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Rising Sea Level May Trigger Groundwater Floods

For coastal communities already worried about the rising sea level that comes with climate change — the ocean is already 8 inches higher, on average, than it was in 1900, and it’s likely to go up another 3 feet by 2100 — scientists from the University of Hawaii have some more bad news.

In a report released Sunday in Nature Climate Change, Kolja Rotzoll and Charles Fletcher predict that in some areas, freshwater bubbling up from underground could more than double the flooding caused by intruding seas alone. “If the underground water table is already close to the surface, it’s going to come out,” Rotzoll said.

Credit: NOAA

That’s the last thing shore-dwellers need. Rising seas are already going to inundate shorelines around the world, and may swamp some island nations entirely. Add storm surges to higher water and you’ve got even more trouble. As Hurricane Sandy showed, a powerful storm surge will be worse if it starts from a higher launching pad, making storm-driven floods all the more devastating. Beyond that, it’s harder for rivers to drain into a higher ocean, so freshwater floods in cities located at river deltas could get more intense.

And now there’s evidently a new threat from underground. Rotzoll said the reason is that seawater is constantly pushing its way inland below the surface, especially in places where the soil is porous. In South Florida, for example, where there’s lots of sand, some coastal communities are already seeing their freshwater wells becoming contaminated with intruding saltwater. But since saltwater is denser than freshwater, most of the invading liquid tends to push underneath freshwater aquifers, nudging them even closer to the surface.

Rotzoll and Fletcher decided to look at what might happen on their home turf of Honolulu, where the soil is largely made of porous limestone, as the ocean continues to rise. It was already clear from earlier studies that pressure from underlying saltwater forces groundwater to rise and fall with the tides, and that the effect grows less, not surprisingly, the farther you get from the shore. 

That gave them a formula that calculates how responsive the groundwater actually is to changes in sea level across their study area, which stretches from around Honolulu International Airport toward Waikiki Beach, and includes significant parts of downtown Honolulu. When they plugged projections for sea level rise over the rest of the century into the formula, they discovered that groundwater in some places would be pushed permanently upward to form permanent lakes.

“This would become even more pronounced during storms,” Rotzoll said. “You’d have steady state of higher water levels, but storm surges or heavy rainfalls would make the flooding worse.”

Strictly speaking, Rotzoll and Fletcher write, “This study focuses on southern Oahu, Hawaii,” which means they aren’t necessarily making any specific claims about other parts of the world.

But they’re coming pretty close. “The problem is applicable to most low-lying coastal lands where the water table (freshwater or otherwise) is near the surface,” they said.  

Related Content 
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Hurricane Sandy Roars Ashore, Threatening Record Surge 
Sandy's Storm Surge Explained and Why It Matters
Ongoing Coverage of Historic Hurricane Sandy
Hurricane Sandy’s Five-Fold Flood Threat, with Local Maps 
How Fujiwara Effect Will Toss Sandy Into U.S. 
How Hurricane Sandy Can Become a 'Frankenstorm'
Sea Level Rising Faster Than Average in Northeast U.S. 
The Future is Now for Sea Level Rise in South Florida 
Why Groundwater is Another Sea Level Rise Concern 
Report: Warming Doubles Extreme Coastal Flood Risk Across U.S.


By a housewife (oak lawn Illinois 60453)
on November 25th, 2012

you know, us “conspiracy theorists” require that the theory cannot be debunked with a single google search! perhaps the global warming crowd should leave the “conspiracy theories” to the experts! lol

“A cat can have kittens in an oven, but that don’t make them biscuits”

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By John d'Oregon (97013)
on December 4th, 2012

The comment and links provided by “housewife” have no relation to this article (I have read the articles at each link). The links discuss the possible cause and effects of groundwater depletion in 4 disparate communities.

The article above discusses the theory that shallow groundwater under porous coastal areas might be pushed to the surface by intruding saltwater from rising sea levels. You could call this groundwater displacment. The article acknowledges the limited nature of the study being described and that more study in porous soil coastal areas is required before the theory can be validated.

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By fernando bettega (Curitiba/Paraná/Brazil)
on November 8th, 2013

...“freshwater bubbling up from underground”... my god! These guys are completely lost. They don’t know nothing about groundwater.

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