Off-stream floodplain storage: numerical modeling and experimental analysis

Floodplain storage should be carefully considered among the structural measures for active flood risk mitigation. Floodplain storage facilities assure an attenuation effect and consequently a significant reduction and delay of the flood peak by allowing temporary storage of the flood volume in an adequate upstream capacity. This paper addresses off-stream floodplain storage. In off-stream floodplain storage, the reservoir is located outside the river bed and equipped with an adequate intake weir, which is usually controlled by downstream waterworks and outlet orifices. The results of an experimental survey conducted in the laboratory of the Department of Civil, Architectural, and Environmental Engineering at the University of Naples Federico II are presented to analyze the hydraulic behavior of off-stream floodplain storage filled by a side weir that is controlled downstream. The experimental results, which were obtained by varying the geometric characteristics of the weir, were compared with those inferred from a one-dimensional numerical model, highlighting small deviations and providing useful information for the overall design of the storage. The obtained results primarily showed that an increase in the storage volume causes an attenuation of the inflow hydrograph, achieving lower outflows and a decrease of the attenuation ratio η, thus increasing the storage efficiency. Finally, experimental data were compared with the results of a previous survey on in-stream storage, highlighting a higher efficiency of the off-stream floodplain storage.