There has been abundant research into the effect of tree roots on stabilizing river banks, and also on the effect of trees on bed-scour after they have fallen into the stream, but there is little research into the effect of fallen trees on bank erosion. Here we develop the hydraulic theory that predicts erosion associated with trees and conclude that individual trees can increase local bank erosion, but multiple trees can reduce overall reach erosion. For consistent bank resistance, the local erosion varies in a non-linear way with the position, angle and blockage ratio of the log. The reach scale effect of the wood depends on the load and distribution of logs through the reach. Erosion effects of instream wood are difficult to measure. We tested the above model of erosion around wood in an ideal setting below a large irrigation dam on the Murray River in SE Australia. The consistent regulated flow, combined with multiple anabranching channels of different sizes, and abundant fallen trees, provides an environment in which we can measure erosion as well as hydraulics in a controlled condition.

Keywords: Bank erosion; Instream wood; Shear stress; Velocity field