| Summary: | 碩士 === 國立成功大學 === 水利及海洋工程學系碩博士班 === 98 === Sediment hazards on hillslope in Taiwan have been widely analyzed in relation to hydrological factors. Despite qualitative and quantitative analysis of sediment hazards can be concluded from rainfall amount, but a more complete assessment of the con-trols on hazards behavior requires the investigation of more detailed rainfall characte-ristics. This study uses the rainfall erosivity index R30 (= kinetic energy E × maximum 30-min rainfall intensity I30) which can simultaneously reflect the responses of soil to precipitation and runoff, to investigate reservoir sedimentation, new landslide rate and the occurrence of debris flows in the Watershed Tseng-Wen River. Additionally, the pseudo-rainfall erosivity R60 (= calculated kinetic energy by hourly rainfall data E60 × maximum hourly rainfall intensity I60) was computed by hourly rainfall data from six rainfall stations, instead of R30 for the lack of 10-min or min rainfall data in many places, establishing the assessment model as R30 = 1.5 R60, which concluded from the Watershed of Tseng-Wen Reservoir and Chenyulan Watershed.
As a Result, the rainfall erosivity index R30 can characterize the behaviors of sedi-ment hazards on hillslope more pertinently then other rainfall indices. In a temporal analysis, annual reservoir sedimentation can be commanded by the R30, associating with significant typhoon events for the correlation coefficient being 0.92. The empiri-cal equation can be represented as reservoir sedimentation RS (104 m3) = 0.67 R30 (10 MJ-mm/ha-hr) – 96, moreover, the sedimentation will be greater than 300 (104 m3) when the R30 exceed 580 (10MJ-mm/ha-hr) in a typhoon event. In a spatial analysis, the rainfall erosivity index R30 was recognized as a key factor to describe new landslide rate induced by a heavy rainfall in a watershed. The results were used to ca-librate the Uchihugi’s empirical model. Debris flows linked to rain are likely to be triggered when the R30 exceeds 840 (10MJ-mm/ha-hr) and totally above 1400 (10MJ-mm/ha-hr). Finally, the case study of Typhoon Kalmegi and Morakot indicated that kinetic energy center accompany the occurrence of catastrophic disasters. In Ty-phoon Morakot, record-breaking rainfall resulted in the R30 distributing over the Wa-tershed of Tseng-Wen River greater than the center of Typhoon Kalmegi, and may be used to explain the deep-seated landslide in Xiaolin village.
This study showed that the approach of taking the pseudo rainfall erosivity index R60 as a variable to estimate the rainfall erosivity index R30 is effective and creative, also reflecting hazards processes in study area very well. Therefore, these methods can still provide information for assessing the effect of rainfall on soil failure, thereby refining hazards prediction, prevention and mitigation.
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