Based on Landsat images, the spatial information of major gulfs along the coast of China in 2010, 2015, and 2020 were extracted to explore the morphological change characteristics. Some evaluation indexes, such as shoreline utilization degree, gulf area, gulf shape index, and gulf centroid, were used to analyze the gulfs' spatio-temporal change features in detail. The research showed that: (1) The shoreline length and structure of major gulfs changed significantly between 2010 and 2020, and the length and retention rate of the natural shorelines decreased. (2) The development and utilization degree of the gulf increased, with the increase rate gradually slowing down, and most of the gulf was moderately developed. (3) The gulf area continued to shrink, with a slow-down shrinking rate, and the gulf area in the Bohai Sea shrunk the most and the fastest. (4) The morphological change trend of the gulf is different from north to south. The north tends to be complicated, while the south to be simple. (5) After 2015, the gulf's plane center of gravity tended to be stable, the displacement distance decreased, and the displacement velocity slowed down. Human activities, such as sea reclamation, are the dominant factors leading to gulfs' morphological change. The national policies and measures for sea reclamation management and control in recent years have initially shown their effectiveness in affecting the gulf's morphology changes. The gulfs along the coast of China should be functionally positioned and divided, with refined classified management to optimize the layout of the "Ecological-Production-Living" space in the gulf areas.

The rapid erosion and deposition of tidal flat in alluvial island in estuaries due to the variations of land-sea sediments discharge has become the focus of urban development. Chongming Island, the largest alluvial island in China, has become an extremely valuable reserve land resource in Shanghai. However, the tidal resource in this island possibly presented new variation due to intensive human activities in drainage basins and estuaries. Accordingly, based on topographic data and measured bathymetric data around Chongming Island from 1984 to 2016, we illustrated the variation process of tidal flats in the study area through ArcGIS analysis and mathematical statistical analysis, and discussed the main factors influencing the geomorphic evolution of tidal flats. The results mainly include: (1) The process of erosion and accretion in each section of tidal flat around Chongming Island was complicated and varied. Totally, during the past three decades, the areas of the northern, southern and eastern beaches increased by 170 km², 8 km² and 51 km², respectively. (2) Variation of the northern tidal flat was affected by land reclamation, which led to the shrinkage of the north branch and promoted the continuous growth of the tidal flat. (3) The deposition of Xinqiao channel and its northward movement to the island directly caused the expansion of the southern tidal flat. The eastern tidal flat was dominated by frequent reclamation projects. Owing to the continuous outward movement of the embankment, the tidal flat expanded greatly in the past decades.

The bay is a unique strategic and ecological space in the coastal zone. Under the influence of global warming and intensified human activities, the shape and ecosystem of the bay have become more sensitive, fragile, and changeable. It is particularly necessary to clarify the temporal and spatial differentiation characteristics of wetland evolution in the bay areas under the influence of human activities and to find out the root cause of the wetland resource endowment. With the increase of satellite remote sensing data and the development of geographic analysis platforms (i.e., Google Earth Engine, GEE), rapid, low-cost, high-precision continuous monitoring of the evolution of the land surface becomes possible. The analysis of the temporal and spatial characteristics of the evolution of the bay wetland is of great significance for clarifying the resource and environmental base of the bay, understanding the comprehensive development potential, and promoting the sustainable development of the bay area. Taking the Sanmen Bay Wetland (SBW) in Zhejiang province as the study area, using Landsat TM/ETM+/OLI remote sensing image data from 1984 to 2019, based on the change characteristics of the spectral index time series, we extracted the information of the study area to examine its dynamic changes. The results show that the area of SBW has changed significantly since 1984, mainly found in the south of Ninghai county, the northeast of Sanmen county, and the southwest of Xiangshan county. In terms of the area, there was a fluctuating downward trend from 1984 to 2002, an accelerated downward trend from 2003 to 2009, and a steady development trend from 2010 to 2019. Human activities, such as reclamation, have caused significant changes in the shape of the bay, and the supply process of coastal zone management policy has become one of the important factors in the evolution of the bay wetland. The research can provide decision support for the construction of the bay area and the sustainable development of coastal areas in the future.

Coastal erosion is one of the major challenges in coastal zones. Coastal landscape, tourism and leisure functions have been seriously affected by coastal erosion in many tourist sand beaches. Artificial nourishment on regular tourist beaches is of great importance for maintaining beach grade and promoting sustainable tourist development. Based on analysis of coastal dynamics, sediment and beach conditions, as well as comparison of remote sensing images and beach profiles, erosion conditions and origins of erosion are examined on the east section of Sanya Bay, Hainan Island, China. Artificial beach nourishment that does not shift coast nature was proposed for restoration and preservation of the beach, concerning the scenic urban area. By analyzing beach landform, dynamics, native and borrowed sand conditions, we designed a beach nourishment project, with the goal of proving resistivity to erosion and increasing the material sediment supply of the beach. Comparison of satellite images and fixed beach profiles before and after beach nourishment shows that the beach was restored and broadened as designed. Profiles measured one and a half years after nourishment show that the nourished beach face and berm were relatively stable, without an obvious increase in the rate of sand loss. The east section of Sanya Bay is a relatively calm area sheltered by the Luhuitou Peninsula. Beach erosion is mostly induced by a decrease in foreign sand supply and enhancement of local dynamic forces due to sandbar reclamation, construction of hard structural revement and artificial islands. Weakening dynamic forces to reduce sand loss and/or adding an effective supply of foreign sediment by artificial measures are necessary ways to restore and protect the beach. However, hard structures like groins and breakwaters will have a negative effect on both the coastal landscape and tourists' safety. The proposed beach nourishment project does not modify the beach nature, which is a reliable choice for beach restoration in Sanya, a famous tropical tourist city in China. Practice of the beach nourishment project indicates that beach nourishment with suitable location, range and borrowed sand following the natural coastline, landform and hydro-dynamics is effective for restoration of beach erosion of the east section of Sanya Bay. However, beach nourishment is not efficacious forever. Sand loss is still inevitable after nourishment, because the hydrodynamics have not changed. Further sustained re-nourishment is necessary to maintain the dynamic stability of the beach. Meanwhile, no strong typhoon hit the area during the surveys after nourishment, further response of the beach to storm waves are to be concerned.