Tidal flats, also known as mud flats, make up coastal wetlands that protect against destructive powers from the ocean such as waves, tsunamis, and hurricanes. Climate change including sea level rise and rapid urbanization have resulted in a “coastal squeeze” between land and sea.

More than 20,000 kilometers of tidal flats have shrunk worldwide since 1984. In the US, human development has made irreversible damages to tidal flats. To date, research investigating patterns of tidal flat loss due to urban expansion has only been conducted in small areas.

For the first time, a new Florida Atlantic University study provides a “big picture” examination that demonstrates the impacts of urban expansion on tidal flat environments across the contiguous US.

Out of 226 seaside counties, researchers selected 156 that were directly by the seaside and examined data from 1985 to 2015 to identify change patterns of tidal flats and urbanization in those counties. They assessed correlations between tidal flat loss and urban expansion using annual maps and a pixel-based approach to track and analyze land cover transitions.

This map shows the intensities of tidal flat erosion in the selected 156 counties between 1985 and 2015. (Image credit: FAU)

Results of the study, published in the journal Science of The Total Environment, reveal how human activities, rather than natural factors, have impacted tidal flat environments in the contiguous US Urban expansions have not only substantially squeezed the space of tidal flats, but also significantly affected the surrounding tidal flat environments over three decades. Degeneration patterns are more significant as they get closer to new urban locations.

Findings reveal massive urban expansions throughout the entire contiguous coastal US, in which the Atlantic Coast shows outstandingly high rates. Several cities have considerably affected nearby tidal flats while expanding urban areas, including Boston; Wilmington, North Carolina; Charleston, South Carolina; Savannah, Georgia; Jacksonville; Palm Bay; Fort Myers; Tampa; Houston, Corpus Christi, Texas; and San Jose, California.

This map shows the intensities of coastal urban expansion in the selected 156 counties between 1985 and 2015. (Image credit: FAU)

In contrast, three major cities—New York City, Miami, and Seattle—have had significant urban expansions, but they do not overlap the large clusters of tidal flat erosions. This finding suggests that tidal flats are not a major land source for urban expansions in these three cities. Instead, New York City, Miami, and Seattle relied highly on other land sources over the three decades.