Analysis of hydrodynamic characteristics during the occurrence of red tide in the Qinhuangdao sea area

The Qinhuangdao coastal area in northern China is a significant site for aquaculture and coastal tourism. However, frequent red tides have adversely affected water quality and fishery resources in recent years. This research investigates the hydrodynamic characteristics of the area by conducting continuous seabed observations during the summer of 2019 and analyzing historical data. The goal is to identify potential impacts on bath water quality and red tide disasters. The findings indicate that Qinhuangdao falls under the diurnal tide sea area category with a small tidal range. The northern and southern regions experience regular and irregular diurnal tides, respectively, with tropic and equinoctial tides occurring every half a month. The prevailing tidal current type is an irregular semidiurnal tidal current, with M2 tidal current prevailing along the coast, exhibiting weak power and slightly higher offshore velocity. During the summer, the residual current velocity is relatively small, mainly flowing northeastward along the isobaths, while the surface residual current in the southeast of Beidaihe flows toward the shore. The area has several rivers with high summer runoff, resulting in pollutants from Tanghe, Daihe, and Yanghe river estuaries being easily transported to the beach, adversely affecting water quality and the environment. Pollutant accumulation on the north side of the bath, near the harbor area, is likely due to weak water exchange and offshore residual current inflow, which may trigger red tide outbreaks.