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

SEDIMENTATION HAZARDSEDIMENTATION HAZARD

AND RESERVOIR PLANNING IN AND RESERVOIR PLANNING IN THE AMHARA REGION:THE AMHARA REGION:

A CASE STUDY ON ANGEREB A CASE STUDY ON ANGEREB WATERSHEDWATERSHED

1. INTRODUCTION1. INTRODUCTION

4. LIMITATIONS4. LIMITATIONS

Absence of Topo map of Gondar area

at scale of 1:50,000 (1237A1-A4)

Absence of previous study documents

(Angereb watershed)

6. OBJECTIVES OF THE 6. OBJECTIVES OF THE PROJECTPROJECT

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

3. METHODOLOGY3. METHODOLOGY

METHODSThe watershed was divided in to 8 main and 5

smaller sub watershedsResults in 260 mapping unitsData (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

… … CONTINUEDCONTINUED

Field observation Questionnaire Focus group discussion Review of secondary data sources Problem tree analysis Microsoft excel, Arc View & Arc Info

Bathymetric measurement

MATERIALSMATERIALSAerial 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)

2. BACKGROUND OF STUDY AREA2. 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

…….BACKGROUND.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)

……BACKGROUNDBACKGROUND

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)

……BACKGROUNDBACKGROUND

TOPOGRAPHY AND MORPHOLOGY

Mountainous & rugged south facing Oval in shape Dendritic drainage pattern Steep ridges at the boundary Numerous convex hills & steep gorges inside

……BACKGROUNDBACKGROUND

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

……BACKGROUNDBACKGROUND

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

……BACKGROUNDBACKGROUND

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

……BACKGROUNDBACKGROUND

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%

……BACKGROUNDBACKGROUND

0.01 0 0.01 0.02 Kilometers

LEGENDBush landForest landGrazing landIntensively cultivatedModerately cultivatedReservoirScrub landSettlementSparsely cultivated

N

EW

S

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

IMPORTANT DRAINAGE PARAMETERIMPORTANT DRAINAGE PARAMETERSub Number of streams and its length (km)TotalMain M.streamDrainage Drainageoverland

watersheds1st 2nd 3rd 4th stream slope(%)Area(ha) densityflowl

a b a b a b a b (km) length(m) (km/km2)length(m)Ingodo 6 2.75 1 2 0 0 0 0 4.4 2.1 13.79 130.00 3.35 149.43 Abamatebo 2 1.00 1 1 0 0 0 0 2.3 2.05 6.5 105.00 2.14 233.33Arbagirifat 1 1.80 0 0 0 0 0 0 1.8 1.8 12.5 87.50 2.06 243.06woleka 1 1.80 0 0 0 0 0 0 1.8 1.8 17.5 132.50 1.36 368.06korebreb 12 11.70 2 3 1 3.9 0 0 19 7.5 7 1474.89 1.28 389.15Angereb 12 10.10 2 2 1 2.5 0 0 15 5.3 2.72 942.33 1.55 321.61Keybahir 16 11.80 2 1 1 4.5 0 0 18 6 2.84 1076.55 1.63 305.84Chultie 9 2.80 1 2 0 0 0 0 4.9 3.05 8.6 197.50 2.46 203.61Makek 4 0.70 2 1 1 1.4 0 0 2.8 2.05 6 104.17 2.69 186.02Debteramesk 6 1.55 1 2 0 0 0 0 3.1 1.75 6 73.75 4.14 120.90Embuaymesk15 11.75 4 2 1 5.5 0 0 20 7 4 1223.31 1.60 312.87Kokoch 25 11.60 4 3 2 2 1 5 22 8.1 4 961.94 2.25 222.16Defecha 4 1.75 1 1 0 0 0 0 2.9 1.7 9.5 237.00 1.22 408.62directly join Angereb11 12.80 0 0 0 0 0 0 0 0 0 361.10 0.00 240.00Total 124.0 71.10 21 21 7 20 1 5 116 7262.82 0.00KEY: a= number of streams b=stream length(km)

