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Post-earthquake debris flows that have occurred in Sichuan Province in southwestern China following the Wenchuan earthquake on May 12, 2008, have caused significant damage and casualties. Previous earthquake-induced landslides produced large amounts of loose material that remained on the steep slopes and in the gullies. As a consequence of heavy rainstorms during the rainy seasons, the existing loose material was transformed into numerous debris flows. Research has shown that the debris flows in the Wenchuan earthquake disaster areas have been characterized by their large scale, high speed, long run-out, and destructive impact. In order to identify the areas potentially at risk and to predict the flow severity, an accurate numerical method is needed to simulate these debris flows. In this paper, we have proposed a smoothed particle hydrodynamics (SPH) modeling technique-a meshfree particle method-to simulate the post-earthquake debris flows in the Wenchuan earthquake disaster areas. The SPH modeling technique introduces a Bingham model to analyze the relationship between material stress rates and particle motion velocity. Compared to traditional numerical methods, the SPH modeling technique is a true meshfree method of a pure Lagrangian nature. It can instantaneously track the motion of each particle, accurately predict the velocity, and naturally handle problems with extremely large deformations. In addition, the SPH method is based on continuum mechanics, and is therefore an efficient method to simulate large-scale debris flows. In this work, first, a viscoplastic fluid was simulated and verified with experimental results in order to evaluate the accuracy of the SPH model. Then propagation analysis of two typical post-earthquake debris flows in earthquake-hit areas was carried out, applying the SPH model. The simulation results showed good agreement with the limited field observation data. Our proposed SPH numerical modeling is able to capture the fundamental dynamic behavior of post-earthquake debris flows and can partially explain these complex phenomena. These simulation results can provide a preliminary scientific basis for hazard assessment and site selection for reconstruction in earthquake-prone areas.

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Flow slides in the municipal solid waste (MSW) landfill are common geological disasters that have the potential to cause loss of life, destruction of property, and damage to the natural environment in the surrounding region. In this work, a mixture of peat, kaolin clay and quartz sand was used as a model test material to simulate MSW. A series of physical model tests on MSW simulant flows was carried out to capture the run-out behavior of the waste and analyze its mobility. The testing assembly consisted of a transparent model box, a steel frame and a highspeed camera. Flow failure was induced by lifting up a baffle to cause the MSW simulant to collapse and flow. Images of the flowing mass were taken by the high-speed camera. The series of images clearly displays the propagation of MSW simulant flows. The final profile of the MSW simulant and the shape of the deposition area were observed and measured. The run-out distances, final deposit shapes, flow depth, velocities and angle of reach showed significant variation between test configurations, indicating the strong influence of moisture content on overall mobility. The test results obtained can aid in the prediction of distal reach, flow depth and maximum velocity of solid waste following a landfill slope failure, which are necessary for hazard assessment and mitigation planning, and also to provide physical data for theoretical and numerical model verification.