2.2.2. Average Slope

According to statistics from the Bureau of Soil and Water Conservation, the most frequent damage from debris flows occurs on Taiwan's mountain slopes above 30 degrees, and especially at 40 degrees or more, while the least damage is suffered where slopes are 15 degrees or less [15]. This is because steep slopes provide greater driving force and also reduce slope resistance, making them conducive not only to the development of shallow landslides but also to the fluidization of landslides and the formation of sloping debris flows [27]. Cheng used GIS and a conditional-probability method to analyze four factors—bare land, eroded gullies, slope, and lithology—and established that, among them, slope had the greatest influence [28]. Kao showed that unstable Index method achieved good accuracy in predicting slope collapses, with 92% of actual collapsed land falling within the areas it identified as being at medium or high risk [29,30]. Liulater used a neural network-like sensitivity analysis to establish that the most important factors in this type of damage were rainfall, slope, slope type, elevation, lithology, fault, slope direction, roads, folds, and erosion gullies [30]. After controlling for rainfall, however, the greatest influence was slope, irrespective of whether Kao's or Liu's analysis method was applied.

### 2.2.3. Geology

The right bank of the main channel of the Chenyulan River consists of Paleogene submetamorphic rock strata, with interbedded argillite, slate, meta sandstone and quartzite, among other types of rock; on the left bank are Miocene sedimentary rock strata, with interbedded sandstone, shale and sand shale. Other strata include platform accumulation, four-sided sandstone layers, and hsichun, shihti, alluvial, nanchung, kueichoulin and kankou formations [31]. The wider area is dominated by thick-bedded sandstone, shale (argillite), and sandstone and shale formed together. When thick-bedded sandstone is subjected to tectonic stress, the rock mass is often cut into large blocks because it is thick and strong, but the density of the fractured surface is low. This type of rock is also relatively easy to weather. When the degree of weathering is slight, shale often forms smaller cuttings; when the degree of weathering is severe, a weathered soil layer forms. Due to the sharp difference in water permeability and resistance between sand and shale interbeds, the interface between them is often a stratum-slip surface, and the exposed area of interbeds

often forms a single-sided mountain topography [32]. Chang and Lin investigated the Chenyulan River after Typhoon Huber and found that the contact between the upper mountain belt and the submetamorphic belt of Taiwan's geological structure was a fault. That is to say, near the Chenyulan River, the rock mass is abnormally broken, and a considerable amount of broken rock and soil accumulates on the surfaces of slopes and in the river itself, which may cause disasters [33].

### *2.3. Rear of the Terrace*

### 2.3.1. Number of Erosion Ditches

Chang showed that erosion ditches are mainly caused by rain, surface runoff and wind, which causes the original soil to loosen or move; this process removes fine particles, and the resultant slope appears to be grooved [34]. Taiwan's Water Resources Agency, MOEA, on the other hand, defined an erosion ditch as a slender, linear drainage route from the top of a slope to its foot, usually caused by incision and erosion by concentrated runoff on the slope's surface. At the same time, the ditch wall is emptied and collapses, forming an obvious drainage pipe [35]. Chang suggested that the degree of slope erosion is a dynamic topographic effect on slope and is judged by the degree of contour curvature on a topographic contour map, supplemented by field surveys. Such curvature can also be used to determine the grade of a slope-erosion gully, i.e., a trough-shaped depression formed by the removal of vegetation by runoff on a hillside, excluding stream beds [36]. Hung noted that the debris-flow disasters caused by Typhoon Huber in the Chenyulan River Basin mainly occurred in the large erosion ditches (some of which are large enough to be named "Stream") and the flat reclaimed land of the community at the intersection and Provincial Highway 21 [37].

### 2.3.2. Number of Collapses from the Rear

When the combination of hydrological and geological conditions exceeds its damage threshold, a hillside will collapse. Hydrological conditions include rainfall intensity, rainfall delay, the soil's water content, pore water pressure, etc. Geological conditions include soil cohesion, anti-friction angle, soil slope, surface vegetation and whether there has been a recent earthquake or not. Tang conducted simulations of the Xiaolin Village disaster using PFC 3D. Their preliminary results show that just 60 s after the landslide was triggered, some of the houses in the village may have been covered by falling rocks or pushed to the opposite bank of the Qishan River [38]. Certainly, at its maximum sliding speed of 50 m per second, the kinetic energy of soil and rock is sufficient to cross the river entirely at this point, and a barrier lake was formed by this process in this vicinity. Ji investigated landslides in Caoling over a period from 1862 to 1999 and identified five large-scale ones linked to earthquakes or heavy rain. The landslides directly or indirectly caused disaster to the Caolingtan dyke breach, and a total of 170 people were killed and injured. Additionally, during the "921" earthquake of 1999, Caoling Mountain collapsed rapidly, its soil and rock moving up to 4 km, and the impact area of the collapse was nearly 500 hectares. Such cases of large-scale rock mass sliding are extremely rare, in Taiwan or anywhere else [39].

