NHSG research: Seacoast roads under new threat from rising sea levels

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New Hampshire roads as far as two miles from the ocean are facing a new threat that can’t be currently seen by drivers: Rising groundwater caused by increasing ocean water levels.

Researchers at the University of New Hampshire have identified specific sections of Seacoast roads — including Route 286 in Seabrook and Gosling Road in Portsmouth — as those most vulnerable to impacts from rising groundwater due to sea-level rise. Without more drastic improvements to the materials at or below the pavement surface, motorists can expect segments of these affected roads to deteriorate more quickly, require more frequent maintenance and experience longer periods of closure as groundwater levels rise, according to the study. The N.H. Sea Grant-funded research, led by UNH professors of civil engineering Jo Daniel and Jennifer Jacobs, was recently published in the journal Transportation Research Record.

“Previous road vulnerability studies have looked at road surface flooding, but groundwater had not been addressed,” said Jayne Knott, Ph.D. candidate at UNH and lead author of the study. “We found that groundwater rises further inland than the surface water effects. In coastal N.H., the effects of surface water flooding on roads occur within a mile of the coast, and groundwater effects occur to more than twice that, all the way to Pease Tradeport,” she said.

Groundwater levels are higher than sea levels, and that drives the groundwater discharge to the ocean, Knott explained. But as sea levels begin to rise, this forces groundwater to slowly rise up to maintain the equilibrium, inching closer to the pavement base layers that need to stay dry to maintain their strength.

“The worst enemy of pavement is water,” said Daniel.  “If the soil and substrate under the pavement get wet, then the strength that we had counted on to carry the traffic isn’t there anymore. So the pavement develops ruts and cracks, allowing more water to get into the underlying layers which makes the situation worse,” she explained. Roads must then remain closed for longer periods of time to dry out, thus impacting commuters and tourists alike.

For the study, the research team examined the cross-section data for the most vulnerable sections of five Seacoast roads — Spaulding Turnpike, Gosling Road, Route 286, Route 101 and Middle Street. Highways are usually built more stout, Daniel said, with thicker cross-sections of materials to withstand heavier traffic, while smaller town roads are sometimes little more than layers of pavement over shallow depths of crushed gravel. The thickness of the pavement base layers provides a buffer that protects the road as groundwater rises, Knott explained. The roads where groundwater is already close to the surface are the ones that will likely be affected first, although local geology, topography, soil type and drainage can also influence this, she said.

Knott then compared the N.H. Department of Transportation (NHDOT) road cross-section data with current and projected groundwater levels given various sea-level rise scenarios ranging from one foot by 2030 to 6.6 feet by the year 2100. The results indicate that although Route 101 and the Spaulding Turnpike will probably not be adversely affected by rising groundwater until late in this century, Route 286 — an emergency evacuation route from the beach — and Gosling Road are likely to be some of the first roads affected.

Ann Scholz, research engineer with the N.H. Department of Transportation (NHDOT), noted that the NHDOT has been working with Knott, Daniel and others to provide data in order to identify vulnerable road sections. Knott’s results will provide an engineering resource to address this upcoming challenge for the NHDOT.

“The NHDOT does not intend to waste taxpayers’ dollars and will use funding for road designs that will be resilient,” Scholz said. “These climate change issues are on our radar. As long as people live and work in vulnerable areas, we will need to make sure they remain accessible,” she added.

For more information, please contact Jo Daniel at jo.daniel@unh.edu or 603.862.3277.

N.H. Sea Grant promotes the wise use, conservation and sustainable development of marine and coastal resources in the state, the region and beyond. Located at the University of New Hampshire, NHSG is part of a national network of programs located in our coastal and Great Lakes states as well as in Puerto Rico and Guam.  

Rebecca Zeiber, N.H. Sea Grant Science Writer