The Influence of Different Methods in Calculating the Effective Rainfall Accumulation on the Critical Time for Debris flow Warning

• Main Authors: Cho-HanChung, 鍾卓翰
• Other Authors: Chyan-Deng Jan
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/89093495637447850592

碩士 === 國立成功大學 === 自然災害減災及管理國際碩士學位學程 === 103 === With steep topography, special geological conditions, typhoon seasons, and over developed hillsides, Taiwan frequently suffered debris flow hazards. Debris flows caused by typhoon Morakot in 2009 resulted in many fatalities, missing, and seriously economic loss in Taiwan. Therefore, it will be necessary to enhance public awareness of many debris-flow hazards and educate people how to react to these hazards. A rainfall-based debris-flow warning system is needed for preventing or mitigating debris-flow hazards. The basic conditions for debris-flow occurrence are the abundant loose soils, steep slope, and the large amount of water. For a specified watershed of a potential debris-flow torrential, the changes of the topographical and geological conditions as well as the loose soil conditions in a period of normal time are negligible compared with the change of rainfall. The rainfall variability played a main inducement to result in hazardous debris flow. The rainfall intensity I and the effective rainfall accumulation are the parameters used for establishing the warning debris flow warning index in Taiwan. This research aims to discuss the effect on the warning time for debris flow occurrence due to different methods used to evaluate the effective accumulated rainfall. Four methods were used to calculate the effective accumulated rainfall in this study. This methods were applied to the rainfall events brought by three severe typhoons, such as Kalmaegi in 2008, Morakot in 2009, and Fanapi in 2010) to assess the critical time for the warning of debris flow in the Gaoping watershed. These four methods were named as A, B, C, and D. All methods A, B, and C consider 7-day antecedent rainfall and daily reduction weighting factor α, but they have different methods to calculate 7-day antecedent rainfall. Method D considers hourly reduction weight factor with an idea of half-life time. The results shows that the effective accumulated rainfall calculated by the Method A has larger amount and this will result in earlier reaching to warning criteria. The initial time for considering the rainfall as a previous rainfall is changed for each 24 hours in Method B. Therefore, the effective accumulated rainfall calculated by the method B has a little drop for each 24 hour. The initial time for considering the rainfall as a previous rainfall is changed for each hour in Method C. Method D has a consideration of half-life time in assessing weighting factor, and the value of weighting factor is changed for each hour. The higher half-life time T the smaller weighting factor in Method D. For T = 72 hours, the effective accumulated rainfall calculated by the Method D is closed to that by Method C. Based on the rainfall data in the Xinfa rainfall station for the events of Typhoons Kalmaegi and Morakot, the effective accumulated rainfall calculated by Methods C and D (T=72 hours) failed to play the role of debris-flow warning. This implies that a single rainfall parameter such as the effective accumulated rainfall is not enough to play the role of debris-flow warning. The rainfall intensity is taken into considered for debris flow warning, by using the rainfall triggering index (RTI) that is the product of the effective accumulated rainfall and the rainfall intensity. The warning time can be earlier to the real time of debris flow occurrence when the rainfall intensity is taken consideration with effective accumulated rainfall.