N.H. to Study How Rising Seas Will Affect Seacoast Highways, Local Traffic | New Hampshire Public Radio

N.H. to Study How Rising Seas Will Affect Seacoast Highways, Local Traffic

Feb 25, 2020

A 2015 study found that 6.6 feet of sea level rise by the end of the century would cause twice daily flooding on more than 40 percent of Route 1A, seen here underwater after the March 2018 nor'easter.
Credit Kim Reed / UNH

State officials are using federal money to look at how rising seas will threaten major highways and connecting routes on the Seacoast.

The project, funded by the National Oceanic and Atmospheric Administration, will build a "vulnerability assessment" for the I-95, Route 1 and Route 1A corridors, and local connector roads, including Routes 101 and 286.

Over the next year or so, researchers and planners will combine traffic models and climate change projections to find areas at a heightened risk of flooding from rising seas and storms.

They’ll assess how those hazards could affect road infrastructure or change how and where people drive.

Senior planner Julie LaBranche of the Rockingham Planning Commission will lead the study. She says their findings will help Seacoast towns prioritize and fund their road maintenance needs over time.  

"It's not like we want them to develop concrete plans that say 'you're going to be doing this 20 years from now,'” LaBranche says. “It's trying to get a handle on locations where improvements will need to be made, and coming up with sort of back-of-the-envelope estimates as to how much money that would be."

The Rockingham Planning Commission did some analysis like this for a report in 2015, based on different climate change scenarios.

They found that 1.7 feet of sea level rise by 2100, for example, would cause flooding on just over 5 miles of the Seacoast's state and local roads. Under a 4-foot scenario, flooding would occur on more than 23 miles of those roads.

Waves crest Route 1A in Rye during the March 2018 nor'easter.
Credit Dan Tuohy / NHPR file

That study says local roadways will be more impacted by flooding than state roadways - especially in Hampton, as well as Portsmouth and Rye. Storm surge greatly added to the miles that would be flooded.

A state report released last year gave a 1 in 20 chance that 4 feet of sea level rise would occur by 2100, even if carbon emissions begin to level off or decline. 1 to 3 feet of sea level rise is deemed "likely" under most scenarios.

That report also notes that sea levels in the coastal New Hampshire area rose about 8 inches between 1912 and 2018.

"This flooding will significantly impact transportation networks and their derived services, including the 18,000 drivers that use [Route 1A] every day in peak summer season," a summary of the new NOAA grant says. 

Route 1 and I-95 sit farther inland and so face less risk, the summary says, but those corridors could still see isolated flooding that would affect drivers -- and could be further impacted by traffic overflowing from a flooded Route 1A. 

The new study will evaluate those potential issues and associated costs. LaBranche says it marks the first time that towns and regional groups like hers have worked with academics and state agencies on this kind of broad climate adaptation effort.

"We've had incredible response … and, I think, recognition that it has to be this way,” she says. “It has to be a collaborative effort – it cannot be each town going it alone and everyone working alone in their individual silos, because it's just not an efficient way to evaluate the problem."

LaBranche says the project could also be a model for future studies looking at inland roads.