Abstract
Groynes are recognized to have a role on preventing bank erosion because they act as flow resistance and decrease velocity near banks. In general, water surface is elevated under larger flow resistance. However, if groynes are installed, river bed at the main channel part is scoured. So, groynes has those two opposite effects on affecting water elevation. Interaction between those two opposite effects seem to have key factor for determining water level though such features have not yet been fully discussed. In this study, we try to clarify the effects of groyne interval on flow resistance by both experimental and numerical approaches under fixed bed conditions. The experimental results showed that the resistance changes according to the groyne interval and it takes a peak at a certain value of the aspect ratio. If the aspect ratio is small, an isolated separated vortex moves along the junction one by one. However, if the aspect ratio becomes larger, multi vortices can exist together at the junction. The numerical results showed that scale of vortex depends on the momentum thickness. If groyne interval is small, momentum thickness increases along the entire interval. Thus, the vortex is amplified as it goes downstream. If the groyne interval is larger, the momentum thickness takes peak within the interval. Thus, after the point where the momentum thickness takes peak, the vortex becomes attenuated. It is also shown that the momentum exchange rate at the boundary between the mainstream and the groyne region related to scale of vortex, which have close influence on determining flow resistance due to groynes.
