SEDIMENTATION HAZARD
AND RESERVOIR PLANNING IN THE
AMHARA REGION:
A CASE STUDY ON ANGEREB
WATERSHED
1. INTRODUCTION
4. 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 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
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
… CONTINUED
Field observation
 Questionnaire
 Focus group discussion
 Review of secondary data sources
 Problem tree analysis
 Microsoft excel, Arc View & Arc Info
 Bathymetric measurement
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)
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
….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)
…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)
…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
…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
…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
…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
…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%
…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
LEGEND
Bush land
Forest land
Grazing land
Intensively cultivated
Moderately cultivated
Reservoir
Scrub land
Settlement
Sparsely cultivated
0.01
0
0.01
0.02 Kilometers
IMPORTANT DRAINAGE PARAMETER
Sub
Number of streams and its length (km)
TotalMain M.stream
Drainage Drainage
overland
watersheds1st
2nd
3rd
4th
stream slope(%)
Area(ha) densityflowl
a
b
a b
Ingodo
6
2.75 1 2
Abamatebo 2
1.00 1 1
Arbagirifat
1
1.80 0 0
woleka
1
1.80 0 0
korebreb
12 11.70 2 3
Angereb
12 10.10 2 2
Keybahir
16 11.80 2 1
Chultie
9
2.80 1 2
Makek
4
0.70 2 1
Debteramesk 6
1.55 1 2
Embuaymesk15 11.75 4 2
Kokoch
25 11.60 4 3
Defecha
4
1.75 1 1
directly join Angereb
11 12.80 0 0
Total
124.0 71.10 21 21
KEY: a= number of streams
a b a b (km) length(m)
0 0 0 0 4.4
2.1 13.79
0 0 0 0 2.3
2.05
6.5
0 0 0 0 1.8
1.8 12.5
0 0 0 0 1.8
1.8 17.5
1 3.9 0 0 19
7.5
7
1 2.5 0 0 15
5.3 2.72
1 4.5 0 0 18
6 2.84
0 0 0 0 4.9
3.05
8.6
1 1.4 0 0 2.8
2.05
6
0 0 0 0 3.1
1.75
6
1 5.5 0 0 20
7
4
2 2 1 5 22
8.1
4
0 0 0 0 2.9
1.7
9.5
0 0 0 0
0
0
0
7 20 1 5 116
b=stream length(km)
130.00
105.00
87.50
132.50
1474.89
942.33
1076.55
197.50
104.17
73.75
1223.31
961.94
237.00
361.10
7262.82
2
(km/kmlength(m)
)
3.35 149.43
2.14 233.33
2.06 243.06
1.36 368.06
1.28 389.15
1.55 321.61
1.63 305.84
2.46 203.61
2.69 186.02
4.14 120.90
1.60 312.87
2.25 222.16
1.22 408.62
0.00 240.00
0.00
Important morphological charac.
S/N
1
2
3
4
5
6
7
8
9
10
11
12
parameters
symbol
Unit
Fomula
Area
A
km2
measured
Perimeter
Pb
km
measured
Axial length
Lb
km
measured
Basin width
W
km
measured
Total no.streamsN
no
counted
Total stream length
L
km
measured
Stream density S f
no/km2 N/A
Main stream length
Lm
km
measured
Main stream slope
S
%
measured
Stream order
Os
no
counted
Over land flow length
Lo
m
Lo=1/2D
Drainage density D
km/km2 D=L/A
values
76.12
37.60
13.20
10.10
153.00
116.20
153/76.123
14.45
5.00
4.00
1/(2*1.53)
116.2/76.123
Result
Remark(s
76.12
37.60
13.20
10.10
153.00
116.20
2.01
14.45
5.00
4.00
327.55
1.53
13 Shapefactor
B
unitless B=Lb 2/A
(13.2)2/76.123
2.29
14 Form factor
Rf
unitless Rf=A/Lb 2
76.123/(13.2)2
0.44
0.5
15 Elongation ratio E
unitless E=1.128A0.51.128(76.12)
/Lb
16
17
18
19
unitless
unitless
unitless
unitless
Circularity ratio Rc
Texture ratio
T
Bifurcation ratio Rb
Compactness co-eff
Cc
/13.2
0.75
2
30.952 )1.00
Rc=4∏A/P c4*3.12*76.12/(
T=N/P b
153/37.6
4.07
Rb =Nw/Nw+1
124/21,21/7,7/1
6,3,7
Cc=P b /P c 37.6/30.95
1.22
LAND FORM OF EACH SUB WATERSHED
Sub watersheds
Area in each slope class
L0
L1
L2
L3
L4
Ingodo
0
0
27.5
0
20.00
Abamatebo
0
12.5
0
17.5
35.00
Arbagirifat
0
0
0
62.5
25.00
woleka
0
72.5
0
0
10.00
korebreb
36.56 140
416.719 55.83 581.64
Angereb
11.67 20.83 240.422 284.2 290.25
Keybahir
0
2.5
181.609 355.4 424.95
Chultie
0
32.5
0
86.67 0.00
Makek
0
0
72.5
27.5
0.00
Debteramesk
0
0
27.5
12.5
21.25
Embuaymesk
0
185
398.328 233.3 296.66
Kokoch
0
55
192.969 17.5
597.09
Defecha
25
40
50
0
82.00
B/n Ingodo&Aba
0
20
27.5
0
22.50
B/nKorebreb&Angereb
10.83 51.67 137.484 17.5
60.00
B/nChultie&Makek
0
0
13.3281 0
0.00
B/n Debtera &Embuaymesk0
0
0
31.25 15.00
B/n Embuaymesk &Kokoch0
161.7 285
8.75
0.00
Total
84.06 794.2 2070.86 1210
2481.34
Percentage
1.12
10.57 27.57
16.11 33.03
KEY:
L=0-3 %
L3=15-30 %
L4=30-50 %
L1=3-8 %
L2=8-15 %
L5=>50 %
L5
82.5
40.0
0.0
50.0
244.2
95.0
112.1
78.3
4.2
12.5
110.0
99.4
40.0
0.0
15.0
0.0
0.0
0.0
983.1
13.1
Total area(ha)
130.00
105.00
87.50
132.50
1474.89
942.33
1076.55
197.50
104.17
73.75
1223.31
961.94
237.00
70.00
292.48
13.33
46.25
455.42
7623.92
101.49
MAJOR CATHMENT FEATURES

