@*災害潛勢-大規模崩塌-劃定方式*@Potential Disaster-Large-scale Landslide-Delimitation Method

Investigation and Assessment of Large-scale Landslide Potential Zones

Investigation of Large-scale Landslide Potential Zones

The occurrence of large-scale landslides is associated with specific topography and geology. Therefore, by analyzing the terrain and geological surveys of slopes, potential areas can be inferred. Except for special geological or topographical conditions (such as dip slopes), the sliding surface of a large-scale landslide does not form abruptly; it evolves over a long period of time. During the development of the sliding surface, micro-topographical signs of collapse are left on the ground surface. Thus, with good tools (such as high-precision digital elevation models) and detailed field verification, locations with landslide potential can be identified.

Due to the dense forests in Taiwan's mountainous areas, it is not easy to directly interpret cracks and surface features caused by landslides. Therefore, high-precision digital elevation data can be used to generate hillshade maps and slope maps to understand the surface features beneath the vegetation, and observe whether they have crown, depression, and bulge zones (Figure 2-1)

大規模崩塌潛勢區地表特徵圖 — Illustration: Figure 2-1 Surface Features of Large-scale Landslide Potential Zone

Historical Investigation of Large-scale Landslides

Since Typhoon Morakot in 2009, to prevent the recurrence of large-scale landslide disasters like the one in Xiaolin Village, relevant agencies conducted interpretations of micro-topographical features in potential large-scale landslide areas, identifying a total of 9,948 areas with the potential for large-scale landslides. Considering the scale of the disaster, the Rural Development & Soil and Water Conservation Agency of the Ministry of Agriculture further focused on 315 medium-to-high-risk areas with safeguard households as the scope of project implementation. In addition to analyzing the frequency and security level of the aforementioned areas, Interferometric Synthetic Aperture Radar (InSAR) analysis was used to assess their activity level, thereby selecting 103 higher-risk large-scale landslide potential zones as performance indicators for the second phase of the Large-scale Landslide Disaster Prevention and Mitigation Plan under Climate Change.

Assessment of Large-scale Landslide Risk Potential Level

The assessment method for the potential of large-scale landslide potential zones is a comprehensive evaluation based on two assessment factors: "Hazard (H)" and "Vulnerability (V)". Specifically, "Hazard (H)" uses the "Grading of Hazard Assessment Factors for Large-scale Landslide Potential Zones," and "Vulnerability (V)" uses the "Grading Table of Vulnerability Assessment Factors for Large-scale Landslide Potential Zones." After scoring and grading separately, a risk level (score) assessment table is then used to comprehensively evaluate the risk potential level.

風險潛勢等級(分數)評估表 — Illustration: Risk Potential Level (Score) Assessment Table

Hazard Scoring and Grading

The large-scale landslide hazard assessment process mainly includes data collection and selection, factor processing and grading (such as slope, distance to geological structural lines, distance to river channels, dip slope terrain characteristics, and activity level, etc.), factor weight scoring, and hazard grading. The grading results are classified into high, medium, and low.

Vulnerability Scoring and Grading

In terms of vulnerability assessment, all important safeguard targets within the potential impact range of a large-scale landslide, such as households, transportation (including railways, highways, and bridges), and critical public facilities, must be considered. After summing up the vulnerability scores of the large-scale landslide potential zone, the grading results are classified into high, medium, and low.

Delimitation of Large-scale Landslide Impact Area

The main purpose of the delimitation of the large-scale landslide potential zone impact area is to provide the operating procedures and methods for various levels of government and the public to carry out disaster prevention and mitigation related to large-scale landslides. Therefore, the most conservative principle is adopted for the investigation, simulation, and assessment of the delimited area. The operating procedures and methods include the following four tasks for the large-scale landslide potential zone: "Application for Addition and Adjustment," "Field Investigation," "Potential Assessment," and "Data Review."

