About 29 percent of the population of the United States lives in coastal counties — more than 41 million in Atlantic counties. This high population density represents a crucial challenge for sustainable development in coastal areas.
Tidal flats, which are part of coastal wetlands, face unprecedented challenges due to increased human activities. They are widely recognized as sentinels of coastal environmental change. Importantly, they are the guardians of beachside communities, as they can largely reduce the ocean’s destructive forces. Without them, coastal communities are more vulnerable.
Currently, there is no effective way to identify and quantify the interactions between urban areas and tidal flats, which are essential for preserving the country’s coastal communities. Moreover, existing research is limited to individual cities, which does not give the big picture.
To tackle this environmental crisis, researchers from Florida Atlantic University have developed a new approach that quantifies patterns of change of mudflats from a spatial and temporal perspective using data collected over time. Their study is one of the first to explore correlations between mudflats and urban areas using this technique.
For the study, the researchers selected and analyzed the annual dynamics of three highly urbanized coastal provinces in the southeastern US, which represent a unique environment: a mudflat system of more than 3,168 square kilometers characterized by numerous sounds, estuaries, as well as the twice-diurnal ebb tide. and flood of the tides.
They separately assessed the spatiotemporal dynamics of tidal flats and urban areas in the counties of Charleston, South Carolina; Chatham, Georgia; and Duval, Florida between 1985 and 2015. They then identified and quantified the tidal flat losses in these three counties, which are directly or indirectly related to urban sprawl, from a geographic lens focused on place and space.
Results of the study, published in the international journal, Soil, verifies and highlights the conflicts between intensified human activities and coastal environments. Importantly, this new approach can be reviewed and applied across the US
Findings showed that tidal flats are under pressure from urban sprawl from both the east and west sides. The west side, in particular, has been rapidly urbanized over the three decades, giving Duval County the largest new urban areas within 2 miles of the coast. Duval County has experienced more erosion and urban growth than the other two counties and also has much smaller tidal flats than the other two counties. It has the largest newly urbanized area towards the coast. Compared to the other two counties, Duval County also has a less stable tidal flat environment, prompting a higher level of public awareness and concern.
Meanwhile, some suburbs in the other two counties are also rapidly urbanizing, including the east and west wings of the city of Charleston and the southwestern side of the city of Savannah. These urbanizations would also have significant consequences for the surrounding mudflats, and the closer places would be subject to a higher environmental pressure.
Observing the spatial overlaps between new urban areas and tidal erosion, the researchers found that the constant shrinkage of tidal flats is hysterical (has a lagging effect) for the accelerated process of urbanization in the coastal area. This hysteresis effect also exists in ecosystem restoration and management, and severe damage can lead to irreversible changes.
“It takes time to observe the ecological follow-up effects of urban sprawl in recent years. Based on our research, the tidal flats in these three provinces are in urgent need of a sustainable management plan in response to the rapid expansion of urban areas,” he said. Weibo LiuPh.D., senior author and associate professor, Department of Geoscienceswithin FAUs Charles E. Schmidt College of Science† “The conflict between humans and the coastal environment calls for immediate public awareness, as well as effective collaborations between legislators, scientists and local authorities.”
The study finds that Charleston County has the largest tidal flats (544.87 square miles annually), followed by Chatham County (343.46 square miles annually) and Duval County (89.49 square miles annually).
Interestingly, in each individual province, the area shares of urban growth and erosion are significantly close to each other. This means that urban sprawl and erosion in the early years would be strongly offset by erosion and urban sprawl in the following years, and thus would not have a significant impact on the total surface of the tidal flats over a long period of time.
“Urban sprawl is considered one of the greatest anthropogenic threats to the tidal flat environment,” said Chao Xu, lead author and a Ph.D. candidate, FAU Department of Geosciences. “While it is challenging to stop the flooding process caused by sea level rise, a more practical solution would be to limit urbanization near the coastline. In this way, mudflats would be more flexible to migrate inland.”
For the study, researchers used a 30-meter annual spatial resolution map collection of urban areas in the contiguous (sub-border) US from 1985 to 2015; an annual map collection with a spatial resolution of 30 meters of tidal areas in the contiguous US from 1984 to 2020; and a coastline shapefile (the most detailed coastline data) provided by NOAA. This created a two-sided distance buffer of 2 kilometers along the coast, which makes it easier to identify, quantify and analyze the interactive dynamics between urban areas and mudflats.
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