Unravelling the multiphase run-out conditions of a slide-flow mass movement

Abstract

In this paper an attempt is made to unravel the run-out characteristics of a mass movement in the Sichuan Province, SW China by means of 1D numerical modelling and calibration on the topography of run-out profiles. The Dagou mass movement started as a rockslide with an initial volume of 480,000m3, which transformed into a debris flow, increasing in volume due to entrainment of loose material in the upper part of the travelling track. The rapid mass movement had a run-out distance of 1380m and a run-out time of about 50s.Numerical calculations were conducted with the depth average shallow water equation to explain the variation in thickness of the debris flow deposits along the run-out track. For the calibration of the first run-out phase, three rheological models were applied, namely the Bingham, Voellmy and Quadratic rheology. Calibration was done on 1) the run-out distance, 2) the run-out time and 3) the goodness of fit with the thickness of the deposits along the track. In addition the erosion constant in the entrainment equation was calibrated on the observed versus calculated run-out volumes. Sensitivity analyses of the resistance parameters for the different rheologies showed that the viscosity, the basal friction, the turbulence term and the resistance factor are the most sensitive ones. It appeared that the variation in thickness along the run-out track can be explained by entrainment of material in the upper part of the track and a change in parametric values during the run-out process. The three rheologies produced a reasonable fit with the observed geometry of the run-out profile, run-out time and run-out volume. It was argued that the Voellmy rheology seems to give the most appropriate explanation for the difference in resistance along the run-out path. The main problem in the simulation was to stop the debris flow on a slope with a gradient around 22°.A reactivation of the mass movement by frictional sliding of the material half way the run-out track was simulated. It occurred 30min after the first run-out phase due to an increase in pore pressure. The sliding material changed into a slow flowing mass that reached a newly built up area after about 1h and moved into Wangong Town over a distance of 80m.