由於河口是聯繫海洋及河川之間的通道，都會區之廢水經由淡水河口匯流入附近海域，必然影響附近生態及環境，因此，若能了解河口環流之動力機制及其對沈積物分佈之影響，對河口海域環境及海岸保護有相當大的幫助。本研究收集了淡水河口及近岸之潮位與海流、淡水河之流量等，配合人照衛星影像來分析水舌之擺盪情況，並建立淡水河口三維斜壓海洋模式，利用數值模式模擬各種物理因素對淡水舌之影響，透過不同大小之河水流量以及外海長期平均流對河口淡水舌的影響分析，以供探討淡水舌之分佈與影響其變動之動力機制。 本研究針對河水流量、潮流、平均流以及風，利用數值實驗，近一步探討，淡水舌變動情形。研究結果顯示，淡水舌厚度約5~6公尺，河口外流淡水在最低潮後2小時水舌向西南擴散範圍最廣，約可達到6~7公里，而最高潮後約1~2小時淡水舌東北向流速最大，直到轉為退潮流後，淡水舌亦轉向東北移動。模擬結果與海流觀測、衛星遙測可見光影像吻合。河水流量在大流量(500 cms，cms=m3/s)時離岸距離比基本模擬(90 cms)遠將近2~3倍，而低流量(30 cms)時，受到潮汐往返的影響，使得淡水被侷限在河口，淡水舌無法在海面上形成。推估淡水河流量大於50 cms時，海面上之淡水舌才比較容易被肉眼觀測到。當淡水河上游因暴雨產生一洪峰流量時，模擬經過17小時的延遲後，淡水舌鹽度開始有劇烈變化，之後歷經70小時左右的鹽度變化，淡水舌之鹽度以及擴散範圍才慢慢恢復至初始情況。外海平均流的作用是將淡水舌持續向平均流的方向推送，這也是將河口污染物質推走的主要機制。利用加入洪峰流量(300 cms)的方法，計算河口淡水滯留時間，得到淡水河口淡水之滯留時間為72小時。透過風應力之數值實驗，在風速10 m/s的強風(東北風)影響下，淡水舌向西大幅延伸，海表面流速向西南方向可增加一倍。在平均風速4.5 m/s的西南風作用下，退潮時淡水舌明顯貼著右側海岸，無法離岸向外擴張，但漲潮期間影響則不大。 Since a considerable amount of sewage from Taipei metropolis is discharged into the lower reach of the Danshuei River, it may significantly affect the estuarine environment and ecosystem. To understand the circulation in the estuary and the adjacent coastal zone is crucial to their environmental protection. For this study were collected tidal levels, ocean currents, river discharges, and satellite images for analysis of the shape and variation of the Danshuei River plume and understanding of its driving mechanisms. A three-dimensional model is adopted to facilitate the investigation of the river plume and estuarine circulation. Results derived from model results and observations indicate that the plume is of about 5 m thick. Two hours after low tide, the plume extends toward the southwest and can reach 6~7 km from the river mouth. The simulation is consistent with that revealed by satellite images. Discharge rate (500 cms, cms =m3/s), distribution of plume extends 2~3 times offshore than the control run (90 cms). During low discharge, the plume extends be visible in the sea. While the discharge is greater than 50 cms, the plume during a high farther can hardly becomes more visible in the sea. During a flood caused by the torrential rain, the model predicts that the salinity in the coastal zone begins to change dramatically 17 hours after the flood began. After about 70 hours, the salinity returns to nearly the initial level. Based on a model run using a peak discharge rate of 300 cms, the model results show that the residence time is on the order of 72 hours. Through the numerical study with wind forcing, at a northeasterly wind of 10 m/s in speed, the plume extends westward prominently and the surface velocity doubles toward southwest. Under a southwester (by wind of 4.5 m/s in speed), the plume was restricted to a narrow band along the right side coast during ebbing tide, but the wind has little effect on the plume during flood tide.
