The Fudu Estuary Sandbar Lagoon is one of the most representative sandbar-lagoon landforms in China, and has undergone drastic evolution in recent years, accompanied by increased coastal engineering activities. The evolution process and its control factors are studied through remote sensing interpretation and coastal sediment transport calculations. During 2010–2021, the sandbar quickly extended at an average speed of 49.5 m/a, but the annual growth has shown a decreasing trend in both area and width, and the shoreline has retreated by 25–45 m. The recent changes are the result of the combined action of natural conditions and human activities. Coastal sediment transport from west to east under the action of W-oriented waves is the natural cause of extension. An estuary dam and artificial island block the sediment transport path, and the material for the new growth of the sandbar comes from the erosion of its west side, which has directly caused the retreat and narrowing of the sandbar. The reduction in sediments from the river further aggravates the shrinkage. It is predicted that the sandbar will continue its eastward extension to connect with the coast in about 2–3 years. The erosion status is unlikely to change before the sediment supply is restored. Measures such as dismantling the estuary dam are recommended.

The contradiction between the development of marine resources and ecological protection in Zhejiang is prominent, which restricts the high-quality and continuous supply of marine ecological products. As a result, the efficient maintenance of the ecological security pattern of inshore waters relies on the scientific division of marine ecological function zones and the management and maintenance of zoning areas. This paper constructs a technical framework for marine ecological function zoning from the aspects of marine ecosystem health, marine ecosystem diversity and economic and social development balance using GIS-based overlay analysis and the three-dimensional matrix discriminant model to divide marine ecological function zones in the managed waters of Zhejiang Province. The results indicate that the managed sea areas in Zhejiang Province are divided into ten marine ecological function areas, including three marine ecological supply areas (54%), four marine mariculture and resting areas (24%) and three marine ecological restoration areas (22%). As per the zoning results of the marine ecological function, this study further proposes protection and management strategies for different zoning units, in the hopes of providing technical support and practical reference for the division and management of marine ecological function areas in other sea areas.

Grain size and element content of surface sediment from Bohai Bay were analyzed to study the sediment migration and diffusion based on grain size trend analysis (GSTA) and discriminant function (DF). The sediment in the southern, central and western part of Bohai Bay mainly originates from the Yellow River, while that in the northern part of Bohai Bay mainly originates from the Luanhe River. The influence boundary between the Yellow River and the Luanhe River is estimated to be at 38°50′ N. In both the southern and northern parts of Bohai Bay, sediment is transported into the bay under the influence of prevailing waves, strongest waves and tidal remnants, resulting in sediment accumulation in western Bohai Bay. The coastal sediment of Bohai Bay is generally in a state of offshore movement, which is consistent with the large-scale coastline retreat in past decades found by previous studies.

Waterline extraction is a key step in applying the Waterline Detection Method (WDM) to Digital Elevation Model (DEM) generation. Cloud interference remains a major challenge for achieving high-quality extraction of waterlines. This study developed an image filtering method termed “Cloud Coverage in Region of Interest” (CCROI). By integrating the CCROI method with the Otsu algorithm and noise smoothing techniques, this study enabled high-quality batch and automated extraction of waterlines within the Google Earth Engine (GEE) platform. Using the WDM, DEMs were established to evaluate recent geomorphological changes in the estuarine tidal flats of the abandoned Diaokou Course (ETFADC). The results confirm that the erosional trend of the ETFADC has persisted throughout nearly 50 years of natural adjustment. In areas distant from oil extraction zones, erosion dominates the high-tide zone, while accretion prevails in the low-tide zone, indicating a slope-flattening process. However, in areas near the oil extraction zone, tree-shaped embankments have acted to inhibit erosion rather than exacerbate it, with strong accretion even occurring in wave-sheltered areas. By enhancing the quality of the selected images and reducing the waterline false detection rate, the CCROI method demonstrates significant potential for time-series studies of small regions.