Synchronizing Time Series Satellite Data and Real-Time Reservoir Level Measurement to Assess Sedimentation in Koka Reservoir of Ethiopia

Abstract

Regular reservoir surveys are necessary to assess the reduction in storage capacity caused by sediment deposition. Optimal water allocation requires up-to-date information on reservoir storage and sedimentation status, yet such information is often limited. This research aimed at assessing the sedimentation of Koka reservoir by integrating time series satellite image data and real-time reservoir water level measurements. The reservoir surface area was extracted from satellite images using QGIS), aided by Automated Water Extraction Index (AWEI) to identify water pixels. Water level records from the reservoir gauging station were combined with surface area to estimate reservoir volume using a prismoidal formula. The resulting estimates were used to update the elevation-capacity curve. The findings show that the current storage capacity at full reservoir level was 780.01 Mm³. This indicates that 869.99 Mm³ of silt has accumulated in the reservoir since initial operation. This corresponds to a loss of approximately 52.73% of the original storage capacity. The mean annual sediment deposition rate over the 62 years of operation is 14.03 Mm³, equivalent to an annual sedimentation rate of 0.85%. This rate is comparable to the global index of annual reservoir sedimentation, which ranges between 0.5% and 1%. The research findings provide up-to-date information on the sedimentation status of Koka. The results support informed water allocation planning for effective reservoir water management. Scheduling sediment flushing during the peak flood periods of the upper Awash River provides an adaptive approach to managing sedimentation in the Koka Reservoir.