Flood hazard Mapping and 1D Hydrodynamic Modeling for Sebeya River in Northwestern Rwanda.

Sebeya catchment in north-western Rwanda has been subject to frequent floods over the years. Despite recent investments in structural measures for flood protection in the catchment, a flood event in May 2023 caused a loss of 97 human lives and damages to around 2,200 buildings. The event urged authorities to invest in additional measures and improve flood management practices to minimize the cost of future flood events. To support the decision-making and planning in flood management practices, this study conducts a flood hazard assessment in the Sebeya catchment using a HEC RAS 1D model based on storm events of 10 to 100-year return periods. A 14 km long reach of Sebeya River was modeled using a 12.5-m resolution digital elevation model (DEM) and under the assumption of steady flow for each daily time step to simulate water surface profiles. The model was calibrated with measurements of 181 daily water levels ) by adjusting Manning’s roughness values and validated against 152 daily water levels ). The RMC-Best Fit Software was then applied to analyze flood frequency and generate flood hazard maps for storm events of 10, 25, 50, and 100-year return periods. The flood hazard maps can support policymakers and decision-makers in integrating flood hazard assessments into planning and development processes, enhancing efforts to effectively mitigate flood hazards in the Sebeya catchment and other regions in Rwanda. Moreover, the results will extend the capacity of the Government of Rwanda to understand flood risk and improve efficiency in investments for flood control measures.

 

Key words:  HEC-RAS 1D; Flood analysis; Steady flow; Flood hazard; Sebeya catchment; Rwanda