RAINFALL CONDITIONS FOR THE INITIATION OF DEBRIS FLOWS DURING TYPHOON MORAKOT IN THE CHEN-YU-LAN WATERSHED IN CENTRAL TAIWAN

Abstract

In the August of 2009, Typhoon Morakot brought heavy rainfall and caused several landslides, floods and debris flows in central and southern Taiwan. This work presents the critical rainfall conditions for the initiation of debris flows in the Chen-Yu-Lan water - shed of central Taiwan during the action of Typhoon Morakot, where had many severe debris-flows events during the actions of Typhoon Herb in 1996 and Ty- phoon Toraji in 2001. Thirty-seven debris flow events induced by the heavy rainfalls brought by Typhoon Morakot in the Chen-Yu-Lan watershed were iden- tified by the examination of aerial photographs and field investigations. The rainfall conditions, including the rainfall intensity, the cumulative rainfall, the rain- fall intensity-duration curve and the return period of the rainfall, for debris-flows initiation were analyzed and compared with the rainfall conditions of histori- cal debris-flows events in the watershed. The average rainfall intensity that triggered abundant debris flows in the watershed were determined and the relation be- tween the density of debris-flows occurrence and the rainfall characteristics were also presented.