Morphodynamics of Coevolving Fluvial and Hyperpycnal Valleys

本論文的主要目的是在研究共同演化的河谷與海谷之形貌動力學。論文主要分為兩個部份：一、在邊界受限制的峽谷中之形貌動力學研究。二、在邊界自由的峽谷中之形貌動力學研究。 論文的第一個部份，我們考慮在邊界受限制的峽谷中之形貌動力學。其中，三角洲及水下的崩蝕現象被選為做更進一步之研究。就三角洲而言，我們以小尺度實驗來探究三角洲在岩盤上，對於入流密度、底床坡度及輸砂量改變之反應及演化。除了實驗之外，我們也推導出包含兩個內部移動邊界之自我相似解析解，並以之預測三角洲在岩盤上之演育過程。為了檢視此理論之預測能力，我們將這個數學的理論與小尺度及大尺度的實驗相互比較，比較的結果非常吻合，也確定了我們的研究是與尺度無關的。對於水下的崩蝕現象，我們也做了小尺度的實驗並提供相關的數學理論，以研究在水下邊界受限制的峽谷中之形貌動力學。經過實驗與理論之比較之後，確定了我們所推導出來的理論，能夠預測水下崩蝕在水平方向上向後侵蝕的速度。此理論在水下崩蝕層的速度剖面上也提供了我們對於此現象更進一步的了解。最後，我們以水下崩蝕受到高密度流影響之實驗作結，實驗的結果也促使了我們做出第二部份的研究。 論文的第二個部份，我們考慮在邊界自由的峽谷中之形貌動力學。我們也以小尺度實驗來探究，在固定降雨量與地體抬升速率之條件下，陸上與海下之動態地形演化過程。我們以高密度流作為主要的傳輸機制，聯結陸上與海下這兩個不同的沉積環境，以便研究沉積物由供應源至消沉區之傳輸過程。在實驗過程中，我們以鹽水來模擬高密度流，並且透過灑水噴頭將之均勻地灑在實驗區域中。為了模擬抬升作用，我們也設計了一個設備來控制侵蝕基準線，透過下降侵蝕基準線來達到如同地體抬升之作用。在以上條件之下，實驗過程中自然形成了陸上與海下的山脊與峽谷系統。在這自然發展的過程中，陸上的山脊與峽谷系統會被迫與海下的系統接合。在完成接合過程後，整個地形演化即達到穩定狀態並且達到同時演化及相互影響的階段。透過影像量測技術，我們得到了一系列完整的數位地形資料，並且將之定性地與現地之地形相互比較。比較之結果將有助於詮釋現地陸上與海下之地形演育。 論文結論的部份，我們回顧各章節所做之內容並統整出貫穿整本論文的兩個重要主題。最後，我們也提出可能的未來研究以完成整本博士論文。The aim of this dissertation is to examine the morphodynamics of coevolving fluvial and hyperpycnal valleys. The dissertation consists of two parts: (i) morphodynamics of constrained valleys, and (ii) morphodynamics of free valleys. In the first part, we consider the morphodynamics in constrained valleys. Deltas and submarine breaching are the two selected subtopics for further examination. For deltas, we use microscale experiments to explore delta formations over bedrock, responding to the condition of inflow density, bed inclination and sediment influx. Similarity exact analytical solutions with two internal moving boundaries are then used to predict the delta formation over bedrock. The theory compared to those experiments in either microscale or Froude scale results in good agreement, confirming that our study is scale independent. For submarine breaching, microscale experiments and relative mechanic theory are also provided for examining the morphodynamics in submarine constrained valleys. The theory catches the trend of horizontal breaching speed and provides an insight into the velocity profile in the breaching layer. Submarine breaching with hyperpycnal flows is used to end the first part of this dissertation, and the experimental results motivate us to study the second part of this dissertation. In the second part, we look at morphodynamics in free valleys. We use microscale experiments to characterize the formation of coevolving terrestrial and submarine dynamic landscapes with a constant rainfall and uplift rate. To link these two different sedimentary environments, hyperpycnal flows are used as a physical transporter to study the sediment source-to-sink processes. During the experimental processes, hyperpycnal flows are simulated using saline water spurting out from two sprinklers above the experimental sedimentary basin, and the uplift force is modeled by a step base level falling device. With above conditions, it yields autogenic terrestrial and submarine valley-ridge systems. During the autogenic processes, the terrestrial system is forced to coalesce with the submarine system. After the completion of this coalescence, the entire landscape reaches a steady-state and coevolving phase. Through imaging measurements of the entire landscape, a series of high quality DTM are acquired and compared qualitatively with field landscapes. The preliminary comparisons are in good agreement and may help to interpret the evolution of terrestrial and submarine landscapes in the field. In conclusion, a summary gives an overview of the works and syntheses integrate the entire dissertation. Finally, possible future works are provided to end this dissertation