Evaluating structural mitigation measures for the Medjerda River: proactive flood hazard assessment in Bou Salem through 2D simulations

Diagram contrasting a 63.38% flooded urban area before protection with a 6.19% area after bypass canal construction, concluding with a recommendation to keep bank roughness below 0.05. Urban centers in semi-arid regions face escalating flood threats exacerbated by complex topography and intensifying climate extremes. To address limitations in existing flood assessments, this research evaluates the hydraulic performance of a future protection project covering both the Medjerda River and the bypass canal simultaneously in Bou Salem, Tunisia. Coupling high-resolution 5 m LiDAR topography and ESA WorldCover data within a fully 2D HEC-RAS hydrodynamic model enabled the simulation of flood dynamics across 10- to 100-year return periods. The integrated megaproject (bypass canal, dikes, dredging) reduces the 100-year flooded urban extent from a baseline of 63.38% to only 6.19%. Furthermore, evaluating operational resilience through Manning coefficient variations reveals high system sensitivity to riparian roughness. If vegetation maintenance is neglected, a roughness increase to n = 0.05 swells the flood extent by 73%, while reaching n = 0.09 causes a 121% increase, effectively nullifying the initial protection. Practically, this study provides quantitative evidence that long-term physical defence reliability inherently depends on continuous riverbank management.