臺灣西部地區岩屑崩滑雨量重現期之門檻值及其差異性之研究

Abstract

本研究以臺灣西部三個集水區流域為對象，包含北部新店溪、中部大甲溪以及南部高屏溪，利用降雨事件前、後之岩屑崩滑圖層，建立各流域內所有斜坡單元之岩屑崩滑紀錄，並利用雨量站的資料，來進行研究區域雨量重現期及斜坡單元之岩屑崩滑雨量分析，以了解各區域之降雨背景條件，以及其岩屑崩滑之雨量特性，進而訂定各斜坡單元之岩屑崩滑雨量重現期的門檻值，並探討地質、地形條件對門檻值之影響，以及新生與復發岩屑崩滑之門檻值差異。 研究結果顯示，各重現期累積雨量以高屏溪最高，大甲溪次之，新店溪較低。在岩屑崩滑雨量重現期門檻值方面，新店溪、大甲溪、高屏溪各有22.2%、32.4%、42.6%的斜坡單元在超過特定雨量重現期門檻值會發生岩屑崩滑，並各有0.1%、5.0%、9.0%的斜坡單元在歷次降雨事件均發生岩屑崩滑，此結果顯示，新店溪集水區流域之抗災能力為三者中之最佳。 地質條件對岩屑崩滑雨量重現期門檻值有很大的影響，岩石單壓強度愈高、岩體不連續面間距愈大之斜坡單元，愈可承受高重現期雨量，顯示斜坡單元地質條件愈好，其抗災能力也愈佳。在地形條件方面，坡度愈陡、高程愈高之斜坡單元，愈無法承受高重現期雨量，此外，坡向朝南者，其可承受之重現期雨量較朝北者低，顯示地形陡高及日照量較多的斜坡單元，其抗災能力較差。 在新生與復發岩屑崩滑方面，新店溪、大甲溪、高屏溪等集水區流域之新生岩屑崩滑雨量重現期門檻各約為35年、20年、15年，而復發岩屑崩滑之門檻則概略均為5 ~ 10年，明顯較新生岩屑崩滑之門檻為低，顯示當岩屑崩滑發生後，再次復發所需之雨量較低。

The rainfall return period threshold (RRP threshold) for debris slide and their differences between Xindian, Dajia and Gaoping watershed are analyze in this study. Records of debris slide in each slope units are established by comparing the mapping layers before and after rainfall events. The rainfall return period and triggering rainfall of debris slide are also analyzed using rainfall data in order to understand the characteristics of rainfall amount of study area and critical rainfall of slope units. Thereafter, the RRP threshold for each slope unit is set up and its relationship with geological/geomorphological conditions as well as the difference between new/recurrent debris slide are discussed. Rainfall amount is higher in Gaoping and lower in Xindian among three watersheds. In Xindian, Dajia and Gaoping watersheds, there are 22.2%, 32.4% and 42.6% slope units that might occur debris slide when rainfall is higher than specific RRP threshold. On the other hand, there are 0.1%, 5.0% and 9.0% slope units in three watersheds that occurred debris slide during every rainfall event. These results indicate that the slopes in Xindian watershed are more resistant to rainfall than others. Slope units with higher uniaxial compressive strength and spacing of discontinuity also have a higher RRP threshold, indicating that the better geological conditions are, the abler resistance to high rainfall intensity. On the contrary, slope units with higher gradient, elevation and south-facing have a lower RRP threshold, showing that steeper, higher, enduring more temperature difference can make slopes more fragile. The RRP threshold for new debris slide in Xindian, Dajia and Gaoping watersheds are 35, 20 and 15 years separately, but are all around 5 ~ 10 years for recurrent debris slide. This means that when a debris slide occurred, a lower rainfall can induce it again.