Mekong River Commission P.O. Box 6101, 184 Fa Ngoum Road, Unit 18 Ban Sithane Neua, Sikhottabong District, Vientiane 01000, Lao PDR Telephone: (856-21) 263 263. Facsimile: (856-21) 263 264 The ISH 0306 Study Development of Guidelines for Hydropower Environmental Impact Mitigation and Risk Management in the Lower Mekong Mainstream and Tributaries Volume 4 – Final Case Study Report Mainstream Dams Assessment Including Alternative Scheme Layouts March 2018 The Final Reports constitutes six volumes: Volume 1: Version 3.0 – Hydropower Risks and Impact Mitigation Guidelines and Recommendations Volume 2: Version 3.0 – Hydropower Risks and Impact Mitigation MANUAL – Key Hydropower Risks, Impacts and Vulnerabilities and General Mitigation Options for Lower Mekong Volume 3: Version 2.0 – Final Knowledge Base and Supporting Documents Volume 4: Version 2.0 – Final Case Study Report – Mainstream Dams Assessment Including Alternative Scheme Layouts Volume 5: ISH0306 Proposed Update of the Preliminary Design Guidance (PDG) and Hydropower Development Strategy Volume 6: Final Closure Report TABLE OF CONTENTS Abbreviations ...................................................................................................................................................... 8 Executive Summary ........................................................................................................................................... 10 1 Introduction ............................................................................................................................................. 33 1.1 Overview.......................................................................................................................................... 33 1.2 Specific Objective of the Case Study ................................................................................................. 33 1.3 Specific Case Study Activities, Outputs and Deliverables .................................................................. 35 1.3.1 2nd Interim Phase ......................................................................................................................... 35 2 1.3.2 Final Phase ................................................................................................................................... 37 2 Case Study ................................................................................................................................................ 39 2.1 The Context of the Study Base Case ................................................................................................. 39 2.1.1 The Historic Conditions and Basin Scale Management ................................................................. 40 2.1.2 The BDP 2030 Scenario and Council Study 2040 Scenario ............................................................ 40 2.1.3 Development of the Base Case for Assessment of Mitigation Effectiveness ................................. 41 2.2 Description of the Mainstream Dams ............................................................................................... 44 2.2.1 The Lao Cascade .......................................................................................................................... 44 2.2.2 The Mainstream Dams below Pakse ............................................................................................ 47 2.3 Description of Risks .......................................................................................................................... 49 2.3.1 Hydrology and Flow ..................................................................................................................... 49 2.3.2 Sediments and Geomorphology ................................................................................................... 50 2.3.3 Water Quality .............................................................................................................................. 51 2.3.4 Fisheries and Aquatic Ecology ...................................................................................................... 52 2.4 Description of Mitigation Options .................................................................................................... 56 2.4.1 2nd Interim Phase Mitigation Options ........................................................................................... 56 2.4.2 Final Phase Mitigation Options .................................................................................................... 62 2.5 Description of Scenarios ................................................................................................................... 66 2.5.1 2nd Interim Phase – Northern Lao PDR Cascade ............................................................................ 66 2.5.2 Final Phase – Council Study Hydropower Sub-Scenarios .............................................................. 72 3 Methods to Assess Mitigation and Economics .......................................................................................... 74 3.1 General Overview ............................................................................................................................ 74 3.1.1 Multicriteria Assessment and Indicator Framework – 2nd Interim Phase – Northern Lao Cascade 74 3.1.2 Phase 4 Multicriteria Assessment Approach: Lower Mekong Dams.............................................. 76 3.1.3 Hydropower Design Criteria of Lao Cascade and Downstream Dams ........................................... 77 3.1.4 Criteria for Different Modes of Hydropower Operation ............................................................... 82 3.2 Sites for criteria assessment ............................................................................................................. 84 3.2.1 2nd Interim Phase – Northern Lao PDR Cascade ............................................................................ 84 3.2.2 Final Phase – Lower Mekong Dams .............................................................................................. 85 3.3 Methods to Assess Hydrologic and Flow Changes ............................................................................ 86 3.3.1 2nd Interim Phase – Northern Lao PDR Cascade ............................................................................ 86 3.3.2 Final Phase ................................................................................................................................... 92 3.4 Methods to Assess Sediment Impact Mitigation ............................................................................... 95 3.4.1 2nd Interim Phase – Northern Lao PDR Cascade ............................................................................ 95 3.4.2 Final Phase ................................................................................................................................... 98 3.5 Methods to Assess Water Quality Impact Mitigation........................................................................ 99 3.5.1 2nd Interim Phase ......................................................................................................................... 99 3 3.5.2 Final Phase ................................................................................................................................. 100 3.6 Methods to Assess Fisheries and Aquatic Ecology Impact Mitigation ............................................. 101 3.6.1 2nd Interim Phase – Northern Lao PDR Cascade .......................................................................... 101 3.6.2 Final Phase ................................................................................................................................. 118 3.7 Economic Evaluation ...................................................................................................................... 121 3.7.1 Valuation of Sediment Trapped/Flushed .................................................................................... 121 3.7.2 Valuation of Fisheries ................................................................................................................ 121 3.7.3 Valuation of energy loss ............................................................................................................. 122 4 Description of Modelling System and Approach ..................................................................................... 125 4.1 Description of Mekong system to be modelled .............................................................................. 125 4.1.1 Geographical subdivision ........................................................................................................... 125 4.1.2 Dominant processes to be modelled .......................................................................................... 127 4.2 Modelling approaches for specific processes and functions ........................................................... 127 4.2.1 MRC Decision Support Framework (DSF) ................................................................................... 127 4.2.2 Rainfall-runoff in the tributaries and the UMB (Chinese dams) .................................................. 128 4.2.3 Sediment input from the various tributaries .............................................................................. 129 4.2.4 Reservoir cascade modelling ...................................................................................................... 129 4.2.5 Flow routing through the main channels