*7.3. Evaluation of Landslide Residual Risk*

Based on the field survey and the aerial images from unmanned aerial vehicles, multiple tensile cracks appeared in the upper part of the trailing edge of the landslide and formed an independent unstable block. In the case of rainfall and ice and of snow melt infiltration, the unstable block would be extremely easy to slide, which poses a threat to agricultural production and road operation. To avoid secondary harm, in this paper, we use the DAN model to predict and analyze the movement trend of the unstable body. According to the results of the field geological survey, the unstable block area ranges from 3 to 6 m, with a volume of nearly 1.2 <sup>×</sup> 104 <sup>m</sup>3.

The selected models and parameters are the same as in the previous landslide dynamic hazard analysis. Figure 13 shows the running velocity of the unstable body, the thickness of the deposit, and the predicted disaster threat zone. Since the slope of the unstable body surface is 30◦, which is relatively gentle, most of the sliding body is deposited in the slide-source area after the unstable body slides. The average thickness of the sliding body in the sliding source area reaches 5 m, the longest distance reaches 300 m, and the maximum moving speed reaches 18.5 m/s. According to the sled model, the farthest distance of the unstable body motion is 362 m. Combined with the calculation results of the DAN-W model and sled model, it can be concluded that, if the unstable landslide body starts, it will be pose a threaten to road operation. Owing to its rapid movement speed, the landside

mass also threatens the safe production of grazing herds of animals. Setting up a warning sign around the landslide to warn herders to locate grazing far from the area is recommended. The local should set engineering measures (such as garbion) around the road to ensure traffic safety.

**Figure 13.** (**a**) The runout distance in the DAN-W. (**b**) Variation of the thickness of the unstable slope in the DAN-W. (**c**) Variation of the velocity of the unstable slope in the DAN-W. (**d**)The sled model for the two different reach angles. (**e**) Maximum extent of the unstable slope runout at different methods.

#### **8. Conclusions**

Based on the geological survey in the field, multi-period historical remote sensing images and aerial images of the drone, combined with the geological conditions of the study area, we analyzed the inducing factors and runout process of the Panjinbulake loess landslide and predicted the secondary disaster. Furthermore, the DAN-W dynamic model and a set of combined basal rheological models (Frictional–Voellmy–Frictional models) can suitably simulate the dynamic hazard effects of the Panjinbulake loess landslide. We analyzed the influence of the landslide movement speed, typical point velocity, accumulation body thickness, and friction coefficient. The simulation results showed that the duration of the Panjinbulake loess landslide was 22 s, the maximum speed was 20.5 m/s, and the maximum thickness of the accumulation body was 5.5 m, which is in line with the actual situation based on the field investigation. The basal rheological model combination and parameters obtained through trial and error can be used to simulate and predict the long runout distance of loess landslides, and it is necessary in strengthening the early identification and prevention of loess landslide hazards using multi-precision observation technology and numerical techniques.

**Author Contributions:** L.Y. analyses, writing and dealing with data; Y.W. analyses the field geological phenomena; W.W. offers the technology of DAN-W; S.Z. analyses the field geological phenomena.

**Funding:** This study was supported by the National Key Research and Development Program of China [No.2018YFC1505404], China Geological Survey (DD20190647, DD20179609, DD20190637).

**Acknowledgments:** The authors are grateful to O.Hungr for supplying a copy of the DAN-W software. We thank MDPI (www.mdpi.com) for its linguistic assistance during the preparation of this manuscript. We also thank Gang Liu, Chinese Academy of Sciences, Feihang Qu, Northwest University and Jie Luo, University of Science and Technology of China, for their kind assistance. Finally, we thank editors and reviewers for their thoughtful review and valuable comments to the manuscript.

**Conflicts of Interest:** The authors declare no conflicts of interest.
