Hydrologic modeling and flood hydrograph reconstitution under an arid climate condition: case of Gabes Watershed, South-Eastern Tunisia

Flash floods are among the most serious natural hazards. They are associated with exceptional rain events, characterized by short durations, very high intensities and rapid flows. The city of Gabes (South-Eastern Tunisia), located at the outlet of Gabes Basin, is characterized by an erratic rainfall distribution and has been exposed to numerous damaging floods. This study is interested in the study of the phenomenon of flash flood under an arid climate condition through the analysis of the case of Gabes Watershed. More specifically, the objective of this study is to reproduce observed historical peak flows and reconstruct recent flood hydrographs at the outlet of Gabes Basin. The Hydrologic Engineering Center's Hydrologic Modeling System (HEC-HMS) was used to simulate flood flows entering the city in case of extreme rainfall to determine early warning indicators under different rainfall patterns. The model was calibrated using Soil Conservation Service and Snyder unit hydrographs in order to find the appropriate transfer function. The calibration process was performed on the basis of two statistical evaluation criteria, the NSE (Nash coefficient) and NOF (objective coefficient). The NSE values for calibration varied from 0.90 to 0.95 while NOF values ranged between 0.05 and 0.22, indicating a reasonably good simulation. To further validate the chosen model, simulation of the June 2014 hydrograph was carried out based on the frequency flow estimated empirically. The corresponding Nash and determination coefficient values were found to be around 0.98, which shows very satisfactory results. In a data sparse environment, the findings of this study are essentially valuable for 1D and 2D hydraulic modeling in order to simulate the variation of the water level during a rainfall event. The generation of water depth may then be transposed into floodable areas. Hydrologic simulations can also be valorized in hydraulic design of flood control structures.

Automated summary

This study focuses on the phenomenon of flash floods under arid climate conditions in Gabes, Tunisia, by simulating historical peak flows and recent flood hydrographs using the HEC-HMS model. Calibration of the model based on statistical evaluation criteria resulted in good simulation results, which can be valuable for 1D and 2D hydraulic modeling to simulate water level variations during rainfall events and identify floodable areas.