Landslides and debris flows caused by an extreme rainstorm on 21 July 2012 in mountains near Beijing, China

An increasing number of debris flows in mountainous regions near Beijing have been documented in the last few decades, some of which were triggered by extreme rainstorms. In this paper, we examine a total of 40 landslides and 16 debris flows triggered by an extreme rainstorm on 12 July 2012. Rainfall records were used to check the triggering conditions. Field reconnaissance and measurements, supported by aerial photo interpretation, were conducted to identify the distribution, mobility, and volume change of landslides. This extreme rainstorm provided a higher triggering condition than the required rainfall threshold in the study area. The rainfall intensity during periods of intense rainfall was comparable to some typhoon events and other extreme rainstorms. Records from seven rainfall stations reveal that the rainfall in valley floor areas was higher than that in mountainous and plain areas. The rainfall-induced landslides were small, and most of them were located in terrain with a slope gradient >25 degrees and pelitic dolostone strata underneath. The watersheds have a small area, short stream length, and the debris flow volume is low because of the limited material resources. The landslide-type debris flows exhibit weak mobility as the debris flow deposits mainly consist of gravels. Although the material resources from shallow landslides are small, the debris flow volume is 1.03-4.05 times the initial landslide volume, indicating that the debris flow entrains sediments. Finally, the entrained sediments per channel length and debris flow volume magnification were positively related to the watershed area and channel length, respectively. Our findings proved that an extreme rainstorm can trigger landslides and debris flows in an area where no such hazards were documented before. It is necessary to identify the site prone to landslides and debris flows, which may pose a hidden threaten for local inhabitants.