A rapid method to identify the potential of debris flow development induced by rainfall in the catchments of the Wenchuan earthquake area

Abstract

In many mountainous areas, the limited space in the valley floors has created a need to construct temporary settlements in zones potentially exposed to debris flow hazards after a strong earthquake. Existing methods for the identification of debris flow catchments based on pre-earthquake catchment characteristics were not applicable. In the earthquake-affected zones, a rapid identification of catchments with a high hazard level for debris flows is then necessary, to provide information for future risk management. With the aim of assessing the potential of debris flow development in catchments in the Wenchuan earthquake area, a detailed analysis of the relationships between catchment morphology and the area of co-seismic rockfalls and rotational or translational landslides was carried out. This paper presents three empirical models to identify debris flow-prone catchments at a regional scale. One hundred and twenty-three catchments in the Wenchuan earthquake area were selected and investigated using high-resolution aerial photography to estimate the area of co-seismic rockfalls and rotational or translational landslides in the catchments. The statistical results show that debris flow-prone catchments have areas higher than 0.5 km2, relief ratios which vary between 36.4 and 119.2 %, mean slopes which range between 25° and 45°, and a catchment relief larger than 0.5 km. But the debris flow catchments are more closely related to the catchment area and catchment relief than the relief ratio and mean slope. So the catchment area and catchment relief were selected as the key factors to establish the applicable identification models. The threshold for co-seismic landslides was defined in terms of areas of co-seismic landslides (A L), catchment area (A), catchment relief (H), and catchment area–relief ratio (A/H). A L–A, A L–H, and A L–(A/H) threshold models were constructed for debris flow-prone catchments in the Wenchuan earthquake area using the empirical data set of debris flow catchments. The validation suggests that the proposed models are suitable for identification of potential debris flows in the Wenchuan earthquake area. The empirical models are very simple, and the data necessary for the input are easily measurable topographic parameters and co-seismic landslides in a catchment. The approach may be applied to identify the potential debris flow in other earthquake areas due to simplicity and the very low cost-benefit rate, when adapting the threshold parameters according to new local conditions.