Flume experiments on scour downstream of wood stream restoration structures

River restoration aims to improve physical natural form and processes of a river. Techniques to control the riverbed, stabilize channel alignment, protect stream banks, and rebuild the natural habitat are an important part of river restoration projects. Rivers can be stabilized and habitat restored through techniques such as rebuilding meanders and pool-riffle sequences and managing large wood. Structures that limit channel width to accelerate the normal flows through the constricted section are referred to as stream deflectors. Single-wing, double-wing and triangular deflectors are the most commonly used types of this measure. Log-frame deflectors consist of a triangular log frame filled with rock. Deflector constructions singly or in series in low gradient meandering streams, divert base flows toward the center of the channel and, under certain conditions, increase the depth and velocity of flow thereby creating scour pools and enhancing fish habitat. Scour characteristics and morphologies downstream of log-frame deflectors have been analyzed at the hydraulic laboratory of the University of Pisa. All experiments have been carried out in clear water conditions. The results showed that the tailwater depth plays an important role on scour characteristics. In addition, it was experimentally proven that using log-frame deflectors instead of log-deflectors result in a better river bank protection. In this case, for all the tested hydraulic conditions, the scour hole never occurred close to the channel bank. Useful empirical relationships have been proposed in order to evaluate the main features of the scour geometry. •Scour morphology downstream of single log-frame deflector and double log-frame deflector were studied.•Results showed that densimetric Froude number, drop height, height of the structure and the tailwater depth are the main parameters to predict the scour parameters.•Results show, that a classification of morphology type can be done in function of the non-dimensional parameters Fd and Δy/hst.•Comparing the results with Pagliara et al. (2015) data, depicts that in case of installing log-frame deflector never occurs scour hole close to the channel bank and just morphology Type C occurs.