Three-Dimensional Tidal Dynamics In A Bended Deep Channel With Multiple Islands

The Luotou strait, located in the Zhoushan archipelago, is in the east-west direction, connected with the Jintang channel and the Cezi channel in the northwest and the Pushen channel, the Fulimen channel in the southeast, among the Damaoshan island, Zhairuoshan island, Daxie island, and Chuanshanban island. It is about 17 km long, and 4.5 km wide (the narrowest about 1.6 km) with water depth varying from 35 m to 116 m. The Luotou strait is one of the most important channels in Ningbo-Zhoushan port core area, and one of the main tidal channels connecting HangzhouBay and East China Sea. For the complicated shoreline and bathymetry, the tidal characteristic in the channel is quite dynamic and requires a detailed investigation.This study applies a boat-mounted Acoustic Doppler Current Profiler (ADCP), a sub-bottom profiler, and a CTD sampler in two field surveys covering spring and neap tidal cycles to obtain tidal current, salinity, temperature, turbidity, etc in the Luotou strait. Meanwhile a 3-D high resolution numerical model based on FVCOM including the Yangtze estuary, Hangzhou bay, the Zhoushan islands and the East China sea is established to study the tidal dynamic characteristics of the Luotou strait.By analyzing the measured and numerical data, the main research results and conclusions are as follows:1) The measured salinity and temperature gradient in surface and bottom layers show that the water is weakly stratified during both spring and neap tide in this curved deep water channel. Due to centrifugal and Coriolis forces, tidal current comes into the bay along the northern side of the channel during flood tide and retreats along the southern side of the channel during the ebb. During the flood, tidal velocity in the lower layer is larger than that in the surface layer, because of the sea water intruding into the bay with a higher density. While during the ebb, fresher and wormer water in the bay retreats into the East China sea in upper layer. The near bottom temperature and salinity vary in a combined frequency of semi-diurnal and higher, which is in consistent with the frequency of current variation. The salinity and temperature variations in this curved tidal channel are highly impacted by the tidal current, residual current, and other aspects.2) Simulation results show that due to the constraints of terrain and coastline tidal current comes into the bay along the northern side of the channel during flood tide and retreats along the southern side during the ebb, as observed in the field. In the lower layer, tidal velocity is constrained to the deep channel. Tidal velocity in the surface is larger than velocity in the lower layer. Because the channel has a large curvature as a main tidal passage in the islands, there is a cross-section circulation phenomenon in the section with a large curvature. The centrifugal force might play an important role in flow pattern.3) Impacted by island, this area has many eddies and circulations. There is a strong residual current in this archipelagic region, which can reach up to 0.62 m/s. Islands, complicated shorelines, and varying bathymetry have major impacts on the nonlinearity of tidal wave. The surface, middle, and lower layers of the Luotou channel have anticlockwise circulations of residual current. The flood residual current is dominant in the northern side of Luotou channel, while the ebb residual current dominant in the southern side of Luotou channel.