土石流對山區及坡地環境危害重大，隨著極端氣候變化威脅漸增，釐清其運動行為及機制能評估其危害及精進規劃情境，提升治理工程功效。本研究期望能針對土石流流動中固液相分離及固相積聚在前端的現象進行探討，初步分析流體組成物佔比對此現象發展的影響。本研究以 Tai et al. (2019) 二相流數值模型為基礎，參考 Ma et al. (2022) 簡化三相流模型中對懸浮微粒與流體黏滯性與阻力關係的描述，探討固液兩相相對速度的阻力為如何影響固液相分離的現象。透過懸浮微粒佔比的改變，土石流間隙流體的黏滯性將受到影響進而有不同程度的阻力抵抗，流體的形貌以及組成濃度分佈將會出現明顯差異。計算中固液兩相運動行為分開討論，成份間各自受到不同的阻力會產生相對速度差，進而造成相位的分離。
本文中藉由平坦地形、平板坡道及小林村事件案例確認可依此理論模型，描述出固液相分離及固相於流動中積聚在流體前緣的現象。接著以凹面渠道為參考地形，模擬不同組成成份及不同粗糙度底床的流動結果，將固相在前現象發生與否統整、繪製成以固相濃度、懸浮微粒在液相中佔比及底床摩擦角為坐標軸的三維相位圖。將相位圖的結果與 de Haas et al. (2015) 實驗室小規模流槽試驗成果進行比較，雖會因有無考慮入滲沖刷機制而有差異，但整體而言對於固相及懸浮微粒濃度增加會提昇固相在前現象發生的結論相同。此相位圖可說明流體組成對於流動濃度分佈的影響，初步歸納此現象發生的條件及特性，為未來相關土石流實驗縮小試誤範圍，並提供配置設計上的參考。
另外在附錄一中，透過 MATLAB 引擎建立一操作介面，可在 MATLAB 等高線地圖中任意選取橫跨河道的兩點，建立指定高度及寬度的地形，以此模擬防砂壩等水工構造物。將此重建過後的地形帶入模式中進行模擬，可快速計算出不同位址、尺寸的構造物對減災的影響，找出最具防災效益的設計，提供未來防災單位應用上的參考。

Solid-phase accumulations at the margin or forefront are often observed in the on-side debris flow events. However, there are rare researches dedicated to exploring this phenomenon via either indoor channel experiments or numerical simulations. Based on the two-phase mixture model proposed by Tai et al. (2019), the present study thoroughly discussed conditions for the development of the solid-phase accumulation by considering the viscosity variation in the interstitial fluid, of which the magnitude, as suggested in Ma et al. (2022), is determined by the concentration of the suspended fine clay/silt. Because of the variation of the clay-concentration, drag forces, which denote the relative resistance between the two phases and the basal drag, are viscosity-dependent as well. We conduct a series of numerical investigations on the development of the phase separation and the solid-phase accumulation at the front section of the debris flow body on an idealized chute. Simulation involves a finite mass released from the state of rest with various initial concentration of clay, where the flow body slides over a curved chute topography. The reference model parameters are based on the validity against the large-scale Hsiaolin landslide event in 2009. The investigation on the phase accumulation phenomenon explores the potential conditions for the occurrence. Consequently, we present a three-dimensional phase diagram that illustrates the presence or absence of the solid-phase accumulation phenomenon based on the associated results. The simulated results closely replicate the flow properties observed in small-scale experiments conducted by de Haas et al. (2015) as well. This implies the fact that the viscosity plays a substantial role in the development of frontal phase accumulation. That is, a higher viscosity enhances the phase accumulation. However, it is also found that excessively high viscosity will diminish the occurrence of a high relative velocity between the two phases. In addition, this study incorporates a simulation tool graphical user interface (GUI) with 3D user-interactive illustration in the appendix. This tool enables the incorporation of virtual artificial structures at arbitrary locations on digital terrain contour maps utilizing the MATLAB engine. This interface enhances the efficiency of evaluating the planned countermeasure or the effectiveness of disaster mitigation arrangement, such as the size or the optimal location of a check dam.

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