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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.  

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