Rainfall condition of triggering debris flows in Beijing mountain regions
TU Jian, MA Chao, YANG Hailong
1. Key Laboratory of State Forestry Administration on Soil and Water Conservation, Beijing Forestry University, 100083, Beijing, China; 2. Beijing Engineering Research Center of Soil and Water Conservation, Beijing Forestry University, 100083, Beijing, China; 3. School of Soil and Water Conservation, Beijing Forestry University, 100083, Beijing, China
Abstract：[Background] Debris flows are one of the mountain hazards in mountain regions near Beijing, and cause serious threatening to local ecological safety. In recent years, the debris flow events increased a lot than last few decades as a result of the double influence of increasing extreme rainstorm events and human being activities. Researches on the triggering rainfall condition are the basis of early-warning and prediction of debris flows.[Methods] Using the distribution of debris flow sites and rainfall stations, 18 rainfall records were selected to analyze the rainfall characteristics. A rainfall event is defined when the hourly rainfall is greater than 8 mm/h in the beginning and ends in less than 8 mm afterwards. The rainfall records and rainfall events were used to identify the rainfall conditions of triggering debris flows in study area. Two rainfall thresholds, including the mean rainfall intensity-duration (I-D) and the accumulative rainfall-duration (C-D), were developed. The formula of rainfall thresholds with single-line method after analysis of the I-D and C-D were built. The Landsat data in year of 1989, 2004 and 2010 in Miyun District from geographic spatial data cloud of Chinese Academy of Sciences were analyzed. Through the ENVY5.1 and ArcMap10.3, the normalized difference vegetation index was calculated. Vegetation coverages in the three periods were obtained with pixel linear decomposition model. The relationship between triggering rainfall condition and vegetation coverage were built by standardized calculation.[Results]The intensified rainfall duration lasted merely 3 h~9 h, during which the mean rainfall intensity ranged from 22.6 mm/h to 50.0 mm/h. Accumulative rainfall ranged from 91.9 mm to 350.0 mm, accounting for 63.9%~100% of total rainfall. Two functions with expressions as I-D and C-D in Beijing mountain regions were identified. Rainfall conditions of triggering debris flow in Miyun at 2009-2011, 2004-2005 and 1989-1991 were identified respectively. Vegetation coverage in Miyun at 2009, 2004 and 1989 were 72.48%, 68.57% and 64.48% respectively.[Conclusions] The results show that rainstorm in Beijing mountain regions characterizes in short duration, high intensity and high rainfall amount. The formula of rainfall thresholds with single-line method after analysis of the I-D and C-D can be used as a reference in forecasting of debris flows. Then, relationship between vegetation cover and the rainfall conditions of triggering debris flows in three stages of Miyun was studied. Results found that the vegetation cover degree in Miyun increased as time went on, corresponding rainfall triggering also increased. This study may provide a suggestion for the debris flow early-warning in the study area.
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2017, Vol. 15 