Dissected catchment
 Circular
 Steep slope cultivation
 Pollutants
 All these results in high

-runoff

-sedimentation

-pollution
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
Average soil loss rates before and after
treatment for each sub watershed
S.WATERSHED
Area
(ha)
Ingodo
130.00
Abamatebo
105.00
Arbagirifat
87.50
woleka
132.50
korebreb
1474.89
Angereb
942.33
Keybahir
1076.55
Chultie
197.50
Makek
104.17
Debteramesk
73.75
Embuaymesk
1223.31
Kokoch
961.94
Defecha
237.00
B/n Ingodo&Aba
70.00
B/nKorebreb&Angereb
292.48
B/nChultie&Makek
13.33
B/n Debtera &Embuaymesk
46.25
B/n Embuaymesk &Kokoch
455.42
Total
7623.92
Average
Soil loss beofre treatment Soil loss after treatment
total(ton)
rate(t/ha)
total(ton)
rate(t/ha)
1271.73
9.78
1059.98
8.15
1300.03
12.38
1300.03
12.38
6509.75
74.40
2993.27
34.21
3087.58
23.30
1758.91
13.27
113184.11
76.74
46699.70
31.66
77646.18
82.40
38387.61
40.74
83649.00
77.70
27735.94
25.76
11860.47
60.05
2985.35
15.12
7245.13
69.55
2528.94
24.28
5070.66
68.75
2020.55
27.40
104902.01
85.75
54833.37
44.82
91009.61
94.61
89141.50
92.67
9298.16
39.23
8906.39
37.58
1271.73
18.17
495.30
7.08
7873.44
26.92
2142.04
7.32
664.67
49.87
221.56
16.62
3092.90
66.87
970.23
20.98
17517.09
38.46
17423.59
38.26
546454.24
301604.26
71.68
39.56
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
SEDIMENTATION PERIODS
120
R.f and Sediment distribution in%
100
80
60
RF
T1
T2
40
T3
T4
T5
20
0
1
2
3
4
Period in 15 days
5
6
ESTIMATED SEDIMENT LOAD IN TWO SCENARIOS
NO
PARAMETERS
SHAWEL,
1999
Angereb
TC, 2004
1
Sedimentation rateEstimation
124000 m3/yr
83389.81 m3/yr
2
Sedimentation rateActual survey
120000 m3/yr
128400 m3/yr
3
Sediment accumulatedEstimation
1.364 Mm3
0.9173 Mm3
4
Sediment accumulatedActual
1.32 Mm3
(25%)
1.412 Mm3
(27%)
…CONTINUED
Service life of Angereb reservoir
Service Life
Conditions
years
up to
With out watershed treatment & with
out flood flushing
6
2010
With out watershed treatment & with
flood flushing
With watershed treatment & with out
flood flushing
With watershed treatment & with flood
flushing
8
2012
7
2011
10
2014
… …CONTINUED
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
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
…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 .
… 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!
THANK
YOU!