Flood plains are by definition subject to periodic flooding. They are generally characterized by relatively flat topography and soil types that were laid down during past inundations by flood waters. If your property is in the 100-year flood plain, there is a 1-in-100 chance in any given year that your property will flood. If it is in the 25-year flood plain, there is a 1-in-25 chance in any given year that your property will flood. The statistical chance of flooding is not changed by an anyone flooding event, but repeated flooding may result in the flood plain being recalculated. A 100-year flood plain is always wider than a 25-year flood plain, and the 25-year flood plain is contained within the 100-year flood plain. The flood-prone areas of the United States cover approximately 150,000 square miles or 94 million acres of land, an area roughly the size of the State of Montana. People living in flood plains are 26 times more likely to experience a flooding disaster than they are a fire disaster during the life of the 30-year mortgage on their homes. The changes in flood plain maps reflect changes in land use (such as increased building activity), changes in the waterways, and flood control improvements (such as detention ponds or other flood control measures). As more lots are covered with more buildings and parking lots, the amount of water that flows into creeks and lakes increases because there is less vegetation to absorb the water when it rains. This is one reason why buildings that were not originally built in a flood plain are now in the 25-year or 100-year flood plain.

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