Sedimentation Problem and Mitigation Measure of Koga Reservoir
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Date
2018-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
AAU
Abstract
Reservoir sedimentation originates from the catchment area and the river system then gradually, settles in
reservoir. It is a major problem that endangers the performance and the life of the reservoirs by reducing
water holding capacity. Inappropriate use of land, rapid population growth, and poor ecosystem
management are the main causes of reservoir sedimentation. Since Koga dam is constructed on the outlet of
degraded watershed, measurements of sediment load during the study have confirmed high rate of
sedimentation. The aim of this study was to estimate sediment yield inflow in to the reservoir, to
investigate the spatial variation of sediment yield and to recommend the appropriate sediment reduction
measures. The ArcGIS interface Soil and Water Assessment Tool (ArcSWAT) was used with sediment
data from rating curves to model the amount of sediment yield inflow in to Koga reservoir from catchment
with an area of 155.75 km², located at the upper Blue Nile Basin, Amhara Region, Ethiopia. Primary data
were used to identify the distribution, the grain size variation of sediment load of the reservoir. Secondary
data were used as input for SWAT and SWAT CUP. For stream flow calibration, the most sensitive
parameters were Base flow alpha factor for bank of storage (ALPHA_BNK.rte); and for sediment
calibration, were SPEXP (Exponent parameter for calculating sediment restrained in channel sediment
routing). The results indicated that stream flow calibration and validation period indicated that R²=0.81,
NS=0.76 and R²=0.77, NS=0.74 respectively and sediment yield calibration and validation period indicated
that R²=0.66, NS=0.61 and R²=0.76, NS=0.70 respectively. Both the calibration and validation result
indicated good agreement between measured and simulated stream flow and sediment yield values. The
simulated result shows that the average annual sediment yield is 158 ton/km²/year at the dam site. Subcatchment
25
and
26
are
the
most
erodible
areas.
The
spatial
variation
of
sediment
ranges
from
1.79
ton/ha
to
62.53 ton/ha. The laboratory result indicated that relatively more clay soils are eroded from the upper
catchment and enter in to the reservoir. These clay soils can be retained by suitable sediment detention
mechanism to be used as nutrient input. For the identified erosion prone areas, Scenarios were developed
for reduction of sediment yield in the Koga reservoir and S5 (changing 50% of agricultural land to forest
mixed land) was the best scenario.
Description
Keywords
Reservoir sedimentatio, Koga dam, ArcSWAT, SWAT CUP, SUFI-2, Calibration, Validation, Grain size variation, Scenario, erosion-prone, Blue Nile Basin