integration of hydropower in the ongoing reservoir

INTEGRATION OF HYDROPOWER IN THE ONGOING RESERVOIR STUDIES OF LAKE TANA SUB-BASIN

By Netsanet Zelalem

Arba Minch University School of Post Graduate Studies August, 2008

INTEGRATION OF HYDROPOWER IN THE ONGOING RESERVOIR STUDIES OF LAKE TANA SUB-BASIN

By Netsanet Zelalem

A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Masters of Science in Hydraulic and Hydropower Engineering

Arba Minch University August, 2008

i

CERTIFICATION

CERTIFICATION The undersigned certify that they have read the thesis: Integration of Hydropower in the Ongoing Reservoirs Studies of Lake Tana Sub-Basin and here by recommended for the acceptance by the Arba Minch University in partial fulfillment of the requirements for the degree of Master of Science in Hydraulic and Hydropower Engineering. _____________________________ (Supervisor) Date _________________________

____________________________ (External Examiner) Date ________________________

____________________________ (Co-supervisor) Date _______________________

____________________________ (Internal Examiner) Date _______________________

Zelalem Netsanet

M.Sc. Thesis

Augus 2008 ,AMU

ii

DECLARATION AND COPYRIGHT

DECLARATION AND COPYRIGHT I, Netsanet Zelalem Cherie, declare that this thesis is my own work and that it has been presented and will not be presented by me to any other University for similar or any other degree award.

Signature ________________________________ Date ________________________________

The thesis is protected by copyright laws and international copyright treaties, as well as other intellectual property laws and treaties. It may not be reproduced by any means, in full or in part, except for short extracts in fair dealing, for research or private study, critical scholarly review or discourse with an acknowledgement, without written permission of the directorate of post graduate studies, on behalf of both the author and University of Arba Minch.

Zelalem Netsanet

M.Sc. Thesis

Augus 2008, AMU

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ABSTRACT

ABSTRACT Currently, there are five reservoirs proposed for irrigation purpose only in Lake Tana sub basin. These are: Gumera-A, Megech, Ribb, Gilgel Abbay-B and Jema. Among these Megech, Ribb, Gumera-A and Gilgel Abbay-B are the focus of this study. At Megech reservoir two alternatives are proposed. The first alternative is for irrigation only and the second is for irrigation and water supply. The proposed irrigation area of: Megech of alternative one is 14622 ha, Megech of alternative two is 7311 ha, Ribb 19925 ha, Gumera 1400 ha and Gilgel AbbayB 12490 ha. Integration of hydropower in the proposed reservoir study is analyzed by using Excel Spreadsheet. A sequential routing technique is used to see options of hydropower generation. Final output from sequential routing shows the estimated firm power generation of Gilgel Abbay-B, Megech alternative two, Gumera-A and Megech alternative one is 5.614 Mw, 0.61 Mw, 0.13 Mw and 35kw respectively without affecting the irrigable area. But, from the analysis one can see as the irrigable area decreases the hydropower generation will increase and vice versa because of the rise of reservoir water level. Economic analysis is done for Megech reservoir to show the economic feasibility of hydropower projects. The result from benefit-cost analysis shows that hydropower projects have B/C ratio of 1.9 which indicates its feasibility, where as for irrigation project B/C ratio of 0.37 and the overall B/C ratio for combined purpose (irrigation and power) of 0.4 which are not economically feasible.

Zelalem Netsanet

M.Sc. Thesis

Augus 2008, AMU

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ACKNOLEDGEMENT

ACKNOWLEDGEMENT I would like to express the deepest gratitude and grateful acknowledgement to my adviser, Dr. Ing. Nigussie Tekle for his constant guidance and constructive criticism with encouragement throughout this thesis work and my co-adviser, Mr. Solomon Tassew for his invaluable advices, suggestions and help. I want to express sincerest appreciation to Professor Dr. JSR Murthy for his valuable comment, suggestions and criticism. I extend my acknowledgement to Dr.Semu Moges who gave me the opportunity to attend this program. There are people who should deserve a great acknowledgment for their contribution to me during research time without whom the study here in AMU would have only been a dream: Nigussie Damte, Selam Yonass, Tsehay Mengistu, Belayneh Siyum, Mulu Sewnet, Sirak Tekleab, Genet Elalla, Derege Beyene and Tedi ‘Esquanch’ are a few of them. A special thank to my brothers Wondu Gudeta and Gashaw Mola and my sister Tsiyon Mihiretu, for their dedication and for taking good care of my family during my study. Grateful acknowledgements are extended to my parents for their love, constant encouragement, moral support and faith that made me who I am. I extend thanks to Awassa TVET College, in which my former employer, for providing financial support during this study and Ministry of Water Resources deserves great appreciation for their financial support to my thesis work. Finally, Sincerest thanks to my wife, Lemlem Estifanos, for her love, understanding, devotion and all she has done for me. She has always been a great source of inspiration, has always been a solid support throughout my work and has made a great difference in my life.

Zelalem Netsanet

M.Sc. Thesis

Augus 2008, AMU

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DEDICATION

DEDICATION

I DEDICATE THIS THESIS MANUSCRIPT TO MY UNCLE ENDAWEKE CHERIE, MY MOTHER TIRU WUBE AND THOSE PEOPLE WHO ARE WORKING FOR HUMANITY AND HUMAN WELFARE.

“For it is by grace you have been saved, through faith-and this is not from your selves, it is the gift of GOD-not by works, so that no- one can boast”Eph.2:8-10.

Zelalem Netsanet

M.Sc. Thesis

Augus 2008, AMU

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TABLE OF CONTENTS

TABLE OF CONTENTS Page CERTIFICATION……………………………………………………………………………………..i DECLARATION AND COPYRIGHT........................................................................................ii

ABSTRACT………………………………………………………………………………….iii ACKNOWLEDGEMENT.........................................................................................................iv DEDICATION…………………………………………………………………………………………v TABLE OF CONTENTS .........................................................................................................vi LIST OF TABLES...................................................................................................................ix

LIST OF FIGURES…………………………………………………………………………...x LIST OF TABLES IN THE APPENDICES..............................................................................xi LIST OF FIGURES IN THE APPENDICES...........................................................................xii LIST OF ABBREVIATIONS AND ACRONYMS ................................................................... xiii CHAPTER ONE ..................................................................................................................... 1 INTRODUCTION................................................................................................................... 1

1.1. Background ................................................................................................................. 1 1.2. Statement of the Problem ......................................................................................... 4 1.3. Objective of the Study................................................................................................ 5 1.3.1. General Objective .....................................................................................5 1.3.2. Specific Objective: ....................................................................................5 1.4. Scope and Limitation of the Study .......................................................................... 5 CHAPTER TWO..................................................................................................................... 6 LITERATURE REVIEW.......................................................................................................... 6

2.1. General ........................................................................................................................ 6 2.2. Irrigation Potentials and Extent of Exploitation in Ethiopia .................................. 7 2.3. Power Potentials and Extent of Exploitation in Ethiopia ...................................... 8 2.4. Power Generation from Irrigation Water Release ............................................... 11 2.5. Turbines ..................................................................................................................... 12 2.5.1. General ...................................................................................................12 2.5.2. Selection of Hydraulic Turbines ..........................................................13 2.6. Review of Previous Studies .................................................................................... 15 2.6.1. Design of Dams in Lake Tana Sub-Basin Projects (Gilgel Abbay, Megech and Ribb )-WWDSE and TAHALE Pvt. Ltd.......................................................15 2.6.2. Gumera Irrigation Project –WWDSE and ICT Pvt. Ltd ............................15 2.6.3. Country Wide Master Plan Studies – EVDSA/WAPCOS (1988-90)........16 2.6.4. Study by BCEOM in Association with ISL and BRG 1999 ......................16 CHAPTER THREE............................................................................................................... 18 STUDY AREA ...................................................................................................................... 18

Zelalem Netsanet

M.Sc. Thesis

Augus 2008, AMU

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TABLE OF CONTENTS

3.1. Abbay Basin .............................................................................................................. 18 3.2. Lake Tana Sub-Basin .............................................................................................. 18 3.3. Project Location and Descriptions ......................................................................... 21 3.3.1. Megech Irrigation project ........................................................................21 3.3.2. Ribb Irrigation project..............................................................................23 3.3.3. Gumara-A Irrigation project.....................................................................24 3.3.4. Gilgel Abbay Irrigation Projects...............................................................26 CHAPTER FOUR................................................................................................................. 28 DATA PROCESSING AND ANALYSIS .............................................................................. 28

