Abstract
Jiaozhou Bay (JZB), located at Qingdao City, north China, is a semi-enclosed shallow bay that has undergone large-scale land reclamation and is suffering from a deteriorated water environment. Long-term evolution of JZB with respect of coastline, tidal prism, tidal dynamics, water-exchange capacity, and pollutant transport from 1863 to 2020 was investigated in this paper, using remote sensing images, historical charts, and a numerical model. The JZB was predominated by natural evolution from 1863 to 1935, during which the coastline barely changed. Thereafter, human intervention became intense and more and more natural tidal flats were replaced by salt ponds, aquaculture area, and reclamation very quickly. As a result, tidal prism, area of tidal flats, and area of JZB decreased sharply by 0.290 km3, 182 km2, and 223 km2, respectively, from 1935 to 2020, corresponding to annual decreasing rates being of 123 times, 10 times, 12 times, respectively, as that of before 1935. A numerical model showed that the residual current in JZB tended to be weaker due to the change of coastline and bathymetry, which is not favoring the water-exchange and pollutant transport, especially in the northeast of JZB. The basin residence time increased from 15.5 days in 1935 to 17.6 days in 2020, because of weaker residual tidal current and smaller tidal prism. Local residence time increased significantly near the area with large land reclamation, especially in the northeast and west of JZB. Distribution of dissolved inorganic nitrogen (DIN), in each year, which is the dominant pollutant in JZB, indicated higher DIN concentration and weaker transport along with reclamation. The research on JZB evolution over the last 150 years can provide useful suggestions for the decision-makers of the local government to improve the marine ecosystem. The systematic method to investigate long-term water environment evolution of JZB can be used to study other semi-closed bays.
Highlights
Coastal areas are the zone where human activities are most intensive
Long-term evolution of Jiaozhou Bay (JZB) over the last 150 years was systematically analyzed in terms of coastline change, tidal prism, tidal residual current, water-exchange capacity, and pollutant transport
Some local residual current eddies disappeared while new eddies appeared at the places with the greatest coastline change
Summary
Coastal areas are the zone where human activities are most intensive. Developed countries pay great attention to the utilization of coastal zones, especially bays. Large-scale land reclamation has changed the natural coastline and the area and tidal prism of bays, resulting in a deteriorated environment. This research emphasizes the long-term response of coastline, tidal prism, water-exchange, and pollutant transport to land reclamation in JZB over the last 150 years, and provides a reference for similar studies in other semi-closed bays with intense anthropogenic activities. MIKE 21, a very popular numerical model for simulating hydrodynamics, especially in bays and coastal zones, was applied here to simulate the tidal dynamics and pollution transport for each historical scenario. Tidal prism can be calculated as the volume between the spring high-water and the spring low-water based on the detailed bathymetric data in section “Coastline and Water Depth Extraction.”. Three runs to investigate the tidal current pattern, residence time, and pollutant transport, respectively, were carried out for each year in 1863, 1935, 1966, 1986, 2005, and 2020
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