Impact of large instream logs on bank erosion

Abstract

© 2018 Dr. Nuosha ZhangBank erosion is a fundamental geomorphic process that also has management implications. A substantial body of research has demonstrated that trees act to reduce rates of river bank erosion. However, when trees fall into a river, they can continue to influence bank erosion rates. This influence of fallen trees on bank erosion processes has not been well researched and is the topic of this thesis. Qualitative observations in the literature suggest that large instream logs can cause local bank erosion by deflecting flow towards the bank, whilst multiple large logs can act as roughness elements that could potentially reduce the bank erosion rate at reach scale. The aim of this research is to develop a more quantitative understanding of the effect of large instream logs (large, dead, fallen-trees) on near-bank flow velocity as a surrogate for bank erosion, considering the form, position, orientation and distribution of the logs. The study achieves this aim by using a combination of hydraulic models, flume experiments, and field surveys of velocity and erosion rates. I developed a predictive hydraulic model to estimate the near-bank velocity changes caused by a single log, and the model produced good agreement with measurements from flume experiments. The two main control variables of the near-bank velocity change, caused by a single log, are distance between the log and the bank, and the blockage ratio. The model estimates the mean velocity in the gap between the log and the bank by solving equations for continuity and equal head loss of flow. The model results suggest that the root plate has more influence on velocity than the log diameter. The findings suggest that a hydraulically isolated log can accelerate the near-bank flow and increase bank erosion. This increase in bank erosion can be reduced by the hydraulic interactions between multiple logs. The effect of multiple logs depends on their spacing and distance between the log and the bank. Logs spaced more than 50 root plate diameters apart behave in the same way as an isolated log. As the logs are spaced more closely together the hydraulic interactions between the logs eventually suppress the accelerated flow. When the logs are close to the bank and spaced less than 17 root plate diameters apart, they can reduce general bank erosion rates. The general predictions of the flume models were supported by measurements of velocity and erosion around logs in anabranching channels of the Murray River in SE Australia, with large, dense, Eucalyptus camaldulensis logs. The field measurements identified more complex interactions between logs and bank irregularities. Finally, these measures and observations were combined to propose conceptual models of the interactions between riparian trees and bank erosion in a reach, as logs decay