4.1 General........................................................................................................................ 28 4.2. Precipitation and Evaporation Data ....................................................................... 28 4.2.1. Precipitation and Evaporation at Megech Dam Site................................29 4.2.2. Precipitation and Evaporation at Ribb Dam Site .....................................30 4.2. 3. Precipitation and Evaporation at Gumera-A Dam Site ...........................30 4.2.4. Precipitation and Evaporation at Gilgel Abbay-B Dam Site.....................31 4.2. 5. Reservoir of Evaporation at Proposed Dam Sits....................................31 4.2.6. Estimating the Missing Data ...................................................................33 4.2.7. Quality checking .....................................................................................34 4.2.8. Aerial Precipitation ..................................................................................36 4.3. Hydrological Data ..................................................................................................... 37 4.3.1. Estimating the Missing Streamflow Data.................................................38 4.3.2. Method of Determining the Flow at Desired site .....................................40 4.3.2.1. Simple Area Ratio Method ....................................................................... 40 4.3.2.2. Data Generation Method .......................................................................... 41 4.4. Other Data ................................................................................................................. 42 CHAPTER FIVE ................................................................................................................... 44 METHODOLOGY................................................................................................................. 44

5.1. Integrating Hydropower ........................................................................................... 44 5.2. Methods of Computing Hydropower Potential ..................................................... 46 5.2.1. Flow-Duration Curve Method ..................................................................46 5.2.2. The Sequential Streamflow Routing (SSR) Method................................46 5.3. Procedures for Computation of Firm and Secondary Power............................. 47 5.4. Selection of Turbine and Power House ................................................................ 53 5.4.1. Selection of Turbine ................................................................................53 5.4.2. Selection of Power House.......................................................................56 5.5. Economic Evaluation of Megech Irrigation Project ............................................. 58 5.5.1. General ...................................................................................................58 5.5.2. Methods of Economic Evaluation............................................................58 5.5.2.1. Benefit-Cost Ratio (B/C ratio) ................................................................. 58 5.5.2.2. Net Present Value (NPV).......................................................................... 59 5.5.2.3. Economic Internal Rate of Return (EIRR).............................................. 59 5.5.3. Cost of Projects ......................................................................................59 5.5.4. Case1: Economic Analysis of Irrigation Project ......................................63 5.5.5. Case-2: Economic Analysis of Hydropower Project................................63

Zelalem Netsanet

M.Sc. Thesis

Augus 2008, AMU

viii

TABLE OF CONTENTS

CHAPTER SIX ..................................................................................................................... 65 RESULTS AND DISCUSSION............................................................................................. 65

6.1. General .................................................................................................................. 65 6.2. Option-1: Without Affecting the Extent of Irrigated Land. .................................. 65 6.2.1. Firm and Secondary Power Potential......................................................65 6.2.2. Discharge Released ...............................................................................68 6.2.3. Rule Curve for Option-1 ..........................................................................71 6.3. Option-2: With Reduction in Proposed Irrigation Land ....................................... 72 6.3.1. Results of Option-2 .................................................................................75 6.4. Results of Economic Analysis ................................................................................ 76 CHAPTER SEVEN............................................................................................................... 77 CONCLUSIONS AND RECOMMENDATIONS.................................................................... 77

7.1 Conclusions ................................................................................................................ 77 7.2 Recommendations .................................................................................................... 78 REFERENCES..................................................................................................................... 80 APPENDICES ...................................................................................................................... 83 APPENDIX – A :Tables........................................................................................................ 84 APPENDIX – B :Figures..................................................................................................... 106

Zelalem Netsanet

M.Sc. Thesis

Augus 2008, AMU

ix

LIST OF TABLES

LIST OF TABLES Page Chapter One TABLE:1. 1.ENERGY DEMAND PROJECTION (1990-2004) ..................................................................................4

Chapter Two TABLE: 2.1. THE OVERALL UNRESTRICTED FULL DEVELOPMENT POTENTIAL OF THE ABBAY BASIN ....................8 TABLE: 2.2. HYDROPOWER POTENTIAL OF ETHIOPIA ...........................................................................................9 TABLE: 2.3. MAIN FEATURES OF THE HYDROPOWER PLANT CURRENTLY IN OPERATION ..................................10 TABLE: 2.4. TYPICAL KAPLAN TURBINE OPERATIONAL RANGE…………………………………………...…...14

Chapter Four TABLE: 4. 1.SUMMARY OF COMPUTED EVAPORATION………………………………………………………..…32 TABLE: 4. 2. RAINFALL STATIONS IN THE STUDY AREA .......................................................................................32 TABLE: 4. 3. REPRESENTATIVE STATIONS OF THE RESPECTIVE PROJECTS .......................................................32 TABLE: 4. 4. STATIONS USED TO FILL MISSING RAINFALL AT BAHIR DAR GAUGING STATIONS ..........................33 TABLE: 4. 5. RELATIVE CATCHMENT AREAS INFLUENCED BY EACH STATION .....................................................36 TABLE: 4. 6. ESTIMATED MEAN MONTHLY RAINFALL AT GILGEL ABBAY-B RESERVOIR .....................................37 TABLE: 4. 7. LOCATION OF STREAM FLOW GAUGING STATIONS IN THE STUDY AREA .......................................37 TABLE: 4. 8. DRAINAGE AREA AT DAM SITES AND STREAM GAUGING STATIONS ...............................................41 TABLE: 4. 9. IRRIGATION WATER REQUIREMENT OF THE PROJECTS ..................................................................43

Chapter Five TABLE: 5. 1. ESTIMATED VALUES OF PARAMETERS FOR GILGEL ABBAY-B AND MEGECH ALTERNATIVE-2 .....49 TABLES: 5. 2A. PARAMETERS USED IN SSR FOR MEGECH ALTERNATIVE-2.....................................................49 TABLE: 5.2B. SAMPLE SSR FORMAT FOR MEGECH ALTERNATIVE-2……………………………..………….50 TABLE: 5. 3. CHARACTERSTICS RANGE OF KAPLAN TURBINE………………………………………………......56 TABLE: 5. 4. INVESTMENT COST OF MEGECH IRRIGATIONPROJECT…………………………………………59 TABLE: 5. 5. COMPARISION OF INVESTMENT COST ON PROPOSED CROPS FOR MEGECH IRRIGATION PROJECT………….………………………………………..…………………………….…60

CHAPTER SIX TABLE: 6. 1. POWER GENERATION FROM OPTION-ONE ............................................................. 66 TABLE: 6.2. ESTIMATED VALUES OF COEFFICIENT USED IN DEVELOPING RELEASE RUEL…….…...…69 TABLE: 6.3. SUMMARISED POWER OUT PUT FOR DIFFERENT SCENARIOS OF IRRIGABLE AREA AND AT 95% RELIABILITY OF RESERVOIR…………………………………………………………....74

Zelalem Netsanet

M.Sc. Thesis

Augus 2008, AMU

x

LIST OF FIGURES

LIST OF FIGURES Page Chapter Two Figure: 2. 1. Schematic Representation of Hydropower Integration on Irrigation projects…………….12 Chapter Three Figure: 3. 1. Location of the study Area…………………………………………………..………………...20 Figure: 3.2. Location of Watershed in the Study Area…………………………..…………………………21 Figure: 3.3. Megech Watershed…………………………………………………..…………………………22 Figure: 3.4. Ribb Watershed………………………………………………………………..………………..24 Figure: 3.5. Gumera Watershed…………………………………………………………………………..…25 Figure: 3.6. Gilgel Abbay Watershed……………………………………………………………………..…26 Chapter Four Figure: 4.1. Location of Meteorological Stations in Lake Tana Sub Basin…….…….………………….29 Figure: 4.2. Double Mass Curve for Gonder Rainfall Station……………………..………………………35 Figure: 4.3. Double Mass Curve for Bahir Dar Station……………………………..………………….…..35 Figure: 4.4. Location of Streamflow Gauging Station…………………………………..…………….……38 Figure: 4.5. Correlation between Gauging Stations at Ribb near Adiss Zemen And Upper Ribb near Debre Tabor…………………………………………………..………….….39 Figure: 4.6. Correlation between Streamflow Gauging Stations at Addis Zemen and Azezo…………………………………………………………………………….……….….39 Figure: 4.7. Correlation between Flow at Dam Site and at Gumea Gauging Station and at Dam Site…………………………………………………………………….…………..………42

