## 3 Overview of the Da'anshan Landslide Disaster  
A rockslide-debris avalanche occurred on 11 August 2018, in the Da'anshan village, Fangshan district of Beijing, China (115\({}^{\circ}\)45\({}^{\prime}\)49.37\({}^{\prime\prime}\) E, 39\({}^{\circ}\)53\({}^{\prime}\)14.67\({}^{\prime\prime}\) N). The rock mass collapsed from the head scarp of the slope and impacted the Junhong Highway below the source area. The Junhong Highway is one of the important highways in the Fangshan District, with a total length of 19.8 km and a road width of 7 m. The violent impact of the rock against the road resulted in severe disaggregation, and the fragmented rock slid down the slope diffusively. Severely weathered rocks and vegetation on the slope were eroded and entrained by the avalanche. The slowly triggered rockslide turned into a rapid debris avalanche. The affected region can be roughly divided into three areas, namely the source area, the avalanche area, and the accumulation area (Figure 4a). The red dashed line outlines the boundaries of different zones; the cyan dashed line represents a vertical profile section, along which data will be extracted in the subsequent simulations. A road with a section width of about 80 m was blocked by the collapsed gravel rocks (Figure 4b,c). By comparing the digital elevation maps before and after the landslide, the mobilized rock in the source area can be estimated to be 3.1 \(\times\) 10\({}^{4}\) m\({}^{3}\). The height difference between the front edge of the accumulation area and the rear edge of the source area is about 176 m. Correspondingly, the sliding mass traveled a maximum horizontal distance of about 232 m (Figure 5). Figure 3: Flow chart of the integrated dynamic model. Figure 4: Aerial and on-site photos of the Da’anshan Landslide. (**a**) Panoramic aerial view and characteristic zoning; (**b**) large boulders on the west side of the blocked highway; (**c**) east side of the blocked highway section. Figure 5: Engineering geological features of the slope. The digital elevation data, which were extracted along one of the main avalanche paths (the cyan dotted line in Figure 4a), were plotted in Figure 5. The angle between the north and this avalanche path is approximately 134.7\({}^{\circ}\) clockwise. In Figure 5, the ratio of height loss to the traveled distance of the landslide mass center is about 0.78. The maximum horizontal avalanche distance (\(L_{max}\)) is about 232 m, the maximum height difference (\(H_{max}\)) is about 176 m, and the \(H_{max}/L_{max}\approx\) 0.82; this ratio is also relatively high for a rock landslide. These ratios reflect that the avalanche distance of this landslide is much smaller than that of a normal landslide. This landslide destabilized gradually and collided with the road below it; kinetic energy was consumed during this process. Da'anshan Township is in the northwest of Fangshan District, covering an area of 62.29 km\({}^{2}\), including about 45 km\({}^{2}\) of forest land. Da'anshan Township has complex terrain and landforms, and the maximum relative elevation difference is about 1371 m. The Da'anshan landslide body is mainly composed of moderately and strongly weathered metamorphic basalt of the Jurassic Nandaling Formation with well-developed columnar joints and gray-white gravelly quartz sandstone of the Permian Shihezi Formation (Figure 5). The slope near the highway contains soft-hard interlayered rock (Figure 6a). Field measurements show that there are mainly three to five groups of obvious structural planes. Local rock masses have a relatively high degree of fragmentation (Figure 6b). The widely distributed fissures in the rock mass cause the water infiltrated by rainfall to seep out of the local surface of the rock mass. Under the influence of recent rainfall infiltration, water seeps from the rock fissures at the foot of the slope (Figure 6c). The Da'anshan area is in a sub-humid, warm-temperature continental monsoon climate zone. It has an average annual temperature of 10-12 \({}^{\circ}\)C and abundant rainfall in the summer.