Debris-flow events and their mitigations in the Qianghuangkeng stream, Tainan, Taiwan

Typhoon Morakot lashed Taiwan with about 2800mm of rain over August 7~10, 2009, and caused catastrophic sediment hazards in the southern and central Taiwan such as landslides and debris flow. The Qianghuangkeng stream watershed is located at Nanhua Township in southern Taiwan where has been channelized which was a 3m width and 5m depth concrete channel about 400m and became more meandering for avoiding pass through the private lands to prevent debris flow inundation after Typhoon Kalmaegi in 2008, but debris flow still occurred during typhoon Morakot and damaged 15 buildings, therefore we need to know the performance of the countermeasure. Base on field investigation and hydraulic numerical model FLO-2D (FLO-2D Software, Inc) simulations, we analyzed the inundation area and depth with channelization countermeasure and natural channel during typhoon Morakot which accumulative rainfall of return period was over 200 years. We also discussed the performance of the countermeasure under designation standard that return period was 50 years. The results show that the community nearby stream was inundated by debris flow during typhoon Morakot no matter channelization countermeasure or natural channel and the inundation area and maximum depth were getting close. In case the return period is 50 years, the inundated area and maximum depth of natural channel are larger than channelization countermeasure. It reveals that the countermeasure is effective against debris flow under the return period of designation, but extreme rainfall such as typhoon Morakot.