Physical Modeling of Sediment Trap for Le Bez River, Switzerland

The Le Bez torrent in the Canton of Bern, Switzerland, is known for flashy and violent floods due to its watershed topography and vegetation conditions. Such floods can bring a substantial amount of sediment and wood into the small downstream village of Villeret resulting in severe flooding. To protect against such events, a sediment/wood trap system composed of two retention basins with two weirs upstream of each of them is under construction a few hundred meters upstream of the village. The downstream basin is equipped with an open check dam with a rack in front of it that aims to retain sediment and wood debris using a combination of hydraulic and mechanical control systems. To study the performance of the retention system a physical model was constructed using Froude similarity rules with a geometrical scaling factor of 1:10. The model is used to simulate different flood scenarios with return periods from 5 to 300 years with respective amount of sediment and wood income into the model. The amounts of wood and sediments transiting the model are measured are then measured to evaluate the efficiency of such trap system. Different geometrical parameters are varied to optimize the design. A vertical clearance in the rack of the open check dam of 1.8d90 was found to be sufficient to guarantee bed load transit during a 5-year flood. The retention system is working properly for a 100-year flood but during the falling limb of the hydrograph spontaneous sediment flushing was observed.