In a large area north of Tampa Bay, subsidence rates have been clocked at up to 6 millimeters (0.24 inches) per year, about twice as much as global sea level rise, from 2015 to 2020 due to groundwater pumping. The Tampa Bay area is also relatively flat, meaning rising seas may overwhelm large swaths of the low-lying area.
Comments by OSFR historian Jim Tatum.
jim.tatum@oursantaferiver.org
– A river is like a life: once taken,
it cannot be brought back © Jim Tatum
Land around the U.S. is sinking. Here are some of the fastest areas.
Imagine Earth’s surface is like a stack of pancakes. The pancakes, or layers of soil and rocks, may appear fairly evenly stacked and fluffy. Over time though, the stack can become compressed, thinner and shorter.
But understanding this slumping land motion is not simple, scientists say. Even within the same city, some regions may be sinking at faster rates than other areas.
“If we look at the data, we don’t see a city as a whole, uniform society. We generally see regions of hot spots where you can see parts of cities subsiding depending on what is happening,” said NASA scientist David Bekaert, who has analyzed land subsidence in the United States and worldwide.
Some land subsidence, Bekaert said, is related to deep natural processes over long periods of time, such as responding to plate tectonic activity or to the retreating of the glaciers from the last Ice Age. Other sinking is linked to human activity, including extracting oil, water or minerals from underground. In cities, buildings can also add weight and push land down.
Many of the fastest sinking places in the world appear in populated areas in southeast Asia largely because of groundwater extraction, but the United States also faces substantial land subsidence. There, you may not notice land settling around you in your daily life, but scientists found that many places are sinking faster than global sea levels are rising — increasing flood risk in coastal cities. (Sea levels are rising at an average global rate of about 3.4 millimeters, or 0.13 inches, per year.)
Gulf Coast: Houston
The western Gulf Coast is experiencing some of the fastest land subsidence rates in the United States.
Analyzing land subsidence rates in large coastal cities, Bekaert and his colleagues found Houston has the fastest peak subsidence rates — about 17 millimeters (0.67 inches) per year from 2014 to 2020 — in the United States. Other research showed parts of Houston lost over 3 meters in elevation in certain areas since 1917.
Such land subsidence can worsen damage during extreme weather events. Researchers found that the majority of areas flooded during 2017′s Hurricane Harvey showed relatively high subsidence rates.
Land subsidence in the Houston-Galveston area is largely caused by groundwater withdrawals. According to the U.S. Geological Survey, removing water from fine-grained silts and clays underground compresses the aquifer and lowers the land surface. Some regions experience high land subsidence rates because of withdrawal of local oil and natural gas reserves.
A smaller portion of land subsidence can be attributed to fault movement in the area.
Gulf Coast: New Orleans
Past data from 2009 to 2012 showed the highest subsidence rates along the Mississippi River near industrial areas in Norco and Michoud — experiencing up to 50 millimeters (2 inches) of sinking a year. To the east of Norco, the Bonnet Carré Spillway, intended to reduce flood risk for New Orleans, experienced up to 40 millimeters (1.6 inches) of sinking a year behind the structure.
While Houston and New Orleans are notable subsiding locations, other places in the Gulf also experience high rates. In a large area north of Tampa Bay, subsidence rates have been clocked at up to 6 millimeters (0.24 inches) per year, about twice as much as global sea level rise, from 2015 to 2020 due to groundwater pumping. The Tampa Bay area is also relatively flat, meaning rising seas may overwhelm large swaths of the low-lying area.
East Coast: New York City
Wei said much of the city’s land subsidence are responses to glacial retreat after the most recent Ice Age. During the last Ice Age, ice sheets weighed heavily on the land and caused the crust beneath places such as Canada and the northeastern United States to stretch and sag. Places on the periphery of those sagging areas, though, such as around New York and the Chesapeake Bay, bulged upward. When the ice sheets began retreating about 12,000 years ago, those sagging areas started to rise back up while the bulging areas are settling back down.
Wei said to think of it like a balloon. If you push down on a balloon, parts near your hand will go down while other parts farther away will go up. Once you remove your hand, the balloon will try to bounce back to its original shape, causing some sections to rise and others to lower. He said the same process plays out on Earth except on “a much larger scale and also a much slower speed.”
“New York City is sinking not because of the weight of the buildings. It’s mainly because of the glacier rebound,” Wei said. “But there are places suggesting the weight of the buildings might [have] contributed to the accelerated rate.”
Mid-Atlantic: Norfolk
Groundwater withdrawals are also exacerbating rates on local levels. Bekaert said historically, paper mills near Norfolk did a lot of pumping, but operations ceased with new regulations. The city is experimenting with a new project that would restore water in the aquifer and reverse long-term pumping.
Norfolk may stand out on the East Coast, but many areas along the coast are also combating high rates of subsidence and sea level rise. The Chesapeake Bay has been experiencing around 4.6 millimeters (0.18 inches) per year of sea level rise, exacerbated by land subsidence.
West coast: California’s Central Valley
Overall, data along the western coast show rates are mostly below 3 millimeters (0.12 inches) per year, except near areas like San Diego, which has experienced large amounts of human-induced sinking.
Further inland in California’s Central Valley, excessive pumping of groundwater for agriculture has caused the land to sink significantly. Since the 1920s, sections of the San Joaquin Valley have sunk by as much as 8.5 meters (28 feet). Groundwater pumping becomes even more problematic during droughts, exacerbating subsidence problems. Data from 2015 showed some spots subsided at rates as high as 600 millimeters (2 feet) per year.
“There can be hot spots within cities, where you see different subsidence rates,” Bekaert said. “Often, they are related to the local human-induced fingerprint that is taking place.”