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Modification of flow and boundary stresses by vegetation cover on bank-toe surfaces
A common goal of stream restoration projects is to limit channel migration and streambank retreat through bank stabilization practices. Bioengineering with various types and species of vegetation is a widely used technique due to its natural aesthetic and low costs, but has lacked quantification for design purposes. The need to improve design and better understand the impacts of riparian vegetation on streambank stability has prompted many research initiatives, although very few studies have focused specifically on the impacts of vegetation growth on the bank-toe, a crucial surface for streambank stability. In this study, a series of flume experiments was conducted to better understand how the flow regime and distribution of boundary stresses are impacted by bank-toe vegetation and to test the hypothesis that during high flow events, certain configurations of vegetation growth may enhance erosion potential on streambank and bank-toe surfaces. In the experiments, two bank-toe slopes of 15° and 30° were used, with varying cover by artificial vegetation. Assuming that high stress zones are associated with erosion potential, results indicate that scour is likely to occur at two regions: along the interface of the vegetation at the base of the bank-toe and main channel flow, and on the unvegetated portions of the bank-toe near the streambank face. Near the streambank face, bed stress is maximized at 53% and 66% coverage for 15° and 30° toe slopes, respectively. At this location, lateral momentum exchanges dominate, particularly for the 15° slope, suggesting that fluvial erosion along the streambank is more likely for shallow-sloped bank-toes. At the bank-toe-main channel interface, vertical momentum exchange dominates for the 30° slope, suggesting that steeper slopes are more prone to scour and vertical incision at this location.