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|Title: ||內陸棚及河口混合與擴散特性觀測研究;Observation of mixing and dispersion characteristics in inner shelf and estuary|
|Authors: ||鐘耀照;ZHONG, YAOZHAO|
|Keywords: ||內陸棚;河口;混合與擴散;觀測;inner shelf;estuary;mixing and dispersion;observation|
|Issue Date: ||2019-04-02 14:21:07 (UTC+8)|
（2）基於Stokes Boundary Layer理論，提出了一個新的內陸棚海域混合與擴散研究模型，作為理論框架。該理論中描述邊界層內速度剖面變化的參數κ1與描述流速反轉時間提前的參數κ2是兩個可用於分析海域混合效率的中間參數。進而本研究設計現場實驗檢驗並建立了一個更具可行性的內陸棚海域混合與擴散行為觀測方法。在桃園觀音近岸海域進行的不同海況下的漂流浮球陣列觀測實驗，證實理論所預測的潮流轉向時間早於水位轉向時間，隨著離岸距離越近時間提前量會增加；沿岸方向之流速會隨著離岸越近而快速下降。以漂流浮球陣列實測資料代入理論擬合得到不同時空下的κ1與κ2組合。這兩個參數顯著相關，κ2=6.02*κ1-0.0008（r = 0.97, p < 0.05）。進一步直接計算得實驗海域的υ，結果發現速度衰減項引致的υ約為相位差異項引致的3至12倍，二者均隨量距離的增加成指數形式降低。透過對比不同海況下的擬合結果，本文發現當波浪較小時，Stokes Boundary Layer理論的解釋度佳，說明此時潮流在近岸混合中扮演主要的角色；當波浪較大時，Stokes Boundary Layer理論的解釋度降低，是由於波浪引致的底邊界層增加了底床粗糙度，額外增強了混合強度所致。
基於溫排水口航次調查之三維溫鹽資料分析顯示溫排水口漲潮期間層化效應強，退潮期間垂向均勻混合。基於溫度漂流浮球量測之水溫資料，在垂向均勻混合時溫排水口的溫度衰減速率e-folding scale是層化效應強時的約2倍。基於漂流浮球陣列觀測資料計算的溫排水口剪力流延散係數，發現漲、退潮期間的剪力流延散係數差異可達26.0%。本文透過溫排水口漂流浮球陣列觀測實驗量化了剪力流延散係數隨著Richardson數變化的趨勢。;As the boundary area between land, sea and atmosphere, the coastal zone is the main channel for terrestrial materials transferring into the ocean. The balance of the material cycle will be changed once the terrestrial materials exceed the capacity of the ocean self-purification, which will cause ecological pollution problems. Among the factors affecting the capacity of self- purification, the most important factor is the mixing and diffusion efficiency of water. The near-shore hydrodynamics as well as the mixing and diffusion characteristics are unsteady in time and spatially non-uniform. As a result, it is very difficult to grasp the detail information of mixing and diffusion behavior by theory analysis or by numerical model. If effective observations can be applied to improve the understanding of the mixing and transport processes of terrestrial materials in coastal waters, the evaluation of diffusion efficiency of terrestrial materials and its interaction with hydrodynamics will be more accurate.
According to the classification of landforms, the coastal zone includes beaches, wetlands, estuaries and lagoons. From the perspective of hydrodynamic characteristics, the coastal zones can be divided into surf-zone, inner shelf, middle shelf and outer shelf. For air-sea flux research, wetlands and lagoons have important impacts; for the study of the mixing and diffusion of terrestrial materials in the sea, estuaries and inner shelf play important roles. The study of mixing and diffusion of water has traditionally started with the exchange rate and residence time of water. However, these coefficients can neither represent the details of the estuary mixing behavior, nor represent the influence of the stratification effect. This paper develops the sea surface drifter array as the main observation tool to carry out researches on a) the mixing and diffusion characteristics of the inner shelf and, b) the influence of the stratification effect on the estuary mixing, specific as follows.