Chapter Five Figure: 5.1. Release Rule Chart for Gilgel Abbay-B Reservoir……..…………………..……….……….51 Figure: 5.2. Reservoir Rule Curve Gilgel Abbay-B reservoir……………………………..……….………51 Figure: 5.3. Range of Specific Speed and Net Head for Different Turbines…………………………………….……54 Figure: 5.4. Cash Flow Diagram for Megech irrigation project……….………………………………...……….……..62

Figure: 5.5. Cash Flow Diagram for Megech Hydropower Project……………………………………….64

Chapter Six Figure: 6.1. Power Duration Curve of Megech (Alternative-2) Reservoir…………………..……….….66 Figure: 6.2. Power Duration Curve of Gilgel Abbay – B Reservoir………………………..…………….67 Figure: 6.3. Annual Monthly Power of Megech (Alternative-2) Reservoir……………..………………..67 Figure: 6.4. Annual Monthly Power of Gilgel Abbay – B reservoir……………………..………………..68 Figure: 6.5. Annual Monthly Release Volume of Megech (Alternative-2) Reservoir………..………….71 Figure: 6.6. Annual Monthly Reservoir Elevation of Megech (Alternative-2) Reservoir......................72 Figure: 6.7. Relationship between Power Generation and Irrigation Area …………………..………....72

Zelalem Netsanet

M.Sc. Thesis

Augus 2008, AMU

xi

LIST OF TABLES IN APPENDICES

LIST OF TABLES IN THE APPENDICES Page TABLE: A- 1. GORGORA RAINFALL STATION ......................................................................................................85 TABLE: A- 2. MAKSEGNIT RAINFALL STATION........................................................................................... 86 TABLE: A- 3. CHILGA (AYKEL) RAINFALL STATION .............................................................................................87 TABLE: A- 4. ZEGE RAINFALL STATION ..............................................................................................................88 TABLE:A- 5. DEBRE TABOR RAINFALL STATION.................................................................................................89 TABLE: A- 6. BAHIR DAR RAINFALL STATION ....................................................................................................90 TABLE: A- 7. DANGILA RAINFALL STATION .........................................................................................................91 TABLE:A- 8. ENGIBARA RAINFALL STATION .......................................................................................................92 TABLE: A- 9. GONDER RAINFALL STATION .........................................................................................................93 TABLE: A- 10. STATIONS USED FOR THE CONSTRUCTION OF DOUBLE MASS CURVE GROUP-I .......................94 TABLE: A- 11. STATIONS USED FOR THE CONSTRUCTION OF DOUBLE MASS CURVE GROUP-2 ......................95 TABLE: A- 12. STREAM FLOW FOR MEGECH NEAR AZEZO STATION .................................................................96 TABLE: A- 13. FLOW AT MEGECH GAUGE STATION ...........................................................................................97 TABLE: A-14. STREAMFLOW STAIONS FOR RIBB NEAR ADDIS ZEMEN…………………………..………......98

TABLE: A- 15. RIBB MONTHLY FLOW AT DAM SITE ..........................................................................................99 TABLE: A- 16. UPPER RIBB GAUGING STATION...............................................................................................100 TABLE: A- 17. GUMERA STATION NEAR BAHIR DAR ........................................................................................101 TABLE: A- 18. MEAN MONTHLY STREAMFLOW AT GUMERA DAM ...................................................................102 TABLE: A- 19. STREAM FLOW OF GILGEL ABBAY RIVER AT GAUGING STATION ..............................................103 TABLE: A- 20. FLOW AT GILGEL ABBAY-B DAM SITE ......................................................................................104 TABLE: A- 21. CLIMATE DATA FOR EVAPORATION COMPUTATION ....................................................................105

Zelalem Netsanet

M.Sc. Thesis

Augus 2008, AMU

xii

LIST OF FIGURES IN THE APPENDICES

LIST OF FIGURES IN THE APPENDICES Page FIGURE: B- 1. DOUBLE MASS CURVE OF THE RAINFALL STATIONS IN THE STUDY AREA ..................................107 FIGURE: B- 2. ELEVATION – AREA – VOLUME FOR RESPECTIVE RESERVOIRS. .............................................110 FIGURE: B- 3. SSR OUTPUTS OF MEGECH RESERVOIR FOR ALTERNATIVE ONE .............................................112 FIGURE: B- 4. SSR OUTPUTS OF MEGECH RESERVOIR FOR ALTERNATIVE TWO .............................................123 FIGURE: B- 5. SSR OUTPUTS OF RIBB RESERVOIR ..........................................................................................132 FIGURE: B- 6. SSR OUTPUTS OF GUMERA-A RESERVOIR ...............................................................................140 FIGURE: B- 7. SSR OUTPUTS OF GILGEL ABBAY-B RESERVOIR ......................................................................151 FIGURE: B- 8. MAP OF GUMERA PROJECT AND CATCHMENT ...........................................................................162 FIGURE: B- 9. ISOHYTAL MAP OF LAKE TANA SUB BASIN..................................................................................163 FIGURE: B- 10. TWO AND THREE DIMENSIONS REPRESENTATION OF THE ELEVATION OF LAKE TANA ...........164 FIGURE: B- 11. LOCATION MAP OF MEGECH DAM AND GAUGING SITE ............................................................165 FIGURE B- 12: LAYOUT AND LOCATION OF THE PROPOSED POWER HOUSE ....................................................166 FIGURE: B- 13. THISSEN POLYGONS OF GILGEL ABBAY WATERSHED ....................................................... 166 FIGURE: B- 14. LOCATION OF MAIN TOWNS DAMS IN THE STUDY AREA ...........................................................167

Zelalem Netsanet

M.Sc. Thesis

August2008, AMU

xiii

LIST OF ABBREVIATIONS AND ACRONYMS

LIST OF ABBREVIATIONS AND ACRONYMS Alt.

Alternative

ARBIMID

Abbay River Basin Integrated Development Master Plan Project

B/C

Benefit Cost ratio

CRF

Cost Recovery Factor

DEM

Digital Elevation Model

DMC

Double Mass Curve

EEPCo

Ethiopia Electrical Power Corporation

EIRR

Economic Internal Rate of Return

EM

Engineering Manual

EMA

Ethiopian Mapping Agency

FAO

Food and Agriculture organization

GWh//year

Giga Watt hour per year

Ha

Hectare

ICS

Inter connected System

ICTPL

Intercontinental Consultants and Technocrats Private Limited

Kw

Killo Watt

LTB

Lake Tana Sub Basin

m asl

meter above sea level

Mm3

Million cubic meter

MW

Mega Watt

NPV

Net Present Value

PDC

Power Duration Curve

RoR

Run off River hydropower schemes

R.W.L

Reservoir Water Level

SCS

Self Contained System

SSR

Sequential Streamflow Routing

SWAT

Soil and Water Assessment Tool

USBR

United States Bureau Reclamation

WAPCOS

Water And Power Consultancy Service

WWDSE

Water Works Design and Supervision Enterprise

Zelalem Netsanet

M.Sc. Thesis

August2008, AMU

1

CHAPTER ONE

CHAPTER ONE

INTRODUCTION 1.1. Background In developing countries, owing to rapid growth in population, it is unquestionable to plan development activities which can ascertain fast improvement on living standard of the nation. It is preferential to plan Multi-dimensional development activities with limited amount of resources with consideration for efficient utilization. This can be achieved by optimum development of water resources. The need for the optimal development of water resources has become more urgent than ever before because water is becoming a scarce resource as a result of the growing demand for various purposes such as hydropower, irrigation, water supply, etc. In Ethiopia, most large dam projects were implemented only for single purpose i.e., either for power, irrigation or water supply. To do this, there are inevitable competitions and conflicts of interest between the different water users, and projects that are often plagued by the lack of a cohesive approach. Now a days, there is a growing recognition that planning considerations extend far beyond the interest of single purpose projects, and needed to be viewed at the river basin multipurpose development aspects, which results in a number of benefits associated with social well-being such as secure water supply, irrigation for food production, hydroelectric generation, flood control, watershed management, improved navigation, etc. that makes the projects economically viable and environmentally acceptable.

