A Model Study Of Tide And Its Residual Current In Xiangshan Bay, East China Sea

As the biggest aquaculture base in Zhejiang province, Xiangshan Bay drawsmuch attention for its protection of marine environment. Hence it is necessary toidentify the circulation pattern in the bay, which is supposed to be the basis of marineenvironment issues. The wind is weak in the bay and the rivers flowing into the bayare rather short, both of which contribute little to the overall circulation pattern. Thetidal residual current must be the dominant dynamic factor in the bay circulation sincethe tides are very strong and the coasts and topography in the bay are complex.However, resiticted by our knowledge of tidal residual current, people still use twodistinct methods to filte the tidal currents and to get the residual current, which are theLagrangian Residual Current (LRC) and the Eulerian Residual Current (ERC),respectively. This thesis aims to identify whether the LRC or the ERC should beregarded as the subtidal transport current in the bay through comparison betweenthem and the observed salinity distribution. This work is believed to benefit not onlythe marine environment protection in the bay, but also our understanding about tidalresidual current.The method used in this thesis is mostly numerical model. To guarantee theprecision of the bottom friction parameterization in the model and to advance ourunderstanding about the dynamics of Xiangshan Bay, the research group did fourtimes field observations in the bay. The data reveals that the dominating componentsof tides in the bay are semi-diurnal with two distinc characters. One is the reverse oftidal elevation asymmetry along the bay, the other one is that the peak of floodingcurrent could match that of the ebbing current, even though the tidal elevation ishighly asymmetric. Furthermore, there are one tiny and one big peaks during floodwhile only one during ebb. With those in-situ measurements, a one-dimensional tidalwave equation was established, in which the main balance is between the pressure gradient force and the local inertial term. The pressure gradient, which presents threepeaks during one tidal period, is the main reason for the different numbers of currentspeaks during flood and ebb.Based on the observation data, the author calculated the bottom drag coefficientaccording to the tidal dynamical balance equation and the turbulent boundary layertheory, which are0.81×10-3and0.17×10-3, respectively. Both of them are smallercompared to other reports in East China Sea, which results from the fine-sediment inthe bay. Although the two coefficients are in the same order of the magnitude, theformer one, which includes the form drag, is almost5times larger than the latter one.The form drag resulting from the complex topography must be fully considered whensimulates tides in Xiangshan Bay.With the observation results to improve the parameterization of the bottomfriction, and the tidal elevation data generated by OTPS as the driving force, thehigh-resolution Xiangshan Bay FVCOM model was established. The model did areasonal job in reproducing the observed tidal elevation and currents, which can beused in the study of tidal asymmetry and tidal residual currents in Xiangshan Bay.Firstly, this thesis discussed the specific role of each nonlinear mechanism whichis responsible for the tidal asymmetry in the bay. The model results suggest that thenonlinear mechanisms can be divided into two different categories, one includes theshallow water nonlinarity and the nonlinear advection, which prefer shorter durationof the rising tide, the other one is the time-varying width, which favors longerduration of the rising tide. The time-varying depth and nonlinear advection dominatearound the bay mouth, resulting in asymmetry of shorter duration of the rising tide.However, the time-varying width caused by large mudflat is the main nonlinearmechanism in the inner bay, which makes the tidal elevation asymmetry invert alongthe bay. The overtide M4generated by the two catagories are out of phase, whichcould counteract each other. The nonlinear bottom friction contributes little to thetidal asymmetry in the bay, while its dissipation effect could make the duration of therising tide shorter. Secondly, this thesis tried to identify the pattern of the tidal residual current in thebay. With particle tracking method and periodical average at fixed location, the modelsimulated the LRC and the ERC, respectively. The former one presents muchdifference with the latter one. The LRC splits into two branches, both of which canconnect the north and south parts of the sea outside of Xiangshan Bay. The eastbranch starts from the east bank of Niubi Channel, turns to northeast at the northwestside of Liuheng Island, and flows out of the bay along the east bank of Fodu Channel.The west branch flows into the bay along the bank of Fodu Channel, turns direction atthe mouth of Xiangshan Channel, and flows out of the bay at the west side of NiubiChannel. However, the ERC is characterized by multiple eddies, which cannotconnect the north and south parts of the sea outside of Xiangshan Bay directly.Furthermore, the direction of ERC at section A3-A5is irregular, while the LRCpresents much more consistent structure with the observed salinity distribution, whichflows into the bay along the east side of Niubi Channel and flows out along the west.Therefore, it is the LRC which can represent mass transportation at subtidal frequencyin Xiangshan Bay and the LRC can keep the mass conservation while the ERCcannot.The overall pattern of LRC is highly correlated with the bottom topography. InNiubi Channel, the LRC flows in and out of the bay along the two deep channelslocated on the two sides of channels, while directs out in the deeper areas and streamsin on the adjacent shoals in the Fodu Channel. The nonlinear advection contributesmost to this pattern of the LRC, while nonlinear bottom friction, shallow waternonlinearity and time-varying width have little influce. The bottom friction could alsoaffect the LRC pattern due to its dissipation effect, which may promote the exchangeof sea water between the two sides of Niubi Channel.