三角洲保存了廣泛的泥砂沉積，形成原因為上游河流夾帶泥砂流入湖泊、水庫或出海口時，會在水上與水下的交界處堆積而成。在不同的時空背景以及環境底下，三角洲的形貌又與河流入流的流況息息相關，若河流為homopycnal流況則通常會沖積出Gilbert三角洲，其topset的坡度較緩，卻具有在達到安息角時停止崩落而較陡峭的foreset以及相對平坦的bottomset。另一方面，當入流流況為hyperpycnal flows時(或turbidity currents)，則會使其三角洲的foreset形貌呈現凹向上的曲線，並且比安息角平緩許多。然而，雖然在此領域已經有不少學者對三角洲的形貌動力學做深入的研究，但在針對不同底床坡度上所發展的異重流三角洲之研究並不多，因此本研究將著重於不同底床坡度對異重流三角洲形貌動力之影響，並且為了強調兩者三角洲在foreset的不同，做了下游坡度較陡的Gilbert delta來與異重流三角洲做定量的比較，最後再搭配實驗以及解析解做詳細的說明。本研究的核心概念為利用小尺度的物理模型實驗，找出大自然界異重流三角洲發展的規則，Lai and Capart(2007, 2009)曾提出異重流三角洲具有自我相似結構的實驗以及理論分析，而Lai(2017)更是將Gilbert delta在不同底床坡度上之實驗以及理論完整的呈現，然而本研究的重點將放在異重流三角洲在不同坡度上的發展，並且針對下游底床坡度較陡的案例與Gilbert delta做定量的比較，此外，不同於以往的理論推導，本研究為了與圖表的無因次化方式一致，在理論假設的部分將自我相似結構以√It做後續的推導，而在實驗紀錄的部分以每五秒做間隔攝影(time-lapse photograph)，將得到的高解析度照片以數位影像處理的方式分析，將其量化，最後與理論做交互的比較與討論，希望能透過此實驗了解異重流三角洲在不同坡度下淤積的情況，期待在未來能應用在解決水庫淤積問題、海岸線的進退以及三角洲在水下體積的預測等等。

A hyperpycnal delta forms at shoreline when sediment-laden flows plung into a receiving basin (reservoir, lake or sea). The foreset of hyperpycnal delta often exhibit concave foresets with maximum inclinations smaller than angle of repose. On the contrary, the foreset of Gilbert delta is relatively steep, controlled by angle-of-repoese avalanching of sediment. In this study, we focus on the morphodynamic responses of a hyperpycnal delta to a two-slope basement condition. We derived a new self-similar analytical solution and validated the solution by a series of phtsical experiments. Our results show that strong self-similarities exist in both hyperpycnal and Gilbert deltas over two-slope basements. These results provide potential applications to reservoir sedimentation and delta in morphology at the river mouth.

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