Flow characteristics in a compound channel with double-layer vegetated floodplains: a numerical study

Vegetation plays an essential role in modifying the flow characteristics of natural channels, such as rivers. Efficient hydraulic modeling of flow in compound channels with vegetated floodplains is necessary to understand and identify natural flow processes. In both natural and artificial channels, there are various types of vegetation with differing densities and heights. The main goal of this study is to investigate the effect of double-layered vegetation on flow characteristics in compound channels using the FLOW-3D model. The results of the numerical model have been calibrated and validated with the results of the physical model of the research of Takuya et al., 2014. Experiments were conducted in 2014 in the hydraulic laboratory of the Akashi National University of Technology in Japan, and in a straight trapezoidal channel with a length and width of 4.8 and 0.8 m respectively. During calibration, the model's estimation error for the depth-averaged velocity was within 4 to 6%, which was reduced to approximately 1.5% during validation. The average error in water depth estimation was around 3%. The numerical and physical models showed good agreement in simulating the flow pattern. The numerical model showed that, for larger floods when vegetation is submerged, the vertical profile of velocity in the floodplain is S-shaped. However, during smaller floods or when short and tall vegetation is emergent, the vertical velocity profile is relatively uniform or logarithmic. The resistance caused by the presence of vegetation in the floodplains leads to a decrease in the flow velocity in the floodplain area of the river and an increase in the capacity of flow transfer in the main channel.