CSIRO LAND and WATER Scoping Options for Mitigating Cold Water Discharges from Dams Dr Bradford Sherman Report to: Agriculture, Fisheries and Forestry - Australia, NSW Fisheries, CRC for Freshwater Ecology, and NSW Department of Land and Water Conservation as part of the NHT Murray-Darling 2001 FishRehab Program. CSIRO Land and Water, Canberra Consultancy Report 00/21, May 2000 Scoping Options for Mitigating Cold Water Discharges From Dams Dr Bradford Sherman CSIRO Land and Water, Canberra Consultancy Report 00/21, May 2000 Report to: Agriculture, Fisheries and Forestry - Australia, NSW Fisheries, CRC for Freshwater Ecology, and NSW Department of Land and Water Conservation as part of the NHT Murray-Darling 2001 FishRehab Program. Cold discharge mitigation final report 3 1 EXECUTIVE SUMMARY ....................................................................................................................... 7 2 THE PROBLEM...................................................................................................................................... 10 3 IMPLICATIONS FOR NATIVE FISH ................................................................................................ 11 4 AUSTRALIAN MANAGEMENT INTEREST IN THE PROBLEM............................................... 12 5 LITERATURE REVIEW ....................................................................................................................... 13 5.1 REALISTIC EXPECTATIONS – RIVER HEAT BUDGET .......................................................................... 13 6 THERMAL POLLUTION REMEDIATION STRATEGIES ........................................................... 16 6.1 PHYSICS OF SELECTIVE WITHDRAWAL ............................................................................................. 16 6.2 DESTRATIFICATION .......................................................................................................................... 18 7 REMEDIATION MEASURES .............................................................................................................. 20 7.1 DESTRATIFICATION .......................................................................................................................... 20 7.1.1 Theory..................................................................................................................................... 20 7.1.2 Experience.............................................................................................................................. 20 7.1.3 Cost......................................................................................................................................... 21 7.1.4 Projected cost for large New South Wales irrigation storages............................................ 21 7.2 SURFACE PUMPS ............................................................................................................................... 22 7.2.1 Theory..................................................................................................................................... 22 7.2.2 Experience.............................................................................................................................. 23 7.2.3 Cost......................................................................................................................................... 24 7.3 SELECTIVE WITHDRAWAL USING MULTI-LEVEL OUTLET (MLO) STRUCTURES.............................. 25 7.3.1 Theory..................................................................................................................................... 25 7.3.2 Experience.............................................................................................................................. 25 7.3.3 Cost......................................................................................................................................... 26 7.4 FLOATING INTAKES (TRUNNIONS) ................................................................................................... 27 7.4.1 Theory..................................................................................................................................... 27 7.4.2 Experience.............................................................................................................................. 27 7.4.3 Cost......................................................................................................................................... 28 7.5 SUBMERGED WEIRS, SUSPENDED CURTAINS .................................................................................... 29 7.5.1 Theory..................................................................................................................................... 29 7.5.2 Experience.............................................................................................................................. 29 7.5.3 Cost......................................................................................................................................... 30 7.6 STILLING BASINS............................................................................................................................... 31 7.6.1 Theory..................................................................................................................................... 31 7.6.2 Experience.............................................................................................................................. 31 7.6.3 Cost......................................................................................................................................... 31 7.7 MODIFIED GUIDE OR RULE CURVES.................................................................................................. 31 7.7.1 Theory..................................................................................................................................... 31 7.7.2 Experience.............................................................................................................................. 32 7.7.3 Cost......................................................................................................................................... 32 8 SUMMARY............................................................................................................................................... 33 9 ACKNOWLEDGEMENTS .................................................................................................................... 33 10 APPENDIX A – CASE STUDY OF BURRENDONG DAM ........................................................ 35 10.1 INTRODUCTION – THE CHALLENGE .................................................................................................. 35 10.2 SITE DESCRIPTION............................................................................................................................. 37 10.3 DESIGN CONSIDERATIONS - RESERVOIR OPERATION ....................................................................... 37 Cold discharge mitigation final report 4 10.4 THE OPTIONS..................................................................................................................................... 39 10.4.1 Selective withdrawal .............................................................................................................. 39 10.5 SURFACE PUMPS ............................................................................................................................... 40 10.5.1 Unconfined surface pump ...................................................................................................... 40 10.5.2 Draft tube mixer ..................................................................................................................... 41 10.6 SUBMERGED CURTAIN ...................................................................................................................... 43 10.7 CAVEATS AND RECOMMENDATION .................................................................................................. 44 11 REFERENCES.................................................................................................................................... 45 Cold discharge mitigation final report 5 List of Figures Figure 1 Selective withdrawal removes water from strata within the water column. It is usually done by replacing one or more bulkheads in a multi-level outlet structure with trash racks to screen out large debris while allowing water to pass through the dam. The thickness of the withdrawal layer depends on the strength of the stratification and the discharge rate................................................. 17 Figure 2 Typical circulation pattern set up by artificial destratification......................................................... 18 Figure 3 Surface pumps send surface layer water downwards into the withdrawal layer adjacent to the outlet. a) The use of a draft tube decreases entrainment of reservoir water and can increase the energy efficiency of the system. b) An unconfined jet issuing from the surface pump is the simplest type of system to construct.................................................................................................. 22 Figure 4 A trunnion intake structure. ............................................................................................................... 27 Figure 5 A submerged curtain forces the release to originate from within the warmer surface layer of a reservoir. ............................................................................................................................................