| dc.description.abstract | 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. | en_US |