Challenges to estuarine freshwater resources and mechanisms of saltwater intrusion under large-scale reclamation projects

Abstract

Hengsha Dongtan, with a reclamation area of ~ 106 km² initiated in 2006, is a key ecological agricultural development zone in Shanghai. However, freshwater intake for agriculture along its northern margin is significantly constrained by saltwater intrusion. Based on three months of field observations and ECOM-si, this study investigates the spatiotemporal patterns and driving mechanisms of saltwater intrusion under varying guaranteed flow rates and cold front conditions. Results demonstrate that during the mid tide after neap tide (MTAN), seaward water flux decreases by 55%, marking a high-risk period for saltwater intrusion. In contrast, saltwater intrusion is weakest during the mid tide after spring tide (MTAS), which favors freshwater intake. Under extremely dry year conditions, the allowable freshwater intake period at three intake points decreases to 12.9, 6.4, and 2.1 days, respectively. Cold fronts intensify saltwater intrusion through the North Channel (NC), significantly reducing the availability of agricultural freshwater. Salt flux decomposition reveals that landward advective transport driven by strong northerly winds is the dominant mechanism of saltwater intrusion. In addition, the Hengsha Shoal reclamation project, currently under construction with a planned area of about 300km², is expected to alter regional hydrodynamics. Simulation results under climatological conditions indicate that the project would weaken saltwater spillover (SSO) from the North Branch (NB) and direct saltwater intrusion from the NC, thereby improving freshwater intake conditions at source areas in the South Branch (SB).