主流和支流土石流沖積扇交互作用：小尺度實驗和水流模擬

Abstract

支流土石流沖積扇與主流之間的相互作用在許多地區引發自然災害方面扮演著關鍵角色。然而，目前關於兩條支流土石流扇與主流之間相互作用的研究仍然相對有限。本研究針對單一支流的受限土石流沖積扇及兩條支流的受限土石流扇，重點探討它們在形成階段的特性、對主流流態的影響，以及主流流動對土石流扇緣侵蝕模式的後續影響。

由於影響土石流扇形成與主流岸蝕的因素十分複雜，本研究採用類比實驗以簡化相關影響變數。實驗中使用塑脂砂以在縮小比例下提升其流動性並模擬輸砂；除此之外也採用Na-CMC溶液作為增稠劑，以模擬土石流的流變特性。本研究探討了單一支流及兩條支流土石流沖積扇的情境，土石流扇的形成皆在無主流流水條件下進行，隨後在無沉積物供給條件下，進行主流驅動的扇緣侵蝕實驗。地形通過雷射掃描進行測量，而水深與流速則透過高速攝影機進行記錄。

此外，研究採用了視域多邊形模擬技術分析土石流沖積扇形成階段，並結合淺水模擬以預測主流的流動模式，將實驗結果與理論模型進行比對。研究結果顯示，攝影測量技術在地形、水深及流速測量方面具有高度的準確性。值得注意的是，在兩條支流土石流扇的情境中，下游支流土石流扇較上游支流土石流扇更容易受到主流引發的侵蝕，這與2024年颱風「凱米」期間洪水事件的野外觀測結果相符。

The interaction between tributary debris fans and the mainstream plays a critical role in triggering natural hazards in many regions. Research on the interaction between two opposing tributary fans and the mainstream, however, remains limited. This study investigates single tributary confined debris fan and two opposing tributary confined fans, focusing on their properties during the build-up phase, their influence on mainstream flow, and the subsequent impact of mainstream flow on fan bank erosion patterns. Due to the complex factors influencing debris fan formation and mainstream bank erosion, this study employs analog experiments to simplify the influencing variables. Plastic sand was used to enhance mobility and transport at a reduced scale, while a Na-CMC solution served as a thickening agent to mimic the rheology of debris flow slurry. Single and two tributary debris fan scenarios were investigated, with debris fans constructed under no mainstream flow conditions and subsequently subjected to mainstream-induced bank erosion under conditions of no sediment supply. Terrain was measured using laser scanning, while water depth and flow velocity were documented using high-speed camera methods. Visibility polygon simulations for fan build-up and shallow water simulations for mainstream flow patterns were employed to compare experimental results with theoretical predictions. The findings demonstrate the effectiveness of photogrammetry in measuring terrain, depth, and flow velocity. Notably, in the two-tributary fans scenario, the downstream tributary fan proved highly susceptible to mainstream-induced erosion compared to the upstream fan, consistent with field observations during the flood event triggered by Typhoon Gaemi (2024).