Sedimentation Hazard and Reservoir Planning in the Amhara Region: A Case Study on Angereb Watershed

1. SEDIMENTATION HAZARD AND RESERVOIR PLANNING IN THE AMHARA REGION:A CASE STUDY ON ANGEREB WATERSHED

2. 1. INTRODUCTION

3. 4. LIMITATIONS • Absence of Topo map of Gondar area at scale of 1:50,000 (1237A1-A4) • Absence of previous study documents (Angereb watershed)

4. 6. OBJECTIVES OF THE PROJECT • To assess sedimentation hazard of the region • To compare the empirical formulas with respect to the actual values so as to aware designers, planners and policy makers

5. 3. METHODOLOGY METHODS • The watershed was divided in to 8 main and 5 smaller sub watersheds • Results in 260 mapping units • Data (such as land use, soil depth, color, infiltration, stoniness etc) were collected from each sub watersheds • Catchment morphology,drainage parameters such as • Soil loss and sediment yields were analyzed based on the worst condition of the factors

6. … CONTINUED • Field observation • Questionnaire • Focus group discussion • Review of secondary data sources • Problem tree analysis • Microsoft excel, Arc View & Arc Info • Bathymetric measurement

7. MATERIALS • Aerial Photo of scale 1:50,000 • Table Stereoscope, Digital Planimeter, Clinometers, • Computer and its accessories • Topo-map of scale 1:50,000 (Code 1237B3) • Others ( like digitizing board)

8. 2. BACKGROUND OF STUDY AREA LOCATION • Found in North Gondar Zone of Amhara Regional State. • Located at about 748 and 180 km from Addis Ababa and Bahir Dar, respectively. • Lie between UTM of N 1394096, N 1407336, E 328073 and E 337991 • Altitude ranges from 2100 to 2870 m.a.s.l

9. ….BACKGROUND • ANGEREB WATERSHED • 29,148 People • 10 Kebeles (7 Rural & 3 Urban) (i.e., Lay Armachiho 5, Gondar Zuria 2 and Gondar city 3 Kebeles) • 103 Villages (49 within & 54 outside)

10. …BACKGROUND CLIMATE AND AGRO-ECOLOGY • Annual rainfall from 711.8 to 1822.42 mm • Mean annual rainfall is 1159.22 mm • Mean monthly temperature from 180C to 220C • Agro-ecology Moist Woina Dega (dominant)

11. …BACKGROUND TOPOGRAPHY AND MORPHOLOGY • Mountainous & rugged south facing • Oval in shape • Dendritic drainage pattern • Steep ridges at the boundary • Numerous convex hills & steep gorges inside

12. …BACKGROUND GEOLOGY • Extensive area of Volcanic rocks • Ashangi Group of the early Tertiary Age • Consist of Basaltic flows • Deeply eroded • Are not permeable • Water moves through fractures • Along the horizontal contacts between the flows

13. …BACKGROUND HYDROLOGY • Abay river basin & Megech river sub basin • Ample rainfall but erratic in nature • Several small streams and springs • Rivers: Angereb, Kaha, korebreb Shenta and Demaza

14. …BACKGROUND SOILS • Dominant soils are shallow Cambisol • underlain by unconsolidated medium sized gravels • underlain by watertight rocky layers • Silty clay loam and silty clay texture • Brown color (dominant) • Soil depths are between 25 and 100 cm

15. …BACKGROUND LAND USE PATTERN (7,624 ha) • Cultivated = 69.1% • Grazing = 4.8% • Forest = 10.6% • Bush = 7.2% • Scrub = 3.9% • Settlement = 4.2% • Wet land = 0.3% • Total 100.00%

16. L E G E N D B u s h l a n d F o r e s t l a n d G r a z i n g l a n d I n t e n s i v e l y c u l t i v a t e d M o d e r a t e l y c u l t i v a t e d R e s e r v o i r S c r u b l a n d S 0 . 0 1 0 0 . 0 1 0 . 0 2 K i l o m e t e r s …BACKGROUND L A N D U S E / C O V E R M A P O F A N G E R E B W . S N W E S e t t l e m e n t S p a r s e l y c u l t i v a t e d

17. IMPORTANT DRAINAGE PARAMETER

18. Important morphological charac.

19. LAND FORM OF EACH SUB WATERSHED

20. MAJOR CATHMENT FEATURES • Dissected catchment • Circular • Steep slope cultivation • Pollutants • All these results in high • -runoff • -sedimentation • -pollution

21. SOIL EROSION HAZARD ASSESSMENT • Erosion hazard - Soil Loss and Sediment Yield Estimation -Using USLE, A= RKLSCP A = Average annual soil loss (ton/ha) R = Rainfall erosivity K = Soil erodibility S = Slope length factor C = Cover factor P = Management factor

22. Average soil loss rates before and after treatment for each sub watershed

23. SEDIMENT YIELD Sr = Sediment yield (ton) at the watershed out let E = total erosion (ton) A = Watershed area (ha) (1/A0.2) = delivery ratio (0.167) Thus, Sr = 546,454.24*(1/7623.92 0.2) = 91,436.19 ton/yr • With trap efficiency 95% (90 to 98%. ) Sr= 91436.19*0.95 Sr= 86,864.383ton/yr (86,864,383.01kg/yr) • Finally it will be 83,389.81 m3/yr with 20% bed load • But the actual (bathymetric survey) result shows the sediment load is 128400 m3/yr

24. SEDIMENTATION PERIODS

25. NO PARAMETERS SHAWEL, 1999 Angereb TC, 2004 1 Sedimentation rate- Estimation 124000 m3/yr 83389.81 m3/yr 2 Sedimentation rate- Actual survey 120000 m3/yr 128400 m3/yr 3 Sediment accumulated-Estimation 1.364 Mm3 0.9173 Mm3 4 Sediment accumulated-Actual 1.32 Mm3 (25%) 1.412 Mm3(27%) ESTIMATED SEDIMENT LOAD IN TWO SCENARIOS

26. Conditions Service Life years up to With out watershed treatment & with out flood flushing 6 2010 With out watershed treatment & with flood flushing 8 2012 With watershed treatment & with out flood flushing 7 2011 With watershed treatment & with flood flushing 10 2014 …CONTINUED Service life of Angereb reservoir

27. … …CONTINUED

28. SERVICE LIFE OF ANGEREB RESERVOIR • Under sluice gate of the dam should be kept open from June 15 to August 20 each year. • Total of 7,869,312m3 of water is expected to be released • The sediment volume is reasonably assumed to be 1% of the water volume, Amounting to 78,690m3

29. 16. CONCLUSION & RECOMMENDATIONS CONCLUSION From the analysis results • It is indicated that • a great deal of runoff at the outlet point • the sediment yield is much more than the calculated value using empirical formula i.e. our estimation of sediment yield is much more less than the actual value

30. …CONTINUED • watershed treatment can reduce sediment loads of the reservoir • Flushing is much more advantageous than catchment treatment • But using both of them is effective in reducing reservoir sedimentation so as to sustain life of the dam .

31. … CONCLUSION AND RECOMMENDATIONS RECOMMENDATION • Mechanism like sluice gate, bypass structures should be provided for earth dams for minimum sediment deposit in each season. • Under sluice gate of the dam should be kept open for some time each year. • There should be some method appropriate for the region in designing and planning reservoirs. • Open for discussion!

32. THANK YOU!