Quantitative Analysis of Rapid Siltation and Erosion Caused Coastline Evolution in the Coastal Mudflat Areas of Jiangsu

Abstract

Muddy coast due to natural and human-induced factors can be variable over a wide range of different spatiotemporal scales. Our limited capability to understand and especially predict this variability can lead to misinterpretation of muddy coastal change information, which hampers informed decision-making and subsequent design and implementation of engineering interventions. Therefore, the research on the evolution of the historical coastline is particularly important. In this study, annual mean coastlines and causes for the variability and possible evolutions are analyzed in a manner that avoids these effects by averaging the instantaneous coastline positions registered during the same year. A set of coastlines is obtained from Landsat MSS/TM/OLI series of remote sensing images taken between 1985 and 2020 in the Google Earth Engine at Jiangsu muddy coast of China Yellow Sea. The coastline is extracted based on the water body index and the Otsu method, and is classified by fractal dimension, annual rate of change, area calculation, center of gravity shift, and artificial index to calculate the temporal and spatial changes in the coastline. The results showed that most of the coastline advanced seaward due to rapid siltation and coastal development in the past 35 years with only a small portion receding landward. The entire coast experienced segmental changes and different evolutionary characteristics and the coastline shifted from rapid erosion to siltation. The silted banks were mainly distributed in the middle section of the coastline in the radial sandbank. The annual change rate in the coast section was 82.21 m/a and the average annual area increased by 18.07 km2. The artificial coastline increased substantially, while the natural coastline gradually decreased due to rapid erosion. From 1985 to 2020, the proportion of artificial coastline in Jiangsu rose from 14% to 77%. The coastline became more curved and the center of gravity of the coastline moved 5.13 km to the northwest by 61°. Before 2000, the driving force affecting the coastline variability was mainly rapid siltation, while the impact of rapid erosion and human activities gradually increased after 2000 with the natural coastline being continuously cut and straightened. Overall, Jiangsu’s muddy coast was the site with the highest anthropogenic pressure and highest vulnerability. However, it is difficult to generalize the results under other conditions, especially on decadal time scales. Scientific and engineering improvements still require more quantitative insight into the physical mechanisms behind the natural and forced shore behavior responsible for the variability to better explain the change process of the coastline and provide a basis for coastal management and sustainable development.

Tidal Flat Erosion Processes and Their Dynamic Mechanisms on the South Side of Sheyang River Estuary, Jiangsu Province

“…C D = 0.40 1 + ln(z 0 /h) 2 (20) in which q b is the volume transport rate of the bedload; C D is the drag coefficient; θ is the Shields parameter, which can be calculated according to Equation (12); ϕ is the angle of repose of the sediment, set to ϕ = 32 • ; and β is the slope of the tidal flat, which can be calculated based on field topographic profile observations.…”

“…According to monitoring data from the China Marine Environmental Quality Bulletin, the proportion of eroded coastline length on the northern side of the SYRE increased from 58.4% in 2013 to 68.3% in 2017, while the average erosion rate decreased from 26.4 m/a to 10.5 m/a. Over time, the eroded coastline has continuously moved southward, especially since 2000, when the coast south of the SYRE also gradually began to erode with progressive intensification of erosion [17][18][19][20][21]. Comparison of satellite images from different times shows that coastline erosion and destruction of aquaculture ponds have occurred in the study area and the adjacent coastline (approximately 4 km length) since 2014, of which the rate of coastline retreat ranged from 12 m/a to 44 m/a during 2014-2019, with an average rate of 23.3 m/a, and the length of eroded coastline was 920 m. However, from 2019 to 2021, the coastline retreat rate increased rapidly from 13 m/a to 193 m/a, with an average rate of 43.5 m/a, and the length of eroded coastline increased to 3840 m, leading to the destruction of aquaculture ponds and rapid coastal retreat (Figure 10).…”

“…On the other hand, due to the 20 km outward protrusion of the coastline of the AYRD and the magnificent subaqueous delta in the early period, the propagation of tidal waves from north to south were blocked, and with the retreat of the Jiangsu coastline, the subaqueous delta of the AYRD flattened, allowing tidal waves propagating from north to south to flow more smoothly, thereby enhancing the tidal flow on the southern side of the AYRD [50] and thus, intensifying the coastal erosion on the south side of the SYRE. Additionally, since 2000, there has been a rapid increase in projects of port construction and reclamation in the central Jiangsu coast [20], which has significantly impacted coastal erosion near the SYRE and its adjacent coastal area. Activities such as dredging of the Sheyang Port (SYP) navigation channels, construction of double guide levees in the SYP area, construction of sluices, and reclamation have influenced the hydrodynamic and erosion-accretion patterns near the SYRE, enhancing the coastal erosion near SYRE [51][52][53].…”

“…The coastal area south of the Sheyang River estuary (SYRE) (Figure 1a) exhibits signs of continuous accretion in the early stage, due to the transport of eroded sediment from the AYRD to the south [15][16][17]. However, since 2000, the coastal area south of the SYRE has also begun to gradually erode, and the intensity of this erosion has been gradually increasing [18][19][20][21][22]. Severe coastal erosion has severely impacted local socioeconomic development and damaged residents' property, necessitating ecological protection and restoration efforts.…”

“…Severe coastal erosion has severely impacted local socioeconomic development and damaged residents' property, necessitating ecological protection and restoration efforts. However, although there is a general understanding of the causes of coastal erosion [14,[18][19][20], the spatial and temporal changes in coastal erosion on the coast near the Sheyang River estuary are complex; thus, there is insufficient understanding of the coastal erosion process and its spatiotemporal variations in this area, which seriously restricts the effectiveness of measures for coastal erosion protection and ecological restoration in the region. Generally speaking, the main methods for researching coastal erosion and accumulation mainly include (1) setting up fixed sections in typical areas for repeatedly monitoring profile elevation, which is analyzed to explore the dynamic change process of coastal erosion and accumulation [15,16];…”