Abstract:
In the present situation, the world wide loss of storage capacity in surface water
reservoirs is due to sedimentation. River inflows are often the dominant source of
nutrients, sediments, and contaminants into a lake or reservoir. Nile tributaries,
originating from the Ethiopian plateau, carry huge quantities of sediment. Reservoirs
built on these tributaries are experiencing alarming loss in capacity due to
sedimentation. Arjo-Dedessa Embankment Dam is under construction on the Dedessa
River involving large investment cost. Therefore, it was important to assess sediment
yield at Arjo-Dedessa reservoir and evaluate its consequences on the reservoir and
sustain the reservoir through a long term optimum sediment management program.
The objective of this study was to assess Sediment yields to Arjo-Dedessa reservoir
and establishing sediment-discharge relationship. The samples were collected from
the Dedessa River across the width and depth from June to August. The stratified
probability sampling technique was used. Field measurements in the Dedessa River
were included suspended sediment load, flow velocity and discharge. The Depth
integration technique sediment sampler was used for collecting sample. Price AA
current meter was used to measure the river flow velocity. The discharge was
calculated using Velocity –area method. Data processing was done using Excel sheet,
and GIS software. Sediment rating curve was representing for the relationship
between sediment concentration and stream flow. The curve was used to derive
sediment concentrations and sediment flux from measured flow. The rating curves
were developed using the regression. The obtained P-value is 0.0012 and the r2
values to evaluate the relationship between discharge and sediment concentration the
results obtained r2 =0.94, r2 =0.90 and r2= 0.94 for Linear, logarithm, and power
curve fit respectively. The r2
value shows that there is a positive and strong
relationship between sediment concentration and discharge. At α = 0.05, from P value, hypothesis test it indicates that, there is the significant relationship between
sediment concentration and discharge. The annual sediment flux 1.25Mtons/year
predicted by the rating curve method. The sediment yield was found
225.73tons/km2
/year. The highest suspended sediment of 0.027 Mtons/day was found
during the monsoon season in august. About 21.60% of annual sediment volume was
transported in July and 58.17% of annual sediment volume was transported in
August. The volume of sediment accumulated in the reservoir every year would be
1.02Mm
3
/ year and is 0.117% per a year considering the trap efficiency 97.08%.
However this will not have significant impact on the economic life of the reservoir.
Maintaining of appropriate vegetative cover and implementing the soil conservation
measure is recommended in the catchment and would reduce the rate of erosion and
the sedimentation.
