Field Investigation And Research Of Debris Flows Trigged By An Extreme Rainstorm On July 21,2012 In Beijing

On July 21,2012,a rare extreme rainstorm occurred in Beijing.The Fangshan district had the longest duration of rainstorm and was damaged most seriously as the rainfall-induced landslides and debris flows.In this paper,a field investigation was carried out to know about the landslides and debris flows in Sancha and Xiazhuang village of Dashiwo town.Aerial sensing images,meteorological data were used to analyze the characteristics and formation process of debris flows from the aspects of geology,geomorphology,source and rainfall conditions.Then we calculated the dynamic characteristic parameters by using morphologic method and rain-flood method and analyzed the erosion according to the changes of debris flow accumulation.The mainly conclusion are showed as follows:(1)Fangshan district was hit by a serious natural adversity on July 21,2012.The total rainfall of 4 rainfall stations Xiayunling,Hebei town,Tuoli and Liangxiang were 337.5mm,535.0mm,391.2mm,308.7mm respectively.The maximum rainfall per hour of 4 stations was 95mm/h,far exceeded the National Red Rainstorm Warning standards,and more than the rainfall record of Beijing in 1989.Using the empirical formula to calculate the critical rainfall threshold of debris flows in the study area,we found when the rainfall intensity up to 40mm/h,the watershed prone to have a debris flow.So it's speculated that the debris flows were occurred about 13:00 in July 21.(2)Under the influence of rainstorm,there were 40 landslides in the study area,and some converted into debris flows,the Sancha and Xiazhuang debris flows were typical.Then through the particle size analysis,we found due to the strong erosion of channel runoff,it left an armouring layer on stream surface,in which the content of clay and gravel was low.And the sediments was dominated by large particles,of which 20mm?50mm gravel accounted for 80%.Overall,the debris flow belonged to low frequency and turbulent debris flow.(3)The resistance to weather of the lithology in study area is weak,and there was a large amount of loose material accumulation in the shallow surface,which provided a great kinetic energy condition and sufficient source for the formation of debris flows in this region.The study shows there were two initiation mechanics of debris flows,including mobilization from shallow landslides like Xiazhuang and from failure of armouring layer due to high rapid runoff produced during intensified rainfall interval,like Sancha.(4)According to the morphologic method,the flux of 4 debris flows in Xiazhuang and Sancha exceeded a-fifty-year flux calculated by rain-flood method.Because the most were shallow landslides in the watershed,limited by source condition,the scale of debris flows were not up to the level of a-hundred-year.In addition,There were volume magnifications that debris flow volume in the accumulation area was 1.04?4 times of landslide volume because of channel erosion.