New research suggests that excessive groundwater usage and damming have changed the natural risk of nuisance floods, for better or worse, in eastern U.S. coastal cities
September 28, 2017 — Nuisance flooding—the kind of sunny-day, nonfatal flooding that closes roads, seeps into basements, and generally causes, well, a nuisance—is on the rise along southern stretches of the U.S. eastern coast and on the decline farther north. The ways that humans are altering the natural flow of water in and on the Earth are partly responsible for these changes, according to an international team of researchers.
“In some of our past work, we showed that there is a high rate of subsidence along the U.S. East Coast,” said Makan Karegar of the University of South Florida in Tampa and the University of Bonn in Bonn, Germany. “We asked ourselves, ‘What comes next? How does this relate to human life, and what are the implications?’” Karegar is lead author on a paper describing this research that was published in Scientific Reports on 11 September.
Karegar and his colleagues measured vertical land motion along the U.S. East Coast using data from the past 2 decades. They found that coastal latitudes that include Virginia, North Carolina, and South Carolina—regions that extract groundwater faster than nature replenishes it—are sinking significantly faster than geologic average. As a consequence, they are experiencing increased nuisance flood rates.
In contrast, the coastlines of Maine and New Hampshire are actually rising, and nuisance flooding in northern U.S. coastal cities has decreased. These regions are ascending in response to water building up behind Canadian dams hundreds of kilometers away, the weight of which presses down on the land directly beneath the reservoirs and pushes peripheral land upward, according to this study. Human-induced changes to local water loads, said Karegar, have altered the natural risk of nuisance floods along the coast.
“We are nowadays able to measure such small rates [of vertical land motion],” said Riccardo Riva of Technische Universiteit Delft in Delft, Netherlands, who did not participate in the project. “This is highly relevant for urban and infrastructure planning.”
Although this research does not consider potentially fatal floods due to severe weather, like the recent hurricane-induced flooding in Texas, Florida, and Puerto Rico, frequent nuisance flooding can indicate areas that may be at risk for catastrophic flooding in the face of future storms.
A Southern Sink and a Northern Rise
On average, the U.S. East Coast is slowly sinking at a rate of about 1.5 millimeters per year, with some variation based on latitude. As northern glaciers melt, they release the pressure they had been exerting on Earth’s crust, which had created a temporary bulge along the coastline. This coastal bulge is now subsiding back to its previous elevation, Riva said, a fact that is well known to geologists.
Karegar’s team, however, used GPS data from 1990 to the present to estimate vertical land motion above or below this average rate. They discovered two regions where elevation changes are significantly different than the geologic average for those latitudes: a southern anomaly spanning Virginia, North Carolina, and South Carolina and a northern anomaly that encompasses the Maine and New Hampshire coasts.
GPS data for the southern anomaly show areas where land is sinking at nearly twice the geologic rate. Not coincidentally, Karegar said, this area has also experienced an increase in nuisance floods over the past 2 decades. The authors suggest that humans have been removing groundwater from this region at a rate faster than nature replaces it, making that section of crust denser and less buoyant and causing it to sink faster.
The modern records also show that there is a lot more variation in the rate of subsidence in this region than other areas of the coast, which the researchers attribute to differences in local groundwater management policies.
In the north, however, the researchers found that the land is rising, not sinking, and that the change does not appear to be related to a shift in groundwater usage like the southern anomaly. The team tested whether something else human-made could be shifting enough weight around to counterbalance the glacier’s slow retreat.
They realized that the northern anomaly sits approximately 800 kilometers from the James Bay hydroelectric project in Quebec, Canada, which has drastically shifted the regional water load with dams around James Bay, Hudson Bay, and the Gulf of St. Lawrence that power hydroelectric stations. Karegar’s team tested how the lithosphere responded to the extra weight using theoretical models, which indicated that the shift in weight from displaced water was likely just right to cause a peripheral bulge near Maine and New Hampshire and reduce the frequency of nuisance floods in those states.
“They have highlighted the importance of vertical land motion in coastal areas,” Riva said. He added that the team has demonstrated that the human contribution to this motion is a critical factor in some coastal regions and that “even a moderate [relative] sea level rise could be enough to largely increase the frequency of nuisance flooding.”
Reducing the Nuisance
The frequency of nuisance flooding has increased 925% in some areas since the 1960s. Frequent nuisance flooding can be an early indicator of regions that will eventually see accelerated land loss and increased catastrophic flooding from hurricanes and tropical storms. The researchers suggest in their paper that their work may help identify regions that will be at risk for increased nuisance flooding, which could lead to more conscientious municipal planning and water management policies.
Karegar plans to conduct a similar investigation into the rate of nuisance flooding in southeastern Texas and the Mississippi Delta, two regions that have seen increased nuisance flood rates in the past decade.