The assessment of coastal vulnerability to erosion is urgently needed due to increasing coastal erosion globally. Based on the coastal characteristics of the Xiamen artificial coastline, which accounts for more than 80% of the coastline in this area, this study provides an integrated approach based on a multi-criteria index. The evaluation index system of the local coastal vulnerability to the erosion of Xiamen includes 12 indexes based on natural (coastal characteristics, coastal forcing), and socio-economic factors (coastal infrastructure, disaster reduction). The spatial differentiation characteristics of the coastal vulnerability to erosion along the Xiamen coast (2018) have been quantitatively assessed with the aid of GIS (Geographic Information System) and RS (Remote Sensing) technology. The results show that the very high vulnerability, high vulnerability, medium vulnerability, low vulnerability and very low vulnerability areas of coastal erosion accounted for 4.6%, 30.5%, 51.6%, 12.5% and 0.8% of the Xiamen coast, respectively. The coastal vulnerability to erosion classes of artificial coasts is significantly higher than those of natural coasts. This difference is mainly controlled by the coastal slope and coastal buffer ability. The results of the evaluation are basically consistent with the present situation. The rationality of the index system and the applicability of the theoretical method are well explained. The evaluation model constructed in this study can be extended to other areas with high ratios of artificial coasts.

Global climate change-induced sea-level rise and storm wave intensification, along with the large population densities and high-intensity human development activities in coastal areas, have caused serious burden and damage to China’s coasts, led to the rapid growth of artificial shorelines development, and formed a “new Great Wall” of reinforced concrete against the laws of nature. After the last ice age, transgression formed the different features of China’s coast. Depending on the types of geological and landform features, coasts are divided into 36 evaluation units, and 10 indicators are selected from natural aspects (including tectonics, geomorphology, sediment, and storms) and aspects of social economy (population, GDP, Gross Domestic Product), and cloud model theory is used to build a coastal erosion vulnerability evaluation index system in China. The results show that high grade (V), high-middle grade (IV), middle grade (III), low-middle grade (II), and low grade (I) coastal erosion vulnerability degrees account for 5.56, 13.89, 41.67, 33.33, and 5.56% of the Chinese coastlines, respectively. The coastal erosion vulnerability of the subsidence zone is significantly higher than that of the uplift zone. Reverse cloud model and analytic hierarchy process calculation show that the main factors that control coastal erosion vulnerability since the transgression after the last ice age are geological structure, topography and lithological features, and in recent years, the decrease in sea sediment loads and increase in reclamation engineering. Mainland China must live with the basic situation of coastal erosion, and this study shows that the index system and method of cloud modeling are suitable for the evaluation of the coastal erosion vulnerability of the Chinese mainland. This study provides a scientific basis for the adaptive management of coastal erosion, coastal disaster assessment and the overall planning of land and sea.

Under the dual effects of global climate change and intensive human development activities, vulnerability to coastal erosion in bay areas is becoming increasingly serious. This study focuses on 15 counties and districts along the coast of the Pearl River Estuary (PRE) Great Bay Area and selects 12 evaluation indices from five perspectives for analysis, including coastal characteristics, hydrodynamic forces, economics, population and coastal reconstruction. The analytic hierarchy process (AHP) method, Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) method, independent weight method, Jenks natural breaks method (Jenks), exposure-sensitivity-adaptation (ESA) model and obstacle degree method are used in conjunction with the above indices to construct a coastal erosion vulnerability evaluation system for the PRE. The results show that coastal erosion vulnerability in the PRE is low in the eastern hilly area and high in the central and western delta areas. Coastal characteristics, coastal lowlands and protection capability are the main controlling elements of erosion. The PRE experienced an era of rapid economic development from 1980 to 2010, and coastal erosion vulnerability gradually increased, with a cost of ecological environment destruction. Then, an era of coastal zone ecological restoration supported by policy protection occurred from 2010 to 2020. Compared with three major bay areas with similar developed economies worldwide, the PRE is characterized by comparatively late but rapid economic development. Notably, the development and utilization efficiency of coastal zones is very high, the duration of damage to the ecological environment is short, and the effects of ecological repair and restoration are obvious. The results of this study provide a reference for economic development and ecological restoration in the bay areas of China and provide scientific guidance for coastal zone development, management and planning.

