北京山区暴雨泥石流激发雨量条件

摘要

泥石流是北京山区常见的一种山地灾害,也是威胁山区生态环境的头号安全问题.分析该区的泥石流激发雨量条件对于该区山洪泥石流预测预报具有指导意义.笔者搜集18场降雨数据,分析该区泥石流的激发雨量特征.结果表明:激发降雨的峰值降雨时段介于3 ～9h,峰值时段平均降雨强度达22.6 ～ 50.0 mm/h,累积雨量达91.9 ～ 350.0 mm,峰值降丽时段雨量占累积雨量的63.9％～100.0％,这说明激发泥石流的降雨过程具有历时短、雨量集中、突发性强的特点.用修正后的李氏法进行雨场分割,得到该区泥石流激发的降雨强度-历时和累积雨量-历时关系.最后,以密云区北部山区为例分析3期植被盖度和泥石流激发雨量条件.归一化植被指数和像元线性分解模型分析表明,1989-1991、2004-2005和2011-2012年植被覆盖度分为64.48％、68.57％和72.48％;相应的泥石流雨量激发条件也随着植被覆盖度的增加而增大,可见植被恢复盖度对泥石流的形成具有一定影响.本文统计分析得出的泥石流激发雨量条件可为该区山洪泥石预测预报和监测预警指标选取提供参考.%[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 earlywarning 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 intensityduration (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 ArcMapl0.3,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.