Geomorphic Evolution of Radial Sand Ridges in the South Yellow Sea Observed from Satellites

The radial sand ridges consist of more than 70 sand ridges that are spread out radially on the continental shelf of the South Yellow Sea. As a unique geomorphological feature in the world, its evolution process and characteristics are crucial to marine resource management and ecological protection. Based on the multi-source remote sensing image data from 1979 to 2019, three types of geomorphic feature lines, artificial coastlines, waterlines, and sand ridge lines were extracted. Using the GIS sequence analysis method (Digital Shoreline Analysis System (DSAS), spatial overlay analysis, standard deviational ellipse method), the evolution characteristics of the shoreline, exposed tidal flats, and underwater sand ridges from land to sea were interpreted. The results demonstrate that: (1) The coastline has been advancing towards the sea with a maximum advance rate of 348.76 m/a from Wanggang estuary to Xiaoyangkou Port. (2) The exposed tidal flats have decreased by 1484 km2 including the reclaimed area of 1414 km2 and showed a trend of erosion in the north around Xiyang channel and deposition in the southeast around the Gaoni and Jiangjiasha areas. (3) The overall sand ridge lines showed a trend of gradually moving southeast (135°), and the moving distance is nearly 4 km in the past 40 years. In particular, the sand ridge of Tiaozini has moved 11 km southward, while distances of 8 km for Liangyuesha and 5 km for Lengjiasha were also observed. For the first time, this study quantified the overall migration trend of the RSRs. The imbalance of the regional tidal wave system may be one of the main factors leading to the overall southeastward shift of the radiation sandbanks.

Sediment Characteristics and Intertidal Beach Slopes along the Jiangsu Coast, China

Tidal flats play an important role in promoting coastal biodiversity, defense against flooding, land reclamation and recreation. Many coastal tidal flats, especially the tide-dominant ones, are muddy. However, the number of studies on the profile shape and surficial sediment distribution of muddy tidal flats is small compared to sandy beaches. Based on high spatial-resolution measurements along the tide-dominant Jiangsu Coast, China, we analyzed the morphology and sediment characteristics of the unvegetated intertidal flats along the Jiangsu Coast. The Jiangsu Coast can be divided into an eroding northern part (north coast) and an accreting southern part (south coast). The beach slope of the north coast shows a southward flattening trend, apart from some outliers related to rocky parts of the coastline. We found alternating very fine and coarse sediment (depending on the local clay content) for different locations along the north coast, which can be explained from consolidation and armoring-induced erosion resistance. In the south coast, we found gradual coarsening of bed surface sediment and gradual flattening of beach slopes to the south. This seemingly unexpected pattern is explained by the flood-dominant current causing landward sediment transport, larger tidal range in the south part, sheltering effect of the Radial Sand Ridges, and contribution of different sediment sources, viz. the Abandoned Yellow River Delta and the Radial Sand Ridges. In the cross-shore direction, the sediment grain size decreases landward. Waves are only of secondary importance for the sediment dynamics at the unvegetated tidal flats along the Jiangsu Coast.

Recent Evolution of the Intertidal Sand Ridge Lines of the Dongsha Shoal in the Modern Radial Sand Ridges, East China

The Dongsha Shoal is one of the largest shoals in the South Yellow Sea and has important marine ecological value. The shoal extends in a south–north direction and is controlled by the regional dominating tidal currents. Recently, due to human activities and some natural factors, the geomorphic dynamics of the Dongsha Shoal has undergone drastic changes. However, few people have proposed quantitative research on the changes of tidal flat morphology, let alone the long-term sequence analysis of sand ridge lines. Hence, we attempt to take the Dongsha Shoal in the Radial Sand Ridges as the research area, and analyze the trends of the long-term morphological evolution of the sand ridge lines over the period 1973–2016 based on a high-density time series of medium-resolution satellite images. The sand ridge line generally moves from southeast to northwest, and the position distribution of the sand ridge line from north to south has gradually changed from compact to scattered. We also found that the geomorphological dynamics at different positions of the sand ridge line are inconsistent. The north and south wings are eroded on the west side, while the central area is eroded on the east side. Most of the sand ridge line is moving eastward. In addition, the change of sand ridge line is affected by multiple factors such as sediment supply, typhoon, reclamation and laver cultivation.

Recent Evolution of Coastal Tidal Flats and the Impacts of Intensified Human Activities in the Modern Radial Sand Ridges, East China

The coastal tidal flats of the modern Radial Sand Ridges (RSRs) are typical silt-muddy tidal flats in Central Jiangsu Province. These tidal flats play a critical role in coastline protection and biodiversity conservation, and against storm surges, but have recently been displaying drastic changes in geomorphic dynamics because of human activities. However, a comprehensive understanding of spatiotemporal changes in tidal flats in RSRs remains lacking. Hence, we employed a novel remote sensing method by obtaining the instantaneous high/low tide line positions from over 112 scenes of Landsat satellite images of the study area from 1975 to 2017, which were used to track the recent evolution of the coastal tidal flats in the modern RSRs over the past four decades. We found that the shoreline of the tidal flats showed an advanced seaward trend, and the waterline of the tidal flat presented a gradual process during different periods. The total tidal flat area in the study area showed an obviously decreasing trend overall, and approximately 992 km2 of the tidal flat was lost. We also found that the coastal tidal flats in the modern RSRs were generally undergoing erosion in the low tidal flats, especially in the Northern Swing and Southern Swing areas, while the high tidal flats showed a slowed accretionary change. Land reclamation was the main factor affecting the reduction in the tidal flat area, as the reclamation area has increased by 1300 km2, with an average of 35.14 km2/year. In addition, the erosion of the tidal flats was associated with a reduced sediment supply. Our findings will provide useful information for local managers and researchers to support future environmental management because increasing demand for land and rising sea levels are expected in the future.