Houston, Texas, is the fastest sinking city in the United States (Photo Credit: RyanJLane via Getty Images)
New satellite data shows that every major city in the United States is partially submerged, affecting 34 million people. However, some of these cities are submerging faster and over much larger areas than others.
The study found that Houston is the city sinking at the fastest rate, with some areas losing ground at 0.4 inches (10 millimeters) per year, while most major cities have localized areas where the ground is sinking faster than 0.2 inches (5 mm).
The reasons why cities are sinking vary, but the most common is groundwater extraction. Lowering groundwater levels are becoming a problem because they increase the risk of flooding and threaten buildings and other infrastructure, according to a study published Thursday (May 8) in the journal Nature Cities.
To combat the flooding problem, researchers have proposed a number of measures, such as flood mitigation, upgrading vulnerable structures, and limiting construction in the most risky areas.
“Instead of simply saying this is a problem, we can act, we can address, we can mitigate, and we can adapt,” said lead study author Leonard Ochenhen, a research fellow at Columbia University’s Columbia Climate School, in a statement. “We need to move toward practical solutions.”
Land subsidence is often seen in coastal areas, where the combination of sinking land and rising sea levels could threaten to inundate cities like New Orleans and San Francisco. But the problem of land subsidence extends beyond coastal areas and could cause problems across the United States, according to the study.
A new study analyzed land subsidence in 28 of the country's most populous cities, all of which have populations of more than 600,000. The researchers used remote sensing data collected by the European Space Agency's Sentinel-1 satellites to create high-resolution maps of vertical land movement.
The maps showed that all 28 cities experienced flooding in at least some portion of each city — at least 20 percent of their urban area. Twenty-five of the 28 cities experienced flooding in at least 65 percent of their area. In the case of Houston, Dallas, Fort Worth, Chicago, Columbus, Detroit, Denver, New York, Indianapolis, and Charlotte, 98 percent of the city was flooded.
Houston, Dallas and Fort Worth have the largest share of land (more than 70%) sinking at 0.1 inches (3 mm) per year. Houston was the hardest hit, with 43% of its land sinking more than 5 mm per year, and 12% sinking at a rate of 10 mm per year, according to the study.
Why do cities sink?
Land subsidence occurs for both natural and human-made reasons. The land is never completely static, as geological processes are constantly changing it. In cities like New York and Nashville, the land is adapting to the loss of the massive ice sheets that pressed down on it during the last ice age about 16,000 years ago. This process, known as glacial isostatic adjustment, causes the land to rise and fall, returning it to its original pre-ice age shape.
However, despite these natural processes, human activity is the biggest contributor to urban land subsidence, according to the study. The researchers noted that 80% of urban land subsidence is due to groundwater extraction. Groundwater extraction can have a variety of negative impacts. For example, extracting water from aquifers with fine-grained sediments creates voids between these sediments, which can then collapse and compact the land below them, pushing the surface downward. In Texas, this problem is exacerbated by oil and gas extraction, according to
Sourse: www.livescience.com
