The effect of instream logs on bank erosion potential: a flume study with multiple logs

Abstract Riparian trees can reduce bank erosion rates, but once a tree falls into a river it can increase local bank erosion. However, the influence of multiple logs, that hydraulically interact, on near-bank velocities has not been investigated. This paper reports flume experiments of the near-bank velocity changes and water level changes produced by multiple in-stream logs with equal and unequal spacing. The results suggest that the near-bank velocity increase caused by a single log can be reduced, and even reversed, by multiple logs. This reduced near-bank velocity mainly results from wake interference between the logs, rather than from the effect of backwater, and it varies systematically with the spacing between the logs. Bank erosion potential can be reduced where logs are spaced under 17 root-plate diameters and where the root-plate is located close to the bank. By contrast, the logs are likely to increase bank erosion when they are within an intermediate distance from the bank and are closely spaced (under 3.3 root-plate diameters apart). The flume results allow us to explore the temporal changes of the potential bank erosion in a reach with various log distributions.