The effect of instream logs on river-bank erosion: Field measurements of hydraulics and erosion rates

This is the first substantial field measurement of river-bank erosion around fallen logs in rivers. Whilst numerous studies have established that living trees can stabilize river banks, and that fallen trees can cause scour of the river bed, knowledge of bank erosion effects from logs is largely restricted to qualitative observations. Recent flume studies suggest that a single log can increase near-bank velocity (and thus erosion) and this increase is related to the blockage ratio of the log and the distance between the log and bank. However, hydraulic interactions between logs can reduce this increase or even decrease the near-bank velocity. These theories, developed in a straight flume, have not been tested in the field. We measured erosion rates (relative to controls) on river banks adjacent to 35 large logs for 2 years, and velocity distributions around 11 logs during a near-bankfull flow in anabranching channels of the River Murray, SE Australia. These channels have abundant large instream logs, consistent bank material, and consistent regulated high flows. The field results generally supported the velocity changes caused by single and multiple logs in the flume studies, with single logs increasing near-bank velocity, but with the hydraulic interactions between successive logs tending to reduce this increase. Flow patterns caused by logs adjacent to curved banks were more complicated as the local effects of logs reinforced or weakened recirculating flows. Instream logs did not change overall, average, bank erosion rates, but they tended to shift the erosion from bank top to bank toe. However, individual logs increased or decreased bank erosion rates in patterns that generally concur with the near-bank velocity changes predicted in flume studies: that isolated logs increased erosion rates whilst hydraulically interacting logs did not increase erosion rates. © 2020 John Wiley & Sons, Ltd.