Deposition pattern of stony and muddy debris flow at the confluence area

Abstract

Debris flow fan affects the river profile and landscape evolution. The propagation of multiple debris flows along a river can cause inundation and breaching risk, which can be exemplified by the Min River after the Wenchuan earthquake, Sichuan province, China. In this work, large flume tests were conducted to examine the interactions between debris flows and water current with the fan geometry, momentum, runout distance, deposited width, the relative water level upstream and dominated stress. The results reveal that stony flow commonly travels at a high speed and forms a long rectangle shape fan, while the muddy flow generally travels at a low speed and forms a fan-shaped depositional area. The stony flow can block a river even when the momentum is close to the water current; the muddy flow can block a river when the momentum is lower than that of water current. In case of complete river damming, the relative water level upstream indicates that the inundation risk from the muddy flow damming river would be higher than the inundation risk of stony flow. The diversion ratio of muddy flow decreases as damming ratio. Comparison of dimensionless numbers reveals that stony flow is dominated by grain collision stress combined with turbulent mixing stress, while the muddy flow is dominated by viscous shear stress over friction stress. The fan geometry, damming ratio, diversion ratio, and the dominated stress all together indicate that stony flow strongly interacts with water current while the muddy flow does not. The results can be helpful for understanding the physical interactions between water current and various debris flows, and debris flow dynamics at the channel confluence area.