The physical mechanics and velocity of debris flow are crucial for debris flow mitigation measures. The two aspects closely relate to the grain composition, density, and flow depth. We present a combined research on Manning coefficient, debris flow classification, and mean velocity using mechanical analysis. Comparison of Manning coefficient reveals that it varies greatly at the same observation site and given event. The reciprocal Manning coefficients for viscous flows in Jiangjia Ravine, China, are the highest among the observation sites at a given flow depth. The stony debris flows in Kamikamihorizawa, Japan, are mainly governed by inertial stress, whereas the viscous debris flows in Jiangjia Ravine and Wudu, China, are mainly governed by friction stress. The reciprocal Manning coefficient of stony debris flows in Kamikamihorizawa, but not of viscous debris flows in China, increases with increasing Savage and Bagnold numbers. The reciprocal Manning coefficient decreases with the Friction number for both viscous and stony debris flows. Based on dimensionless parameters, we proposed debris flow classification from the perspective of physical mechanics including friction- and inertial stress-dominated flows. Finally, a new debris flow mean velocity equation was developed considering the characterizing diameter parameters (D10, D50), density, flow depth, and channel gradient. This equation performs well and could be updated in the future if the observed data of friction- and inertial stress-dominated flows are available. The results of this work can help strengthen the resistance of debris flows in different flow regimes.

An investigation on 152 gullies along the Daheba River in the Tongde sedimentary basin was performed. Debris flows develop in gullies with an excess topography ZE, which represents the sediment availability, above a critical threshold value. Debris-flows in the Daheba watershed are supply-unlimited, i.e sediment is abundantly available from the steep erodible gully banks. Debris flows consist of a head and a body. The body propagates faster than the head and constantly supplies it with sediment. The body and head propagate in an intermittent way through the transient storage of sediment on the riverbed and its subsequent remobilization. Although the main sediment supply is provided by bank collapse, debris-flow events also incise the gully bed. The growth and incision of debris-flow gullies in supply-unlimited watersheds is mainly controlled by the frequency of occurrence of debris flows, which is closely related to ZE. With growth of the gully drainage area, ZE and the debris-flow frequency initially increase, until they reach maximum values in gullies with a drainage area of intermediate size, which are assumed to be the morphologically most active gullies. With further growth of the gully drainage area, ZE and the debris-flow frequency decrease, which opposes the development of debris flows and leads to a more stable gully morphology. The observations indicate and explain the upstream migrating incision of the Daheba watershed. The lack of available sediment in the mountain reach is supposed to limit the further upstream migration of the reach of most active debris flows.