Flood Inundation Assessment in the 9th District of N'Djamena, Chad, Using an Integrated HEC-HMS-HEC-RAS Framework

Abstract

Flooding is a major natural hazard in rapidly growing African cities, where hydrological variability, urban expansion, and limited drainage capacity can increase exposure. N'Djamena, the capital of Chad, is recurrently affected by flooding associated with the Chari-Logone River system. The Chari-Logone watershed forms the principal active drainage area of the Lake Chad Basin and covers approximately 613,000 km². This study evaluates an integrated HEC-HMS, HEC-RAS, and Geographic Information System workflow for reconstructing historical flood conditions in neighbourhoods of the 9th District of N'Djamena. Hydro-climatic conditions for 2021-2023 were analysed, and the 2023 monthly discharge series at the N'Djamena station was used for HEC-HMS calibration. The calibration produced a Nash-Sutcliffe efficiency (NSE) of 0.50, a root mean square error (RMSE) of 562.93 m³/s, and a Pearson correlation coefficient (r) of 0.86. These statistics indicate moderate calibration performance, with a substantial overestimation of the principal discharge peak. HEC-HMS outputs were subsequently used in HEC-RAS to generate flood-inundation maps for 2021, 2022, and 2023. The mapped patterns were qualitatively consistent with historically affected areas, although no independent spatial accuracy metric was available for flood extent or depth. The modelling framework therefore provides a useful basis for historical inundation assessment and planning support, while its predictive application requires independent hydrological and spatial validation.