PRINCETON, NJ — A four-year flood study of Harrys Brook found that no additional homes fall inside the 100-year floodplain, a Princeton engineer told the Mayor and Council during the Monday meeting.
The study covers the stretch of the brook between Snowden Lane and Harrison Street. It was the only part of a major stream in Princeton that had not been mapped to determine base flood elevation, officials said. FEMA's flood insurance rate map labeled it Zone A, which means the flood limits were an estimate.
Princeton hired The Nader Group in 2022 to study the brook. The firm did a detailed survey and ran flood models to set the base flood elevation, which is how high water would rise in a 100-year storm.
Jim Purcell, Princeton's assistant municipal engineer, said the biggest change is at Snowden Lane. FEMA's 2016 map assumed the culvert there could not handle a 100-year storm and that the brook would flow over the road. The study found otherwise.
"It turns out that not only will that culvert carry the 100-year storm, it also carries the 500-year storm," Purcell said.
As a result, the new map shows a smaller floodplain just upstream of Snowden Lane. Purcell said there were some minor changes farther along the brook, all upstream of Gordon Way, but none put more structures at risk than before.
FEMA has issued a Letter of Map Revision, and the new flood insurance rate map takes effect Feb. 7, 2027, Purcell said.
The study also set the limits of the 500-year flood, which is a 0.2 percent chance in any given year. Princeton defines its floodplain by that line. Anyone building within it needs a local floodplain development permit.
Some residents have asked whether they now need flood insurance, Purcell said. He said FEMA makes that call, and because the homes were already outside the 100-year floodplain, the federal government does not require it. A mortgage lender can still require it. In that case, he said, homeowners can ask FEMA for a letter showing their homes are outside the floodplain.
Purcell said three wooden footbridges cross the brook on private property. They would be overtopped in a 100-year storm and could collapse, he said.
The mapping accounts for the bridges, Purcell said. FEMA did not raise concerns about them, but Princeton did.
"We in Princeton have some concerns. We would like to have them removed," he said.
If the bridges are removed, the study would need to be updated because the flood map assumes they cause some water to back up, he said.
Mayor Mark Freda asked whether a collapsed bridge could block a culvert downstream. Purcell said that is the concern.
Purcell said the bridges were built by property owners without permits and that people use them to cross the brook. Owners would be responsible for repairs and would need permits to do them, he said.
Council member Brian McDonald said several families on Wheatsheaf Lane have had flooding in storms as small as a 10-year event. He asked what steps the town can take to help those residents.
Purcell said permits within the floodplain require flood mitigation. He also pointed to Princeton's stormwater rules, which require on-site stormwater management for property owners who add more than 400 square feet of impervious surface. He said the town has no large-scale flood mitigation plan right now.
Council member David Cohen said the stormwater rules are "a rearguard action" meant to slow the rate at which flooding is getting worse. He said it is a lot to expect the town to make things better for residents who live in or near floodplains.
Purcell said the town is moving away from the terms "100-year storm" and "500-year storm." A 100-year storm means a 1 percent chance of happening in any given year, he said. A 500-year storm means a 0.2 percent chance.
A resident who lives in the Harrys Brook floodplain area, thanked the engineering staff for being responsive. He said storms of that size can happen back to back. He asked the town to show the Princeton Sewer Operating Committee's easement along the brook on the tax maps. The easement covers the Harrys Brook sewer interceptor.
He also said he disagrees that the culvert on Harriet Drive can handle a 500-year storm, saying he has photos and video of water backing up there.
Freda told the resident to send his recommendations to Purcell, who would share them with the council.
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