Connectivity at transitions of local river widenings

Local river widenings have been carried out at many water bodies in Switzerland over the past three decades. Local widening has proven to be an efficient tool in many ways, both from an ecological and hydraulic engineering perspective. However, the effects of local widening usually do not remain local. In addition to potentially desirable effects (e.g., bed stabilization in headwaters), in some situations local widenings can also have a destabilizing effect on adjacent river reaches. In both transition zones, the discontinuity of slope often requires large-scale and extensive streambed stabilization measures. A conventional and widely used method is to stabilize the transition zones with solid block ramps. However, riprap ramps, riprap ramps with riffle-pool structure or random stones, and unstructured block ramps are considered more ecological variants than solid block ramps. Meandering ramps also significantly improved fish passage compared to the classically design. The aim of the research project is to assess which geometries and construction methods optimize the discharge capacity in the transition zones in the event of a flood; and promote combinations of flow velocities, turbulence, and water depth to allow fish passage.