Sunday, November 11, 2012

Flooding from sea level rise will double due to groundwater rise: UH scientists

As the sea level continues to rise, low-lying parts of Hawai`i will face attack on two fronts.

We can plan for driving through vast pools of brackish water throughout much of low-lying Honolulu, including much of Waikīkī.

(Image: Flooding in Māpunapuna. Credit: University of Hawaii.)

 The rising seas will push shoreward, but at the same time, groundwater will back up and further raise the water level, says a new study: “Assessment of groundwater inundation as a consequence of sea-level rise.”

The report, published in the journal Nature Climate Change, was written by Kolja Rotzoll of the Water Resources Research Center at University of Hawaii, and by Charles “Chip” Fletcher, of the University of Hawai`i School of Ocean and Earth Science and Technology.

“Besides marine inundation, it is largely unrecognized that low-lying coastal areas may also be vulnerable to groundwater inundation, which is localized coastal-plain flooding due to a rise of the groundwater table with sea level,” they write.

Their work suggests that sea level rise alone can’t be isolated as the source of flooding.

“The flooded area including groundwater inundation is more than twice the area of marine inundation alone,” they write.

Looking just at Honolulu, their work concludes that, particularly at high tide, there will be massive regions of flooding—ponds and lakes of standing brackish water that could represent 10 percent of all the low-lying land within a kilometer of the coast as sea level rise reaches one meter.

In many parts of the Islands, groundwater is already very near the surface in the lowest lying areas like Waikīkī and other parts of Honolulu. In some areas, including parts of Māpunapuna, the streets already flood during the highest tides. 

“With groundwater tables near the ground surface, excluding groundwater inundation may underestimate the true threat to coastal communities,” said Rotzoll.

 “Finding that the inundated areas double when including groundwater inundation in coastal flooding scenarios will certainly be a surprise for everyone assessing the effects of (sea level rise) without considering the local groundwater table,” Rotzoll said.

The authors recommend continuous groundwater level monitoring in the coastal zone, along with “rigorous planning” for the impacts of brackish water inundation.  And they recommend residents of coastal communities everywhere study their own potential situations.

“We hope other coastal communities use our research as the basis for conducting their own localized analysis,” Rotzoll said.

Fletcher, an associate dean at the University of Hawai`i and a coastal geologist who has been studying sea level rise and erosion issues statewide, said the planning won’t be easy.

“This research has implications for communities that are assessing options for adapting to (sea level rise). Adapting to marine inundation may require a very different set of options and alternatives than adapting to groundwater inundation,” Fletcher said.

The paper: Rotzoll, K. and Fletcher, C.H. Assessment of groundwater inundation as a consequence of sea-level rise. Nature Climate Change, DOI: 10.1038/NCLIMATE1725

The abstract can be viewed here.

Fletcher’s website on Hawaiian sea level rise impacts includes a virtual fly-through of those impacts. 

© Jan TenBruggencate 2012

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