Runout distance of debris flows : experimental and numerical simulations

Abstract

The main objective of this paper is to study the runout distance of debris flow by experimental simulations and to compare the runout distance observed in experiments with numerical predictions. In the experimental study, we design a new flume for modelling the real scale debris flows, by ensuring the equivalence of three constitutive scaling parameters (Bagnold number, Savage number and friction number) and four geometric scaling factors (velocity factor, flow factor, yield stress factor and viscosity factor) between the model tests and real events. The first three parameters reflect the relative importance of dispersive force, the friction force and the viscous force at the microscopic level, while the latter four factors ensure that the same macroscopic governing equations are applicable to both model and prototype. Debris materials collected by the Geotechnical Engineering Office (GEO) of the Hong Kong Government from the site of the Tsing Shan Debris Flow of 1990 are used in our study. It is found that the seven scaling parameters and factors of experiments are comparable to those of the Tsing Shan Debris Flow. The development of debris fan and the maximum runout distance of debris flows are investigated as functions of various parameters (including slope gradient, deposition board gradient, granular content and cross section shape of flow channel). For experiments with a flat deposition board, a slope angle increase from 26o to 32o of the flume leads to 84% increase in the runout distance. However, a further 6oincrease (from 32o to 38o) in the flume slope only gives a 42% increase in the runout distance. In addition, the runout distance can increase by 59% on average when the flume cross-section is changed from rectangular to triangular for various debris materials with flume slope at 26o and deposition board slope at 0o. The experimental simulations results are then compared to the predictions of various theoretical models, including the dispersive model (Takahashi and Yoshida, 1979), constant friction slope model (Hungr et al., 1984), sled model (Heim, 1932) and DAN model (Hungr, 1995). The DAN model prediction (Hungr, 1995) is found to be more agreeable to our experimental results than the predictions by other models. In the DAN model simulations, the Voellmy constitutive model is found to yield agreeable results if proper values in friction and turbulence coefficients are employed.