當上游河川挾帶大量土砂流入湖泊、水庫或出海口時，粗顆粒泥砂會在河川與下游水面之交界處(shoreline)堆積形成三角洲，而河川含砂濃度的高低決定了三角洲發展的形態。在台灣或其他季風型氣候國家，往往遭受季節性的暴雨或颱風侵襲，導致河川之含砂濃度提高，當其流入下游之湖泊、水庫或出海口時，會潛入水下形成高密度異重流(hyperpycnal flow)，進而形成異重流三角洲(hyperpycnal delta)。而在長時間尺度下，水下之異重流不會持續存在，當颱風或暴雨結束後，上游之河川會逐漸恢復至含砂濃度較低的狀態，此時形成的三角洲稱為吉伯三角洲(Gilbert delta)。Lai and Carpart (2007, 2009)曾提出異重流三角洲具有自我相似性之實驗與理論分析；Swenson et al. (2000)與Lorenzo-Trueba et al. (2009)也相繼提出吉伯三角洲包含兩個移動邊界隨時間發展之理論描述，後者更與實驗比較取得一致的結果。然而，前人之研究大多針對三角洲在單一坡度底床上的發展，並未考慮現地岩盤地形的複雜變化，且少有研究指出在河川含砂濃度交替變化下，由異重流三角洲與吉伯三角洲交替構成之三角洲堆積系統的實驗與理論分析。本研究以鹽水模擬豪雨形成之異重流，清水模擬一般河川之常流情況，在上游提供穩定之輸水量及輸砂量，兩者於固定時距下交替入流；此外，本研究採用五種不同之雙坡度一維渠道以簡化並模擬現地岩盤之條件，目的為透過小尺度模型實驗，探究入流密度交替變化及雙坡度岩盤對三角洲發展的影響。實驗中透過縮時攝影(time-lapse photograph)每5秒記錄三角洲之發展歷程，並以數位影像處理獲得三角洲之底床高程及兩個移動邊界隨時間變化之軌跡。將水面線對齊岩盤坡度之轉折處(slope-break)，並以其定為三角洲發展之原點，實驗結果顯示在入流密度的交替改變下，三角洲之演化仍具有高度的自我相似性，且於不同入流密度之事件下，愈後期之發展愈能被理論準確地描述，此結果在未來有機會結合現地資料，協助水庫庫容量之評估、三角洲淤積體積之預測、及河口海岸線消長之演化等。

When a river with high sediment concentration flows into a lake or a reservoir, it may produce a hyperpycnal flow, and form a hyperpycnal delta. On the contrary, the delta formed by a river with low sediment concentration that closes to the receiving basin defined as a Gilbert delta. Lai and Capart (2007, 2009) reported that hyperpycnal deltas possess self-similarity with the validation between experiments and theory; Lorenzo-Trueba et al. (2009) also proposed a theory with regard to two moving boundaries in the evolution of a Gilbert delta. However, most of the previous studies do not consider the slope variation in field basements and the change of river density in response to the changing climate. In this study, we simulate hyperpycnal and homopycnal flows by using salt water and clean water respectively. At the upstream, these two kinds of flows are provided alternatively under constant time period. In the experiments, five different bedrock slopes are selected to understand delta evolution. Our results show that strong self-similarity exists in delta even though the inflow density change alternatively. These results provide potential applications in the fields.

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