*6.3. Typical Point Velocity Analysis and Accumulation Body Thickness Analysis*

Combined with the actual situation of the field geological survey, the four points with horizontal distances X = 140, 200, 250, and 350 m were selected as the typical points for analysis. The calculation results based on the DAN-W software and F–V–F rheological model are as follows.

(1) The point X = 140 m is located at the landslide shear outlet (Figure 4b). Within 0–10 s, the speed at this point increased linearly from 0 to 17.5 m/s (Figure 9a), which indicates that the acceleration of the sliding body during the starting process increased, reaching 1.75 m/s2. Also, typical negative terrain was present, and the lower part of the raised bedrock was exposed. This made it so that, after 12 s, the residual sliding body moved to the point where it was hindered by negative terrain with trough shape, the speed gradually attenuated, and the final thickness of the accumulation body at this point was 0.5 m (Figure 9b).

(2) The point X = 200 m is located in the debris flow area (Figure 4b). After the landslide slid for 7 s, it reached this point. Within 7–17.5 s, the speed at this location was always relatively faster, and the average speed reached 17.5 m/s (Figure 9c). In this area, the sliding body had a rapid speed and exposure to spring water, which provides conditions for the occurrence of a high speed and long runout landslide. At 8 s, the thickness of the accumulation body reached 4.53 m. After 17.5 s, the speed at this point gradually decreased, and the final thickness of the accumulation body reached 0.5 m (Figure 9d).

(3) The point X = 250 m is located at the foot of the slope (Figure 4b). After the sliding body slid for 9 s, it reached the foot of the slope. Due to the steep slope, the peak speed reached 19 m/s (Figure 9e), and the thickness of the accumulation body reached 5 m (Figure 9f). Scattering phenomenon was appeared at the foot of the slope where in the interval t = 20–24 s, and there was an increase of debris

flow depth reaching a depth higher to the front passage. In addition, this was also a turning point for the speed of landslide movement. From this point on, the speed of the sliding body began to decrease due to the sudden slowing of the slope and the friction of the sliding body against the sandy gravel of the river terrace.

(4) The point X = 350 m is located at the foot of the opposite slope (Figure 4b). After the sliding body crossed the river terrace, it reached this point at 16 s; then, the speed dropped to 11 m/s (Figure 9g), and the thickness of the accumulation body reached 1.5 m (Figure 9h). From this point on, the sliding body began to hit the opposite side of the mountain. Because of this and even though a considerable amount of energy had been consumed, the sliding body continued to climb 20 m before stopping, indicating that a high-speed and long runout landslide has incredible energy and is severely devastating.

**Figure 9.** Typical point velocity and variation trend of the accumulation body.
