Reservoir sedimentation and estimating dam storage capacity using bathymetry survey: a case study of Abrajit Dam, Upper Blue Nile basin, Ethiopia

This specific study was conducted on the Abrajit reservoir in east Gojjam, Amhara region, Ethiopia. The general objective of the study was to investigate reservoir siltation and determination of the remaining valuable life span of the reservoir using ArcGIS 10.5 version and Golden surfer 16 latest version. A bathymetric survey using echo-sounding equipment (Garmin GPSMAP421s) was conducted and a topo map of reservoir data was used for assessment of the sediment volume. The accumulated sediment volume was estimated by subtracting the triangular interconnected network (TIN) map of the initial bed level from the currently measured bed level. The result of the research revealed that to date, 343,700.1 m3 bulk of sediment was accumulated in the reservoir that reduces 20% of the total reservoir capacity. In the results from the bathymetric investigation and software analysis, annually 28,641.675 m3 year−1 amount of sediment is coming from the catchments to the reservoir that contributed 1.66% of annual reservoir volume reduction. The measured recent storing capacity for the Abrajit Dam was 1,388,870 m3. From a comparison of remaining reservoir storage and annual sediment load, the reservoir will not serve more than 12 years. The estimated specific sediment yield (SSy) was found to be 4733.387 ton km−2 year−1. Therefore, to improve the estimated life span of the reservoir, watershed management is very important and further study is required to identify the area where watershed conservation will focus in the future. Upon the findings, the reservoir volume is running to reduce and it cannot serve all the demand what it designed for during the dry season. As a result, operation curves should be mandatory to give water service for the critical water needs, and regular monitoring of sediment accumulation in the reservoir is recommended.