Numerical Simulation Of Tidal Dynamics In The Zhoushan Archipelago Waters

Thousands of islands in the Zhoushan Archipelago(ZA)region cause the complex coastline and form many narrow channels with deep water,which have a great impact on the local tides and tidal currents.Based on a high resolution three-dimensional ROMS model with a two-way nesting,this paper simulates the tides in the Hangzhou Bay and ZA region.After validation for the model results,the spatial structure of tides and tidal currents are presented.Tides in this area are dominated by irregular semi-diurnal tides around the islands,and regular semi-diurnal tides in the relatively open area.The tidal currents are mainly appear as regular semi-diurnal.Affected by topography,the amplitudes of M2 and S2 constituents are significantly reduced in the channels,while there is no obvious effect on the K1 and O1 tides since their amplitudes are much smaller than that of M2 and S2.The current velocity increases rapidly in the channels due to the sharp reduction of cross-sectional area,forming jet-like flows.Near the islands,there are many eddies,showing the strong tidal non-linearity.Therefore,the tidal energy dissipation in this area is high and with most hotspots in the channels.About 46.4% of the total incoming tidal energy is dissipated in the region and 67.3% of the dissipated energy is consumed by the bottom friction.Analysis of momentum balance averaged over flood or ebb period demonstrates that the primary balance near the islands is between the horizontal advection(HADV)term and the pressure gradient force(PRE),while in the area away from the islands,these two terms become less important.Moreover,the spatial distributions of HADV and PRE terms are basically consistent during flood and ebb period,respectively,with positive and negative values interlaced around the islands,indicating there is a great fluctuation in tidal current and water level around the islands,also there are many low or high value centers of water level among the islands throughout the entire tidal cycle,which may be mainly caused by the Bernoulli effect,and the eddies affected by spatial change in water depth may be another reason.Finally,we discuss the generation mechanism of M4 and M6 constituents in this area by six additional numerical experiments.It is found that these two shallow water constituents are mainly generated by the nonlinear advection,the bottom friction plays a minor role,and the interactions between different constituents also have some influence.