Characteristics of landslides and debris flows triggered by extreme rainfall in Daoshi Town during the 2019 Typhoon Lekima, Zhejiang Province, China

Climate change and rapid urban expansion can foster landslide disasters and cause severe damage in areas that are traditionally considered relatively safe from such hazards. In this regard, a recent event in Daoshi Town, China, is a good example that showcases the combined effect of extreme rainfall and urbanization in generating a first-time landslide disaster for a developing community. On 10 August 2019, Typhoon Lekima made its landing in Zhejiang Province, China. It generated a rainstorm close to Daoshi Town, inducing severe and unexpected damage from shallow landslides and debris flows (7 deaths, 25,560 affected and a direct economic loss of 1.5 hundred million dollars). In this region, similar rainfall has never been experienced before, and landslides have seldom been reported in the past. Field investigations and measurements were conducted to identify the landslide distributions, size characteristics, and mobility. A landslide inventory map, including 251 shallow landslides and 82 debris flows, was compiled to study the geological and topographical factors related to widespread failure. The results showed that shallow landslides and debris flows mainly occurred in the north-central part of the study area, in which the base was mainly composed of strongly weathered limestone. The elevation, mean slope, and slope aspect also contributed to the occurrence of landslides. In addition, 32.4% of the disasters were located close to buildings and roads, highlighting a relevant predisposing role played by human activity. This typhoon-bound rainstorm gave rise to a higher triggering condition than the normally required rainfall threshold in the study area. Under this circumstance, landslides were easily triggered and were able to transition into debris flows due to liquefaction. Field observations demonstrate that the erosion of slope toes caused by debris flows also facilitated the occurrence of other shallow landslides. A changing climatic setting is a never-before-experienced phenomenon for the study area. This research improves the understanding of landslide development in the area and prepares basic data and information for further studies and mitigation strategies.