Human activities for exploitation and utilization of coastal zones have transformed coastline morphology and severely changed regional flow fields, underwater topography, and sediment distribution in the sea. In this study, single-beam bathymetry coupled with sediment sampling and analysis was carried out to ascertain submarine topography, geomorphology and sediment distribution patterns, and explore sediment provenance in Qinzhou Bay, China. The results show the following: (1) the underwater topography in Qinzhou Bay is complex and variable, with water depths in the range of 0–20 m. It can be divided into four underwater topographic zones (the central (outer Qinzhou Bay), eastern (Sanniang Bay), western (east of Fangcheng Port), and southern (outside of the bay) parts); (2) based on geomorphological features, the study area comprises four major submarine geomorphological units (i.e., tide-dominated delta, tidal sand ridge group, tidal scour troughs, and underwater slope) and two intertidal geomorphological units (i.e., tidal flat and abrasion platforms); (3) sandy sediments are widely present in Qinzhou Bay, accounting for 70% of the total sediments. From the mouth of the Maowei Sea to the central and northern part of Qinzhou Bay, the sediments gradually become coarser, shifting from sandy mud to muddy sand, and then to fine sand and medium–coarse sand, especially inside the trench. The detrital minerals contained in the sediments mainly consist of quartz, feldspar, ilmenite, leucosphenite, tourmaline, and detrital minerals, whereas the clay minerals are dominated by kaolinite, followed by illite and smectite. The sediment provenance is mainly terrigenous input from near-source river. With sea reclamation and dam construction, outer Qinzhou Bay has experienced enormous morphological variation of its coastline. Human activities for exploitation and utilization of coastal zones have transformed coastline morphology and severely changed regional flow fields, underwater topography, and sediment distribution in the sea. Together with the channel effect where the velocity of ebb tide is greater than that of flood tide, the underwater topography is characterized by increased scale and height difference of troughs and ridges as well as enhanced offshore deposition.

Different types of pockmarks, including single pockmarks, circular pockmarks, elongated pockmarks, chain-type pockmarks, and compound pockmarks, were identified in coastal areas around Fujian, China. The sediments associated with pockmarks were mainly silty clay to clay, with a small quantity of silt with fine sand. The sulfate content in the pore water in the sedimentary layers associated with pockmarks decreased with depth from the surface, whereas the free methane content increased with depth. The interaction between sulfate and methane is well known, but differences in the sulfate–methane transitional zone (SMTZ) were observed in different areas with different hydrologic characteristics. The sedimentary SMTZ of the offshore Zhe-Min mud wedge was shallow, at 50–70 cm below the seafloor. The sedimentary SMTZ was moderately deep (90–115 cm) in the central bay area and deep (180–200 cm) in the sandy area offshore. This variability in SMTZ depth reflects different amounts of free methane gas in the underlying formations, with a shallower SMTZ indicating a higher free methane content. The free methane had δ13C values of −26.47‰ to −8.20‰ and a biogenic hybrid genetic type. The flux of sedimentary gas from the pockmark surfaces, calculated according to Fick’s formula, was 2.89 to 18.85 L/m²·a. The shape, size, and scale of the pockmarks are directly related to the substrate type and the gas production of the underlying strata and thus vary with the sedimentary environment and development stage. Therefore, different types of pockmarks, in various phases of development, are associated with different sedimentary and dynamical conditions. A single circular pockmark is formed by a strong methane flux. As the intensity of methane flux weakens, the pockmark becomes elongated in the direction of the water flow because of long-term erosion induced by regular hydrodynamic forces. Finally, under a weak intensity of methane flux and the influence of complex hydrodynamic conditions, pockmarks merge to form large-scale compound pockmarks.