As climate change pushes seas to rise ever higher, shallow groundwater could become a more costly hazard across the Atlantic coast, particularly in South Florida where the water table sits just feet below the surface, according to a new study from the U.S. Geological Survey.
Up to 70% of residents between North Carolina and Florida, and a trillion dollars in property, could be impacted by the end of the century with just over three feet of sea rise. South Florida represents the vast majority of that risk, with about 7 million people and $750 billion in property expected to face increasing risks as groundwaters rise, the study found.
“We often get fixated on overland flooding. It's more dramatic. It happens during hurricanes,” said Patrick Barnard, a research geologist at USGS and lead author on the study. “But that water table is going to rise through time and it's going to amplify those overland flooding impacts.”
READ MORE: Coastal flooding is getting more common, even on sunny days
That means that in addition to fixating on elevations in low-lying South Florida, we also need to pay attention to the hidden water table.
The study was launched after Hurricane Florence slammed the Carolinas in 2018 and triggered devastating flooding as the slow-moving storm dumped more than 30 inches of rain on some areas. Heavy rain in the days before had left the ground saturated, fueling widespread flooding that shut down major highways for days.
As planners struggle to prepare for more water, the researchers wanted to look at the effect of combined risks, Barnard said.
“There's also beach erosion. There's subsidence and there's also rising groundwater tables,” he said.
That last risk proved to be the greatest in South Florida, where impacts from groundwater are already being experienced, from failing septic tanks to more severe inland flooding. Until now, most studies and forecasts have focused on coastal flooding worsened by high tide and hurricane storm surges. That has helped steer resilience work to shorelines, where homes are being elevated, aging coastal pumps are being replaced and seawalls elevated. Yet rising groundwater remains a mostly uncalculated risk even as it compounds flooding from storm surges and heavy rainfall.
That’s because it can be complicated and vary widely from place to place, Barnard said, making such studies costly.
“Everything that's happening is happening underground. So we don't know where every fracture is and exactly how things are flowing through the ground unless we have very, very detailed studies,” he said, noting that the study included 24 authors with different areas of expertise, from a nonprofit research institute in the Netherlands to the U.S. Army Corps of Engineers in Mississippi.
Many areas also lack a thorough accounting of underground infrastructure or information on the depth at which those features begin to fail.
This study, he said, represents a sort of first draft.
“We can do a fairly simple study like this to identify areas that are going to be most at-risk of rising water tables and the hope is that could lead to more detailed studies,” he said.
And those areas could account for billions in property and millions of people feeling the impacts, like saltwater contaminating drinking water and soil as well as damage to septic tanks, underground utilities, roads and foundations.
With just over three feet of sea rise, the number of people living in areas where the water table is about 6.5 feet below the surface is expected to reach 10 million, with a $1 trillion in property value.
Dangers from hardening shorelines, subsidence
The study also found that hardening shorelines to protect residents from flooding could have a downside – major beach loss.
Historic rates of beach renourishment will likely do little to stop shores from migrating landward. Yet efforts to protect coasts with seawalls or other hard infrastructure will likely lead to an 80% loss of beaches with about three feet of sea rise.
“This presents a challenge to coastal managers,” the study said: defend shorelines to protect residents and communities at the expense of beaches or allow shores to move and wrestle with the economic and social consequences of losing coastal communities.
The researchers also looked at subsidence, but found Florida mostly spared from sinking land with the exception of Miami Beach, where parts of the island were once covered with mangroves and wetlands later filled for development. Subsidence occurs when land gradually sinks. That typically happens in Florida when soils break down or compact, or when sinkholes open up.
In a 2020 study, Florida International University geophysicist Shimon Wdowinski found localized pockets of subsidence on the Beach but concluded about 97% of the beach was stable. The USGS study found about 1.3 million people currently live in areas where subsidence exceeds less than a tenth of an inch per year.
“The take-home message is that groundwater hazards are as or more important than any of the other hazards,” Barnard said. “As you know living in South Florida, it's an everyday issue dealing with water tables and high tides and it's just going to get worse with future sea level rise.”
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