Author(s): Andris Wyss; Isabella Schalko; Volker Weitbrecht

Linked Author(s): Isabella Schalko, Volker Weitbrecht

Keywords: Bridge pier; Field test; Flood risk assessment; Large wood; River engineering

Abstract: Recent floods demonstrated an increased transport of large wood (LW) in rivers. LW is, therefore, prone to accumulate at river infrastructures, for example at bridge piers. Such LW accumulations can increase the upstream water level and may pose additional structural risks. To predict the probability of LW accumulations at bridge piers, various studies have investigated the phenomenon using flume experiments with a model scale factor in the range of λ = 10…50. However, up to now no field tests at the prototype scale (λ = 1) have been performed to validate their results. To close this gap, the accumulation process of individual logs at a single bridge pier was investigated in a field test at the prototype scale. The objectives were to validate the results of recent flume experiments conducted at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at ETH Zurich and to improve the process understanding of LW accumulations at bridge piers. The field test was performed at the River Glatt in Switzerland (Canton Zurich). A total of 65 wooden logs with an average log length L = 4 m and diameter dl = 0.2 m were added to the river 10 m upstream of a circular bridge pier with a pier diameter of dp = 1.2 m. Similar to the flume experiments, the logs were added perpendicular to the main flow direction to hit the bridge pier at its centerline. We recorded and analyzed the transport and accumulation behavior of each log. A comparison of the prototype log movements with the flume experiments revealed that the accumulation process is very similar for both scales. The field observations are supported by a comparison of the wood accumulation probability. In the field, a total of 24 logs accumulated at the bridge pier. The resulting accumulation probability of p = 24 / 65 = 0.37 corresponds well to the predicted value of p = 0.44 based on the flume experiments. In addition to the validation of scale model tests, the field test provides new insights in the accumulation process of single logs at bridge piers. Based on the field observations, we analyzed the maximum duration between the impact and the separation of the logs from the pier. Furthermore, we derived a simple criterion based on the acting forces and moment equilibrium to explain why a log accumulates at a bridge pier. The results of the field test can, therefore, contribute to a better understanding and predictability of LW accumulation processes at river infrastructures and their associated risks.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022106

Year: 2022