Evaluation of the accuracy of digital bathymetric models derived from USVs in shallow sandy beaches: Implications for morphodynamic monitoring

High-resolution bathymetry is fundamental data for physical geography and for understanding coastal geomorphological processes, including sediment transport, erosion, and the evolution of the underwater landscape. Unmanned Surface Vehicles (USVs) represent an efficient and precise alternative for data acquisition in shallow coastal areas. This research evaluates the precision and accuracy of a USV, equipped with a single-beam echo sounder and RTK GNSS, in generating Digital Bathymetric Models (DBMs), which are key elements for morphodynamic modelling. The study was conducted at Ensenada del Esparto beach, Spain, a sandy environment with intermediate morphology subject to significant storm dynamics. To determine the optimal density, six DBMs were created from surveys with varying transect spacings: 5 m, 10 m, 15 m, 25 m, 50 m, and 100 m. The precision analysis using standard error metrics (ME, MAE, and RMSE) established a clear correlation between data density and the geomorphological fidelity of the model. High-density models (5 m to 15 m) showed minimal errors, while sparser models (50 m and 100 m) exhibited a significant increase in error and a tendency to generalize and flatten the seabed topography. It was identified that a spacing between 15 m and 25 m offers the optimal balance between the precision required for sediment process studies and survey efficiency. We conclude that the USV is a robust tool for geomorphological monitoring and that optimising the survey design is critical to ensuring the quality of DBMs necessary for coastal hydrological and sediment transport models. • USV enables cost-effective high-res bathymetry in challenging areas. • Determining optimal survey spacing is crucial for model accuracy. • Intermediate line spacing balances data quality and survey efficiency.