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Details of a Floodplain

Flat, occasionally flooded land next to a river is called a floodplain.  A floodplain is a natural and beneficial part of a river system.  It gives the river a place to temporarily expand.  By combining complimentary uses, a floodplain allows humans and nature to interface without jeopardizing one another.  Examples of positive floodplain stewardship between people and nature are recreational uses (golf course, parks, etc.)—these types of features provide people with open space without compromising a river's ability to sprawl over its natural course. 

Special Flood Hazard Areas

The floodplains that are designated on Flood Insurance Rate Maps as Special Flood Hazard Areas may or may not look like the description given above.  More often than not, the Special Flood Hazard Areas do not contain any characteristics that appear different from the surrounding geography. This is because the Special Flood Hazard Areas are a statistical probability notion per FEMA's regulations.  A line on a Flood Insurance Rate Map separating what is "in" the floodplain from what is "out" of the floodplain does not always correspond to a recognizable ground feature or vegetation zone. It really just serves as an approximate dividing line between areas that have different statistical probabilities of being flooded.

People who have lived through floods know that floods are not always statistically oriented. Thus, the dividing line on Flood Insurance Rate Maps between the Special Flood Hazard Areas and the rest of the region is an engineer's model of the watershed runoff characteristics, which determines how friction at areas such as bridges and culverts is going to affect the water level during a 100-year flood. 
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