Federal flood maps for New Hampshire — which determine, among other things, cost and availability of flood insurance — are being upgraded.
That makes sense because scientific models of possible flooding are changing, because the world is changing, especially the parts covered with asphalt.
“Water modeling incorporates numerous factors including soil permeability, changes in ground cover. So, if there have been more parking lots, that’s going to [mean] more water is diverted rather than seeping into the soil,” said Ela Schmuhl, director of public affairs for the New Hampshire Department of Business and Economic Affairs, which is helping with the new maps.
“It’s not uncommon for base flood elevation to have raised 10 feet in an area that has experienced rapid development,” she said. New buildings, roads and pavement “will have a very large impact on the water level.”
In other words, maybe the reason you’ve started getting two feet of water in your basement after a storm isn’t just that storms are getting bigger due to climate change but also that the nearby Target has put asphalt on an area the size of a golf course.
I mention this not because parking lots are grossly overbuilt, although they are, but to remind myself that it’s hard to figure out where water will go. Which is why flood maps are so important as well as so controversial.
An open house about the process had been planned for Thursday, Nov. 13 in the Concord city council chambers with the participation of representatives from the Federal Emergency Management Administration, or FEMA. The session has been indefinitely postponed, as FEMA employees are currently unavailable due to the government shutdown, but staff from the city’s planner’s office and the New Hampshire Department of Business and Economic Affairs will still be available to anyone who has questions.
As you would expect, updating flood maps is a long and detailed process of gathering data, holding hearings and looking at records of each watershed, the term for an area that drains into a particular river. Tech improvements help, such as LIDAR, which allows topographic mapping at an astonishing detail.
Different watersheds are at different stages. New maps for the Merrimack watershed of Merrimack County, including the Concord region, will become effective on January 23, 2026, whereas the Piscataqua Salmon-Falls watershed of Rockingham County got their preliminary maps last month.
Project estimates indicate that by 2030 all active mapping projects in New Hampshire should be complete, although with the Trump administration’s destructive habits, there’s no certainty about that.
You can play with — by which I mean “study” — the proposed maps via a nifty online tool with one of those sliders that lets you compare the current vs. proposed map for many areas, including Concord. It’s on the arcgis.com website, but the easiest way to find it is to search “Flood Map Changes Viewer.”
The thing you’ll notice are blue areas, which indicate a 1% annual flood hazard — formerly called the 100-year flood — and orange/brown areas indicating 0.2% annual chance, formerly the 500-year flood. Seeing whether your property is in those areas is pretty straightforward.
But flooding isn’t a binary thing, either a yes or a no; what also matters is the depth of water that could invade your property.
That is the base flood elevation, which is also written on the online maps once you figure out how to find it. That figure is particularly important if you want to build a new structure in a floodplain or make improvements equal to at least 50% of the value of the existing structure, in which case you’ll have to raise it above the base flood elevation.
Raising something 10 feet to avoid future floods is not for the financial faint of heart, hence all the pushback when flood maps change.
There is, of course, an over-arching issue here: climate change. All the topo maps in the world won’t help you if 12 inches of rain falls in a day and the superheated atmosphere is making such deluges more likely.
That’s no reason not to plan based on what we know, of course, but keep it in mind the saying that financial folks like to use: “Past performance is no guarantee of future results.”