Zelalem Netsanet

M.Sc. Thesis

August2008, AMU

2

CHAPTER ONE

Multipurpose reservoir operation involves various interactions and trade-offs among purposes, which are sometimes complementary but often competitive or conflicting. Reservoir operation may be based on the conflicting objectives of maximizing the amount of water available for conservation purposes and maximizing the amount of empty space for storing future flood waters to reduce the downstream damages which is one of the objectives of Lake Tana irrigation projects. Dams which are planned to be constructed in Lake Tana sub basin are: Megech, Ribb, Gumera-A and Gilgel Abbay-B, rivers are originally single purpose, only for irrigation. However, these dams will have importance to generate electricity immediately at the designed distance downstream of the dam; it can supply water for the downstream-irrigated land and also for domestic water supply so that these dams serve for multipurpose uses. Eighty-three percent of Ethiopians currently lack access to electricity, with 94 percent still relying on fuel wood for daily cooking and heating [18]. This represents only 17% of the total population of the country has electricity access. This electricity access is almost entirely concentrated in the urban areas, but although 85% of the populations live in the rural areas, less than 1% has access to electricity service. The majority of the populations, primarily living in rural areas, lack a number of facilities as a result of poverty and insufficient access to energy. The Ethiopian government is therefore pursuing plans and programs to develop hydropower and irrigation in an effort to substantially reduce poverty and create an atmosphere for social change. It has been shown that access to electricity, including rural electrification, is a key to poverty reduction in Ethiopia.

Zelalem Netsanet

M.Sc. Thesis

August2008, AMU

3

CHAPTER ONE

Integrating hydropower schemes on the proposed Reservoirs have many advantages: ¾ Raising income level of rural population. ¾ Reduction of firewood consumption consequently decreasing deforestation and soil degradation. ¾ Promotion of local industry and facilitation of job opportunity for rural residents and mitigation of population drifts towards urban areas. ¾ Multi-purpose projects give multi use for the community around the project sites thus increases the interest of dwellers towards the development of future water resource projects in the area. ¾ The availability of electricity can support advanced development methods such as tele-education, familiarizing schools which are found in rural areas with new technologies and it could provide access to distant information and support for farmers and other entrepreneurs.

Zelalem Netsanet

M.Sc. Thesis

August2008, AMU

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CHAPTER ONE

1.2. Statement of the Problem Ethiopia is a country endowed with a large hydropower potential but to develop as single purpose may require huge financial resources which cannot be handled with the Ethiopian existing condition. In the case of rural electrification, it is very expensive and revenues are frequently poor; however, if we integrate with other projects it needs only small amount of additional capital cost for full development [2]. If we see areas considered in this study, there is no access to electricity till this time. The author of this paper tries to show the opportunity of generating hydropower to the nearby villages of the respective irrigation projects and the benefits gained from optimal design and operation by integrating hydropower with irrigation projects. Table1. 1.Energy demand projection (1990-2004) [26] Since 90% of the total power is contributed from hydropower, we need to find ways

Year

Total

(G.C)

Demand

to meet the immense power demand in

(GWh/Year)

the near future.

1990

5500

With reference to 1990 G.C Ethiopia needs

2000

13400

2.5 times by year 2000

2010

35000

9.7 times by year 2020

2020

53400

17 times by year 2040 table1.1 illustrate this

2030

72700

fact.

2040

93700

Zelalem Netsanet

M.Sc. Thesis

August2008, AMU

5

CHAPTER ONE

1.3. Objective of the Study

1.3.1. General Objective ¾ Integrating hydropower with irrigation projects which are planned to be implemented in Lake Tana sub-basin (i.e., on Megech, Ribb, Gumera-A and Gilgel Abbay-B).

1.3.2. Specific Objective: ¾ To show the possible options for hydropower integration in the proposed reservoirs based on alternative irrigable land. ¾ To quantify the firm power and secondary power of the respective project sites. ¾ To develop reservoir operation rules for the respective project sites.

1.4. Scope and Limitation of the Study There are around six irrigation projects that are proposed to be implemented around Lake Tana sub basin; but this study focuses only on four reservoirs (Megech, Gumera-A, Ribb and Gilgel Abbay-B) Preliminary economic analysis and turbine selection is done for Megech only because of time constraint. Since the projects are at pre-feasibility some data such as project cost, agricultural costs are adopted from projects’ draft report and from master plan and hence some of the data may be changed through time.

Zelalem Netsanet

M.Sc. Thesis

August2008, AMU

6

CHAPTER-2 LITRATURE REVIEW

CHAPTER TWO LITRATURE REVIEW 2.1. General The optimum development of water resources is considered to be key element in the socio- economic development of a country. As hydropower does not consume or pollute the water it uses to generate power, it leaves this vital resource available for other uses. At the same time, the revenues generated through electricity sales can finance other infrastructure essential for human welfare. This can include drinking

water

supply

systems,

irrigation

schemes

for

food

production,

infrastructures enhancing navigation, recreational facilities and ecotourism. There are contradictions between power generation and irrigation. Taking water away from a river or from a reservoir for irrigation will result in a reduction in the water flow for power generation. Moreover, in most cases the supply schedule for irrigation does not coincide with that for power generation. However, we can make use of the drops of head from the irrigation reservoir to develop power generation, or make use of the tail water of the power station for irrigation, i.e., to let the flow first pass through the water turbine, and then go on for irrigation.

Zelalem Netsanet

M.Sc. Thesis

August 2008, AMU

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2.2. Irrigation Potentials and Extent of Exploitation in Ethiopia Ethiopia’s total land area is estimated at 1.13Million km2. Out of this about 66 % is considered as arable land (i.e. suitable for crops). Out of this total arable land 27.9 Million hectares or 22.8% of the total land area is already cultivated. 10.3% (12596900 ha) and 12.5% (15287500 ha) is intensively and moderately cultivated respectively. In the Ethiopian context, the irrigation sub-sector is classified as small (less than 200ha), medium (200 to 3000ha) and large-scale (over 3000ha) schemes. Ethiopia’s irrigation potential has been estimated to be in the order of 3.5 million hectares. The total area currently irrigated by modern irrigation schemes in Ethiopia is approximately in the order of 160000ha, i.e. 4.6% [25]. Currently, the government emphasis is to develop the sub-sector to fully tap irrigation potential of Abbay basin as shown in table 2.1, by assisting and supporting farmers to improve irrigation management practices and the promotion of modern irrigation systems like the projects that will be implemented around Lake Tana sub basin is one part among the activities. The net command area of Gumera-A, Megech of alternative one and two, Ribb and Gilgel Abbay-B accounts 14000ha, 14622ha (7311ha + Gonder water supply) 19925ha and 12490 ha respectively. For efficient use of Megech reservoir two alternatives are planned: One alternative is increasing the command area by 100% of 7311ha. The second alternative, the reservoir used for irrigating 7311ha command area as well as supplying domestic water supply for Gonder town. These two cases are considered in our analysis as alternative one and alternative two of Megech reservoir.

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M.Sc. Thesis

August 2008, AMU

8


Table: 2. 1. The overall unrestricted full development potential of the Abbay basin at each sub-basin [31,5] S.No

Sub-Basin Name

Sub-Basin 2

Area(km )

Agriculture

Maximum

Identified

Suitable

Irrigable

Irrigable

2

2

Land(km )

Land(km )

Area(ha)

1

Lake Tana

15054

10497

4639

113669

2

North Gojjam

14389

10330

4245

11716

3

Beshelo

13242

8538

3474

-

4

Weleka

6415

3903

1973

-

5

Jimma

15782

6819

6408

11687

6

South Gojjam

16762

11414

5459

19789

7

Mugar

8188

5885

3384

-

8

Guder

7011

3990

3990

8040

9

Fincha

4089

3048

1165

17358

10

Didessa

19630

18235

14809

52617

11

Angar

7901

6684

4177

26563

12

Wombera

12957

9222

3916

2357

13

Dabus

21032

18978

8513

8816

14

Beles

14200

11358

2908

138720

16

Dinder

14016

1975

59555

17

Galegu/Rahad

23160

4655

1794

54995

Total

199812

147572

72829

525957

2.3. Power Potentials and Extent of Exploitation in Ethiopia Ethiopia possess abundant water resources and hydropower potential, second only to the Democratic Republic of Congo in all of Africa, yet only three percent of this potential has been developed. Ethiopia's economically exploitable hydropower potential is in order of more than 139,244Gwh/year as shown in table 2.2. However, only 3 % of the total potential has been utilized so far. This enormous potential classifies Ethiopia as one of the world's leading countries in hydro potential.

