## 4 Establishment and Grading of Evaluation Indicators  
### Slope Unit Division  
In this experiment, a combination of regular grids and slope units is used as the evaluation unit. The risk assessment employs a 10 m \(\times\) 10 m regular grid as the initial evaluation unit. After standardizing the data of each evaluation factor to a uniform grid unit and calculating the disaster susceptibility, spatial statistical tools are used to summarize the evaluation results within the range of slope units and express them in the attribute table of the slope units [29]. First, areas with flat terrain and extremely low geological hazard potential were excluded to focus on regions with potential risks. For small slope units that were difficult to identify by the naked eye at a 1:2000 scale, adjacent units were merged to ensure the practical relevance of the division. In addition, slope units crossing multiple administrative villages (or communities) were subdivided based on administrative boundaries to facilitate management. Each slope unit was required to possess similar geological environmental conditions, disaster-inducing mechanisms, and disaster modes to improve the evaluation accuracy. Watershed divides or valleys served as the upper and lateral boundaries of the slope units, while the lower boundaries were defined by the extent of the potentially affected disaster-bearing bodies or river exteriors. Micro-topographic features and the connectivity between units were also taken into account to achieve scientifically reasonable boundary delineation. In regions that had experienced or were prone to gully-type debris flow disasters, relevant slope units were merged, and exclusive evaluation units were delineated according to debris flow evaluation methods. Furthermore, the scale of slope units was controlled according to practical conditions to ensure the evaluation precision and to facilitate risk management efforts by government authorities. The division of slope units was specifically carried out in three steps. The first step involved preliminary division based on DEM data. Using the ArcGIS platform, hydrological analysis was conducted, including operations such as generating depressionless DEMs, extracting flow directions, calculating flow accumulation, generating river networks, and delineating catchment areas. Ridgelines and valley lines were extracted, and catchment and reverse catchment areas were integrated. Combined with terrain, lithology, vegetation coverage, and residential distribution characteristics, the preliminary slope units for Huangshi Town were divided, and the preliminary division map was generated (Figure 3) [30,31]. The second step is the refined division. With the aid of high-precision optical images and DEM digital elevation base maps, and by comprehensively considering the distribution characteristics of disaster-bearing bodies, the preliminarily divided slope units are optimized. The refinement and adjustment are mainly focused on the areas with threatened objects. Finally, 387 slope units are divided, among which there are 350 units in the key investigation area and 37 units in the general investigation area, and a refined division map of the slope units is drawn (Figure 4). The last is the field correction link. During the ground comprehensive investigation stage, field investigations are carried out according to the finely divided slope units, and corrections are made in combination with the actual situations, such as the distribution of disaster-bearing bodies, topography and geomorphology, and slope structures. For slope units where disaster-bearing bodies are concentrated, have a large scope and obvious topographic changes, they are split; for units where disaster-bearing bodies are scattered, adjacent and have a small area, they are merged to reasonably control the number of disaster-bearing bodies within the unit, which is convenient for risk prevention and control.