Log-Frame Deflectors Scour Morphology in Curved Channels

In the last decades, low-impact structures have become more and more popular in river restoration projects. In fact, these types of structures are very flexible and are characterized by limited costs. In the meantime, they are able to preserve the natural contexts in which they are located and can be easily implemented, repaired or rebuilt in the case of a failure due to excessive floods and/or erosions. Therefore, in these perspective, wood hydraulic structures appear as a valid alternative to traditional concrete sills, weirs and deflectors in order to control localized scour phenomena and, at the same time, to enhance water quality and natural meandering processes. In particular, woody in-stream grade-control structures have already been employed to control riverbed evolution, to protect riverbanks, and to improve aquatic habitats. Their role in controlling localized scour phenomena is particularly effective in correspondence with stream bends, where localized scour phenomena are more significant because of flow acceleration. Among low-impact structures, log-frame deflectors made of a triangular log frame filled with rocks, result to be particularly suitable for such scope. Usually, log-frame deflectors, constructed at the outer bends in meandering streams, protect the riverbank by diverting base flows toward the center of the channel. Under certain hydraulic conditions, they modify both the depth and velocity of flow, resulting in scour pool formation and fish habitat improvement. The main purpose of the current study is to analyze the effect of the stream curvature and the position of log-frame deflectors on the scour morphology. Therefore, a dedicated flume was built in the hydraulic laboratory of the University of Pisa and a series of experimental tests have been conducted under different hydraulic conditions and geometric configurations. The results showed the important effect of the channel curvature and the structure position on the maximum scour depth value and scour morphology.