Block ramp failure: morphological variations and effects on the downstream scour

The presence of an hydraulic structure in a river affects the natural sediment threshold process creating localized discontinuity in river morphology. A correct structure design has to solve two main problems: it has to assure the structure stability in order to preserve its hydraulic functionality and it has to tend to minimize the structure impact on the river body in order to not cause morphologic variations which can be incompatible with the natural biological processes. For this reason in the last few decades block ramps have been extensively studied as they have the peculiarity to be both eco-friendly structures and assure a greater energy dissipation than other traditional structures. Recent studies analyzed both the scour process downstream of them and their stability, highlighting that four main failure phases can be distinguished: incipient motion, local failure, global and ultimate failure, respectively. In case of a failure the ramp configuration changes and some blocks are transported downstream. Although the block ramp can loose its original hydraulic functionality, the blocks transported downstream can have a protective effect on the scour hole in the stilling basin. The aim of this paper is to analyze both the ramp morphological variations and the reduction of the scour hole depth in the stilling basin. Experimental tests were carried out at the University of Pisa with different ramp beds and stilling basin materials for two different ramp toe configurations. Simple relationships were developed in order to evaluate the maximum scour depth and the ramp morphology.