Understanding Subsidence: How America's Cities Are Beginning to Sink

A new study released last month is raising alarm bells about the risk that millions of Americans face as some of the country's major cities continue to sink. Here is a look at what metro areas are sinking the fastest and what can be done to reverse the trend.

New Study Paints Dire Picture of America's Sinking Cities

A study out of Virginia Tech detailed that 28 major American cities are subsiding, raising the threat of flooding, failure of key infrastructure, and foundation damage. While the sinking is a slow process, it still presents a host of dangers with the greatest risks targeting coastal urban areas.

The results of the research were published in the journal Nature Cities. The researchers looked at the 28 most populated cities in the country to determine the level of subsidence. The findings detailed that at least 20% of the land area was slowly sinking. What is most concerning is that 25 out of these 28 cities showed at least 65% or greater of the land area affected by the subsidence. Scientists believe that groundwater extraction is the primary culprit to blame.

The state of Texas is experiencing the most significant threat of subsidence. Houston, Dallas, and Fort Worth are all distinguished as being the fastest sinking cities in the country. Large swaths of land in these urban corridors are sinking at rates of over 5 millimeters per year, equating to almost two inches every decade.

Looking at Houston specifically, 42% of the metro area is sinking at this rate. Some of the hardest-hit zones are topping a sinking rate of 10 millimeters per year.

New York City is also experiencing a rapid level of subsidence with 26% of the population in the Big Apple reportedly exposed to the threat. Its counterpart on the West Coast, Los Angeles, is also dealing with widespread sinking. The areas near Long Beach on the coast and industrial zones came in with the highest levels of subsidence.

The major urban areas in the Desert Southwest also ranked high on this list. For instance, the Northgate neighborhood of Las Vegas is demonstrating worrisome subsidence. Phoenix also has high levels of sinking due to the land compaction that happens as water is withdrawn from desert aquifers.

Moving into the heart of the nation, cities such as Chicago and Columbus are not sinking at the fastest speeds when compared to areas closer to the coast. However, the percentage of land area affected by subsidence is higher. The scope of the ground movement inherently raises the long-term threats to roads and structures.

The study found that over 33 million Americans are currently building lives on sinking ground across the 28 cities. Eight metro areas account for more than 60% of this total.

Why Subsidence is Such a Concern

Due to the slow-moving and nearly invisible nature of subsidence, it does not generate the headlines that natural disasters such as tornadoes and hurricanes demand. However, experts warn that the impacts can be just as severe. Subsidence slowly weakens foundations, creates cracks in roadways, overwhelms storm drainage systems, and more. Because of the slow-moving nature of subsidence, the damage is often not noticed until it is too late to reverse.

What is causing this sinking? Experts believe that the excessive pumping of groundwater is largely to blame. The water that is trapped in confined aquifers causes the land above it to compact together and begin to sink. The researchers of this latest study found that San Diego, New York, and Houston demonstrated an exceptionally strong link between sinking land and groundwater decline.

The issue has been observed for quite some time in New York City. According to the New York City Panel on Climate Change, sea levels in the Big Apple are expected to rise between 8 and 30 inches by 2050. This level is forecast to increase as high as 75 inches by the end of the century. The rising sea levels and the increased prevalence of storms at the hands of climate change put the city at a higher risk for land subsidence.

Not only does climate change cause more stormy activity, but it also increases the frequency of flash flooding events. New York City's location makes it vulnerable to the dual threat of heavy rainfall and storm surge.

Now that experts are getting a better handle on the severity of this problem, the question becomes how to stop the sinking. The answer starts with doing a better job at using groundwater across America's cities. Some of the most damaging consequences can still be avoided with the appropriate actions.

This includes implementing techniques such as managed aquifer recharge that replenishes the water supplies underground. City planners can also take steps to limit the amount of extraction in areas shown to be at a higher risk of subsidence. Some areas have already updated construction codes to recognize the shifting ground. Improving drainage practices in coastal areas, building up the infrastructure, and creating green flood zones are also good strategies to mitigate the threat of subsidence.


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