Flood Risk
100-Year Flood Explained: What It Means and Why It Can Happen Twice
A plain-English guide to the 100-year flood idea, annual chance, flood maps, development, and changing rainfall risk.
Quick answer: A 100-year flood means a flood level with about a 1 percent chance of being equaled or exceeded in any given year, based on current analysis.
Not a schedule
The term does not mean the flood happens exactly once every century.
Two 100-year floods can happen close together.
Mapped risk
Flood maps depend on river data, rainfall assumptions, terrain, drainage, and development.
Local flooding can occur outside mapped zones.
Planning
Insurance, elevation, drainage, and evacuation planning should account for more than the phrase 100-year flood.
Risk is about probability and consequences.