Delineation of the Flood Prone Zones Along the Medjerda River Downstream of Sidi Salem Dam in Tunisia

Abstract

The surge of extreme flooding events in the lower valley of the Medjerda between 1993 and 2005 raised people’s awareness to their effects. The main goal of this study is to delineate flood prone zones in response to the release of water volumes from Sidi Salem Dam, Tunisia. In order to achieve this goal, it was necessary to quantify the inflow flood hydrograph and characterise the geomorphology of the Medjerda River downstream the dam. The US Army Corps of Engineers HEC-RAS program was used to estimate the extent of floodplains associated with the released waters from the dam of Sidi Salem during historical and potential flooding events. The flood conveyances were calculated using surveyed topography, and HEC-RAS which was calibrated using site specific field data. The model output included water surface profiles. These profiles, coupled with a digital elevation model, were utilized to generate flood maps. Results showed a reduction of the main channel conveyance and overflows even for modest discharges. Data analysis proved that there were minor changes in the riverbed profile and the shape of the river’s cross-sections resulting in slower flow rates and larger sediment load depositions. This variability in the geomorphology of the river suggests that the flow friction factor varied as well. It is therefore crucial to monitor this friction factor variability in order to adjust the hydraulic models for better performances and forecasting capabilities with respect to standard models forced with static friction parameters. The produced flood maps are of great importance for several end-users particularly those in charge of: i) flood control and mitigation, ii) hydraulic structure design and deployment along major rivers, and iii) urban planning in order to avoid construction in flood-prone areas. The accuracy of these maps can be further improved by taking into account the morphological changes of the riverbed during each flooding event.