New York City is sinking 1-2 millimeters a year. The northern part of Tampa, Florida, is sinking up to 6 millimeters a year. This phenomenon, called subsidence, is one that many coastal cities are facing. Urban areas are sinking while sea levels are rising, resulting in a heightened risk of floods. While the impacts may sound drastic, the issue is not always top of mind, even for officials in coastal cities, said Steven D’Hondt, a professor of oceanography at the University of Rhode Island.

“Probably most coastal city managers are aware of sea-level rise, but I think not very many of them are aware of the sinking of the shore,” said D’Hondt, who has co-authored research papers on the topic.

Local governments can take steps to minimize future flooding by enacting policy and operational changes now, say researchers and city officials.

Engineers plan for the fact that every building causes the ground beneath it to sink somewhat in the first one to two years after construction, D’Hondt said. The more significant factor in sinking cities worldwide, however, is people removing groundwater for drinking, bathing, industrial and other uses.

Cities have sunk throughout history, D’Hondt said. For example, much of Roman-era Alexandria, Egypt, has been underwater since the second or third century AD. The modern city of almost 6 million people is expected to sink by 2100. Subsidence is also a problem in non-coastal cities such as Mexico City, where parts of the city are sinking as much as 20 inches per year.

In the near term, cities can slow the rate of sinking by no longer removing water from the ground, D’Hondt said. But they would need to find other ways to meet their water needs. For example, Tampa would need to find another source for one-third of its water, he said.

Adding water back underground can slow or stop subsidence, but it generally won’t significantly reverse it, D’Hondt said. The combination of lower buildings and higher sea levels means coastal cities must adapt to a greater likelihood of floods.

Read the full article about coastal cities are flooding by Adina Solomon at Smart Cities Dive.