Sediment replenishment as a measure to enhance river habitats in a residual flow reach downstream of a dam

Floodplains downstream of a dam, where the natural flow regime is replaced by a constant residual flow discharge, often lack sediment supply and periodic inundation due to the absence of natural flood events. In this study, a flood with a one-year return period was released from an upstream reservoir and combined with sediment replenishment. The aim was to enhance hydraulic habitat conditions in the Sarine river downstream of Rossens dam in Western Switzerland. A special configuration of sediment replenishment was applied. It con- sisted of four sediment deposits distributed as alternate bars along the river banks, a solution which was previously tested in the laboratory. The morphological evolution of the replenish- ment and of the downstream riverbed were surveyed including pre- and post-flood topog- raphy. The hydro-morphological index of diversity (HMID) was used to evaluate the quality of riverine habitats in the analyzed reach. It is based on the variability of flow depth and flow velocity. The combination of the artificial flood with sediment replenishment proved to be a robust measure to enhance sediment dynamics.