Important morphological characImportant morphological charac..S/N parameters symbolUnit Fomula values Result Remark(s)

1 Area A km2 measured 76.12 76.122 Perimeter Pb km measured 37.60 37.60

3 Axial length Lb km measured 13.20 13.204 Basin width W km measured 10.10 10.105 Total no.streamsN no counted 153.00 153.006 Total stream lengthL km measured 116.20 116.207 Stream density Sf no/km2 N/A 153/76.123 2.01

8 Main stream lengthLm km measured 14.45 14.459 Main stream slopeS % measured 5.00 5.00

10 Stream order Os no counted 4.00 4.00

11 Over land flow lengthLo m Lo=1/2D 1/(2*1.53) 327.5512 Drainage densityD km/km2 D=L/A 116.2/76.123 1.53

13 Shapefactor B unitless B=Lb2/A (13.2)2/76.123 2.29

14 Form factor Rf unitless Rf=A/Lb2 76.123/(13.2)2

0.44

15 Elongation ratio E unitless E=1.128A0.5/Lb1.128(76.12)0.5/13.2 0.75

16 Circularity ratio Rc unitless Rc=4∏A/Pc24*3.12*76.12/( 30.952 )1.00

17 Texture ratio T unitless T=N/Pb 153/37.6 4.07

18 Bifurcation ratio Rb unitless Rb=Nw/Nw+1124/21,21/7,7/1 6,3,7

19 Compactness co-effCc unitless Cc=Pb/Pc 37.6/30.95 1.22

LAND FORM OF EACH SUB WATERSHEDLAND FORM OF EACH SUB WATERSHED Sub watersheds Area in each slope class

L0 L1 L2 L3 L4 L5 Total area(ha)Ingodo 0 0 27.5 0 20.00 82.5 130.00Abamatebo 0 12.5 0 17.5 35.00 40.0 105.00Arbagirifat 0 0 0 62.5 25.00 0.0 87.50woleka 0 72.5 0 0 10.00 50.0 132.50korebreb 36.56 140 416.719 55.83 581.64 244.2 1474.89Angereb 11.67 20.83 240.422 284.2 290.25 95.0 942.33Keybahir 0 2.5 181.609 355.4 424.95 112.1 1076.55Chultie 0 32.5 0 86.67 0.00 78.3 197.50Makek 0 0 72.5 27.5 0.00 4.2 104.17Debteramesk 0 0 27.5 12.5 21.25 12.5 73.75Embuaymesk 0 185 398.328 233.3 296.66 110.0 1223.31Kokoch 0 55 192.969 17.5 597.09 99.4 961.94Defecha 25 40 50 0 82.00 40.0 237.00B/n Ingodo&Aba 0 20 27.5 0 22.50 0.0 70.00B/nKorebreb&Angereb 10.83 51.67 137.484 17.5 60.00 15.0 292.48B/nChultie&Makek 0 0 13.3281 0 0.00 0.0 13.33B/n Debtera &Embuaymesk0 0 0 31.25 15.00 0.0 46.25B/n Embuaymesk &Kokoch0 161.7 285 8.75 0.00 0.0 455.42Total 84.06 794.2 2070.86 1210 2481.34 983.1 7623.92Percentage 1.12 10.57 27.57 16.11 33.03 13.1 101.49KEY:L=0-3 % L3=15-30 % L4=30-50 %L1=3-8 % L2=8-15 % L5=>50 %

MAJOR CATHMENT FEATURESMAJOR CATHMENT FEATURES

Dissected catchmentCircularSteep slope cultivationPollutantsAll these results in high -runoff -sedimentation -pollution

SOIL EROSION HAZARD ASSESSMENTSOIL EROSION HAZARD ASSESSMENT

Erosion hazard

- Soil Loss and Sediment Yield Estimation- Soil Loss and Sediment Yield Estimation

-Using USLE, -Using USLE,

A= RKLSCPA= RKLSCP

A = Average annual soil loss (ton/ha)A = Average annual soil loss (ton/ha)