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August 2008, AMU

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Ethiopia is also fortunate in having a combination of vast water resources and suitable topography that permit much of this potential to be developed at a remarkably low-cost. There has been little financial and institutional support to develop smaller scale power projects in conjunction with other projects, irrigation for our case. Yet distributed power generation through Pico (≤ 10 kW), micro (11 kw-500 kW), mini (501 kw-1000 kW) and small hydropower (1 Mw – 10Mw) could be an attractive option for meeting rural energy needs in many areas of Ethiopia [11].

Table: 2.2. Hydropower potential of Ethiopia [9, 11, 22] S.

Basin Name

No.

Basin

Coverage

Annual

Hydropower

Area

%

Runoff

Potential

(Bm3)

MW

Gwh/year

Km2 1

Blue Nile

192853

17.1

52.62

12854

78820

2

Omo-Gibe

74912

6.6

17.96

3662

22454

3

Baro-Akobo

73958

6.6

11.81

2244

13760

4

Genale-Dawa

172681

15.3

5.88

1512

9270

5

Tekeze

81034

7.2

8.20

690

4230

6

Wabi Shebele

207497

18.4

3.16

887

5440

7

Awash

113604

10.1

4.6

729

4470

8

Rift Valley Lakes

51664

4.6

5.63

130

800

9

Danakel

66489

5.9

0.86

==

==

10

Aysha

4717

0.4

0.02

==

==

11

Ogaden

82157

7.3

0.86

==

==

12

Mereb

6065

0.5

==

==

==

Total

1,127,631

100

111.6

22, 708

139,244

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M.Sc. Thesis

August 2008, AMU

10


Presently, the Ethiopian electric power system is essentially based on very few medium sized hydropower plants, with minor contribution from thermal. The total firm energy production capacity of the existing hydropower schemes is about 2,338 Gwh/year (see table 2.3), which accounts for 90% of the total electric energy produced in the country.

Table: 2. 3. Main features of the hydropower plant currently in operation [25, 9, 8, and 22] No

Basin

T ype

Plant

of the

Name

schem

System

I ns t al le d

Firm

Ca pac ity

Energy

( MW)

Year of

Production

Commiss ion

Gwh/year

1.

Koka

Awash

Storage

ICS

43.32

80

1960

2.

Awash-II

Awash

RoR

ICS

32

135

1966

3.

Awash-III

Awash

RoR

ICS

32

135

1971

4.

Finchaa

Abbay

Storage

ICS

134

618

1973

5.

Melka

Wabishe

Storage

ICS

153

311

1988

Wakana

belle

6.

Tis Abbay-I

Abbay

RoR

ICS

11.4

55

1964

7.

Tis Abbay-II

Abbay

Storage

ICS

73

331

2001

8.

Gilgel Gibe-I

Omo-

Storage

ICS

184

622

2003

RoR

SCS

0.35

1.2

1991

RoR

SCS

5

47

1992

-

SCS

0.8

2.8

1994

Gibe 9

Yadot*

10

Sor*

Baro – Akabo

11

Dembi*

Total

2,338

* Small hydropower plants in the SCS

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M.Sc. Thesis

August 2008, AMU

11


The Ethiopian electric supply system is characterized mainly by two distinct divisions: the Inter-Contained System (ICS) and a number of Self-Contained System (SCS). The ICS receives most of its energy from relatively large hydro generating stations and supplies the main demand centers. The existing situation in hydropower development, in connection with the available potential and demand, leaves much to be desired. According to the Ethiopian Electric and Power Corporation (EEPCO) energy forecast, the country would face severe shortage of hydroelectric energy for many years to come unless new plants are added to the system. The study indicates that the power generation needs to grow at annual rate of about 10% to fulfill the demand [9]. To cope up with this energy shortage as we have seen these days, special attention has recently been given to the development of hydropower sector. This study has its own role for the contribution of maximizing power coverage.

2.4. Power Generation from Irrigation Water Release It is possible to plan a power generation plant using release for: irrigation water demand, river maintenance, water supply and using reservoir overflow. When water for irrigation, river maintenance flow, water supply and any other water utilization, if any, are discharged from the dam to the river through a water utilization outlet pipe to immediately downstream of the dam are all available for power generation by utilizing the outlet pipe. This concept of hydropower integration can be further illustrated in fig.2.1 below.

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M.Sc. Thesis

August 2008, AMU

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I QI is irrigation discharge QM is river maintenance QF is water over flow during full water level QO is the summations of water supply and additional flow release for power maximization or other uses. P is power house I is stream inflow

Figure: 2. 1. Schematic representation of hydropower integration on irrigation projects [10]. Even though, the release at the downstream of reservoir sites have a great range of fluctuations during irrigation period, power integration concept can be achieved by adjusting additional release(Qo) as shown in fig 2.1. 2.5. Turbines

2.5.1. General Hydraulic turbines are machines which use the energy of water and convert it into mechanical energy. The most common types of hydraulic turbines are broadly classified in to the following two types: 1. Reaction Turbines. These turbines use the available energy partly converted in to kinetic energy and substantial magnitude remains in the form of pressure energy. These turbines obtain the motive force by deflection of water under pressure in a closed passage formed by the turbine blades. The most common reaction turbines listed under this category are: Francis, Kaplan, Propeller and Driaz. 2. Impulse Turbines. These turbines use all the available potential energy which is converted in to kinetic energy with the help of contracting nozzles. The impulse wheel or runner is usually a solid disc or hub upon which are mounted buckets that are designed to split the jet and cause it to turn through nearly 1700 while

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sliding over the inner surface as the bucket travels away from the nozzle. Pelton wheel and Turgo-impulse belong to this category. The details of these turbines is found in [19, 14, 17] and in any hydraulics and hydropower books.

2.5.2. Selection of Hydraulic Turbines The selection of turbine for an efficient use is based on the estimated head and discharge available to generate power. To produce a given power at a specified head for the lowest possible cost, the turbine and generator unit should have the highest speed practicable. However, the speed may be limited by mechanical design, cavitation tendency, vibration and loss of overall efficiency. In addition, greater speed requires the turbine to be placed lower with respect to the tail water, which generally increases excavation and structural costs. The highest speed need to develop high specific speed which is useful in reducing the runner size and power house dimensions. The selection of turbine as mentioned above and from the procedure of selecting turbine as mentioned in chapter four are mainly based on net head over the turbine and discharge through the turbine. The Kaplan turbines are fairly suitable for the purpose of three main reasons: • Relatively small dimensions combined with high rotational speed • A favorable progress of the efficiency curve • Large overloading capacity Since both head and discharge are highly variable and the net head in the considered reservoirs are less than 55m Kaplan turbine is found suitable for all projects.

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M.Sc. Thesis

August 2008, AMU

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This turbine works for net heads ranging from 2 m up to 50m; even it can range up to 60m [6, 17]. Kaplan turbine offers also an advantage with its large range of capacities up to 500 m3/s. Table: 2. 4. Typical Kaplan turbine operational range [24] Hmin/Hmax Qmin/Qmax

Turbine Type Kaplan

~0.4

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~0.4

M.Sc. Thesis

August 2008, AMU

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2.6.

Review of Previous Studies

Because of the importance of the Abbay, very many studies had been carried out in the past, concerning the basin. Though, there were not very specific studies for the development of specific project sites in lake Tana - sub basin except the recent progress works under taken by consultant of water works, design & supervision enterprise and TAHAL Engineering Ltd., only water exclusively, all the studies under taken, pertaining to Lake Tana, or Abbay [29].

