EROSIVE PROCESSES DOWNSTREAM OF ARCH SHAPED SILLS IN VEGETATED CHANNELS

Low-head eco-friendly structures (e.g., rock sills) are commonly used in river restoration projects in order to control the sediment transport and to enhance bed stabilisation. A formation of a scour hole, subsequently followed by a dune, generally occurs downstream of these structures. In addition, it is quite common that trees and other plants can grow in correspondence with emerging portions of dunes, especially during prolonged dry seasons and in mountain regions. These elements, although natural, constitute an obstacle for flow passage which can significantly modify flow characteristics and equilibrium morphologies. Nevertheless, it is quite rare that they are removed from river bed. In fact, mountain streams are generally characterized by a low maintenance, resulting in an uncontrolled vegetation growth in the channel bed. The presence of such vegetation may have a significant impact on the ensuing scour morphology and represents an important area of investigation. In fact, to the best of authors’ knowledge, no studies are present in the literature dealing with the combined effects of both rock sills and rigid wood elements presence in terms of stilling basin equilibrium scour morphology. Thus, a dedicated experimental campaign was conducted by simulating natural wood elements in rivers with cylindrical steel elements located downstream of arch shaped rock sills. Tests were carried out by varying both hydraulic conditions and steel element density. Furthermore, different steel element spatial configurations were tested allowing to evaluate their effect on scour characteristics. Preliminary experimental evidences showed that the density of vegetation placed downstream of the structure, as well as the inflow conditions (discharge and tailwater) are the main governing parameters of the resulting stilling basin morphology. In particular, this study provides a preliminary scour typology classification along with insights on scour dynamics. This study can be a valid tool to optimize the design of such structures.