Four Decades of Systematic Sedimentation at Kiri Dam, Nigeria (1982–2025): Dual-Epoch Bathymetry, Hypsographic Change, and Sedimentological Controls on Water-Supply Reliability

This study examines sediment dynamics at Kiri Dam, Nigeria (1982–2025), through a two-epoch bathymetric reassessment to measure hypsographic change and assess implications for water-supply reliability. In 2025, a high-resolution bathymetric survey was performed in accordance with IHO S-44 standards using Trimble RTK-GNSS and ADCP depth soundings, achieving a crossline mean absolute deviation of ≤0.12 m and a vertical RMSE of ≤0.05 m. Results were compared with the 1982 design dataset to evaluate elevation-banded losses. At NTWL (170.5 m A.M.S.L.), live storage capacity decreased from 615.00 to 344.15 MCM (−44.0%), while water spread contracted from 106.36 to 67.02 km² (−37.0%). Over 60% of storage loss occurred on depositional benches between 161 and 167 m, increasing the elevation–capacity gradient and intensifying volumetric penalties relative to areal shrinkage (elasticity ≈1.19). Multi-season analyses of particle-size distribution confirmed silty clays and clay loams dominate mid-bench and outflow zones, while sandy sediments at the right bank served as coarse-textured controls. Geochemical assays revealed moderate enrichment of Pb (16–27 mg/kg), Cu (12–19 mg/kg), and Zn (63–72 mg/kg) in fine-grained benches, with low hydrocarbon residues (ΣPAHs ≤0.8 mg/kg, ΣPCBs ≤0.04 mg/kg). A preliminary cost–benefit analysis indicated hydraulic sluicing ($0.5–1.5/m³) to be more feasible than mechanical dredging ($3–8/m³), with catchment stabilisation offering benefit–cost ratios of 1:1.3–2.0. Recommended monitoring includes resurveys every 5–10 years, event-triggered campaigns following ≥Q10 floods or >5% shoreline change, and seasonal PSD and geochemistry sampling.