Dam Sedimentation Assessment: A Case Study of Opa Dam, IleIfe, Osun State, Southwest Nigeria

Small reservoirs are mainly utilized for domestic purposes, livestock watering, fishing and irrigation.Due to their importance in water resource provision, planning and management, there is a need to know the capacity and useful life of reservoirs for harvesting the optimum quantity of water.Traditional techniques of estimation of the reservoir capacity and sedimentation such as inflow-outflow technique, hydrographic survey, integrated bathymetric survey are uneconomical, clumsy, tedious, involve enormous manpower, and lack of equipment.GIS and remote sensing technique has provided new opportunity to compute the loss of storage capacity and sedimentation in Opa dam in Ile-Ife, Osun State, Nigeria.Satellite imageries covering the study area were acquired for 2002 and 2022; Landsat 2002 (ETM+) and Landsat 2022 (OLI/TIRS) to determine the extent of shoreline changes during the period of 20 years.The imageries were processed using ERDAS IMAGINE 2014 and AcrGIS 10.8 for land use/land cover and the Normalized Difference Water Index (NDWI), the reservoir's shoreline extractions from the NDWI using slicing and reclassification operations.The results of this study show a decrease in the surface area of the dam between 2002 and 2022, from 40.1 to 22.50 hectares.The surface area of associated farmland/shrubs, secondary/re-growth forest and built-up changed from 187.47 to 148.42, 51.86 to 63.45 and 139.10 to 163.98 hectares respectively, between 2002 and 2022.The storage capacity of Opa dam in 2022 is estimated to be 1,232,444.05m 3 .The loss in reservoir gross capacity due to sedimentation for a period of 44 years since the time of impoundment in 1978 with initial capacity of 2.81 Mm 3 was estimated to be 1,577,555.95m 3 , which translate to 56.14 % gross capacity loss and 35,853.54m 3 capacity loss per year.If the current trend persists, it suggests that Opa dam may be filled up in the next 44 years from 2022, however the useful capacity of the dam may be lost much earlier than projected.