Measures to Reduce Large Wood Accumulation Probability at Bridge Piers

Large wood (LW) in rivers is defined as logs with a diameter ≥ 0.1 m and a length ≥ 1.0 m. During floods, the number of transported logs increases, potentially leading to accumulations at river infrastructures. Bridge piers are in particular prone to LW accumulations, which can result in an upstream backwater rise and local scour. To reduce the flood hazard, measures are required to decrease the accumulation probability of LW at bridge piers.

The objective of this study was to examine and evaluate existing and new measures at bridge piers regarding their accumulation risk reduction effect. The flume experiments were conducted at the Laboratory of Hydraulics, Hydrology, and Glaciology (VAW) at ETH Zurich in the course of the interdisciplinary research project WoodFlow – Management of LW in Swiss rivers. During the experiments, the approach flow conditions (inflow Froude number and flow depth) and log lengths were varied systematically to test the efficiency of (1) LW fins and (2) bottom sills including various configurations. The resulting accumulation probability p was then compared to the setup without measures.

The tested configurations of a LW fin did not decrease the accumulation probability p. For selected approach flow conditions, p even increased for a LW fin. Based on the tested parameter range, LW fins cannot be recommended as an efficient measure to reduce p. Bottom sills, in contrast, are a promising measure to reduce p for a defined range of boundary conditions. The installed sills lead to enhanced turbulence and increased surface waves. The best results to reduce p were obtained with two consecutive sills, leading to a reduction of p by 30%. However, the efficiency of bottom sills strongly depends on the approach flow and the sediment transport conditions.

The results of this study can be used to design and improve measures to reduce the LW accumulation risk. In contrast to retaining LW with retention structures, LW can be safely guided downstream, thereby preserving its relevant ecological role in rivers.