Laboratory Experiments To Investigate The Effects Of Bank-Toe Vegetation On Distributions Of Stresses On Streambanks

Following events of channel incision and widening, bank toes are bare and leave vertical faces susceptible to further retreat. Further bank retreat can be reduced with the establishment of riparian vegetation on the bank toe material. This vegetation protects the bank toe by imposing a roughness onto the channel that minimizes total flow energy. Although a fully vegetated bank toe would be the most beneficial in terms of increased roughness and reduced hydraulic scour of toe material, it is uncertain as to how well the toe is protected in situations where there is only partial vegetation coverage. In particular, cases may exist where vegetation has propagated at the waters edge, but has been removed at the bank face by a recent failure. The aim of this study was to investigate distributions of shear stresses under such conditions, where water may be channeled towards the retreating bank by the vegetation at the waters edge. Laboratory experiments were carried out in a flume consisting of an artificially vegetated banked toe and main channel section. Experiments were conducted in which three-dimensional velocity measurements were collected with an acoustic Doppler velocimeter along the width and up the water column at various cross-sections upstream, downstream, and through the vegetated section. Each test had a different vegetation arrangement, starting with a fully vegetated toe, gradually reducing vegetation density until no vegetation remained. Rows of vegetation were removed parallel to the direction of flow, starting at the bank-ward edge of the toe and progressing towards the channel. These experiments were performed twice, with a 15-degree and 30-degree bank-toe angle, respectively. Preliminary processing of data through one vegetated section shows that flow velocity is reduced through the vegetation and increases in the main channel. Velocity profiles indicate that main channel flow follows the logarithmic law but flow through vegetation does not. Through the vegetation, velocity is somewhat uniform throughout most of the water column, but peaks occur between 10-20% of the flow depth. As more vegetation is removed from the bank-ward side of the toe, flow in that region becomes logarithmic and the velocity matches that of the main channel. Vertical and lateral Reynolds stresses are mostly uniform across the width of the flume except for a significant peak occurring 10 cm from the vertical face. For the vertical stresses, this peak in stress appears for the case of 4 rows of vegetation removed from the top of the bank (6 cm from the top of the toe) for both bank angles; however, the peak is approximately 4 times higher for the 15-degree bank than the 30-degree bank. Therefore, there may be higher potential for toe erosion to occur at lower bank-toe angles where the toe material is partially vegetated. Peak lateral stresses are of about the same order of magnitude between the two bank angles. Further data processing of the remaining cross sections will examine the spatial variability of these results throughout the vegetated section and upstream and downstream of the vegetation.