Systematic Sedimentation at Kiri Dam, Nigeria (1982-2025): Two-Epoch Bathymetry, Hypsographic Losses, and Sedimentological Controls on Water-Supply Reliability

This study presents the inaugural two-epoch comparison of Kiri Dam Reservoir in Nigeria, contrasting the 1982 design survey with a geodetically controlled hydrographic resurvey conducted in 2025. The methodologies employed adhered to IHO S-44 (2020) standards, including Trimble RTK-GNSS control, ADCP depth soundings, crossline calibration, uncertainty propagation, TIN and raster surface generation, and hypsographic volume integration. At NTWL (170.5 meters above mean sea level), live storage has decreased from 615 million cubic meters to 344.15 million cubic meters (a reduction of 44.04%), and the surface area has declined from 106.36 to 67.02 square kilometres (a decrease of 37%). These losses are concentrated in the 161–167 meter benches, as confirmed by PSD analyses of silty clay and geochemical fingerprinting indicating organic-rich fines. Regression analyses reveal strong linear relationships for elevation–capacity, with R² values of 0.996 in 1982 and 0.992 in 2025. The results suggest diminished irrigation reliability and limited flood buffering capacity, emphasising the need for targeted sediment management and periodic resurveying.