Case study of a gully with 7 repeated debris-flow events in 24 years

This article focuses on a gully where seven debris flow events occurred successively over a period of 24 years. The gully is located in the mountainous area of central Taiwan, with a catchment area of 55.85 hectares, an elevation ranging from 720 to 1470 m, an average slope of 80%, and forest covered 80% catchment area. From 2000 to 2023, seven debris flow events have occurred in this gully, and in response to these events, the authorities have constructed various mitigation structures in the area. In addition, 217 earthquakes of intensity 2 or greater occurred from 1995 to 2024, including three earthquakes of intensity 5 and one of intensity 7. This study aims to understand the characteristics of induced debris flow events in this gully by analyzing the rainfall data, seismic sequences, and mitigation structures.

The results of the study show that: (1) The hydrological parameters, including rainfall depth, duration and intensity, exhibited significant variation among the seven debris-flow events. For instance, the 3-hour rainfall depth varies by more than 10 times. (2) Earthquakes with intensity below 5 do not show a significant correlation with the occurrence of debris flow events in this gully, but earthquakes with intensity 7 caused a significant decrease in the occurrence threshold of debris flow; (3) In about 5 years, the decreasing of debris flow occurrence thresholds by the intensity 7 earthquake gradually returned to pre-earthquake conditions; (4) Mitigation structures have a certain degree of disaster suppression against normal rainfall. However, the suppression of debris flow disasters induced by extreme rainfall is limited.