Experimental study on the mitigation effects of deflection walls on debris flow hazards at the confluence of tributary and main river

To mitigate debris flow hazards in correspondence of a stream confluence, deflection walls can be built to modify the confluence angle and reduce the negative blockage of the main channel. However, the effectiveness of such deflection walls has yet been fully investigated. Therefore, we conducted two series of flume experiments with and without streamflow in the main channel to investigate how deflection walls influence debris flow deposition and flow depth variations at the confluence. The results indicate that the deflection angle controls the magnitude and shape of the deposit at the confluence. After the initial sediment pulse into the main channel, part of the sediment was transported downstream and the sediment deposits at the confluence were partially eroded and reshaped. The efficiency at which sediment is being trapped on the fan increases linearly with the increase in deflection angle. Overall, with the change of confluence angle from 90 degrees (i.e., perpendicular to the main channel) to both larger and smaller angles, both the water wave height on the opposite bank and the upstream backwater height decreased, and the obstructed ratio (width ratio) and affected area of deposit at the confluence decreased. Our results indicate that deflection walls can effectively mitigate debris flow hazards at a stream confluence.