Extreme Flood Dynamics and Vulnerability of Semi-Arid Plains: 2d Hydrodynamic Modeling in the Medjerda Basin

Semi-arid plains are highly exposed to extreme hydrological hazards. During major flood events, river overflow directly threatens soil stability and structure. To quantify this fragility, analyzing the spatial propagation of flows and velocities over the existing topography is essential. This study investigates the physical impact of a 100-year flood event from the Medjerda River on the Bou Salem plain (Tunisia). By employing a two-dimensional hydrodynamic model (HEC-RAS) coupled with a high-resolution LiDAR Digital Elevation Model (DEM), the inundation footprint and velocity fields were mapped. The simulation outcomes highlight a near-total submergence of the existing land parcels. Moreover, the spatial analysis indicates a drastic drop in flow speeds once the water breaches the main riverbed, leading to very slow-moving currents across the floodplain. This prolonged water stagnation constitutes a major risk, severely exposing the lands to physical degradation. Ultimately, this approach proves that a precise spatial assessment of flood parameters namely flow velocity and water depth is a fundamental step to estimate spatial losses and guide effective flood protection strategies.