Author(s): Keisuke Yoshida; Tadaharu Ishikawa
Linked Author(s): Keisuke Yoshida
Keywords: Large-scale horizontal vortices; Flood flow in river; Quasi-3D flow model
Abstract: Large-scale horizontal vortices are induced by shear instability near the boundary between a main channel and a floodplain in a compound open-channel. It is well known that this kind of coherent structure plays a key role in both macro-scale flows and corresponding bed processes in rivers. Thus, it is important to predict such an organized fluid motion in the river management. We carried out a quasi-3D numerical simulation of the flood flow occurred in the Tone River on September 17,1998. The governing equations for flow simulation are depth-averaged Reynolds approximation of one continuity equation and momentum equations. These equations are derived by the Galerkin weighted residual method. The computation domain is 20km-long, mid-lower reach of the river. The parameters of this numerical model were adjusted by trial and error, so that the streamwise free-surface flow calculated by the simulation was almost consistent of that by the image correlation method applied to the aerial photos taken during this flood. The numerical results showed that the large-scale horizontal vortices longitudinally develop and decay according to the hydraulic conditions such as a channel curvature, a bed roughness and a relative water depth. In addition, our numerical model was shown to be reasonable, because the simulated vortices have the almost same spatial scale as the shading patterns observed on the free-surface in the aerial photos taken during this flood.
Year: 2010