Design of a diversion structure for the management of residual risks using physical model tests

As revitalization and protection against floods are essential in our time, the town of Delémont in Switzerland has set up a major project named “Delémont Marée Basse”. Part of the project, the residual risk plans to be managed by a lateral overflow bypassing the town via the railway platform. A physical model of the area is built at the scale of 1:32 in order to test and optimize the control structure. The main objective of the latter is to protect the town against extreme flood discharges. The main objectives are to derive 15 m3/s during a 300-year flood (150 m3/s) and avoid overflow for the 100-year flood (110 m3/s). After a major calibration phase of water lines, different geometries of the control structure are tested. The retained configuration is optimized for giving results consistent with the requirements of the project. The model was then used to define the minimum overflow area required, the influence of the support piles of the control structure on water lines and diverted discharge, as well as the influence of an upstream apron on Patouillet Bridge. In addition, qualitative tests examined bed load transport impact and driftwood behavior. Similarly, velocity measurements integrate the risk of erosion around the control structure and scour around the bridge piers. Finally, in preparation for the planning of future interventions, overflow discharge values and water lines are recorded under various scenarios.