R = Rainfall erosivityR = Rainfall erosivity

K = Soil erodibilityK = Soil erodibility

S = Slope length factorS = Slope length factor

C = Cover factorC = Cover factor

P = Management factorP = Management factor

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

S.WATERSHED Area Soil loss beofre treatment Soil loss after treatment(ha) total(ton) rate(t/ha) total(ton) rate(t/ha)

Ingodo 130.00 1271.73 9.78 1059.98 8.15Abamatebo 105.00 1300.03 12.38 1300.03 12.38Arbagirifat 87.50 6509.75 74.40 2993.27 34.21woleka 132.50 3087.58 23.30 1758.91 13.27korebreb 1474.89 113184.11 76.74 46699.70 31.66Angereb 942.33 77646.18 82.40 38387.61 40.74Keybahir 1076.55 83649.00 77.70 27735.94 25.76Chultie 197.50 11860.47 60.05 2985.35 15.12Makek 104.17 7245.13 69.55 2528.94 24.28Debteramesk 73.75 5070.66 68.75 2020.55 27.40Embuaymesk 1223.31 104902.01 85.75 54833.37 44.82Kokoch 961.94 91009.61 94.61 89141.50 92.67Defecha 237.00 9298.16 39.23 8906.39 37.58B/n Ingodo&Aba 70.00 1271.73 18.17 495.30 7.08B/nKorebreb&Angereb 292.48 7873.44 26.92 2142.04 7.32B/nChultie&Makek 13.33 664.67 49.87 221.56 16.62B/n Debtera &Embuaymesk 46.25 3092.90 66.87 970.23 20.98B/n Embuaymesk &Kokoch 455.42 17517.09 38.46 17423.59 38.26Total 7623.92 546454.24 301604.26Average 71.68 39.56

SEDIMENT YIELDSEDIMENT YIELD

Sr = Sediment yield (ton) at the watershed out letSr = Sediment yield (ton) at the watershed out let E = total erosion (ton)E = total erosion (ton) A = Watershed area (ha)A = Watershed area (ha)(1/A(1/A0.20.2) = delivery ratio (0.167)) = delivery ratio (0.167)Thus, Sr = 546,454.24*(1/7623.92 0.2)Thus, Sr = 546,454.24*(1/7623.92 0.2) = = 91,436.19 ton/yr91,436.19 ton/yr With trap efficiency 95% (90 to 98%. )With trap efficiency 95% (90 to 98%. ) Sr= 91436.19*0.95Sr= 91436.19*0.95 Sr= 86,864.383ton/yr (86,864,383.01kg/yr)Sr= 86,864.383ton/yr (86,864,383.01kg/yr) Finally it will be Finally it will be 83,389.81 m3/yr with 20% bed load with 20% bed load But the actual (bathymetric survey) result shows the

sediment load is 128400 m3/yr

SEDIMENTATION PERIODSSEDIMENTATION PERIODS

0

20

40

60

80

100

120

1 2 3 4 5 6

Period in 15 days

R.f

an

d S

ed

ime

nt d

istr

ibu

tion

in%

RF

T1

T2

T3

T4

T5

ESTIMATED SEDIMENT LOAD IN TWO SCENARIOSESTIMATED SEDIMENT LOAD IN TWO SCENARIOS

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%)

……CONTINUEDCONTINUED

Service life of Angereb reservoir Service life of Angereb reservoir

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

… … ……CONTINUEDCONTINUED

SERVICE LIFE OF ANGEREB RESERVOIRSERVICE 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

16. CONCLUSION & 16. CONCLUSION &

RECOMMENDATIONSRECOMMENDATIONS CONCLUSION

From the analysis resultsFrom the analysis results It is indicated that It is indicated that a great deal of runoff at the outlet point a great deal of runoff at the outlet point the sediment yield is much more than the the sediment yield is much more than the

calculated value using empirical formula i.e. our calculated value using empirical formula i.e. our estimation of sediment yield is much more less estimation of sediment yield is much more less than the actual valuethan the actual value

……CONTINUEDCONTINUED

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 .

… … CONCLUSION AND RECOMMENDATIONSCONCLUSION 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!

THANK YOU!