A study of US coastline has provided the first ‘big picture’ of nationwide tidal flats erosion and urban development

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.

These guardians of the coastlines, however, are under threat due to natural and human events.

Climate change including sea level rise and rapid urbanisation have resulted in a ‘coastal squeeze’ between land and sea.

More than 20,000 kilometres 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 had a share of tidal flats greater than 1% as of 1985.

They examined data from 1985 to 2015 to identify change patterns of tidal flats and urbanisation in those counties, of which 76% were by the seaside and assessed correlations between tidal flat loss and urban expansion.

They used annual maps of tidal flats and urban expansions and a pixel-based approach to track and analyse land cover transitions.

They overlapped the maps to conduct a series of correlation assessments between urban expansion and tidal flat erosion over the 31-year study period from 1985 to 2015.

Results of the study 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.

Tidal flats surrounding new urban areas have undergone considerable degeneration and the patterns appear more significant as they get closer to new urban locations. 




Clusters of tidal flat erosion


The researchers identified some clusters of tidal flat erosion that do not greatly overlap the new urban areas.

This is likely because sediment discharge by major rivers or river sloughs, which is one of the critical factors in maintaining the extent of tidal flats, is likely slowing down or stopping the process of tidal flat shrinkages.

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

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

The researchers provide two important key takeaways from this research:

First, local officials should ensure that water channels are unobstructed. Water channels are waterways between two landmasses that are critical for transporting sediment and deposition.

Second, they urge leaving sufficient space for the migration of tidal flats. The sustainable management for tidal flats should particularly acknowledge hydrodynamical factors as well as the ongoing crisis of sea level rise. 

Chao Xu, PhD, 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, said: “Tidal flats have been severely inundated due to sea level rise.

“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.”


The study is published in Science of The Total Environment.

Image 1: Tidal flats are areas that are alternatively and periodically exposed to or inundated by the tides. Credit: Chao Xu, PhD.

Image 2: A beachfront in Hawaii demonstrates how an urban area touches and affects the coastal environment. Credit: Chao Xu, PhD.

Image 3: The map shows the intensities of tidal flat erosion in the selected 156 counties between 1985 and 2015. Credit: Florida Atlantic University.

Research Aether / Earth Uncovered