Modeling of rainfall-runoff process using HEC-HMS model for an urban ungauged watershed in Tunisia

Floods are among the most devastating natural disasters in small urban watersheds, resulting in loss of lifes, enormous damages to properties, and causing a serious threat to the economy. Appropriate modeling can be a useful tool to prevent and reduce such flood risks. Nevertheless, due to the lack of runoff observations, flash flood prediction remains one of the challenges of hydrological modeling in ungauged basins. The main objective of this study is to develop a rainfall-runoff modeling using HEC-HMS software to estimate the flood peak discharges and reproduce hydrographs related to the main events that occurred in a small urban ungauged watershed located in North-East of Tunisia. A GIS environment was used to generate geospatial information of the study area and compute hydrologic parameters. Among many existing methods, the Soil Conservation Service, developed by the United States Department of Agriculture, is selected for runoff estimation through hydrological modeling due to its accurate results and its limited data requirements. In Tunisia, urban watershed monitoring does not exist, thus, the model can't be calibrated with field measurements. Thus, the Rational Formula was adopted for approximate estimation of flood peak discharge for different return periods. Results showed that the rainfall event of 2003 generated a peak flow with a 100 years return period, while in 2019, the simulated peak flow corresponded to 20-years return period. Given the scarcity of observed data, these findings are useful to recognize the magnitude of the extreme rainfall events and to design the appropriate stormwater structures in urban areas.