陸棚含沙異重流之非靜水壓數值研究

Abstract

本研究利用三維高解析度沿岸海洋模式“SUNTANS”配合Chen et al. [2013]所設置之大尺度的理想地形來模擬並分析含沙異重流(hyperpycnal plume)在不同斜坡上的運動行為。為特別了解含沙異重流在潛入區附近的動力情形，我們設置河口附近的網格較為細密。另外，為了模擬真實的洪水案例，我們將河道的入流設置為2 m ，河川的泥沙濃度為50 g 。 經過初步比對，模擬的異重流速度剖面圖與捲水係數和前人的實驗結果一致。而模擬的結果顯示，非靜水狀態的壓力(nonhydrostatic pressure)在潛入區相當重要，若僅利用靜水狀態近似(hydrostatic approximation)來模擬此一區域異重流的運動行為則結果會與實際的物理量有所差異。此外，我們探討了不同坡度下異重流的運動情形並且發現在潛入區所計算得到的捲水效應在坡度越陡的狀況下會顯得更加強烈。透過能量收支分析，我們初步了解了含沙異重流在不同坡度下運動時的各種能量貢獻情形與分佈，並比對在利用靜水壓近似及考慮非靜水壓力時模擬結果的差異。

A three-dimensional high-resolution nonhydrostatic coastal model, SUNTANS, is used to study the dynamics and structure of turbidity currents on sloping continental shelves with an idealized domain setup (Chen et al. 2013). To focus on the plunging region, the grid resolution is refined near the river mouth. The sediment concentration is specified as 50 g at the river inflow with 2 m inflow velocity to simulate the real mountainous river systems during different flood events. The model derived vertical velocity profiles and entrainment rates are consistent with the prior laboratory experiments. Compared with the hydrostatic modeling, our results show that the nonhydrostatic effect is especially important in the plunging region, while it is insignificant in the body region. We also apply three different slopes of the continental shelves in the simulation to understand how the slope change will affect plunging dynamics of the turbidity current. Moreover, the entrainment rate near the plunging region is examined with different slopes and the analysis of energy budget is conducted to investigate the spatial and temporal evolution of kinetic and potential energy associated with plunging.