Author(s): Stefano Pagliara, Michele Palermo, Deep Roy
Keywords: Scour morphology; Log-frame structures; Shrubs; Curvature;
Abstract: River restoration projects generally include the design of low-head structures in order to control global sediment transport and to create localized erosion areas in which fish species can rest. Low-head structures have become more and more popular in the last decades due to their ability to conjugate hydraulic functioning and to minimize their impact on the surrounding ecosystem. Double winged log frame is particularly adapted for mountain stream restoration, as they can contribute to deflect river flow and, at the same time, to create natural pools. Nevertheless, in such natural contexts, it is relatively frequent that vegetation grows, thus contributing to modify flow structure and characteristics. According to the in-situ conditions, different vegetation types can grow in the river bed, including shrubs. Therefore, it appears fundamental to understand the combined effect of the presence of both hydraulic structures and vegetation in the river, in order to throw light on the erosive mechanisms which can occur. In this perspective, an experimental study was undertaken, and several tests were conducted in a dedicated curved channel, by simulating both a double winged log frame (made of wood) and a downstream vegetation growth. Namely, double winged log-frame structures were placed at two different positions in the curved channel and the downstream shrubs were simulated by using tangles of plastic thread. More specifically, different plastic thread configurations were tested by varying both their location and planar distribution on the original bed level. Their effect on the equilibrium scour morphology was analysed in terms of the resulting scour typologies and compared with the corresponding reference configuration (i.e., base configuration without simulated vegetation), under same hydraulic conditions. It was noticed that the structure position, the river curvature and the plastic thread distribution play a fundamental role in the scour process. In general, the increase of plastic thread concentration results in a significant deformation of the river bed morphology, which can also have a prominent impact on river bank stability. Therefore, this paper aims to furnish preliminary insights on such a relevant topic.