Morphological characteristics of eco-friendly protected basins downstream of block ramps in river bends

Block ramps are hydraulic structures used to control the sediment transport in rivers and to dissipate flow energy. Moreover, they serve as useful structures to maintain and preserve ecological habitat and to facilitate fish migration. In the past, scour features of both protected and unprotected stilling basins of block ramps have been studied for straight river branches. Sometimes, these structures are placed at channel curves where the flow distribution is characterized by a significant asymmetry, resulting in a scour formation in correspondence with the channel bank, that needs to be carefully predicted and controlled to minimize the risk of a structural failure. Apparently, so far there are no studies focusing on the scour mechanism occurring in protected stilling basins downstream of block ramps in river bends. Hence, this paper aims at investigating the effect of stilling basin protection on the scour process under a combination of different hydraulic conditions, block ramp configurations and locations of the protection structure along the stilling basin. To this end, tests were carried out by placing the block ramp in a curved channel and varying both the flow discharge and the downstream tailwater depth. The downstream stilling basin was protected by introducing a rock sill at various distances from the ramp toe. It has been found that the equilibrium scour morphology mainly depends on hydraulic conditions and it is also influenced by the position of the rock sill in the basin. A classification of different morphological configurations is proposed as well. Finally, an empirical equation is provided to estimate the maximum scour depth under the tested configurations.