Influence of large woody debris on sediment scour at bridge piers

Large woody debris (LWD) reduces the flow area, deviate the flow and increases the velocity in correspondence of the bridge pier, therefore increases the maximum scour hole depth and accelerates sediment removal. Logs and drifts accumulated on bridge piers are of different dimensions. According to logs characteristics and river morphology, drift accumulations can either extend downstream the bridge pier or they can accumulate totally upstream. This paper aims to analyze the effect of drift accumulation planimetry on bridge pier scour. The experimental investigation has been carried out at the PITLAB hydraulic centre of Civil Engineering Department, University of Pisa, Italy. Drift accumulation was characterized by different relative longitudinal lengths, flow area occlusions, length of longitudinal drift and downstream planimetrical positions relative to the pier center. The experimental investigation has been carried out in clear-water conditions. Several pier sizes, channel widths and sediment materials have been tested. Maximum scour hole in presence of drift accumulation have been compared to the maximum scour hole for an isolated pier. Finally, data were compared with previous literature findings, which highlight the effect of the downstream extension of drift accumulation on bridge pier scour. New relationships have been proposed to predict the effect of drift accumulation on bridge pier scour, both in terms of relative maximum scour and temporal scour evolution.