Due to advanced silting, the Foum El-Gherza dam has lost more than two thirds of its initial capacity (47 hm3) and is no longer able to withstand the floods that threaten its downstream section. Indeed, the damage recorded in recent years has confirmed the vulnerability of the man-made structures located on both banks of the Oued Labiod. Thus, we believe that the hydrological behaviour of the catchment area is no longer influenced by the dam. Therefore, the hydrological study involved a critical analysis (homogeneity tests) of the hydro-climatic data in order to highlight the characteristics of the historical events recorded during the period 1950–2019. The frequency study of the maximum daily rainfall and floods recorded at the dam allowed us to determine the rainfall and flow rates of the return periods (10, 25, 50, 100, 500 and 1000 years). The flooding of the 28 October 2011 were used as a standard for the calibration of the model calculated by the HEC-Ras software. After validation of the model, a prediction of the water levels and flood extent was made for the selected return periods. The results obtained show that a part of the town of Seryana (district located on the edge of the right bank) suffers from flooding proportionally to the return periods of the floods. In addition, some agricultural areas bordering the Oued are also affected by the floodings. The hazard modelling maps can be considered as a basis for a flood risk prevention plan (PPRI) and as a decision support tool.