Analysis of accretion and erosion changes in sandy coasts of Haizhou Bay in the past 40 years

Based on 47 remote sensing images from 1984 to 2022 and four sets of nautical charts from 1980 to 2014 in the Haizhou Bay, Jiangsu, East China, we calculated and analyzed the variations in the exposed intertidal area at three tidal levels (100, 150, and 200 cm), as well as the changes in underwater isobaths, and the spatial patterns of erosion and accretion. Furthermore, we discussed the influencing factors that drove these changes in the sandy coast. Results indicate that: ① From 1984 to 2022, the exposed intertidal area exhibited five fluctuations of increase-decrease. Using the second-phase construction of the Ganyu Port as the boundary, the intertidal area was decreased significantly in both the north and the south directions. Prior to the port construction in 2011, three fluctuations occurred, with a slight decrease in the north while remained stable in the south. After 2011, two fluctuations were observed, with a notable reduction in the north and a moderate decrease in the south. The decline in the north was mainly caused by reclamation and port construction, while in the south it was less affected by human activities. Aside from the impact of human occupation, both the north and south regions showed slight increases before 2011. After 2011, the north region continued to increase slightly, while the south region experienced a significant reduction in area. ② From 1980 to 2003, all underwater isobaths showed a landward retreat. The north region is characterized by weak erosion mainly, transitioning from weak erosion nearshore to weak deposition southward, while the south offshore region showed weak erosion. Between 2003 and 2014, the trend of isobath changes reversed, in which the 2 m and 10 m isobaths advanced seaward, while the 5 m isobath continued to retreat landward. The Ganyu Port construction and its southern extension experienced varying degrees of erosion, while localized nearshore erosion was observed in the north region. Other areas exhibited mostly deposition, with the most pronounced accumulation occurring offshore. ③ The weakening of hydrodynamic conditions caused by the construction of Ganyu Port's breakwaters and the obstruction of sediment transport from north to south were the primary factors leading to the relative stability and slight deposition in the northern intertidal zone, while the southern intertidal zone experienced significant erosion after the port construction. Regarding the underwater coastal slope, the spatial pattern of weak erosion in the north and weak deposition in the south before the port construction may be associated with the net sediment transport trend from northeast to southwest. After port construction, the southward extension of erosion from the port area and the overall deposition in other regions were likely influenced by dredging and reclamation projects.