Williamson County, TN: flood risk, in FEMA's own numbers
On FEMA's National Risk Index, Williamson County scores at the 88.2th percentile for flood hazard, rated "Relatively Moderate" — a middling flood hazard by national standards. The hazard here is driven by inland rivers and flash flooding rather than the coast — FEMA's model puts a flood event here at about 2.2 times a year. That's a county-wide figure, not a per-address one — FEMA's parcel maps are a live lookup, not a bulk file.
Who actually carries the flood insurance FEMA requires
Homes with a federally backed mortgage inside Williamson County's mapped high-risk flood zone (the Special Flood Hazard Area, or SFHA) are legally required to carry an NFIP flood policy. In practice, 86.3% of them do — 687 active policies against 796 structures FEMA counts in that zone, leaving a real compliance gap. Looking beyond the mandated zone, 2.4% of all residential structures in Williamson County carry flood coverage voluntarily or otherwise — relevant because most NFIP claims nationally come from outside the mapped SFHA.
Where Williamson County ranks
Among the 3144 US counties FEMA scores for flood hazard, Williamson County ranks 389 — 1 being the highest hazard nationally — and 272 by population among all 3144 counties covered here, where 1 is the largest. Williamson County, Tennessee covers 582.911 square miles and counts roughly 269,136 residents.
What to do next if you own or are buying in Williamson County
Across all of Williamson County's 53,519 residential structures, 1,284 carry an NFIP policy today — a county-wide count that still can't tell you one address's own mapped zone.
Where Williamson County's figures come from
Every figure on this page traces to a named FEMA dataset, documented in the methodology guide. If you're unsure why any of this insurance requirement exists at all, the SFHA explainer covers it from first principles.