Establishing Sediment-Discharge Relationship for Assessment of Sediment Yield to Reservoir: Case of Arjo-Dedessa Reservoir, Ethiopia

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.