Regional Calibration of SCS-CN Model for Ungauged Basins and Flood Modeling Using GIS and HEC-HMS/RAS: A Case Study for the Sidi Salem-Mdjez El Beb Section (Medjerda Valley-Tunisia)

The current study aims to investigate flood events on downstream of Sidi Salem dam (Sidi Salem-Medjez El Bab section) in Medjerda valley exploitation the 2D capabilities of the HEC-RAS. The 2003 winter flood was used as a case study to assess the modeling methodology. Firstly, a hydrological simulation at the Siliana watershed to estimate the flow at its outlet was carried out, which is subsequently considered to be one of the entrances to the Medjerda flows and an ungauged watershed. These simulations were carried out by the HEC-HMS program, which used and calibrated the SCS-CN technique of runoff estimate.. The Siliana watershed's variables are carefully calibrated to simulate discharge at the outflow accurately. The hydrological model results showed a good agreement with the observed flows presenting a Nash equal to 78% and 79% for the 2003 and 2012 flood event respectively. The calibrated discharge of Siliana watershed outlet was used as inlet boundary condition for the 2D floodplain simulation. All 2D HEC-RAS numerical modeling of the flood events of 2003, 2005, and 2012 were done using topographic data by inserting the DEM of the research region with a 30 m grid. Soil data, land use, and land cover data were obtained from The National Soil and Land Cover Dataset created by Ministry of Agriculture, and finally, river geometry and cross-section were derived by merging measured survey data with the DEM-delineated cross-section. The 2D numerical simulation results obtained allowed visualizing the extent and intensity of flooding (flow rate and water height). For the 2003 and 2012 flood events, the HEC-RAS model presented a NASh of 86% and 81% respectively in terms of flow rate at the Mdjez Elbeb GP5 hydrometric station.