Effect of instream logs on bank erosion potential: a flume study with a single log

Abstract

It is well established that riparian trees reduce bank erosion. However, fallen trees can increase bank erosion as the flow is deflected and accelerated between the log and its root-plate and the bank face, increasing the boundary shear stress and eroding the bank. This erosion contributes to hydraulic variability but can also be a concern in restoring wood loads in rivers. In this first quantitative study into this phenomenon, we develop a theoretical hydraulic model to estimate the near-bank velocity around a single log using principles of continuity and energy loss for a range of log characteristics (morphology, angle, distance from the bank) and flow characteristics. Flume experiments support the theoretical model and suggest that a single log can almost double the near-bank velocity. The fractional channel area taken up by the log (blockage ratio) and the distance between the log and the bank are key factors governing the near-bank velocity. The smaller the angle of the log with the bank the lower the erosion rate, but also the further downstream the erosion extends. As the morphology of the log changes after falling into the channel, bank erosion potentially develops quickly, but slows over time; and migrates downstream.

Keywords:

• Instream wood
• near-bank velocity
• flow deflection
• bank erosion

Acknowledgements

We also thank the Department of Civil, Environmental and Mining Engineering of the University of Western Australia for providing access to their flume. Additional laboratory support was provided by Matt Arpin, Arnold van Rooijen, Maryam Abdolahpour and Joey Voermans. Thanks to Monty Rutherfurd for making the log models used in the experiments. The first author was supported by a postgraduate scholarship provided by the Australian government.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

We acknowledge the generous financial support provided by the NSW Department of Primary Industries, in particular we would like to thank Ben Berry for his consistent assistance and enthusiasm for this work.