Mekong River Commission Office of the Secretariat in Vientiane Office of the Secretariat in Phnom Penh 184 Fa Ngoum Road, Ban Sithane Neua, 576 National Road, no. 2, Chok Angre Krom, P.O. Box 6101, Vientiane, Lao PDR P.O. Box 623, Phnom Penh, Cambodia Tel: (856-21) 263 263 Fax: (856-21) 263 264 Tel: (855-23) 425 353 Fax: (855-23)425 363 [email protected] www.mrcmekong.org The ISH 0306 Study Development of Guidelines for Hydropower Environmental Impact Mitigation and Risk Management in the Lower Mekong Mainstream and Tributaries 2nd Interim Report - Final November 2016 Volume 2 – Version 2.0 – Hydropower Risks and Impact Mitigation MANUAL – Key Hydropower Risks, Impacts and Vulnerabilities and General Mitigation Options for Lower Mekong Document history Version Revision Description Issue date Issued by First Interim Phase 1 1 First Draft – Version 1.0 9 October 2015 Leif Lillehammer 2 2 Second Draft – Version 1.0 1 Dec. 2015 Leif Lillehammer 3 3 Final – Version 1.0 11 Dec. 2015 Leif Lillehammer Second Interim Phase 4 4 Draft Final Version 2.0 23 Sept. 2016 Leif Lillehammer 5 5 Final Version 2.0 7 Nov. 2016 Leif Lillehammer The Draft Second Interim Report constitutes 4 volumes: Volume 1: Version 2.0 – Hydropower Risks and Impact Mitigation Guidelines and Recommendations Volume 2: Version 2.0 – Hydropower Risks and Impact Mitigation MANUAL – Key Hydropower Risks, Impacts and Vulnerabilities and General Mitigation Options for Lower Mekong Volume 3: Case Study Report, Version 1.0 – Modelling, Scenarios and Impact Mitigation Assessment Volume 4: Knowledge Base Report, Version 1.0 – Structure, Usage and Update on Regional and International Practise 2 Contents Preface ..................................................................................................................................................... 7 Abbreviations .......................................................................................................................................... 9 1 Description, Purpose and How to Use the Manual ....................................................................... 13 2 Hydrology and Water Resources – Status, Risks and Mitigation .................................................. 15 2.1 Status and Overview .............................................................................................................. 15 2.1.1 Introduction ....................................................................................................................... 15 2.1.2 Stream Flows ..................................................................................................................... 15 2.1.3 Water Use and Abstraction ............................................................................................... 18 2.1.4 Floods and Droughts .......................................................................................................... 19 2.2 Risks, Impacts and Vulnerabilities ......................................................................................... 20 2.3 Hydrology and Water Resources Mitigation Measures ........................................................ 28 2.4 Indicators and Monitoring .................................................................................................. 29 3 Geomorphology and Sediments – Status, Risks and Mitigation ................................................... 30 3.1 Status and Overview .............................................................................................................. 30 3.1.1 Importance of sediments and sediment transport in river systems ................................. 30 3.1.2 Catchment geomorphology ............................................................................................... 31 3.1.3 Sediment transport in the LMB ......................................................................................... 34 3.1.4 Recent changes to sediment transport in the LMB ........................................................... 35 3.1.5 Other activities affecting sediment transport ................................................................... 37 3.1.6 Observed geomorphic Changes associated with Altered Sediment Delivery ................... 40 3.2 Risks, Impacts and Vulnerabilities ......................................................................................... 41 3.2.1 Annual and inter-annual flow changes .............................................................................. 42 3.2.2 Short-term flow fluctuations ............................................................................................. 43 3.2.3 Loss of river connectivity and creation of impoundments ................................................ 44 3.2.4 Intra or inter-basin transfers ............................................................................................. 45 3.3 Identification of LMB specific geomorphic risks associated with HP developments ............ 45 3.3.1 Zone 1: Chiang Saen to upstream of Vientiane ................................................................ 45 3.3.2 Zone 2 – Upstream Vientiane to Kong Chiam ................................................................... 46 3.3.3 Zone 3 Kong Chiam to Kratie ............................................................................................. 48 3.3.4 Zone 4: Kratie to Chaktomuk and Tonle Sap .................................................................... 49 3.3.5 Zone 5 Delta ...................................................................................................................... 51 3.3.6 Interaction between HP impacts and other catchment developments ............................ 52 3.3.7 LMB Specific challenges associated with sediment management .................................... 53 3 3.4 Geomorphology and Sediments Mitigation Measures ......................................................... 54 3.4.1 Background information from the MRC Preliminary Design Guidance ............................ 54 3.4.2 General approaches to geomorphology and sediment mitigation ................................... 55 3.4.3 Master Plan & Feasibility ................................................................................................... 56 3.4.4 Detailed design .................................................................................................................. 60 3.4.5 Construction phase ............................................................................................................ 72 3.4.6 Operational phase ............................................................................................................. 73 3.5 Indicators and monitoring ..................................................................................................... 76 4 Water Quality, Nutrients and Reservoir Stratification .................................................................. 77 4.1 Status and Overview .............................................................................................................. 77 4.1.1 Introduction and importance of water quality in river system ......................................... 77 4.1.2 Water quality trends in the LMB ....................................................................................... 77 4.1.3 Changes to sediment and flow and impacts on water quality .......................................... 85 4.2 Risks, Impacts and Vulnerabilities ......................................................................................... 87 4.2.1 Annual /inter-annual flow changes ................................................................................... 87 4.2.2 Daily or short-time period flow changes ........................................................................... 88 4.2.3 Loss of river connectivity ................................................................................................... 89 4.2.4 Conversion of river to lake ................................................................................................ 90 4.2.5 Inter intra-basin transfers ................................................................................................. 92 4.2.6 Other risks associated with hydropower developments................................................... 92 4.3 Water Quality Mitigation Measures ...................................................................................... 93 4.3.1 General .............................................................................................................................. 93 4.3.2 Background information from the MRC Preliminary Design Guidance ............................ 93 4.3.3 Master Plans and feasibility .............................................................................................. 94 4.3.4 Detailed design .................................................................................................................. 97 4.3.5 Construction .................................................................................................................... 102 4.3.6 Operational Stage ............................................................................................................ 103 4.4 Water quality indicators and monitoring ............................................................................ 103 5 Fisheries and Aquatic Ecology ..................................................................................................... 106 5.1 Status and Overview ...........................................................................................................