沖積扇之計算與實驗形貌動力學研究

Computational and Experimental Morphodynamics of Aggrading Fans

陳慈愔(Tzu-Yin Chen)

指導教授 ： 卡艾瑋 ； 洪啟耀

國立臺灣大學/工學院/土木工程學系/博士(2022年)

https://doi.org/10.6342/NTU202204140

摘要

沖積扇是地質學家和工程師感興趣的高地和低地過渡地帶常見的地貌。沖積扇可容納來自陡坡集水區的材料，並提供適宜居住的緩坡空間。然而，由於洪水或土石流引起的沖積扇演變也對沖積扇上或周圍的居民和基礎設施構成嚴重風險。
為了研究沖積扇的堆積行為和相關風險，本論文結合計算模擬和實驗及現地觀察。本論文提出了兩種計算模型，並根據實驗室實驗和現場數據評估它們的性能。第一個計算模型模擬受地形控制的靜態沖積扇形貌。該模型將三維形貌問題轉化為一系列二維可視多邊形問題，由此藉由計算幾何學之工具進行計算模擬。而第二個計算模型模擬了沖積扇的動態堆積過程。該模型假設沖積扇局部之坡度受材料性質(摩擦角與凝聚性)限制，以此模擬出在實驗室與現地共同可觀察到的沖積扇特徵，如陡峭的鼻狀扇趾。這兩種模型都有其廣泛的應用，儘管它們被簡化並且無法描述沖積扇表面的一些詳細動態特徵。為了研究這兩個模型中未包含的形貌動力特徵及其影響因素，本論文在實驗室進行了一系列持續土石流沖積扇實驗。我們將實驗結果與現地觀察的沖積扇資料一同分析討論，發現細顆粒含量和流動事件時間尺度會影響渠道化、撕脫動力學以及隨之而來的扇面複雜性。本論文的結果表明，沖積扇形態可以在不同的尺度上考慮：在較廣的尺度上，沖積扇可以通過旋轉表面進行模擬；在較近的尺度上，沖積扇表現出局部結構特徵，如鼻狀扇趾和土石流渠道。前者的形成可以通過材料之凝聚性進行模擬，後者的形成、大小和撕脫動力學與土石流之材料組成和流動時間尺度有關。論文提出的結果和方法適用於沖積扇演化預測、地形資料判釋、災害評估以及其他地景地貌的探討和研究。

關鍵字

沖積扇形貌 ； 旋轉面 ； 計算形態動力學 ； 土石流 ； 組成律

並列摘要

Alluvial fans are common and often dense-populated landforms in the transitions of high and low lands. The evolution of alluvial fans due to floods or debris flows poses severe risks to residents and infrastructure on or around the fans. To assess and mitigate the hazards, it's important to have a better understanding of fan morphology and morphodynamics. The present thesis combines computations and observations to investigate how material properties affect the morphology of aggrading fans. On one hand, I propose hypothetical influencing material properties and develop two computational models to realize and examine them. On the other hand, I conduct a series of alluvial fan experiments and compare the results with field observations to generalize some process characteristics of fan morphodynamics. The results of this thesis implicate that alluvial fan morphologies can be considered on various scales: on a broader scale, alluvial fans can be simply modeled by surfaces of revolution; on a closer scale, alluvial fans exhibit local structure features, like snout-like toes, whose formation may be modeled by the cohesion effects in the flows, and channels, whose formation, size, and avulsion dynamics relate to flow composition and time scales. The results and methods proposed in the thesis are applicable to the alluvial fan evolution prediction, topography data interpretation, hazard assessments, and also the exploration and investigation of other geometric landscapes.

並列關鍵字

Alluvial fans ； morphology ； surface of revolution ； computational morphodynamics ； debris flows ； granular-fluid flows ； constitutive laws

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