Original data · Maryland
Maryland’s basement flood risk, off the map
We pulled every FEMA basement flood-insurance claim on record for the 4 Maryland counties in our data, 1,223 in all, and sorted them by whether the property sat inside the mapped 100-year floodplain. 64% were filed outside it, where coverage isn’t required and almost nobody carries it. And the rain is intensifying: heavy-rain days around Gaithersburg are up 40% since the late 1990s.
The rain that overwhelms drainage is rising
Heavy-rain days (an inch or more, the storms that outrun drainage) per year, late 1990s versus the recent decade. From ERA5 climate reanalysis.
| City | 1990s → now (heavy days/yr) | Change |
|---|---|---|
| Gaithersburg | 7.5 → 10.5 | +40% |
| Baltimore | 9.9 → 12.6 | +27% |
| Rockville | 7.9 → 10 | +27% |
Where the flood claims land, by county
Maryland counties with at least 100 basement flood claims, ranked by the share filed outside the mapped floodplain. 670 of these claims were caused by rainfall or snowmelt accumulation, seepage rather than a river cresting.
| County | Cities | Basement claims | Median claim | Outside floodplain |
|---|---|---|---|---|
| Montgomery County | Gaithersburg, Rockville | 442 | $3,842 | 92% |
| Baltimore city | Baltimore | 498 | $5,000 | 52% |
| Frederick County | Frederick | 145 | $7,807 | 52% |
| Washington County | Hagerstown | 138 | $4,868 | 37% |
Why the outside-zone share is a floor
These are National Flood Insurance Program claims, so every one is a property that had flood insurance. Inside the floodplain a federally backed mortgage effectively requires it; outside, almost nobody buys it. So the pool is heavily weighted toward flood-zone properties, and out-of-zone basements are badly under-represented before we start counting.
That means 64% is the share we can prove, not the share that happens. The real proportion of Maryland basement flooding outside mapped zones is almost certainly higher. This is not a rate of flooding, and nobody should use it as one. Geography is county, not city: FEMA redacts the city field, so a county is not any single town within it.
The structures built to hold it back are aging
And the dams meant to control these floods are old. From the Army Corps of Engineers’ National Inventory of Dams:
Most of them were built for the rainfall and the flood risk of decades ago, and a good share are now past the working life they were meant for. The storms feeding them have grown heavier since: heavy-rain days around Gaithersburg are up 40% since the late 1990s. 166 of the state's dams are locally owned, so the cost of keeping them standing lands on local budgets.
Source: US Army Corps of Engineers National Inventory of Dams (nid.sec.usace.army.mil), updated 2026.
Sources & methodology
Flood claims: FEMA openFEMA FimaNfipClaims (redacted), filtered to buildings recorded with a finished or unfinished basement and classified by rated flood zone (A or V zones are the Special Flood Hazard Area). Rainfall: ERA5 reanalysis via the Open-Meteo archive, daily precipitation 1995-2024; a heavy-rain day is 25.4mm (1 inch) or more. National context: across 129,000+ basement flood claims in 123 US counties, 29% were filed outside the mapped floodplain. Flood-control infrastructure: US Army Corps of Engineers National Inventory of Dams, updated 2026.
WatertightHub (2026). Maryland basement flood claims by flood zone, and heavy-rain trends. Analysis of FEMA NFIP, ERA5, and USACE National Inventory of Dams data. watertighthub.com/basement-flooding-outside-flood-zones/md
Free to republish with attribution and a link (CC BY 4.0). Per-county cuts and the rainfall-vs-river cause split available on request.
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