Woody debris blocking conditions at bridges in mountainous streams

Abstract

When debris flows occur, bridges in mountainous streams may become dangerous when blocked by woody debris. When bridges are blocked with accumulated woody debris, high flow depths can cause the flow to spread widely. However, not all bridges become blocked with woody debris. Many studies have examined bridges blocked with woody debris for gentle slopes, but few studies have been conducted on steep mountain streams with supercritical flow. To better understand the interaction of woody debris and bridges across steep streams, we conducted laboratory experiments using one-pier bridge model and considered factors for the blockage of bridges by woody debris. We used straight rectangular channel flume 7 m in length, 0.3 m width, and with a variable slope. We supplied steady water from upstream end. We supplied woody debris model to the upstream end of the flume at approx.1 second. We set the bridge model 1.5 m upstream from the downstream end of the flume. We used ABS plastic material with a specific weight of 1.05 for the experimental woody debris. We varied the flume slope, water discharge, supply of woody debris, length of wood, height of bridge piers and Froude number. When the woody debris reached the bridge model, the total time of woody debris to pass through became longer from the supplied upstream condition due to rotational motion and diffusion. Because the total time changed, the amount of woody debris per unit time at the bridge was smaller than supplied condition. When examining the blocking conditions, we applied the woody debris condition at bridge model. From the results, we proposed methods to estimate the threshold condition of woody debris blocking at bridge from dimensional analysis. We applied parameters combining the experimental conditions of the bridge model, woody debris model, and hydraulic conditions, as well as the amount of woody debris per unit time required for bridge blocking.