Author(s): Daisuke Harada; Shinji Egashira
Linked Author(s):
Keywords: Driftwood; Convection-diffusion equation; Flood flow; Riverbed deformation; Driftwood accumulation
Abstract: A huge amount of driftwood from upstream influences the flood flow in downstream areas when landslides occur in mountainous areas due to severe rainfall. The present study proposes a method to simulate the behavior of driftwood based on a convection-diffusion equation with erosion/deposition terms as well as with an accumulation term. In order to develop this model, the following points are investigated in particular. The erosion and deposition rate of driftwood is formulated by using its similarity to the erosion and deposition of bed sediment; the rate is treated as the function of relative water depth and size of driftwood. The functional form illustrates that driftwood entrainment does not occur at depths any shallower than the threshold water depth, and the driftwood deposition does not occur at depths any deeper than the threshold water depth. The accumulation of driftwood around the bridge is formulated in terms of flood flow velocity, driftwood concentration within the flow body and O-function, and it is treated to reduce part of the flow region. The convection-diffusion equation of driftwood is solved numerically using a depth-averaged 2-D flow model and a 2-D sediment transport process model. The developed model is employed to conduct numerical simulations of the field hazard. As a result of the numerical simulation, the validity of this model is tested using field hazard data. The study concludes that both sediment supply from upstream and driftwood accumulation significantly affects the flood flow.
Year: 2018