The flood hazards shown on NFIP maps are based on the best information available at the time the maps were prepared. In many areas, hydraulic and hydrologic studies were conducted to reflect the long-term projection of flood risk. Because of the infrequent occurrence of flood events and the relatively short history of the NFIP, Special Flood Hazard Areas (SFHAs) are not based only on past flooding occurrences. The fact that a flood hasn't occurred within memory doesn't mean one won't happen soon.
The 100-year flood is a relatively rare event (1% chance in any given year), but structures located in the floodplain have a significant chance (26%) of suffering flood damage during the term of a 30-year mortgage. For these reasons, flood insurance is required as a condition of receiving Federal or federally-backed financial assistance.