UNIVERSITY OF ZIMBABWE FACULTY OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING AN ANALYSIS OF THE CAHORA BASSA DAM WATER BALANCE AND RESERVOIR OPERATIONS AND THEIR FLOODING IMPACT ON UPSTREAM SETTLEMENTS Mabvuto Phiri M.Sc. Thesis in IWRM Harare, July 2011 THE UNIVERSITY OF ZIMBABWE FACULTY OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING In collaboration with AN ANALYSIS OF THE CAHORA BASSA DAM WATER BALANCE AND RESERVOIR OPERATIONS AND THEIR FLOODING IMPACT ON UPSTREAM SETTLEMENTS By Mabvuto Phiri SUPERVISORS Dr. Eng. H. Makurira Mr. W. Gumindoga A thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Integrated Water Resources Management (IWRM) July, 2011 DECLARATION I, Mabvuto Phiri, declare that this research report is my own work. It is being submitted for the degree of Master of Science in Integrated Water Resources Management (IWRM) in the University of Zimbabwe. It has not been submitted before for any degree of examination in any other University. Date: ____________________ Signature: _____________________ iii Table of contents DECLARATION ------------------------------------------------------------------------------------------------------------------------ III TABLE OF CONTENTS --------------------------------------------------------------------------------------------------------------- IV LIST OF TABLES -------------------------------------------------------------------------------------------------------------------- VI LIST OF FIGURES -------------------------------------------------------------------------------------------------------------------- VII LIST OF EQUATIONS----------------------------------------------------------------------------------------------------------------VIII LIST OF SYMBOLS -------------------------------------------------------------------------------------------------------------------- IX LIST OF APPENDICES ----------------------------------------------------------------------------------------------------------------- X DEDICATION------------------------------------------------------------------------------------------------------------------------- XI ACKNOWLEDGEMENTS ----------------------------------------------------------------------------------------------------------XII ABSTRACT --------------------------------------------------------------------------------------------------------------------------- XIII CHAPTER 1: INTRODUCTION ------------------------------------------------------------------------------------------------------- 1 1.1 BACKGROUND ----------------------------------------------------------------------------------------------------------------- 1 1.2 PROBLEM STATEMENT ------------------------------------------------------------------------------------------------------ 2 1.3 JUSTIFICATION ---------------------------------------------------------------------------------------------------------------- 3 1.4 RESEARCH OBJECTIVES ------------------------------------------------------------------------------------------------------ 3 1.4.1 Main Objective: --------------------------------------------------------------------------------------------------------- 3 1.4.2 Specific Objectives: ----------------------------------------------------------------------------------------------------- 3 CHAPTER 2: LITERATURE REVIEW ------------------------------------------------------------------------------------------------ 5 2.1 DAMS AND RIVER FLOW REGIME ------------------------------------------------------------------------------------------------ 5 2.2 FLOODING ------------------------------------------------------------------------------------------------------------------------ 6 2.3 WATER BALANCE ----------------------------------------------------------------------------------------------------------------- 7 2.3.1 Components of the Water Balance --------------------------------------------------------------------------------- 8 a) Areal Rainfall -------------------------------------------------------------------------------------------------------------------- 8 b) Gauged Flows-------------------------------------------------------------------------------------------------------------------- 9 c) Ungauged Flows ---------------------------------------------------------------------------------------------------------------- 9 d) Outflows ------------------------------------------------------------------------------------------------------------------------ 10 e) Evaporation -------------------------------------------------------------------------------------------------------------------- 10 f) Discharge and Change in Storage ------------------------------------------------------------------------------------------ 10 2.4 GIS AND REMOTE SENSING BASED RAINFALL-RUNOFF MODELLING ----------------------------------------------------------- 10 2.4.1 Drainage Basin Area Ratio Method ------------------------------------------------------------------------------- 11 2.5 CATCHMENT PARAMETER COMPARISON --------------------------------------------------------------------------------------- 11 2.6 CAHORA BASSA DAM ----------------------------------------------------------------------------------------------------------- 12 2.6.1 Operational Objectives ----------------------------------------------------------------------------------------------- 12 2.6.2. Description of the Operating Rule--------------------------------------------------------------------------------- 12 CHAPTER 3: MATERIALS AND METHODS -------------------------------------------------------------------------------------- 14 3.1 STUDY AREA ------------------------------------------------------------------------------------------------------------------ 14 3.1.1 TYPES OF FLOODS AFFECTING THE STUDY AREA ------------------------------------------------------------------------------- 15 3.1.2 Effects of the Floods in the Study Area --------------------------------------------------------------------------- 16 3.1 3 Climate ------------------------------------------------------------------------------------------------------------------- 17 3.2 METHODS USED FOR THE DIFFERENT SPECIFIC OBJECTIVES --------------------------------------------------------------------- 17 3.2.1 The Water Balance Equation---------------------------------------------------------------------------------------- 17 3.2.2 Objective 1: Analyse flows into and out of the Cahora Bassa------------------------------------------------ 17 Inflows ---------------------------------------------------------------------------------------------------------------------------------- 17 Outflows ------------------------------------------------------------------------------------------------------------------------------ 19 3.3.3 Objective 2: Determine the contribution from the ungauged catchments -------------------------------- 20 Rainfall-Runoff Modelling --------------------------------------------------------------------------------------------------------- 20 HEC-HMS model --------------------------------------------------------------------------------------------------------------------- 21 Gauging Ungauged Catchments --------------------------------------------------------------------------------------------------- 25 3.2.4 Objective 3: Investigate link between the operation of the Cahora Bassa reservoir and flooding frequency in the riparian areas -------------------------------------------------------------------------------------------- 26 CHAPTER 4: RESULTS AND DISCUSSIONS -------------------------------------------------------------------------------------- 27 4.1 EXTENT OF UNGAUGED AREA --------------------------------------------------------------------------------------------------- 27 4.2 WATER LEVEL AND LAKE SURFACE AREA---------------------------------------------------------------------------------------- 28 4.3 CAHORA WATER BALANCE ------------------------------------------------------------------------------------------------------ 30 4.3.1 Inflows ------------------------------------------------------------------------------------------------------------------- 30 Gauged Inflows ----------------------------------------------------------------------------------------------------------------------- 30 Direct rainfall on Lake Surface ----------------------------------------------------------------------------------------------------- 32 4.3.2 Volume of Outflows --------------------------------------------------------------------------------------------------- 33 Discharge ------------------------------------------------------------------------------------------------------------------------------ 33 Evaporation---------------------------------------------------------------------------------------------------------------------------- 34 4.4 UNGAUGED FLOW CONTRIBUTION --------------------------------------------------------------------------------------------- 34 4.4.1 HEC HMS Model Calibration and Validation --------------------------------------------------------------------- 34 4.4.2 Catchments Comparison --------------------------------------------------------------------------------------------- 38 4.4.3 Model Simulations ---------------------------------------------------------------------------------------------------- 41 4.5 THE WATER BALANCE ----------------------------------------------------------------------------------------------------------- 43 4.6 RELATIONSHIP BETWEEN RESERVOIR OPERATIONS