本研究的目標為探討高屏溪河口峽谷區域之異重流 (hyperpycnal turbidity current; HTC)之成因，由於 HTC 較難以現場觀測方式進行研究，所以以 ROMS (Regional Ocean Modeling System) 數值模式模擬河水夾帶沉積物流入海後，河口沖淡水與泥沙出 海後與高屏溪出海口及峽谷海底地形間的交互作用。為給定模式中河流開放邊界中的沉 積物特徵，於高屏溪上游攔河堰進行逐時河水濁度變化觀測，並取水樣分析懸沙濃度與 沉積物粒徑分佈，再進行數據分析與品管後，求出高屏溪濁度與懸沙濃度之間的率定曲 線。本研究目標為了解河川泥沙出海後與海底地形間的交互作用，模擬河水夾帶沉積物 流入海後，源自於河川的沉積物與淡水會如何移動。由於 HTC 較難以現場觀測方式進 行研究，因此本研究以 ROMS (Regional Ocean Modeling System) 數值模式建構高屏溪 出海口及高屏海底峽谷周圍海底地形。在懸浮沉積物部分，為了貼近真實情況，本研究 最主要的模擬例子是以上述之率定曲線推估懸沙濃度，以此計算出 2015 年蘇迪勒颱風 期間高屏溪的懸沙濃度低於 20 g/l 以及懸浮沉積物 D50 粒徑(15.15μm) 之沉降速度 (settling velocity = 0.18 mm/s) 輸入數值模式中進行模擬。在模擬結果中，觀察到沉積物 隨著河流出海後會漂浮在海水表層形成表層羽狀流(surface plume)，亦會在河口與海底 峽谷之間大陸棚的海水底層中慢慢匯聚成 HTC 後沿著海底峽谷斜坡流入峽谷並且往深 海移動。本研究認為在高屏溪出海口周圍海底地形與高屏溪粒徑分佈有利於 HTC 形 成，因為海底峽谷地形限制(confine)HTC 的路徑與寬度，加上海底峽谷陡峭的斜坡，使 HTC 有足夠重力(gravitational force)得以持續往深海方向移動。
由於真實河川中含有不同粒徑的懸浮沉積物，為了探討不同粒徑之沉積物對於 HTC 的產生與維持會造成何種影響，我們嘗試了數個不同沉降速度的例子。在數值模 式模擬結果中可以歸納出:河水中沉積物的粒徑越小，河水流出海後底層的懸沙濃度越高，因為粒徑大的沉積物容易堆積在河道中。但是如果懸浮沉積物粒徑小至 4μm，大部 分沉積物會漂浮在海水表層形成表層羽狀流，導致底層的懸沙濃度降低。並且當河川中 沉積物沉降速度不同時，所產生 HTC 的垂直厚度與懸沙濃度也不同，但是 HTC 發生的 臨界懸沙濃度皆小於 20 g/l。

This research aims to understand the mechanisms controling the formation of the hyperpycnal turbidity current (HTC) around Gaoping river mouth and Gaoping submarine canyon in Taiwan. When river-borne sediment concentration exceeds Ccrit=40~45 g/l, the bulk density of the river plume becomes greater than that of the ambient seawater and the plume descends to the seabed as HTC (Mulder and Syvitski 1995). Observational evidences during the passage of tropical cyclones show the occurrence of HTC in the Gaoping Submarine SW Taiwan (Liu et al. 2012), which also broke subsea cables in the canyon (Gavey et al. 2016). We estimated the real time suspended sediment concentration (SSC) of Gaoping River (GPR) through calculating the amount of sediment load within a certain time period with the rating curve for the SSC at the GPR Weir. The SSC of GPR during Typhoon Soudelor was lower than 20 g/l, according to our field data. The observed grain-size distribution and SSC and the bathymetry of GPR and Canyon are applied to the model to examine the possibility of the occurrence of HTC. Because the submarine canyon confines the path and width of HTC, plus the steep slope of the submarine canyon, so that HTC has enough gravitational force to keep moving in the deep sea. There are several grain sizes of sediments in real rivers. In order to investigate the impact of sediment grain size on the occurrence of HTC, the cases with different settling velocities were simulated in the numerical model. The simulation results of numerical model are summarized in the following: When the grain size of suspended sediment is large, the SSC around the river mouth is small. The reason is that the sediments with larger settling velocity is more likely to settle on the river channel. The SSC of HTC increases while the grain size of riverine sediment decreases. However, if the grain size of sediment is smaller than 4μm, most of suspended sediments will flow on the sea surface as the surface plume and there will be no HTC. Furthermore, when the settling velocity of the suspended sediments in the river is different, the vertical thickness and SSC of the produced HTC are also different.

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