(1) This study developed the miniature sea surface drifters with the commercial low-cost GNSS positioning chip. On account of shallow water depth and small hydrodynamic temporal-spatial scale of inner shelf or estuaries, this study improves the positioning accuracy, increases the sampling frequency, lower the production cost, reduces the volume and weight, and improves the data post back method and interaction operation system of sea surface drifter. Aiming at the problem of positioning jumping under low speed, this study established a data processing and quality control method based on Auto Regressive Integrated Moving Average model. The influence of this problem is eliminated and the data quality of sea surface drifter is improved after applying this QC method. All the drifters are calibrated in the 200-m length water channel and the RMSE is ~0.02 m/s compared with the Acoustic Doppler Velocimeter which is installed in the middle of the water channel.
(2) Based on Stokes Boundary Layer theory, a new research framework of inner shelf mixing and diffusion is proposed. The parameter κ1 describing the change of velocity profile in the boundary layer and the parameter κ2 describing the current reversal time are two intermediate parameters that can be used to analyze the mixing efficiency of the sea area. And then this study designed the field experiments and establisded a more feasible observation method of mixing and diffusion behavior in inner shelf. This paper conducted several drifter observation experiments under different sea conditions in the coastal waters of Guanyin, Taoyuan City. It was found that the time of tide reversal is earlier than that of water level reversal, and the time difference increase as the distance to the coast decreases. The alongshore current velocity decreases rapidly as it gets closer to the coast line. Above-mentioned two parameters, κ1 and κ2, were fitted by the field drifter data. These two parameters were significantly correlated, κ2=6.02*κ1-0.0008 (r = 0.97, p < 0.05). Further, the eddy viscosity coefficient of the study area was calculated based on these two parameters. It is found that the eddy viscosity coefficient caused by the velocity attenuation term is about 3 to 12 times that of the phase difference term. Both of them decrease exponentially with increasing distance to the coast line. By comparing the fitting results under different sea conditions, this study found that the Stokes Boundary Layer theory has a good interpretation when the waves are small (Hs < 0.3 m), indicating that the tide also plays a role in coastal mixing. When the waves are large, Stokes Boundary Layer stays at the lower degree of interpretation.
(3) This study selects the Danshui estuary and thermal discharge outlet of No.3 nuclear power plant as the research areas to explore the influence of stratification effect on mixing at different temperial and spatial scales. A number of voyage surveys were conducted with the OR-2 research vessel around Danshui estuary. The 2202 voyage conducted in October 2016 was selected as a representative example of strong stratification condition and the 2225 vyage conducted in March 2017 was selected as an example of verticle well mixing condition based on the 3-D water temperature and salinity observation data. It is found that the decay rate of nutrient concentration under weak stratification condition is about 2 times of that under strong stratification condition in the Danshui estuary. The decay rate of DIN concentration of Danshui estuary is 6% to 46% larger than that of the Pearl River estuary, and the dispersion coefficient of Danshui estuary is one to two orders of magnitude larger than that of an equivalent estuary. The stratification effect of the Danshui estuary will hinder the upward transfer of nutrients and the downward transfer of dissolved oxygen, and then changes the nitrogen cycle of estuary, which will lead to ecological problems such as the decline of primary productivity in the estuary. This study provides observational evidence.
The analysis of the 3-D water temperature and salinity data shows that the stratification effect of the water around thermal discharge outlet is strong during flood tide and weak during ebb tide. The temperature decay rate under weak stratification condition is 2 times faster than that under strong stratification condition based on the e-folding scale. The shear dispersion coefficient is calculated based on the drifter data. And it is found that the difference of dispersion coefficients under different stratification strength can reach 26.0%. This study quantify the variation trend of dispersion coefficient with the increasing Richardson number.
|Appears in Collections:||[水文與海洋科學研究所] 博碩士論文|
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