Runout Scaling and Deposit Morphology of Rapid Mudflows

Prediction of runout distance and deposit morphology is of great importance in hazard mitigation of geophysical flows, including viscoplastic mudflows. The major rheological parameters of mudflows, namely, yield stress and viscosity, are crucial factors in controlling the runout and deposition processes. However, the roles of the two parameters, especially in mudflows with high inertia, remain poorly understood and are not accounted for in runout scaling relations with source volume. Here we investigate the effects of flow rheology on runout scaling and deposit morphology using small-scale laboratory experiments and three-dimensional numerical simulations. We find that yield stress and viscosity both influence flow velocity gained during downslope propagation of mudflows, which is strongly correlated with the runout distance; the role of yield stress is more significant than viscosity. High yield stress and low viscosity lead to an elongated deposit, where longitudinal propagation is more significant than lateral spreading. In contrast, high viscosity promotes the dominance of lateral spreading of the deposit, while low yield stress and moderate viscosity produce an initial elongate deposit, followed by a secondary surge that spreads laterally near the head of the deposit. Following appropriate scaling relations for viscosity and yield stress, a general scaling function is proposed to incorporate flow properties in the well-known correlation of runout distance and source volume. Our findings regarding the inertia effects and the roles of yield stress and viscosity enhance our understanding of mudflows, muddy debris flows, and other viscoplastic geophysical flows.