Addition and Adjustment Operations

Addition Operations
Adjustment Operations

Field Investigation Operations

Preparation of Basic Data
Detailed Field Investigation Operations
Map Data Editing

Potential Assessment Operations

Hazard Assessment
Vulnerability Assessment
Potential Level Assessment

Data Review Operations

Holding Information Sessions
Holding Review Meetings

Flowchart for Delimitation of Large-scale Landslide Potential Zone Impact Area

The preliminary delimitation of the impact area is conducted using the scale and topographic data of the large-scale landslide potential zone, involving two tasks: "Identification of Possible Disaster Types" and "Preliminary Delimitation of the Impact Area." "Identification of Possible Disaster Types": Based on the scale and location of the large-scale landslide potential zone, the possible disaster types are classified and identified, which can be divided into (1) Gravity Deposition Type; (2) Debris Flow Type; and (3) Landslide Dam Type. "Preliminary Delimitation of the Impact Area": Based on the analysis results of possible disaster types, corresponding calculation formulas are used to calculate the reference dimensions of the impact area, which are then fine-tuned according to the terrain.

1. Gravity Deposition Type Delimitation Method

For the delimitation of the impact area of the Gravity Deposition Type, the delimitation should be carried out by referencing the potential sliding distance L_runout and delimiting the potential deposition width W_Max. The calculation process is shown in Figure 2-3.

Illustration: Figure 2-3 Flowchart for Delimitation of Gravity Deposition Type Impact Area
- Potential Sliding Distance L_runout： The potential sliding distance of a landslide can be determined through the equivalent friction coefficient.
  Where H is the elevation difference of the slope crown (m), V is the volume of the large-scale landslide potential zone (m³), and (A_L) is the area of the large-scale landslide potential zone (m²),
- Potential Deposition Width W_Max： Determined based on the landslide width W_L and the deposition width W_Max. W_Max = 1.5 ~ 2W _L. In response to disaster prevention needs, it is recommended to delimit the potential deposition width as two times the landslide width, W_Max = 2W _L
2. Debris Flow Type Delimitation Method

The delimitation of the debris flow type impact area refers to the current method used by the Rural Development & Soil and Water Conservation Agency of the Ministry of Agriculture for delimiting the impact area of potential debris flow streams. The delimitation is mainly based on the location of the valley mouth and/or overflow point and the angle of the alluvial fan. The radius length of the alluvial fan is calculated using the Ikeya formula, but the potential sediment runoff in the formula should consider both the runoff from the catchment area and the potential sediment from the large-scale landslide potential zone. Additionally, the river channel range between the potential zone and the deposition fan needs to be considered (Figure 2-4).

Illustration: Figure 2-4 Flowchart for Delimitation of Debris Flow Type Impact Area
- Debris Flow Alluvial Fan Length L: The radius length of the alluvial fan can be calculated using the Ikeya formula.
  log⁡(L) = 0.42×log⁡(V×tan⁡ θ_d) + 0.935 Where L is the debris flow alluvial fan length (m)、 V is the sediment runoff volume (m³)、 θ_d is the slope of the alluvial fan (°).
- The sediment runoff volume can be calculated by taking the larger value from the following methods.
  Maximum Sediment Runoff from Catchment Area:
  , where A_W is the catchment area (km²).
  
  Landslide Earth Volume:
  , where A_L is the area of the large-scale landslide potential zone m² .
3. Landslide Dam Type Delimitation Method

The delimitation of a landslide dam needs to be carried out under the conditions where a landslide dam is likely to form. A landslide dam can only be determined to have formed when the soil body conditions meet the formation criteria. The deposition range of the natural dam can be assessed based on the dam body's deposition shape (Figure 2-5).

Illustration: Figure 2-5 Flowchart for Delimitation of Landslide Dam Type Impact Area
- Calculated to determine the natural dam's downstream deposition surface length L_d and the natural dam's upstream deposition surface length L_u, The sum of the two is the natural dam deposition length L_B, and its calculation formula is as follows:
Where L_B is the natural dam deposition length (m), L_d is the natural dam's downstream deposition surface length (m), L_u is the natural dam's upstream deposition surface length (m), H_D is the minimum deposition height of the landslide dam's natural dam (m), ∅_d is the natural dam's downstream deposition surface slope (°), ∅_u is the natural dam's upstream deposition surface slope (°), θ_r is the river valley slope (°).