### *2.4. Preservation-Factor Assessment*

Preservation factors include households, schools, hostels, public buildings (if residential), roads, bridges, farmland, orchards and other such sites. The Bureau of Soil and Water Conservation noted that the streams' debris-flow potential should be evaluated and prioritized according to the formula (natural potential factor affecting the risk level of debris flow × 50%) + (preservation hazard factor × 50%) [15]. The individual scores for the following three factors were added together to obtain the hazard degree score for each preservation object. (1) Building factor: The more buildings there are, the more people live in them, so the damage score is higher. (2) Traffic factor: Damage to the bridge is more harmful to the traffic, so a higher score is given. (3) Effective factors of on-site remediation: After many disasters, there have been many remediation facilities for potential debris flows. **3. Methods** 

If the remediation facilities are effective, damage to preservation objects by such flows can be reduced. This study used Li's Chenyulan River terrace map data, modified to reflect the current shape of river terraces there, and purposively selected 40 potential river terraces for

prioritized according to the formula (natural potential factor affecting the risk level of debris flow × 50%) + (preservation hazard factor × 50%) [15]. The individual scores for the following three factors were added together to obtain the hazard degree score for each preservation object. (1) Building factor: The more buildings there are, the more people live in them, so the damage score is higher. (2) Traffic factor: Damage to the bridge is more harmful to the traffic, so a higher score is given. (3) Effective factors of on-site remediation: After many disasters, there have been many remediation facilities for potential debris flows. If the remediation facilities are effective, damage to preservation objects by such

### **3. Methods** further analysis [1]. An analytic hierarchy process was used to analyze the strength of the

This study used Li's Chenyulan River terrace map data, modified to reflect the current shape of river terraces there, and purposively selected 40 potential river terraces for further analysis [1]. An analytic hierarchy process was used to analyze the strength of the mutual influences of the various elements, as well as of the high-level elements on the low-level elements; the levels of risk to each focal river terrace were derived through weighting the latent perception factors [40]. mutual influences of the various elements, as well as of the high-level elements on the low-level elements; the levels of risk to each focal river terrace were derived through weighting the latent perception factors [40]. *3.1. Questionnaire Design* 

### *3.1. Questionnaire Design* In this study, following the methodology laid out by the Bureau of Soil and Water

*Appl. Sci.* **2022**, *12*, x FOR PEER REVIEW 7 of 19

In this study, following the methodology laid out by the Bureau of Soil and Water Conservation, the priority-order score of the potential unearthed rock flows was calculated according to the formula set forth in Section 2.4 above. Therefore, the risk-scoring method for the river terraces in this study equals (the potential factor of river terraces × 50%) + (the preservation hazard factor of the river terraces × 50%) [15]. The questionnaire design can be divided into the two hierarchical-structure diagrams—one for latent factors and the other for preservation factors, shown in Figures 2 and 3. Conservation, the priority-order score of the potential unearthed rock flows was calculated according to the formula set forth in Section 2.4 above. Therefore, the risk-scoring method for the river terraces in this study equals (the potential factor of river terraces × 50%) + (the preservation hazard factor of the river terraces × 50%) [15]. The questionnaire design can be divided into the two hierarchical-structure diagrams—one for latent factors and the other for preservation factors, shown in Figures 2 and 3.

**Figure 2.** Evaluation conditions of latent factors and hierarchy of related factors. **Figure 2.** Evaluation conditions of latent factors and hierarchy of related factors.

**Figure 3.** Evaluation conditions of preservation factors and hierarchy of related factors. **Figure 3.** Evaluation conditions of preservation factors and hierarchy of related factors.

