Abstract

A well-validated 2D hydrodynamic and salinity transport model is established to simulate the influence of the Deep-Water Channel (DWC) project on the salinity front in the North Passage (NP). The main finds: (1) The DWC increases the salinity due to the salt-water intrusion from the South Passage (SP) and ebb-tidal dominant in the NP. (2) The NP salinity front shows a double-front pattern, the positions of the two fronts both move upstream after the DWC. (3) After the DWC, the upstream front weakens while the downstream one strengthens.

Introduction

The Yangtze River, has a length of 6380 km and a catchment area of 1.8×106 km2, making it the most important river in China. Annually it discharges a tremendous amount of fresh-water (~9.004 × 1011 m3) into the Yellow and East China Seas (Fig.1A), and the fluvial flow rate can reach 60,000 m3/s in summer. Under these conditions, there will be an obvious plume phenomenon at the Yangtze River Estuary (YRE). The phenomenon such as increasing eutrophication and harmful algal blooming as well as hypoxia often occur in the Yangtze River plume area (Wu et al., 2014). The Yangtze River plume has a large range and can extend for hundreds of kilometers, there will be a front at the center of the plume, which is often called the salinity front. The hydrodynamic environment around several kilometers near the front will change greatly, and the position of the front is usually determined by the maximum salinity gradient (Mao al et., 1963). The density difference near the salinity front will form a barrier effect, which will affect the matter transport and exchange. The observations show that the migration of the underwater delta is consistent with the variations of the salinity front in the YRE, which indicates that the salinity front has a certain influence on the sediment transport and the underwater delta evolution. (Chen et al., 2001, Mao et al., 1963). Abundant microorganisms and pollutants gather near the salinity front, which is conducive to the development of harmful algal and has a serious impact on the water environment (Ning et al., 2004, Huang et al., 2011). The observations show that the spatial distribution of higher chlorophyll-a concentration corresponds well with the distribution of the salinity front (Kim et al., 2009). The increase of pollutants, suspended sediment and harmful algal near the salinity front aggravates oxygen consumption and hinders oxygen exchange, which is considered to be a major factor causing the formation of hypoxia regions and can cause a massive fish mortality around the YRE (Rabouille et al., 2008, Moon et al., 2009). According to these previous studies, the runoff and tide are major forces controlling the salinity front, however, the salinity front is also affected by human activities, storms and climate change (Wu et al., 2011, Wu et al., 2014, Kao and Lagerloef, 2014).

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