2.6.1. Design of Dams in Lake Tana Sub-Basin Projects (Gilgel Abbay, Megech and Ribb )-WWDSE and TAHALE Pvt. Ltd Studies related to Megech and Ribb at feasibility level such as Hydrological investigation, dam hydrological study, design of dams & appurtenant structures, socio-economic study & resettlement planning, environmental impact assessment, watershed management studies are undertaken by water works design and supervision in association with TAHAL Engineering Pvt.Ltd. Important updated data such as crop water requirements, extent of irrigation area, domestic water supply and reservoir data are taken from these studies reports. 2.6.2. Gumera Irrigation Project –WWDSE and ICT Pvt. Ltd The meteorological and hydrological, irrigation and drainage, hydrological studies and hydrogeological investigations of Gumera Irrigation Project at feasibility level of study is undertaken by water works design and supervision in association with Intercontinental Consultants and Technocrats Pvt. Ltd. From these studies, I have taken irrigation water requirement data, extent of irrigation area and reservoir data of Gumera - A reservoir.

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August 2008, AMU

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2.6.3. Country Wide Master Plan Studies – EVDSA/WAPCOS (1988-90) These studies were carried out during 1988-90 for country wide water and land resources development, covering all river basins.

These were primarily desk

studies for identifying potential irrigation and the study addressed the Abbay basin also, along with other basins. The USBR studies were reviewed in detail for the Abbay basin, and subsequently this study came out with modifications in the hydropower sites based on country wide data collection and analysis, coupled with detailed map studies.

2.6.4. Study by BCEOM in Association with ISL and BRG 1999 The study entitled “Abbay River Basin integrated development Master Plan Project” has been carried out with the following water resources oriented objectives. To prepare water allocation and utilization plans under alternative development scenarios and to generate data, information and knowledge that will contribute to the future water allocation negotiations with downstream countries. The hydrological and hydro meteorological studies are more relevant for review in this section. These aspects have been covered in Section II, volume III of the master plan study report. The basic climatic features, the climate data of the Abbay river basin are discussed first. The rainfall data procured by the study was for 173 stations; the data length varied from 3 to 40 years, with associated monthly gaps. The data for other climatic factors were available for 108 stations.

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All data and analysis as well as typical design for structures were presented in the Phase 2 report - Section II - Volume V - Water Resources Development - Part 1 Irrigation & Drainage). Necessary data extracted from this report and additional details are presented in Appendixes E and G of the master plan. Data of irrigation dams, irrigation structure, drainage and farm input and livestock information is surveyed well in this master plan part and I use this information for economic analysis part.

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August 2008, AMU

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CHAPTER-3 STUDY AREA

CHAPTER THREE STUDY AREA

3.1. Abbay Basin The Abbay Basin is perhaps the most important basin in Ethiopia. It accounts for about 17.5% of Ethiopian land area, 25% of its population and 50% of its annual average surface water resources. In the Lake Tana, it has the country’s largest fresh water lake, covering an extent of 3000 km2. The Abbay has an average annual runoff of about 50 Bm3. The river of Abbay contributes on an average, 62% of Nile river flows in Aswan dam. [29]

3.2. Lake Tana Sub-Basin The Lake Tana sub-basin is located at the headwaters of the Abbay (Blue-Nile) basin (see fig. 3.1). The drainage area of the lake is 15,319 square kilometers, of which 3000 square kilometers is the lake area. It has maximum dimensions of 78 km in length, 67km in width and 14m in depth. The geographical location of the Tana sub-

basin extends from 10.95oN to 12.78oN latitude and from 36.89oE to 38.25oE longitude. Based on the rainfall pattern, according to the study of National Meteorological Service Agency (NMSA, Jan 1996) the year is divided into two seasons: a rainy season mainly centered during the months of June to September, and a dry season from October to March. In the southern parts of the basin the months of April and May are an intermediate season where minor rains often occur.

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The mean annual rainfall at Bahir Dar (south portion of the basin) is 1450mm, 1200mm at Addis- Zemen (Eastern portion) and 1050 at Gondar Air Port meteorological station (northern portion), indicating the spatial variation of rainfall in the basin. Rainfall distribution, both in time and space, decreases northwards in the basin (see Fig B-9 in appendix-B). Of the total annual rainfall, 70% to 90% occurs during the June to September rainy season. The mean annual flow at the outlet of Lake Tana is about 3.5 billion cubic meters and it varies from a maximum of 7 billion cubic meters to a minimum of 1 billion cubic meters in high and low water years respectively. The topography of Lake Tana sub basin (LTB) ranges from flat to cliff. The flat areas dominantly cover the low-lying areas LTB plains and a gently sloping terrain lies in between the low-lying and highland areas. The very steep terrain lies mainly along the boarder of the sub basin and mainly on mount Guna on the east, Armachiho on north and Sekela high lands on the southern part of the basin. The altitude in the basin ranges from 1772 to 4100 m asl. at the bed of Lake Tana and the eastern extreme of the sub basin (mount Guna) respectively. In Lake Tana sub – basin there are about six proposed irrigation projects at different level of studies. These are: Gumera-A, Megech, Ribb, Gilgel Abbay-B, Jema and North East Lake Tana. Among these Megech, Ribb, Gumera-A and Gilgel Abbay are the focus of this study (see fig. 3.1 and 3.2).

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Figure: 3. 1. Location of the study area

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Figure: 3. 2. Location of watershed of the study area

3.3. Project Location and Descriptions

3.3.1. Megech Irrigation project The Megech River originates from the high mountain ranges located to the North and Northeast of Gonder town, in the Gonder Administrative Region at an elevation of nearly 2500 m. The catchment area of Megech River up to its entrance to with Lake Tana is about 700 km2 whereas the catchement area upto the damsite is 432.5 km2. The river traverses a length of 55 km before it meets the Lake Tana (see fig. 3.3).

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Figure: 3. 3. Megech Watershed The project commands an irrigation area of 7311 ha and proposed to expand to 1422 ha. The project irrigation command located about 14 km downstream of the storage dam.

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August 2008, AMU

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3.3.2. Ribb Irrigation project The Ribb River originates from the high range of mountains located to the east of Lake Tana. The river traverses a length of 114 km before it enters Lake Tana. The Ribb dam site is located on the Ribb River 47 km from the origin. The riverbed elevation at the dam axis is about 1,874 m at coordinates E 37° 59’ 45” and N 12° 02’ 30”. Approximate longitudinal section of the Ribb watershed along the main river course shows that it is characterized as a steep mountainous watershed up to the Ribb dam site (at about 50 km course), below this point the river slope gets flatter (See Fig.3-4). The Upper Ribb watershed (844 Km2) is characterized as a mountainous, wedge shaped and a steep sloped (3.6%) watershed. The highest elevation of the watershed is about 4,100 m in its south eastern part, where at the dam site the elevation drops below 1,900 m. There is hydrological characteristics similarity between the Ribb dam site (685 km2) and the Megech dam site watersheds. It should be mentioned that 50 Km downstream of the Upper Ribb gauging site , the Ribb river slope gets flatter with low velocity and deposition of suspended sediment in the river course and over the banks in case of excessive flooding.

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August 2008, AMU

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Figure: 3. 4. Ribb Watershed

3. 3.3. Gumara-A Irrigation project The project area is located in Fogera & Dera woredas of South Gonder Administrative zone of Amhara Nation Regional state (ANRS). It falls between Latitude 110-45’ and 110-55’N and longitude 37030’ and 37050’E. The project area is situated at a distance of about 35 km from Bahir Dar and at about 42 km from Debre Tabor. Woreta and Anbesame towns, the capitals of Fogera and Dera woredas, respectively are very close to the project area. Gumara River is one of the main streams on the east side, flowing into Lake Tana. The river along with the tributaries originates from the high mountain ranges to the east of Lake Tana. The town Debre -Tabor is in the vicinity of the origin. The general elevation in this zone is 3,050 meters.

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The river flows generally in westerly direction for a length of 98 km till Lake Tana (see fig.3.5). The catchment area from the head to Tana is 1,893 km2. Map of Gumera project is shown in appendix Fig. B - 8.

