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Coastal squeeze: America's tidal flats are threatened by urban expansion

Coastal wetlands, known as tidal flats or mud flats, stand as nature’s protective barrier against the mighty forces of the ocean, including waves, tsunamis, and hurricanes. However, these coastal guardians now face alarming threats due to both natural and human-induced causes.

A recent study highlights the phenomenon of “coastal squeeze,” a consequence of rising sea levels owing to climate change, coupled with rapid urbanization. 

Massive losses

Globally, there’s been a significant reduction in tidal flats, with more than 20,000 kilometers disappearing since 1984. The United States has particularly seen its share of irreversible damages due to human development.

Despite the critical importance of tidal flats, past research has largely focused on localized patterns of loss attributed to urban expansion. Now, a study from Florida Atlantic University provides a larger view of how urban sprawl has affected tidal flat environments across the contiguous United States.

How the study was conducted

The researchers zeroed in on 156 out of 226 seaside counties that, in 1985, had a tidal flat area greater than one percent. These counties, with 76 percent situated by the ocean, were scrutinized from 1985 to 2015. The objective was to distinguish the link between urbanization and tidal flat degradation.

By utilizing annual maps of tidal flats and urban expansions, along with a pixel-based approach, the experts meticulously tracked land cover transitions. The overlapping of these maps facilitated a detailed analysis of the correlation between urban growth and tidal flat erosion over the 31-year timeline.

What the researchers learned 

Their findings, as detailed in the renowned journal Science of The Total Environment, present a grim picture. It is abundantly clear that human activities, especially urban expansions, have not just squeezed the space for tidal flats but have also profoundly altered their surrounding environments over the past three decades. 

The degradation pattern of tidal flats surrounding new urban territories becomes glaringly evident as the distance to these urban areas diminishes.

For clarity, the studied counties were divided into four geographically distinct zones: Northern Atlantic Coast (Zone A); Southern Atlantic Coast (Zone B); the Gulf Coast (Zone C); and the Pacific Coast (Zone D). With the exception of the Great Lakes, all of the shorelines of the contiguous U.S. are seashores.

Disturbing trend 

A disturbing trend emerged from the study: vast urban expansions across the entire coastal U.S., with the Atlantic Coast registering alarmingly high rates. 

Eight areas, chiefly from this region, stood out including Prince William County, Virginia; Horry County, South Carolina; the City of Newport News, Virginia; Gloucester County, Virginia; Richmond County, New York; New Hanover County, North Carolina; the City of Hampton, Virginia; and Duval County, Florida.

Furthermore, cities like Boston, Wilmington, Charleston, Savannah, Jacksonville, Palm Bay, Fort Myers, Tampa, Houston, Corpus Christi, and San Jose showed significant urban growth, inevitably affecting adjacent tidal flats. 

Silver lining

Interestingly, tidal flats did not contribute to the expansion of three major cities: New York City, Miami, and Seattle. Instead, these cities drew on other land resources.

Upon further investigation, the largest tidal flat clusters were found from South Carolina and Georgia in Zone B, followed by the eastern shore of Delaware in Zone A, Southern Coastal Texas in Zone C, and the San Francisco Bay Area in Zone D.

However, there’s a silver lining. In some regions, tidal flat erosion does not coincide with new urban expansions. The sediment discharge from major rivers or river sloughs, vital for maintaining tidal flat extents, may be counteracting the shrinkage.

Study implications 

The message from this research is loud and clear. Local authorities must prioritize keeping water channels – vital corridors for sediment transportation and deposition – free from obstructions. There’s also an urgent need to reserve ample space for tidal flat migration with the ongoing sea level crisis. 

“Findings from our study provide important implications for coastal land use and planning to sustain tidal flats,” said study senior author Dr. Weibo Liu, an associate professor in the Department of Geosciences within FAU’s Charles E. Schmidt College of Science. 

“Our study provides worthwhile data for scientists and lawmakers alike that will contribute to helping to develop policy and programs that address how massive urban expansion has tremendously undermined the environment of tidal flats along the U.S. coast.”

Study lead author Dr. Chao Xu is an FAU graduate from the Charles E. Schmidt College of Science who is now an assistant professor of practice in the Department of Geosciences at Texas Tech University. 

“Tidal flats have been severely inundated due to sea level rise,” said Dr. Xu. “If we don’t leave some inland spaces for tidal flats to move around, they will likely disappear, which will have dire consequences for beachfront communities.”


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