It has been known that the shear instability generates large horizontal vortices at the junctions of main channel and flood plains for two-stage channels. The present laboratory tests show that the horizontal vortices appear for flow with small relative depth. For channels with bank vegetation, horizontal periodic vortices also appear for small relative depth, and it was seen that the vortices in the main channel is predominant.
Numerical computations were performed by employing SDS-2DH turbulence model, and the results obtained agree reasonably well with the experiments. The model can predict the wavelength of horizontal vortices, the lateral distributions of the Reynolds stress, temporally-averaged longitudinal flow velocity, etc. The numerical simulation, which was performed for lateral transport of suspended sediment, has revealed that the transport is closely correlated with the large scale horizontal vortices.