When purchasing a property, you may here the term “100 year flood plain”. But what does that actually mean? Counties and cities conduct flood hazard mapping as part of their planning process and insurance companies use that information to formulate your insurance premiums. It is the statistical chances of the property having an extreme hydrologic event. Thus in one year, a property with a 100 year flood plain will have a 1% chance of having an extreme flooding event in a given year whereas a 500 year flood plain will have a 0.2% chance of having an extreme hydrological event. Flood plains statistics are based on historical data and are ever-changing.