Rainfall Thresholds for Homeland Security in Mountainous Townships of Taiwan during Typhoon Morakot

Abstract

Global warming is causing rapid changes in weather patterns. Such changes have brought extreme rainfall in Taiwan and caused numerous hazards in recent years. Typhoon Morakot made landfall in Taiwan on 8 August 2009 and caused 677 deaths, with 22 missing. Its extreme rainfall was concentrated in the mountainous areas of southern Taiwan in Chiayi, Tainan, Kaohsiung, and Pingtung counties. The rainfall caused the most severe natural hazards in the southern mountainous areas of Taiwan in recent years. This study performs a statistical analysis of hazards and rainfall distribution in the mountainous areas of Chiayi County during Typhoon Morakot. The results of the analysis show that the number of hazards in the mountainous areas increased with the rainfall. As rainfall increased over a threshold value, the number of hazards also increased abruptly. The cumulative number of hazards and the time of rainfall duration showed a power-law distribution over a threshold rainfall. A novel rainfall threshold index, the DI t or Disaster Intensity in a time interval for torrential rainfall-induced hazards caused by typhoons is proposed herein to indicate the intensity of increasing hazards during Typhoon Morakot. The proposed index could be extended to future typhoons to aid in hazard warnings for homeland security. The threshold rainfall can be used as an index for hazard prevention and emergency response in the studied mountainous townships and other areas.