EFFECT OF FLEXIBLE VEGETATION ON STILLING BASIN EROSION DOWNSTREAM OF LOG-FRAME STRUCTURES

Log-frame structures are commonly used as eco-friendly low-head structures to control sediment load in rivers thereby enhancing the process of stabilization of channel bed. These structures reduce the channel width thus causing local flow acceleration. Nevertheless, they contribute to create suitable habitat for fish species. Among the different types of log-frame structures, double winged log structure is widely used in this regard. In this study, for the first time, we investigate the effect of flexible vegetation on the downstream stilling basin equilibrium morphology in correspondence with river bends. Namely, dedicated tests were conducted in a curved channel by placing plastic fibres (simulating natural vegetation) downstream of the wood control structures. In particular, we tested different vegetation density and spatial distribution in order to understand their effects on sediment transport dynamics. Preliminary analyses allowed to establish that vegetation deeply modifies scour features. In fact, based on experimental evidences, a preliminary classification of the different scour typologies is proposed. In general, it was noticed that flexible vegetation causes a remarkable decrease in the maximum scour hole depth, as well as an overall modification of the scoured area in comparison to that obtained in reference tests conducted in the absence of vegetation under the same hydraulic conditions. Furthermore, an important role is also played by the channel curvature. Namely, the effect of secondary flows exacerbates localized scour processes, especially close to the outer river bank. In this perspective, the role of flexible vegetation appears again to be fundamental in protecting bank erosion and in avoiding excessive scour depths which could eventually lead to a bank failure. This paper represents the first attempt to understand the complex dynamics characterizing the erosive mechanism in correspondence with low-head structures in vegetated channels and aims to throw lights on this under-investigated topic by furnishing preliminary tools which could be useful for a correct design of such structure typologies