不同重現期距下土石流影響範圍之劃定

近年來，每逢颱風豪雨季節，土石流已成為台灣地區經常發生之坡地災害之一，為此我們已發展出一個數值模式Debris-2D來模擬土石流的影響範圍。然而對於同一個土石流潛勢區而言，不同重現期距的降雨，會導致不同的土石流災害規模，在防救災規劃的考量也會有有所不同。因此，若欲利用數值模式模擬不同降雨頻率下的土石流影響範圍，必須先估計出各個重現期距降雨下的土石流體積量。本研究利用水土保持技術規範中的公式來估計不同重現期距下的降雨強度，式中的「降雨延時」，係以過去的歷史災害中「由開始降雨到土石流發生所經過的時間」來代入，接著將計算出的降雨強度，以合理化公式來估算某個重現期距所對應的洪峰流量，並將此洪峰流量乘上降雨延時，當做是土石流中的「水量」，配合平衡濃度的觀念，可反推出不同重現期距下的土石流體積量，將之輸入數值模式中即可進行模擬。文中將以2009年莫拉克颱風時曾發生嚴重土石流災害的高雄市六龜區新開部落為例，說明此方法如何劃定該地區不同重現期距下的土石流影響範圍。

關鍵字

土石流影響範圍； Debris-2D數值模式；歷史災害之降雨延時

並列摘要

In the past 20 years, debris flows caused great casualties and severe property losses in Taiwan. For disaster prevention, the hazard zone mapping of debris flow becomes more and more important. Although many empirical formulas has been proposed for engineering, but it seems to be not accurate enough. Therefore, we have developed a numerical model Debris-2D to simulate the in-situ debris flows. The boundary layer concept is used to simplify the governing equations(Huang,2003). This numerical model is verified with analytical solutions, laboratory flume tests and historical debris flow events. The simulated results show some good agreements with the measured data. However, rainfalls with different return periods will lead to different scales of debris flow, which will need different planning for evacuation and shelter. In this study, we first introduce the rainfall duration in a historical debris-flow disaster event into the empirical formula to compute the rainfall intensities and peak flows of different recurrence intervals. The rainfall duration means the time elapsed from start of the rainfall to the occurrence of debris flow. Next, the amount of water in debris flow for a certain return period can be estimated conservatively as the product of rainfall duration and the corresponding peak flow. Then the debris-flow volumes of different return periods can be obtained according to the concept of equilibrium concentration. A case study in Xin-Kai Tribe, where a severe debris flow occurred during Typhoon Morakot, shows how this approach can be applied in debris-flow hazard zone mapping.