Scour problems downstream of low-head hydraulic structures

Low-head structures are widely used in river restoration. Their function is to regulate the sediment transport and at the same time they can assure optimal habitat for fish species in the river. They strongly affect river morphology because of the erosive processes occurring downstream of them. Thus, a correct design has to take into account several aspects, i.e. technical, economic, environmental, etc. The present chapter aims to present the most recent achievements in scientific literature regarding the design criteria for low-head structures. In particular, low-environmental impact structures will be analyzed and illustrated. In the last decades, the environmental sensibility has increasingly forced hydraulic engineers to propose design solutions which can conjugate both hydraulic functioning and environmental impact minimization. This chapter proposes a synthesis of criteria to predict scour characteristics of the stilling basin downstream of several low-head structure typologies. Namely, the scour process downstream of block ramps will be discussed and the effect of both stilling basin geometry and ramp configuration will be analyzed, for both clear water and live bed conditions. Furthermore, the erosive process downstream of rock grade control structures and stepped gabion weirs will be illustrated along with relationships to predict the characteristic lengths of the scour hole and dune.