Under Water

University of Hawaii scientists have peered into Oahu’s distant future.

And it’s watery.

Low-lying areas of the coast can expect serious flooding as rising seas push up the water table, creating an effect called "groundwater inundation," the UH researchers warn.

In fact, freshwater sources will contribute more to coastal flooding than rising seas, they conclude.

The findings, reported online last week in the journal Nature Climate Change, describe a factor that has largely been unrecognized in earlier assessments of sea level rise from global warming.

And it could lead to a host of related problems, from beach erosion to increased soil saturation that could undermine buildings.

"It is likely that future urban settings will be characterized by standing pools of brackish water, maximized at high tide," the report said. "This may affect traffic, walkways, and any movement in urbanized coastal areas. Eventually, runoff from rainstorms will encounter few drainage options. Travel, commerce, and emergency services may be impacted and groundwater inundation may change the character of the land surface in ways that work against public health and safety."

All that won’t happen for several decades, say report authors Kolja Rotzoll, a postdoctoral researcher at the UH Water Resources Research Center, and Charles H. "Chip" Fletcher, associate dean of the UH School of Ocean and Earth Science and Technology.

That gives government officials and urban planners time to figure out mitigating measures.

But any solution is likely to be expensive.

Businesses in Mapunapuna experienced a similar phenomenon, so-called "tidal flooding," for years before an $865,900 drainage project was finished last year.

"This research has implications for communities that are assessing options for adapting to SLR," said Fletcher, referring to sea level rise. Adapting to groundwater inundation may require "a very different set of options and alternatives" than adapting to seawater inundation, he said.

Earlier research has estimated that a warming planet could raise sea levels by 7 inches to 18 inches by 2050 and by 3 feet by the end of the century.

As seas rise, they will lift Oahu’s aquifer, a lens of fresh water that rests atop seawater in a complex of underground sedimentary deposits that range from porous limestone to less permeable alluvium, or caprock.

The water table will eventually break out above the land surface, "creating new wetlands and expanding others, changing surface drainage, saturating the soil, and inundating the land depending on local topography," the report says. "Flooding will start sporadically but will be especially intense seasonally when high tide coincides with rainfall."

In Mapunapuna, water only 2 feet deep topped curbs and flooded businesses. The brackish water also has a corrosive effect on the chassis of any car that drives through.

A graphic accompanying the research report shows much of Kakaako, Ala Moana, Waikiki and Moiliili flooded with a 3-foot sea level rise. The flooded area is about 10 percent of the land a kilometer or less from the sea or waterways like the Ala Wai Canal.

Of the flooded area, 58 percent is attributable to groundwater rather than seawater, Rotzoll and Fletcher say.

"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 SLR without considering the local groundwater table," Rotzoll said. "We hope other coastal communities use our research as the basis for conducting their own localized analysis."

The problem is applicable to most low-lying coastal lands where the water table is near the surface, the scientists say.

Global estimates suggest that more than 20 million people live below normal high-tide levels and more than 200 million people are vulnerable to flooding during temporary sea-level extremes, they say.

The effects won’t be felt abruptly, Rotzoll said in a telephone interview.

"It will take a while, and it will show up at high tide first," he said. "It’s pretty much the same as what happened in Mapunapuna."

Fletcher and Rotzoll will be presenting their findings at the meeting of the American Geophysical Union Dec. 3-7 in San Francisco.