模擬台灣易山崩地區之崩塌面積及沉積物輸出

Abstract

山地小型河川的大量沉積物輸出為近年來的研究熱點，尤其在全球暖化的影響下可能導致輸出量增加而益發重要。其中，台灣的高輸砂特性被認為是由於高強度降雨所引發的山崩、土石流所造成。然而，從降雨-崩塌-輸砂這一連串的反應卻少有研究同時考慮並加以量化。因此，本研究建立一概念型的降雨-崩塌-輸砂模式，其中包含崩塌地預估模式，搭配崩塌地帶來的沉積物輸出模式，應用在易山崩的曾文水庫集水區。結果顯示：本模式解釋了颱風事件中降雨及崩塌面積不相關的現象；而崩塌帶來的土砂輸出量透過「沉積物儲存量(sediment storage)」可有效模擬沉積物輸出，並探討崩塌土砂對沉積物輸出的影響。崩塌地模式結果顯示，模擬崩塌地面積與沉積物輸出的效率係數(EC)分別為0.88、0.89，我們發現莫拉克颱風之後的沉積物輸出明顯高於前期，即使降雨量較低，說明了沉積物儲存量在輸砂中扮演的重要性。透過沉積物儲存量與沉積物輸出的比較分析得知：兩者都隨著雨量的增加而有明顯地增加，若以輸出效率（輸出量/儲存量）而言，中強度降雨的颱風會有最大的輸出效率，顯示其為是主導集水區土砂輸出重要的驅動力。此模式將有助於崩塌地面積及輸砂量控制因子的參數化，藉此了解各因子的影響程度，期望未來可以應用在推估崩塌地面積與儲存量的演育。

Magnified consequence from rainfall intensification, landsliding to subsequent sediment discharge is an important issue in landslide-dominated regions. However, a few works simultaneously synthesize this consequence due to the time-variant sediment supply in the consequence. The sediment supply responded to the landslide area triggered by rainstorms also regulates sediment transport which strongly depends on stream power. Thus, the conditional interaction between available sediment and sediment transport is rarely discussed. In this study, a model which can simultaneously simulate time-series landslide area and subsequent sediment discharge was proposed and applied onto the landslide-dominated Tsengwen Reservoir watershed in southern Taiwan. Our landslide model considers not only rainfall effect but also pre landslide area to explain the unclear relationship between rainfall and landslide area. Furthermore, the sediment transport model incorporates the status of available sediment, thus the stream transport power and available sediment could be coupled. The result shows that the simulated time-series landslide area and the sediment transport agreed with the observation with the EC of 0.88 and 0.89, respectively. Reactivated ratio of previous landslide area was up to 73% which indicated the highly-frequent reoccurrence of old landslide in landslide-dominated regions. Even the small rainstorm can transport considerable sediment when the sediment supply is sufficient. We argued that quantifying sediment transport should couple not only with water discharge but also available sediment, which is rarely considered in calculating sediment transport. Finally, the scenario analysis of the model reveals that moderate typhoon has the high efficiency in sediment export. Although sediment supply and sediment transport significantly increase with the magnitude of rainstorms, the increment of sediment transport was less than the sediment supply during strong rainstorms. Finally, the quantifying the impact of available sediment on sediment discharge could improve our understanding in sediment transport and help to do the assessments when considering climate change.