巨額土砂匯入對和社溪河相演變之影響

Abstract

Macro-sediment impulses induced by particular typhoons and rainstorms are the main reason for serious sediment disasters in the Heshe River. For example, the sediment input during Typhoon Morakot in 2009 which accounted for 95.8% of the annual sediment discharge is the most serious of these disasters. Since Typhoon Morakot, the sediment input has decreased, as there have been less serious typhoons and rainstorms, thus, transforming the river morphology from a braided river into meandering river. In addition, river bends and topographical notches restrain sediment from moving downstream and store it in these locations. These factors have indirectly increased the erosion density of the river banks by 2.5 to 10.5 times.特定之颱風及豪雨事件為和社溪集水區內巨額土砂進入河道造成相關土砂災害之主 要因素；尤以2009 年莫拉克颱風其間之輸砂量，佔該年總輸砂量之95.8％最為嚴重。然而，莫拉克颱風過後並無發生規模較大之颱風及豪雨事件，因此進入和社溪之土砂漸減，使得因土砂堆積造成之辮狀河川逐漸走向蜿蜒。此外，河川轉折點跟隘口處都會抑止土砂往下游運移並使土砂集中於此處，間接造成河岸淘刷密度增加2.5 至10.5 倍