Figure: 3. 5. Gumera Watershed The cultivable command Area (CCA), which represents the cultivable area has been assessed as 14,000 ha which works out to 84% of the gross command area. The command area starts downstream of the diversion weir which is proposed on River Gumera about 28 km downstream of proposed dam. In the initial reach the command area is only on one side i.e. on only on the right bank of River Gumera but after 5.90 km the command area starts on the left bank of the river also [28]

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3.3.4 Gilgel Abbay Irrigation Projects Gilgel Abbay River regarded by some hydrologists as source of the Blue Nile (Abbay) River originates at 3520 masl near Sekela village. The stream drains from its source in the north-westerly direction towards Lake Tana (~1786 m asl). It receives its major tributary Jemma River before it crosses the Dangila-Bahirdar road near Wetet Abay town. The other major tributary, Koga River, is joining down stream of the road crossing. Both are right bank tributaries. Most of the catchment area is cultivated agricultural land. The Gilgel Abay catchment located at North Western highlands of Ethiopia belongs to one of the highest rainfall receiving area.

Figure: 3. 6. Gilgel Abbay Watershed

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There are two projects proposed for irrigation using Gilgel Abbay river i.e Gilgel Abbay-A and Gilgel Abbay–B. In this study we consider only Gilgel Abbay-B. The net irrigation area proposed in the master plan is 12490 ha [4]. This project area is located in the Gilgel Valley, between Wetet Abay and Lake Tana between altitudes of +1830 and +1860 m asl (see fig. 3.1).

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CHAPTER-4 DATA PROCESSING AND ANALYSIS

CHAPTER FOUR DATA PROCESSING AND ANALYSIS 4.1 General It is very important to collect adequate and quality data to do successful research work on any field of study. The types of data collected from various organizations for this specific study are as follows: The meteorological data, Stream flow data and topographic Maps for the respective projects are collected from Ethiopian National Meteorological Service Agency, Ministry of water resource, Ethiopian Mapping Agency and Master plan study and from hydrology final report of the respective projects. The range of all data considered for analysis in this paper is 20 years period (1987-2006).

4.2. Precipitation and Evaporation Data Among the meteorological stations in the Lake Tana sub basin(see fig 4.1), the following meteorological stations have relatively better records: Bahir-Dar, Debre Tabore, Maksegnit, Zege, Dangila, Gonder, Gorgora and Enjibara from which rainfall, maximum and minimum air temperature, evaporation, relative humidity, sunshine hours and wind speed are collected. The above stations are grouped further to fill the missing rainfall data of representative Stations (see table 4.3) and used for consistency checking based on altitude and average area rainfall distribution from Isohyetal map developed for the area (see appendix fig.B-9)

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Figure: 4. 1. Location of meteorological stations in Lake Tana sub basin 4.2.1. Precipitation and Evaporation at Megech Dam Site Stations which have a record of more than 20 years available rainfall data around the Megech watershed are Gorgora, Gonder, Maksegnit and Chilga. These stations are used to fill the missing rainfall data of Gonder station, which is taken as the representative station for the Megech project area because of rainfall pattern and geographical proximity (see fig. 4.1 above and B-9 in appendix). Evaporation over Megech reservoir is determined from Gonder station which has better metrological data.

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4.2.2. Precipitation and Evaporation at Ribb Dam Site Zege, Bahir Dar and Debre Tabor stations, which have nearly the same altitude, have mean rainfall ranging from1300mm-500mm. These stations are used to fill the missing data of the specific stations and the rainfall over the reservoir is estimated based on Debre Tabor rainfall station (see fig. 4.1 and B-9 in appendix). The location of the key meteorological and hydrological stations around the Ribb dam site is shown in fig. 4.1and fig.4.4. The elevation of Ribb dam site is about 1870m asl which is less than the altitude of Debre Tabor (2612m asl) and hence could not be used for analysis of evaporation over Ribb reservoir. Since Gonder (1967m asl.) meteorological station is at about the same elevation as Ribb reservoir, this data has been adopted for evaporation rate computation over the reservoir.

4.2. 3. Precipitation and Evaporation at Gumera-A Dam Site The data available ranging from 1987 to 2006 were collected from the observed raw data at gauging stations of Bahir Dar, Debre Tabor, Zege, Wereta which are located around the project area of Gumera which have relatively long years data and having slight variations of rainfall distribution. Among the stations Bahir Dar is the ideal representation of the command area for analysis by the same reason as of Megech and Ribb dam site. The other climatic data like the temperature at various resolution levels, mean wind speed, mean relative humidity and mean sun shine hours were available on comparatively long term basis at Bahir Dar.

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4.2.4. Precipitation and Evaporation at Gilgel Abbay-B Dam Site The meteorological stations in watershed Gilgel Abbay having better records are: Dangila , Engibara and Merawi. Since the rainfall patter in the watershed is highly variable, the rainfall over Gilgel Abbay-B is estimated from aerial precipitation (see section 4.2.8). Evaporation over the reservoir of Gilgel Abbay is determined from Bahir Dar and Dangila meteorological stations as shown in section 4.2.5.

4.2. 5. Reservoir of Evaporation at Proposed Dam Sits Evaporation over open water bodies such as lakes, reservoirs etc. can be determined indirectly by one or more of several methods such as water balance, energy balance, Penman- Monteith's formula and pan evaporation technique. For the present Study the Penman – Monteith method was selected to determine the monthly evaporation rates at four relevant climate stations: Debre Tabor, Bahir Dar, Dangila and Gondar. Open water evaporation (E) is estimated from potential evapotranspiration (ETo) which is calculated using FAO CROPWAT version 4.3 program which uses the Penman-Monteith method and then applies an aridity correction factor. The estimated potential evapotranspiration which is computed by CROPWAT program is converted to open water evaporation from the principle as stated in [16]. It says, with a depth of water higher than 5m and if it is clears of turbidity, the conversion factor ranges from 0.65 to 1.25 for temperate climate condition. For Ethiopia, the aridity correction factor is estimated to be 1.2 [29, 30].

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Table: 4.1. Summary of computed evaporation value (mm) Project Name

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Megech

152

147

179

169

176

146

121

126

147

154

145

142

Gumera-A

133

140

180.1

193.8

206.1

174

291.8

261.8

147.0

186.9

142.4

129.2

Ribb Gilgel Abbay-B

152

147

179

169

176

146

121

126

147

154

145

142

133

140

180.1

193.8

206.1

174

291.8

261.8

147.0

186.9

142.4

129.2

Table: 4. 2. Rainfall stations in the study area S.No

Station

Latitude

Longitude

Altitude

Name

(Deg. and min.)

(Deg. and min.)

(m asl.)

1

Enjibara

10058’

36054’

2670

2

Gorgora

12015’

37018’

1830

3

Gondar

12033’

37025’

1967

4

Zege

11041’

37019’

1800

5

Maksegnit

12022’

37033’

1450

6

DebreTabore

12014’

37002’

1800

7

Aykel(chilga)

12032’

37003’

2150

0

0

8

Bahir Dar

11 36’

37 25’

1770

9

Dangila

11017’

36054’

2140

10

Woreta

11055’

37041’

1980

Table: 4. 3. Representative stations of the respective projects S.No

Name of

Stations used for filling missing

Representative station

reservoirs Records 1

Megech

Gorgora, Gonder, Maksegnit and

Gonder

Chilga 2

Ribb

Zege, Bahir Dar and D/Tabor

Debre Tabor

3

Gumera

Bahir Dar, Debre Tabor, Zege,

Bahir Dar

Wereta

Zelalem Netsanet

M.Sc. Thesis

August 2008, AMU

33


4.2.6. Estimating the Missing Data Twenty years of rainfall data were collected at seven stations in the sub-basin. Various methods are available to estimate missing rainfall records of gauged stations. The methods used for the analysis of data in this study are the normal-ratio method. The normal-Ratio method (NRM) is used where the mean annual precipitation of any of the adjacent stations exceeds the station in question by more than 10% and it is given as Px =

Nx n

⎛ PA P P ⎞ P ⎜⎜ + B + C ..... n ⎟⎟ -------------------------------------- (4.1) Nn ⎠ ⎝ N A N B NC

Where: PX is the precipitation for the station with missing records PA, PB, PC…Pn are the adjacent stations precipitation NA, NB, NC, NX & Nn are long-term mean annual precipitation values at the respective stations. Sample calculation of estimating the missing rainfall of Bahir Dar rainfall station in Month of April of year 1991 is shown as follows: Table: 4. 4. Stations used to fill missing rainfall at Bahir Dar gauging stations S.No

