大規模崩塌地形衍育之數值模擬－甲仙案例

Abstract

大規模崩塌有著歷時短、毀滅性高等之特性，針對潛在大規模崩塌邊坡進行深入研究有其重要性。邊坡可能經過一地形變動過程後，逐步轉變成具大崩塌潛勢之坡體，若能探討邊坡地形衍育之形成過程與機制，藉由現地殘存的跡象來重塑邊坡的過去，不僅加深對邊坡現況的了解，更能有助吾人進一步地推測邊坡未來可能發展。本研究提出一套可用於推測大規模崩塌邊坡場址以往曾經歷的地形衍育之研究方法。先於潛在大規模崩塌研究場址之鄰近區域，尋找具相似地質與水文條件者，以推測場址經歷地形衍育前之可能原始地形面，繼而推演該原始地形面逐漸衍育至現今場址地形之可能過程與機制，以建立地形衍育之概念模型，任一可接受之概念模型都絕不得違背運動條件與力學條件。透過極限平衡切片法與顆粒流運動模擬，可佐證地形衍育模型中各階段衍育過程力學機制之合理性，並能嘗試以多期的變化階段配合情境模擬以解釋案例場址現今地貌之來由。本研究以一個位在高雄甲仙區的潛在大規模崩塌場址為案例，認為該邊坡原始地形為一具上凸下凹地形之山坡，可能因暴雨或地震誘發坡頂發生大規模平面滑動，因而留下現今之崩崖面；隨後受到旗山溪下切與側刷作用影響，邊坡地形再發生局部之變化。位在旗山溪河道攻擊段之北側邊坡，經歷了多期的中、小型崩塌，終演變至現今之地貌。基於此衍育概念模型，本研究除了檢驗過去各期坡壞與運移/堆積之合理性，並評估現況之邊坡穩定性，研判此場址再度發生大規模崩塌之機會不高。

A large-scale landslide may fail in a sudden and result in devastating consequences. The hillslope of a potential large-scale landslide could experience a series of geomorphological evolution in the past. The geomorphological evolution may be resulted from previous landslides followed and their runout. By exploring the past geomorphological evolution of a potential large-scale landslide site, it will not only be beneficial to better understand the site condition at present, but also help to predict the possible instability of the slope in the future.

This thesis aims to develop a method for exploring the conceptual geomorphology-evolution model of a site with large-scale landslide potential. The method starts with finding a reference hillslope adjacent to the site with potential of large-scale landslide. The reference hillslope should have similar geology and geohydrology conditions with the studied site with potential large-scale landslide. Next, possible conceptual model for geomorphological evolution is nominated and checked whether the model is admissible in kinematics. The admissible conceptual model then is examined by using the limit equilibrium method and run-out simulation; this process can be conducted stage by stage, if the geomorphological evolution contains several evolution phases. Each evolution phase is simulated successively for a scenario of heavy rainfall or earthquake as a triggering factor.

With the aforementioned method, a slope site in the Jiasian district, Taiwan was taken as a case study for studying the geomorphological evolution of a hillslope with large-scale landslide potential. A conceptual model was proposed to explain the geomorphological evolution of this slope. The landform of this hillslope was originally in a convex-concave form. A large-scale plane failure on the upper slope took place probably triggered by either a heavy rainfall or a large earthquake. The sliding mass moved down and deposited in the downslope region; the colluvium covered the downslope area all the way toward the toe of the slope. Afterwards, the channel incision of the Qishan River by the toe of the hillslope gradually developed; river bank scouring was also in progress. The incision and bank scouring resulted in several local slope failures in the lower part of the slope. Consequently, the hillslope gradually turned into its current appearance after experiencing these landform-change phases. The slope stability of this site was further assessed according to the simulated results from the conceptual model. The likelihood of the recurrence of a large-scale landslide in the future is relatively low.