Small-Scale Modeling of Flexible Barriers. II: Interactions with Large Wood

Abstract

During strong floods, rivers often carry significant amounts of sediment and pieces of large wood (LW). When bridges and hydraulic structures are unable to allow LW to pass through, it becomes necessary to trap LW through specific wood retention structures (e.g., flexible barriers). This paper presents a comprehensive analysis of the interactions between LW and flexible barriers using small scale models. A dimensionless criterion is first proposed to compute blockage probability of single logs. It is based on experiments varying log size and shape, channel slope (2%, 4%, and 6%), water discharge, and barrier bottom clearance. Based on runs using six mixtures of hundreds of logs, an equation is secondly provided to compute flow depth at a barrier accounting for the head losses related to large numbers of logs. Conditions leading to the release of LW when the barrier is severely overwhelmed are also studied. The deformation measured on the barrier proves to be lower with LW-laden flows than under full hydrostatic loading of a barrier obstructed by a plastic sheet. Overall, we demonstrate that flexible barriers are very relevant structures to trap LW. A companion paper shows how to design and manufacture a small scale flexible barrier in mechanical similitude with the prototype scale.

Small-Scale Modeling of Flexible Barriers. I: Mechanical Similitude of the Structure

“…The top cable elongation gives an indication of the barrier lowering, which is associated with the barrier overflowing. Unfortunately, we were unable to measure this lowering during the experiments (using image-based techniques or intrusive sensors), and, in particular, those where logs and water reached the top cable and passed through the barrier (see the companion paper, Piton et al 2022).…”

“…This number is relevant when dealing with a freesurface flow. The Reynolds number similitude was relaxed but for all experiments with woody debris, turbulent flows with Re ¼ v f • w B • h=ν > 2500 were maintained, with ν as the water kinetic viscosity (Piton et al 2022).…”

“…To a lesser extent, this lower barrier elongation may also be attributed to the presence of the thin plastic film, even though it was folded. Nevertheless, this discrepancy is not critical for the application case where the main focus is placed on the 100% relative loading and slightly above, which is associated with barrier over-topping by the woody debris (Piton et al 2022).…”

“…7. The woody debris mainly consisted of large logs (Piton et al 2022). The water discharge was increased stepwise from 2 to 8 L=s.…”

“…Thus, this loading may be used in view of validating the reduced scale barrier for this application, which is also likely for other applications where high levels of barrier obstruction are expected. In addition, this difference in deformation may be considered as an indicator of the barrier obstruction, as discussed in the companion paper (Piton et al 2022).…”