### *3.2. Questionnaire Survey Subjects*

*3.2. Questionnaire Survey Subjects*  The participants in the questionnaire survey were mainly professors from the fields of land, water conservancy, soil and water conservation, geology, and environment and disaster prevention, employed by National Taiwan University, Chung Hsing University, Kaohsiung University, Tamkang University, Hua Fan University, Pingtung University of The participants in the questionnaire survey were mainly professors from the fields of land, water conservancy, soil and water conservation, geology, and environment and disaster prevention, employed by National Taiwan University, Chung Hsing University, Kaohsiung University, Tamkang University, Hua Fan University, Pingtung University of Science and Technology, and Chaoyang University of Science and Technology.

### Science and Technology, and Chaoyang University of Science and Technology. *3.3. Statistical Results of Questionnaire Recovery*

*3.3. Statistical Results of Questionnaire Recovery*  A total of 23 questionnaires were sent out and 17 were returned, resulting in a response of 79.9%. After eliminating five questionnaires due to repeated answers or omitted, unanswered items, which led them to have a consistency index (C.I.) and a consistency ratio (C.R.) greater than 0.1, 12 valid questionnaires remained for analysis. The C.R. value achieved a margin <0.1, indicating a strong degree of consistency among the A total of 23 questionnaires were sent out and 17 were returned, resulting in a response of 79.9%. After eliminating five questionnaires due to repeated answers or omitted, unanswered items, which led them to have a consistency index (C.I.) and a consistency ratio (C.R.) greater than 0.1, 12 valid questionnaires remained for analysis. The C.R. value achieved a margin <0.1, indicating a strong degree of consistency among the pairwise comparisons, and proved it did not require a statistically significant sample size [40]. Shrestha et al. pointed out that AHP is usually used to survey people who have knowledge about the topic under investigation and a large sample size is not needed [41].

### pairwise comparisons, and proved it did not require a statistically significant sample size **4. Results**

river terrace (0.306).

Level two

[40]. Shrestha et al. pointed out that AHP is usually used to survey people who have knowledge about the topic under investigation and a large sample size is not needed [41]. **4. Results**  Based on AHP principles, the scale indicates the level of relative importance from equal, moderate, strong, very strong to extreme level by 1, 3, 5, 7, and 9, respectively. The Based on AHP principles, the scale indicates the level of relative importance from equal, moderate, strong, very strong to extreme level by 1, 3, 5, 7, and 9, respectively. The intermediate values between two adjacent comparisons are denoted by 2, 4, 6, and 8. The diagonal of the matrix of the comparison is equal to 1, since each criterion is compared to itself. When the number of alternatives is n, a total of n(n − 1)/2 comparisons are made [40]. As shown in Table 1, our expert respondents' ranking of our three categories of latent-sensing factors was in front of river terrace (0.352) > river terrace itself (0.342) > behind river terrace (0.306).

intermediate values between two adjacent comparisons are denoted by 2, 4, 6, and 8. The diagonal of the matrix of the comparison is equal to 1, since each criterion is compared to In the experts' evaluation of the preservation factors of river terraces, as shown in Table 2, the ranking was households (0.599) > traffic (0.292) > farmland (0.109).

itself. When the number of alternatives is n, a total of n(n − 1)/2 comparisons are made [40]. As shown in Table 1, our expert respondents' ranking of our three categories of latent-sensing factors was in front of river terrace (0.352) > river terrace itself (0.342) > behind

**Weight** 

In front of river terrace 0.352 0.352 River terrace itself 0.342 0.342 Behind river terrace 0.306 0.306

**Overall** 

**Weight Rank** 

Latent factor assessment

**Table 1.** Evaluation results, river terraces' latent-sensing factors.

**Level Evaluation Project Inter-Level** 

Level one Latent factor assessment 1.000 1.000


### **Table 1.** Evaluation results, river terraces' latent-sensing factors.

### **Table 2.** Evaluation results of river terraces' preservation factors.


### *4.1. Distribution Method*

Due to the large gaps between the various factors, to avoid extreme values, we first used statistical methods to find the average value and standard deviation of each factor and set reasonable parameter ranges Xmax and Xmin. If a parameter was greater than Xmax, Xmax was used, and if one was less than Xmax, Xmin was used. Then, an interval mapping method was conducted with AHP weightings to determine the score for each factor. In this study, the maximum and minimum range (0.1–1) of the interval mapping method was multiplied by the overall weight of each factor in the AHP to obtain the maximum and minimum range values.
