mEMG96 under Scenario A (baseline) and Scenario D. As Australia’s national science agency and innovation catalyst, CSIRO is solving the greatest challenges through innovative science and technology. CSIRO. Unlocking a better future for everyone. Contact us 1300 363 400 +61 3 9545 2176 csiroenquiries@csiro.au csiro.au For further information Environment Dr Chris Chilcott +61 8 8944 8422 chris.chilcott@csiro.au Environment Dr Cuan Petheram +61 467 816 558 cuan.petheram@csiro.au Agriculture and Food Dr Ian Watson +61 7 4753 8606 Ian.watson@csiro.au