沉積物沉降速度對於高屏海底峽谷異重流結構影響

Hyperpycnal flow structure in Gaoping submarine canyon -sensitivity to settling velocity

蘇政緯(Cheng-Wei Su)

指導教授 ： 陳世楠

國立臺灣大學/理學院/海洋研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.01482

摘要

此研究利用三維的海洋數值模式 (Regional Ocean Modeling System, ROMS) 探討當高屏溪口達到異重流產生條件時(河口流量固定3000 C.M.S，沉積物濃度為60 g/L)，不同沉積物沉降速度對於高屏海底峽谷內異重流質量分布、質心位置及速度垂直結構的影響。首先我們將所使用之數值模式與實際觀測相互驗證，包括海面高度變化、速度斜壓結構、溫度垂直剖面。驗證結果皆呈現良好關係性，也顯示出此模式能解析基本流場特性。
實驗中當沉降速度在0、0.1、0.5 mm/s範圍內，從表層及底部沉積物濃度分布比較結果顯示，大部分沉積物濃度分布於底部並沿著峽谷內往深海傳輸，此潛流特性與異重流符合。然而，當沉降速度達1 mm/s，高屏峽谷內底部無高濃度沉積物分布，顯示並非所有的異重流會潛入峽谷內。而無法潛入峽谷內主要是受到懸浮沉積物沉降至底部所需的沉降時間(Ts)縮短，而減短平流距離(D=U* Ts;U為異重流出河口的平均速度)，當平流距離比河口離峽谷口的距離小的情況下，異重流無法潛入峽谷內。此外，我們也探討異重流的再懸浮機制以及侵蝕作用，對其維持懸浮沈積物濃度與流場強度的影響。在忽略再懸浮機制的條件下，我們以簡化的數學模型預測峽谷內異重流的懸浮沈積物總量，我們發現此預測皆明顯低於數值實驗結果，因此，我們推測再懸浮與侵蝕是異重流在峽谷內維持動量的重要機制。
異重流具鼻狀的速度垂直結構是重要的特徵(近底床有較強的流速)。我們將異重流標準結構比對實驗中速度垂直剖面，定義出異重流維持其結構的最遠位置，並量化質心位置。兩者結果顯示高屏海底峽谷內，異重流結構維持最遠位置與沉積物質心位置相近。特別的是，我們發現所估計的平流距離遠大於質心位置，代表著平流距離並不是決定質量中心的主要因素。因此我們更進一步檢視異重流流速斜壓結構與質量通量，發現兩者在空間中有明顯梯度變化之區域，皆與峽谷地形彎曲或坡度變化處相關。綜合上述，我們認為異重流的速度結構主要可能受到峽谷內地形影響，造成速度的變化而改變底床剪應力、垂直紊流擴散、以及質量通量在空間上的分布，進而影響質量分佈與質心位置。

關鍵字

異重流結構 ； 高屏峽谷 ； 沉降速度

並列摘要

We use a 3D hydrodynamic model (ROMS) to investigate the structure and mass distribution of hyperpycnal flows in the Gaoping submarine canyon. We focus on the sensitivity to sediment settling velocity (ws = 0 to 1 mm/s). To simulate hyperpycnal flow events, suspended sediment concentration of 60 g/L and river discharge of 3000 m3/s is specified at the Gaoping river mouth. The numerical model is first validated by comparing model-derived sea-level, vertical profiles of velocity and temperature against observations. Basic flow characteristics is reasonably reproduced by the model.
From our numerical experiments with ws of 0, 0.1, and 0.5mm/s, hyperpycnal discharge plunges when entering the coastal ocean. The suspended sediment is concentrated near the bottom (within 1 m) and is transported offshore following the Canyon topography. These features are consistent with hyperpycnal flow. However, when ws exceeds 1 mm/s such that the sediment advective distance (D=U* Ts, U is average outflow speed, Ts is the settling time) becomes shorter than the distance between river mouth and canyon mouth, most of the hyperpcnal discharge deposits on continental shelf before reaching the steep canyon head.
We use a simple mass balance model to illustrate the importance of resuspension/erosion processes in maintaining hyperpycnal flow in the canyon. The simple model is based on a balance of mass input from the river and mass removal due to deposition. Without considering resuspendsion processes, the simple model significantly underpredicts the suspended mass in the canyon, suggesting that appreciable amount of sediment is resuspended by the hyperpycnal flow itself to maintain its gravitational force.
We also quantify the offshore penetration of the hyperpycnal flow. The bottom-intensified velocity structure (i.e. nose-shaped) obtained from prior laboratory experiments is used as a reference. The penetration distance is defined as the location beyond which the hyperpycnal flow deviates from the nose-shaped structure. The estimated penetration distance is consistent with the center of mass. The positions where hyperpycnal flow loses the nose-shape structure and where divergence of mass flux elevates appear to lock in with topographic features (such as canyon bending, slope changes). These results suggest that the structure and the fate of hyperpycnal flow in the Gaoping cayon is greatly influenced by the canyon topography.

並列關鍵字

hyperphycnal structure ； Gaoping submarine canyon ； settling velocity

參考文獻

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