Assessment of the Impact of Agricultural Activity on Siltation of Meka Dam Reservoir in the Chancho Catchment Using Bathymetric Survey and GIS, Ethiopia Oromia Region Nekemte

Reservoirs are used worldwide supply water for domestic use, irrigation, hydropower generation, and flood management services, Sedimentation affects the sustainability of reservoir operations and overtime, sediments build-up in reservoirs displaces usable storage volume, which in turn negatively affects the beneficial uses of reservoirs and degrades aquatic habitat. An agricultural activity in the chancho watershed is a major issue which increases siltation in the Meka dam reservoir. The bathymetric survey approach is used in the study which is based on a simple comparison of reservoirs morphology at two different time periods, first at the time of the construction of the dam and second at the time of the survey. Bathymetric mapping involves two types of necessary measures: the position and depth measurements. Silt level was directly measured by eco-sound detector. Bathymetric map of the reservoir was done at predetermined 10 m horizontal interval to collect the representative silt level. Echo-sounder was used to measure the depth of silt from reservoir water surface. This device measures the depth of water by sensing and recording the depth of water from surface to top of silt level. Accordingly, 3025 points across the reservoir were surveyed with echo-sounder The deepest measure was found to be 15.01 m, the shallowest depth is 0.47 m and the average depth of Meka reservoir was found to be 6.67 m. The reservoir capacity was determined by ArcGIS tools. The amount of water that can be stored above the original bed level in Meka dam reservoir is found to be 119,960,000 m3 and the amount of water that is stored above the current level of silt in the reservoir is found to be 9,276,000 m3. The half-life of the Meka reservoir is expected to reach within 42 years. Since its construction in, 2007, it’s found that the storage capacity loss of Meka Dam was 17.86%. Therefore, appropriate watershed management strategy should be enforced to alleviate the problem of storage capacity loss of the reservoir due to sedimentation that arises from agricultural activity in the catchment.