Development of mobilization criterion for debris flows and applicability assessment at a regional scale

Abstract

This research attempts to develop a debris flows mobilization criterion and assess its applicability at a regional scale area with the debris-flow spreading assessment in which slope failure analysis, debris-flow initiation analysis and debris-flow spreading analysis are sequentially applied. In addition, suitable methods for the slope failure analysis and the debris-flow spreading analysis are described for a reasonable modelling of debris flow events in Korea. Firstly, the identification of reasonable methods for the three analyses applied to the debris-flow spreading as-sessment was carried out: (1a) Change in pore water pressure, observed from devices installed at a study site, was compared to the results from an infiltration analysis considering influential factors on infiltration. From the study, a slope stability analysis associated with the rainfall infiltration using the modified van Genuchten-Mualem model for relative permeability and two-phase flow of water and air were identified as an appropriate approach for the study site. (1b) Topographical characteristics in actual slide and debris flow occurrence areas were analyzed through a comparison analysis to identify relationships between debris-flow initiation and each topographic index. Then, a mobilization criterion for debris flows based on the topographical characteristics was suggested using an Artificial Neural Network model combined with a modified threshold for a relationship between slope and upslope contributing area. (1c) The most appropriate combination of algorithms for the debris-flow susceptibility assessment that produced the highest average of efficiency and sensitivity for each classification, based on the geological conditions and the types of soil texture, was examined by applying a confusion matrix. Secondly, sequential applications of the three proposed methods were performed to assess and validate the ap-plicability of the debris-flow spreading assessment for the area of Umyeon Mountain, Seoul, Korea: (2a) The infiltration analysis was conducted, and changes in pore water pressure and pore air pressure obtained from the infiltration analysis were used to predict safety factors. The success rate of the slope stability analysis and the area under the curve of the cumulative frequency diagram was calculated as 96.2 % and 92.3 %, respectively. (2b) The debris-flow mobilization criterion was applied to potential areas of slope failure, predicted by the slope stability analysis, to detect debris-flow initiation areas. The total prediction success rate of the debris-flow initiation analysis was calculated as 93.04 %. (2c) The debris-flow susceptibility assessment utilizing the appropriate schemes was performed with debris-flow source areas determined from the debris-flow initiation analysis. The appropriate schemes, based on the classification according to geological conditions, with a combination of Holmgren’s algorithm with an exponent of 4 and a proportional method with a 5-m resolution DEM and the SFLM with a travel angle of 11.7$\circ$ and a maximum velocity limit of 28 m/s produced the highest average efficiency and sensitivity larger than 90 %. Therefore, the debris-flow spreading assessment with the sequential applications of three methods proposed in this study shows a good applicability for the area of Umyeon Mountain.