Morphodynamic modelling of sedimentation rates in a large riverine reservoir

Reservoir sedimentation poses a critical global challenge to freshwater storage capacity and aquatic ecosystems. Because periodic topo-bathymetric surveys are rarely available, combining process-based numerical modeling with remote sensing and sediment core analysis offers an effective approach for quantifying sedimentation rates and predicting reservoir evolution. This study presents a 2D depth-averaged, morphodynamic model (Delft3D-Flexible Mesh) for Lake Seminole (USA), a shallow reservoir at the confluence of the hydrologically distinct Chattahoochee and Flint Rivers. These tributaries are hydrologically distinct and show different sedimentation signatures. The heavily regulated Chattahoochee features an active subaerial delta, whereas the natural Flint exhibits subaqueous sedimentation. The model integrates dam operations, fraction-specific sediment transport derived via Sentinel-2 imagery, and invasive vegetation extent. The model is calibrated using in-situ flow velocities, suspended sediment concentrations, and 137 Cs-derived accumulation rates. The simulations accurately capture the contrasting depositional patterns, estimating a total accumulation of 5.1⋅10 7 m 3 (aligning with the 7.3⋅10 7 m 3 field estimate). This corresponds to a 0.38% yearly storage loss since impoundment in 1954. While the lake-wide average sedimentation rate of 1.5 cm yr -1 (near the measured 1.23 cm yr -1 ) indicates sedimentation is not an overall critical issue, deposition is highly heterogeneous. Accumulation reaches 10 cm yr -1 at the Chattahoochee delta front, suggesting that common whole-lake metrics miss important spatial information. Furthermore, calibration reveals invasive vegetation fundamentally alters local hydrodynamics and sediment pathways. By resolving spatial depositional dynamics, this integrated framework provides a robust tool for investigating deltaic evolution, predicting sedimentation hotspots, understanding upstream management impacts, and guiding sustainable reservoir operations.