Investigation of hydrodynamics along an embankment generated by a nearby riparian vegetation patch

Typhoon Hagibis in 2019 caused overflow of embankments. The overflow breached the embankment of the Toki River in Saitama Prefecture, Japan, at a point where its height was relatively low. A field survey showed that grass or woody debris piled up in riparian bamboo vegetation patches growing near the breach location during the flooding. In view of this, a series of flume experiments and numerical simulations with a two-dimensional (2D) k−ɛ Reynolds-averaged Navier–Stokes (RANS) turbulence model were carried out to understand the flow hydrodynamics along the open channel embankment with a vegetation model (VM) and a debris model (DM). Several configurations of the VM gap (i.e. the gap between VM and the flume wall: d) and VM angle (i.e. angle of VM to the flume wall: θ) were considered. The gap was normalized (d_nd) with respect to the cross-stream width (w) of the VM (d_nd = d/w). When considering the maximum water depth, velocity and bed shear along the side wall, it is recommended that the vegetation patches that satisfy d_nd > 0.3 be maintained and that 60°, 90° and 120° patch orientations be avoided.