灾害孤岛潜势风险分群与降雨警戒值探讨

台湾本身地貌特性以及近年来强降雨事件发生频繁,促使山区地带发生复合式灾害,如崩塌、泥石流与山洪等,因此灾害孤岛效应有必要仿效泥石流以及洪水灾害,制定出参考警戒值以及警戒线,将有助于防灾决策、救难资源投入以及灾害风险判释。本文针对台湾发生过灾害孤岛效应的114个村进行分析,且着重于首次发生灾害的台风降雨事件。首先通过群集分析并依照各样本的6个灾因指标进行分类,群集结果显示可分为8个群,并再以象限分布描述各群的潜势相对风险。采用2004—2015年台风降雨事件的观测雨量数据,并利用距离反比权重法,得出各村的累积降雨量(R)与最大时雨量(I)。整合各群集样本雨量数据后,使用台湾水土保持局使用的RTI模式概念,计算出各群集之RTImin、RTI30、RTI50和RTI70,以绘制低风险区(0～30%)、中风险区(30～70%)和高风险区(70%～100%),其中着重探讨灾害孤岛事件的下限值RTImin,以便可得知最易发生灾害的群集。结果显示,各群的潜势风险反映RTImin的效果良好,如潜势风险越高则其RTImin值越低,即雨量驱动灾害发生的条件较低。群集中,第1与2群为低风险群、第3、5与6群为中风险群、第4、7与8群为高风险群,仅第5群无法透过RTImin验证潜势风险关系,故本文挑选的6项灾因指标,为辨别灾害孤岛效应潜势等级的重要因子。

Clustering and rainfall triggering index (RTI) setting of potential isolated villages

Due to worldwide climate change,the frequency of extreme rainfall events and rainfall distribution in Taiwan have gradually increased and changed. As a result,the occurrences of various sediment disasters are raised rapidly in mountain areas. The villages in mountain area often become isolated area due to road closures which is called "Disaster Isolation Event". It means that villagers could not beevacuated to safe places through the road network. To adapt disaster isolated event caused by extreme rainfall events, this study aimed to evaluate the rainfall triggering index(RTI) and potential area of isolated villages. Firstly,114 isolated villages impacted by multi-hazards during 2001 to 2016 were investigated as acase study. We collected the characteristics of disaster isolation event including the potential disaster,road network and rainfall events. Secondly,Cluster analysis was applied to group the 114 villages into eight clusters which have similar characteristics. Finally,we set up the rainfall triggering index with different probability and divided it into low risk(P=0%~30%),medium risk(P=30%~70%) and high risk(P>70%). The results reveal that the proposed method has effectively formulated the rainfall triggering index of disaster isolation event.