Live-bed scour downstream of block ramps for low densimetric Froude numbers

Sediment load plays a major role in the morphological evolution of rivers. Therefore, the analysis of the sediment load interaction with hydraulic structures is of main importance in order to enhance the preservation of fish habitats and river morphological characteristics. The present study analyzes the scour mechanisms downstream of a block ramp in live bed conditions, when the sediment supplied by the approaching flow is balanced by the sediment transported out of the scour hole. Experiments were performed in a model flume and the effect of the approaching sediment concentration on the scour geometry was analyzed. It was observed that the scour features depend deeply on the approaching sediment concentration and four main profile configurations were distinguished. The experimental data were analyzed and empirical relationships were developed in order to evaluate the depth and length of a scour hole, the dune height and the distance of the transversal section of maximum dune height from the ramp toe for different hydraulic and geometric conditions. It was also proved that the dynamic equilibrium shape of a scour hole does not depend on the sediment load time history.