Preliminary Studies On The Features Of Tidal-dynamic Structure In A Typically High Density Mariculture Coastal Bay-Observation And Simulations

Sanggou Bay is a main Chinese mariculture sea area along the Yellow Sea. In resent years, there has an alternative feature of tidal-dynamic structure induced by large-scale and over-heavy mariculture. Therefore further researches on the feature as well as the mechanism of the effect of mariculture on tidal-dynamic structure are fundamental to understand the relationship between carrying capacity and environment in this bay. In this paper, one of the main purposes is to analyze the feature of the vertical structure of tidal current from field observing, another is to discuss its mechanism numerically through a modified 1-D double-drag hydrodynamic model.In the present study, two companies of field observation are performed in spring and summer of 2006 respectively. There are totally 5 study stations schemed in all kinds of aquaculture region as follows: Xunshan, Chundao, Anchor station, middle of bay mouth and middle of inner bay. Basic to water level data in both Xunshan and Chudao and all ADCP and ADV data, harmonic coefficients of 6 main components of tide and tidal current and elliptic elements of tidal current are achieved by means of the quasi-harmonic analysis. Features of the harmonic coefficients of tide, vertical structures of tidal current, total kinetic energy of water column, both friction velocities and drag coefficients in tidal upper boundary layer and bottom boundary layer, distributions of eddy viscosity and the shear stress, TKE dissipation rate and the Reynolds stress near the sea bed in the two seasons are analyzed. As a result, Sanggou Bay is dominated by irregular semidiurnal tide and regular semidiurnal tidal components with tidal current flowing back and forth. Only tide speed is decreased by large-scale mariculture, rather than the character of tidal wave. The vertical velocity gradient occurs in upper water column because of plenty of mariculture organisms and culture structures. It changes the vertical structure of tidal current evidently to form a peculiarly tidal upper boundary layer. In the layer, the speed obeys law of the wall, whose logarithmic profile is obtained on integration. The drag coefficient of upper boundary layer C DS exceeds that of bottom boundary layer C DB, but both of them are of 10-2 magnitude, 1 order of magnitude higher than that in natural area. The roughness length is of 10-2~10-1m magnitude, 3~4 orders of magnitude higher than that in natural area. Significant 1-hour earlier-ebb-and-earlier-flooding appears in surface. In the entire bay, the peak tidal current occurs 1 hour earlier in the bay mouth than that inside. Eddy viscosityνT is 1~2 orders of magnitude higher than that in natural area, and TKE dissipation rateεis 1~2 orders of magnitude stronger as well.On the other hand, a modified 1-D double-drag hydrodynamic model is established to simulate the vertical structures of both tidal current and the corresponding shear stress in the kelp culture region, whose input parameters can be obtained from field measurements. Based on this model, the effects of mariculture on the vertical structure of tidal current are discussed through adjusting the values of the averaged drag coefficient in tidal upper boundary layer C DS, the averaged drag coefficient in bottom boundary layer C DB or eddy viscosityνT. As a result, tidal-current speed has a nonlinear tendency to change with the shear stress, because of the nonlinearity in shallow sea. Mariculture and bottom stress have an effect on the vertical structures of both tidal current and eddy viscosity. When the aquaculture density becomes lower, the aquaculture drag decreases resulting in the upper-layer water flows rapidly, thus having a thinner tidal-current upper boundary layer and a thicker bottom boundary layer. When the bottom stress is gradually lower and lower, the bottom-layer water speed up, so the bottom boundary layer is thinner and the upper boundary layer is thicker. The increasing aquaculture density blocks the outflows but speed up the inflows. On the contrary, the increasing bottom stress blocks the inflows, while speed up the outflows. The magnitude and the vertical structure of tidal current are affected by the eddy viscosity.