MULTIMODAL CHARACTERISATION OF ERELU RESERVOIR BED

ABSTRACT This study presents a comprehensive multimodal assessment of the Erelu Reservoir, combining bathymetric surveying, side-scan sonar (SSS), GNSS-referenced topographic measurements, and multi-temporal satellite imagery to evaluate morphological changes and sedimentation dynamics. Depth measurements conducted between February 2022 and August 2025 ranged from 0.60 to 5.70 m, with a mean depth of approximately 3.01 m, while estimated storage volumes varied from roughly 2.16 × 10⁶ m³ to 2.69 × 10⁶ m³. The navigable surface area correspondingly decreased from about 95 ha to 71 ha, indicating progressive infilling and a loss of storage capacity. Analysis of side-scan sonar imagery revealed three primary acoustic facies: medium-sand channels, soft-mud deposits, and coarse backscatter patches. Anthropogenic features, including a canoe wreck and a buried pipeline, were also identified and found to influence local sediment deposition and flow patterns. Land-use and land-cover (LULC) assessment indicated a shift from predominantly agricultural and vegetated landscapes in 1984 to extensive urbanisation by 2024, closely correlating with increased sediment influx. The integrated bathymetric, sonar, and LULC results demonstrate a clear linkage between catchment geomorphodynamics and reservoir degradation. These findings underscore the importance of routine hydrographic and sonar monitoring, the implementation of catchment erosion-control measures such as reforestation, vegetative buffer strips, and contour farming, and targeted dredging of depositional zones. Future research should incorporate sediment-core analyses and hydrodynamic modelling to refine source attribution and predict the remaining reservoir lifespan while accounting for current uncertainties arising from single-beam sampling density and interpolation methods.