Rainfall characteristics of You-Kuang-Chi watershed on Typhoon Kalmaegi

Abstract-During the period of Typhoon Kalmaegi on July 18th, 2008, the rainfall amount accumulated for 2 days in Chia-Hsien (2) Rainfall Observation Station of Water Resources Agency (WRA) was up to 924 mm, which was close to 934 mm, the rainfall amount of the return period for the past 200 years. The maximum rainfall in three hours of the short duration reached 405 mm, which was much more than that measured in the station over the years. It was believed that the heavy rainfall of the short duration caused by Typhoon Kalmaegi resulted in the major disasters in the Chia-Hsien area. The results of the study also showed that the major disaster type in the area belonged to the disaster caused by the stream overflow. In particular, the heavy rainfall brought by Typhoon Kalmaegi caused the overflow in the short cross-sections of midstream of the You-Kuang-Chi, resulted in the flood, and destroyed the revetment. According to the hydraulic analysis of the present study, many cross-sections in the You-Kuang-Chi were below the required standard of the 50 year return period; many locations have not been managed yet. Therefore, the total cross-section management of the whole river is needed. On the whole, the disasters in the midstream of the You-Kuang-Chi were caused by the overflow of the river course because of the shortage of its cross-sections, leading to river bank erosion and revetment destruction. Thus, this study suggests that the management strategy for the You-Kuang-Chi in the Chia Hsien area should focus on restructuring the river cross-sections.