Which strategy should be adopted for the delimitation of the hydraulic public domain? The case of a watershed characterised by dam overflows and inter-basin water transfers (Boudouaou-Algeria)

The dams and reservoirs built in the valleys play a key role in water management and contribute to a society's quality of life. But they are also a potential risk, with often tragic downstream consequences. Keddara dam on the Boudouaou wadi, in central Algeria, is just an example. Its historical water level record shows that it has overflowed on several occasions, resulting in a real danger of flooding for people living downstream, particularly those living in the town of Boudouaou. In this respect, hazard mapping represents a very effective means of prevention, through the delimitation of the Public Hydraulic Domain (PHD). To do this, we were interested in the predetermination of extreme flood flows and hydraulic simulation in a natural environment. In fact, the particularity of our study lies in the strategy adopted for the case of an ungauged catchment, led by a dam located in its upstream part, and characterized by water inflows outside the catchment, This situation of the dam has led to devastating spills, which leads us to consider the entire Boudouaou basin to determine the flow rates and thus the hazard zones downstream the basin. In order to solve this problem a very high spatial resolution data was used, with Digital Terrain Model and Digital Surface Model of 1 m, as well as orthorectified satellite image of 50 cm. On the other hand, the peak flow data for different return periods were estimated using empirical formulas, using frequency maximum precipitation and the physical characteristics of the basins. However, roughness data, bridges as well as details of all hydraulic structures are the subject of several field visits. The results of the hydraulic simulation obtained after calibration and validation of the HEC-RAS model, by using the peak flows caused by the overflow of the Keddara dam on the one hand, and the Boudouaou sub-basin downstream on the other hand, seem to give better results for the various retained recurrences (high, medium and low), i.e. simulated water levels correspond exactly or almost exactly to the heights measured in the field.