Debris Flow and Large-Scale Landslide Disaster Prevention Information Network

1. Delimiting the Affected Area: Determine Vertex A of the affected area—defined as the valley exit, the alluvial fan apex, or a point with a slope of 10 degrees. From Point A, a fan-shaped area is projected downstream, based on the maximum 『debris flow』 expansion angle (105 degrees). Finally, the contour line B with a slope of 2 degrees within the fan area is used as the 『debris flow』 terminus boundary. The area enclosed by the fan-shaped region and line B is the final 『debris flow』 affected area.
2. Alluvial Fan Radius Length: The radius length of the alluvial fan is calculated using Ikeda's formula, as shown below:
   Log(L) = 0.42 × Log(V × tanθα) + 0.935
   Where V is the Sediment Discharge Volume (90% Non-Exceedance Probability) $(m^{3})$, ; $V=70,992A^{0.61}$; A is the area of the 『catchment area』 above the overflow point $(km^{2})$; L is the Deposition Length (m); $\theta_{\alpha}$ is the slope of the flow zone.

1. Using the preliminary 105-degree fan-shaped area estimated by Ikeda's formula as the base map, the overflow points are repositioned based on field survey results (e.g., valley mouth, obstructions, or areas of sudden slope decrease). The boundary is then revised according to the local topography, excluding areas that 『debris flow』 cannot possibly traverse. If the fan radius calculated by Ikeda's formula is insufficient to cover the entire area of protected households, the radius length is extended based on the field survey's findings. Furthermore, if the field investigation reveals additional overflow points near protected households that are not on the base map, the affected area for the stream is expanded according to the aforementioned field delimitation principles.
2. If the field investigation determines that there are no protected households near the stream's overflow point, the 『debris flow』 affected area will not be delimited for that stream.

1. Preliminary Delimitation of the Affected Area
2. Field Revision of the Debris Flow Affected Area
3. Indoor Editing of the Debris Flow Affected Area

Based on the three essential conditions for 『debris flow』 occurrence—sufficient sediment, water, and slope—the most significant influencing factors are selected for scoring: landslide magnitude, slope factor, sediment deposition status, lithology factor, and vegetation factor. Scores are assigned by considering the impact relationship between each factor and the occurrence potential, as shown in the table below, with a maximum score of 100. The 『debris flow』 occurrence potential level is categorized as High, Medium, or Low. A score of 46 points or below is classified as Low Occurrence Potential, 46 to 62 points as Medium Occurrence Potential, and above 62 points as High Occurrence Potential.

Note 1: A refers to the Taitung Rift Valley, Eastern Coastal Range; B refers to Schist, Gneiss; C refers to Metamorphosed Sandstone; D refers to Sedimentary Rock; E refers to Lateritic Terrace; F refers to Igneous Rock; G refers to Basin and Plain.