Physical modeling of the Bez sediment retention system

Mountain streams often have higher sediment flows than lowland rivers because of the gradient and concentration of runoff during heavy rainfall. This increased sediment supply can have consequences on the river and its surroundings by erosion or deposition and lead to overflowing. The contribution of floating wood can also be more important if the watercourse is in a forest zone. Driftwood is tree trunks and branches that end up in rivers and streams during floods. They can cause important damages to the direct surroundings of the watercourse and to the infrastructures, in particular by causing logjams under bridges and during road crossings. It is therefore necessary to take measures to minimize the risks linked to these natural phenomena. The municipality of Villeret in the canton of Bern wishes to protect itself against the flooding of the torrent “Le Bez” and plans to do so with a project including a sediment retention system with two alluvial dumps and a wooden bar screen. These facilities should provide sediment and driftwood retention during large floods (Q > HQ5) and maintain sediment transport downstream during small floods (Q ≤ HQ5). Maintaining a certain amount of sediment supply is, in fact, vital for the eco-morphology of the watercourse but also to limit bed erosion.