Name of the stations

Long term mean annual rainfall (mm)

Station precipitation (mm)

1

Bahir Dar

Nx = 933

Px

2

Zege

N1 =1520

65.7

3

Debre Tabor

N2 = 1465

63.4

4

Enjibara

N3 = 2370

102.5

Zelalem Netsanet

M.Sc. Thesis

August 2008, AMU

34


⎛ P1 P ⎞ P ⎜⎜ + 2 + 3 ⎟⎟ ⎝ N1 N 2 N 3 ⎠

Px =

Nx 3

Px =

933 ⎛ 65.7 63.4 102.5 ⎞ + + ⎜ ⎟ 3 ⎝ 1520 1465 2370 ⎠

Px = 40.35 mm 4.2.7. Quality checking A time series observation data is relatively constant and homogeneous if the periodic data are proportional to an appropriate simultaneous period. This proportionality can be tested by double mass curve analysis (DMCA) in which accumulated rainfall data is plotted against the mean value or the sum of all stations. In DMC analysis the graph is plotted between the cumulative rainfalls of a single station as ordinate and the cumulative rainfall of the group of stations as abscissa. The DMC for Bahir Dar and Gonder is shown in fig. 4.2 and 4.3 respectively. The other rainfall stations were analyzed with this method and there was no break in the slope of the curve which implies that there is no inconsistency in all stations. The DMC constructed for the stations were shown in appendix fig.B-1.

Zelalem Netsanet

M.Sc. Thesis

August 2008, AMU

35


Double Mass Curve Gondar

20000

15000

10000

19820

17930

16174

13507

10947

8126

6254

4603

0

2900

5000

876.3

CummulativeAnnual Rainfall,Gondar

25000

Group M e an Annual Cum m ulative Rainfall

Figure: 4. 2. Double mass curve for Gonder rainfall station Double Mass Curve B/Dar 35000 30000

B/Dar

25000 20000 15000 10000 5000

Group Mean Annual Rainfall

Figure: 4. 3. Double mass curve for Bahir Dar station

Zelalem Netsanet

M.Sc. Thesis

August 2008, AMU

35546

33525

31709

30066

28476

26876

25235

23301

21270

19477

17469

15632

14173

12439

10528

8898

6473

5002

3373

1640

0


36

4.2.8. Aerial Precipitation For hydrological application, it is often necessary to compute estimate of mean aerial precipitation for a watershed from rain gauge observations. The precipitation at one geographical point may not be representative of the precipitation on a large area. For most hydrological analysis, the area distribution of precipitations required for the representative portion of the watershed. As we have seen in table 4.3, except Gilgel Abbay-B reservoir three of the four reservoirs assign representative stations but for Gilgel Abbay because of the rainfall patter (see in appendix –B, fig.B-9) greatly varies. Hence, the precipitation over Gilgel Abbay-B reservoir is estimated from aerial precipitation developed using Thiessen polygon (see fig.B-13 in the appendix). The rainfall gauging stations used for developing Thiessen polygons are Merawi, Engibara and Dangila.

Table: 4. 5. Relative catchment areas influenced by each station Item Area

Dangila

Engibara

Merawi

Total Area

Km2

829.8

838.5

2164.7

3833

%

21.6

21.9

56.7

100

Mean monthly rainfall at Gilgel Abbay-B Reservoir is estimated using the following equation and the values are shown in table 4.6. Pi = (

P1 A1 + P2 A2 + P3 A3 + ...Pn An ) ………………………… (4.2) A1 + A2 + A3 + ... An

Zelalem Netsanet

M.Sc. Thesis

August 2008, AMU

37


Table: 4. 6. Estimated mean monthly rainfall at Gilgel Abbay-B reservoir Stations Dangila Enjibara Merawi G/Abbay.B

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

1.80 8.73 0.20 2.4

2.5 7.7 1.3 3.0

26.1 34.7 26.4 28.2

45.3 66.7 10.6 30.4

141.0 207.3 171.3 172.6

265.4 366.0 310.8 313.0

356.4 502.8 490.9 464.4

371.8 533.8 367.1 404.6

252.7 410.1 291.0 308.7

130.8 196.9 136.4 148.4

29.2 51.8 11.4 24.1

5.5 16.1 5.6 7.9

4.3. Hydrological Data Streamflow records are obtained from the Hydrology Department of Ministry of Water resources for the following gauging stations. The length of streamflow data used in this paper is 20 years mean monthly data. Table 4.4 shows the location of gauging stations from which streamflow data is collected.

Table: 4. 7. Location of Streamflow gauging stations in the study area Station

River

Location

No. 1007

Megech

Near Azezo

1005

Ribb

Near Addis Zemen

1006

Gumera

2002 1009

North

East

Watersheds

(Degree)

(Degree)

Area(km2)

12.48

37.45

462

12

37.72

1592

Near Bahir-Dar

11.83

37.63

1394

G/Abbay

Near Merawi

11.37

37.03

1664

Upper Ribb

Near Debre Tabor

12.05

37.96

844

Zelalem Netsanet

M.Sc. Thesis

August 2008, AMU


38

Watershed

Figure: 4. 4. Location of Stream Gauging Stations 4.3.1. Estimating the Missing Streamflow Data To fill the missing recorded streamflow gauging data, various methods are available. The missing values were filled with multiple station correlations. In order to correlate stations, the stations should have related physiographic, climatic and drainage characteristics of the catchments, geographic proximity also considered.

Zelalem Netsanet

M.Sc. Thesis

August 2008, AMU

39


Ribb at Addis Zemen

Correlation b/n Ribb at A.zemen and Upper Ribb 40 35 30 25 20 15 10 5 0 -5

y = 0.5668x - 0.3022 2 R = 0.9656

0

10

20

30

40

50

60

70

Upper ribb

Figure: 4. 5. Correlation between gauging stations at Ribb near Adiss Zemen and Upper Ribb near Debre Tabor.

Correlation Between Megech and Ribb 100 y = 2.4854x - 0.2833 R2 = 0.9644

Ribb

80 60

Correlation Between Megech and Ribb

40

Linear (Correlation Between Megech and Ribb)

20 0 0

5

10

15

20

25

30

35

Megech

Figure: 4. 6. Correlation between streamflow gauging stations at Addis Zemen ( for Ribb) and Azezo (for Megech)

Zelalem Netsanet

M.Sc. Thesis

August 2008, AMU


40

4.3.2. Method of Determining the Flow at Desired site

4.3.2.1. Simple Area Ratio Method The area ratio method is commonly used to determine the flow at the required sites from the main or tributary rivers stream gauge values. This method uses the drainage area to interpolate flow values between or near gauged sites on the same stream. Flow values are transferred from a gauged site, either upstream or downstream to the un-gauged site. [20] The recommended guidelines for area ratio method to assess the available stream flow for the potential assessment purpose can be estimated as n

Qsite

⎡ A ⎤ = ⎢ site ⎥ Q gauge ---------------------------------------- (4.3) ⎢⎣ Agauge ⎥⎦

Where: - Qsite - discharge at the reservoir site Qgauge-discharge at the gauge site Asite - drainage area at the reservoir site Agauge - drainage area at the gauge site n – Varies between 0.6 and 1.2 This method is used under the following conditions. If the Asite is within 20% of the Agauge ( 0.8 ≤

Zelalem Netsanet

Asite ≤ 1.2 ) then n = 1 to be used [21]. Agauge

M.Sc. Thesis

August 2008, AMU

41


Table: 4. 8. Drainage area at dam sites and stream gauging stations S.No

Reservoirs

Agauge

Asite

(km )

(Km )

ASite AGauge

2

2

1

Megech

462

424

0.9

2

Ribb

1592

685

0.430 B/C = 52,129,826/27,418,207= 1.9 >1 EIRR = 18.37 % > 10 % Each of the above methods shows Megech hydropower project is economically feasible. . The above benefit cost analysis is shows for individual projects but the overall benefit cost analysis is shown below: BT=179.35*14622*3.790787+(7738500+10741.7*14622)*9.8628*0.683013=1110187361birr CT= (5710638+14622*32566.8)*3.790787+(856596+10399*14622)*9.86288*0.683013 = 2,856,861,283 birr. BT/CT = 111018736/2,856,861,283 = 0.4