# Comparisons of Dynamic Landslide Models on GIS Platforms
## 4 Results and Discussion
### Model Selection  
Based on [PERSON] classification [27], the materials include rock, debris (coarse soil), and earth (fine soil). Rock is a solid mass of geological materials, debris is scattered material (large rock fragments), and earth is a cohesive, plastic, clayey soil. These terms are neither geological nor geotechnical [32; 34; 35; 73; 74] but are related to the size, shape, quantity, and properties, which help us select a suitable description. A rock is a solid mass and an aggregate of minerals that can be considered a discontinuous rigid body in landslide dynamics due to its characteristics. \"Earth\" is neither a geological term nor a geotechnical term, and it describes construction material or agricultural soil [75]. Earth is defined  
Figure 7: Yigong landslide simulation using different descriptions: (**A**) at 50 s using Eulerian method; (**B**) at 100 s using Eulerian method; (**C**) at 200 s using Eulerian method; (**D**) at 50 s using depth-averaged SPH; (**E**) at 100 s using depth-averaged SPH; (**F**) at 200 s using depth-averaged SPH.  
as a material in which at least 80% of particles are smaller than 2 mm [27]. Debris is a mixture of large and small blocks of rock, and debris motion involves multi-phase flow (20% to 80% of particles >2 mm). The properties of debris encompass the characteristics of both rigid bodies and fluids. Particle size is one of the critical factors to consider in the description of motion. When the material volume is small, and the quantity of material is large, the Eulerian description including the depth-averaged model is generally suitable for describing these motions. The Lagrangian description is suitable for large-volume and small-quantity discrete rigid body movement. The Eulerian-Lagrangian method may be suitable for the rock and soil aggregate movement.  
In addition, the motion types include falling, sliding, spreading, and flowing. In falling and toppling, collisions and shearing are the major contact processes, and shearing is the main contact process in sliding, flowing, and spreading. This indicates that the movement type determines the force model in a given situation. Collision and friction are critical forces to change the movement state for falling and toppling. However, dry friction and viscosity need to be considered in sliding, spreading, and flowing. Therefore, landslide classification can guide the selection of models of landslide dynamics (Table 4).  
During the movement, the movement forms transform into each other. Fragmentation is a key process in rock movement and influences the whole process. Fragmentation can cause changes in movement form from falling to flowing [76]. As the number of debris increases due to fragmentation, the interactions in the debris become increasingly complex. From an energy perspective, fragmentation can result in energy dissipation [62] and drag reduction during fragmentation. After fragmentation, small debris has a lubricating effect, and large debris can be transported for a long distance. Therefore, fragmentation in long-runout landslides is very complicated [77]. The movement process gradually transforms from falling and bouncing to sliding and flowing. Therefore, a single-phase model is not suitable for these processes. We should build an episode-based multi-phase model on GIS platforms to describe different states. The best method is a rigid body model in the initial stage and a flow-like model after fragmentation. Therefore, we may use various model forms to obtain more accurate results in landslide simulation.