Impacts of rainfall variability and the earthquake on debris-flow occurrences

• Main Authors: Wen-ShunHuang, 黃文舜
• Other Authors: Chyan-Deng Jan
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/21579551870233697933

博士 === 國立成功大學 === 水利及海洋工程學系碩博士班 === 100 === This study analyzed the variability in the number of rainfall events and effects of the Chi-Chi earthquake related to debris-flow occurrences in the Chenyoulan stream watershed located in central Taiwan. Rainfall data between 1970 and 2009 measured at three meteorological stations nearby/in the watershed were collected and used to determine the corresponding regional average rainfall for the watershed. Data of debris-flow events between 1985 and 2009 was collected and used to study their dependence on regional average rainfall. For detecting the impacts of the variability and the Chi-Chi earthquake on the study area, landslides areas were estimated by comparing the data of 11 images taken before and after the rainy events and Chi-Chi earthquake, and hydrological data measured at gauging stations at the downstream of the watershed from 1972 to 2009 as well. The results showed that landslides had an increasing trend with time. After the Chi-Chi earthquake in 1999, the landslides area greatly amplified and nearly tripled to the landslides areas caused by Typhoon Herb in 1996. Due to there was no obviously high precipitation after the earthquake, the abrupt increase in landslide areas was most likely attributed to the Chi- Chi earthquake. The variations in gradients and areas of watersheds that triggered debris flows have changed after the earthquake. The lowest gradient and areas of watershed for triggering debris flows occurred during Typhoon Toraji in 2001. But after 2005, the critical conditions for triggering debris flows gradually return to that before the earthquake. Rainfall characteristics, , , , and were used to analyze the variation in the number of rainfall events in the four decades. The variations in the number of rainfall events obviously changed in last decades. The variation of the higher rainfall events in 2000-2009 is significantly greater than that in other decades. The maximum 24-h regional rainfall was also used to analyze the number of rainfall events that triggered debris flows , and the probability of debris-flow occurrences as well. The variation trends in , , and over recent decades under three rainfall conditions ( 〉 20, 230, and 580 mm) related to debris-flow occurrences were analyzed. In addition, the influences of the Chi-Chi earthquake on and were presented. The results showed that the rainfall events with 〉 20 mm during the earthquake-affected period (2000-2004) strongly responded to the increases in the average number of rainfall events that triggered debris flows and the average probability of debris-flows occurrences, especially within the period (2000–2004) when the lowest number of rainfall events occurred. The number of rainfall events of 〉 230 mm (the lower boundary for the rainfall ever triggering debris flows before the Chi-Chi earthquake), and 〉 580 mm (the lower boundary for extreme rainfall ever triggering numerous debris flows) in the Chenyoulan stream watershed increased after 2000. The increase in the number of extreme rainfall events with 〉 580 mm augmented the number of rainfall events ever triggering numerous debris flows in the last decade. The empirical relationships are presented for the average annual number of rainfall events triggering debris flows and the probability of debris-flow occurrences related to the average annual rainfall events in the past decades. The results indicated that the increase in both the number of rainfall events that ever triggered debris flows and the probability of debris-flow occurrences was greater in the last decade (2000–2009) than in 1990–1999. The spatial and temporal distributions of debris flows are demonstrated by three extreme rainfall events before and after the Chi-Chi earthquake. The results showed that the variations in spatial distributions do not change much, but the total rainfall required to trigger debris flows sharply decreased after Chi-Chi earthquake. The annual variations in river discharge and suspended sediment discharge were evaluated. The maximum of the daily mean discharge and instantaneous discharge both have slight increasing trends. The variations of suspended sediment discharge in 1972-1979 and 1980-1989 are less than the averaged value in 1972-2009 at the same discharges; 1990-1999 and 2000-2009 are obviously larger than the averaged value (1972-2009) at the same discharges. The variations of suspended sediment discharge in last two decades might attribute to rainfall variability. Significant difference between the suspended sediment discharge before 1999 and that after 1999 was found, and the suspended sediment discharge gained after 1999 increases 5 times more than that before 1999 at Q 〈 50 cms, while that gained after 1999 increases 2 times more than that before 1999 at Q 〉 50 cms.