Planner Speak: What is a flood plain exactly?

When the roads get wet and slippery I know that my car can hydroplane. Can it flood plain too? What is a flood plain exactly?

A flood plain (n.) is the area next to a stream or river that may be covered with water during a flood.

The Federal Emergency Management Agency (FEMA) maps flood plains based on the risk and type of flooding. One line that FEMA draws is the boundary of the 100-year flood zone. Every year there is a 1% chance that flood waters could reach or pass the boundary line of the 100-year flood zone. Don’t let the name mislead you. It is possible for a lot of the land within the 100-year flood zone to flood on a fairly regular basis, far more often than once every hundred years. FEMA considers these lands to be Special Flood Hazard Areas, with a high risk of flooding.

FEMA also maps the boundary line of the 500-year flood zone. Similarly, every year there is a .2% chance that flood waters could reach or pass this boundary line. FEMA considers these lands to have a moderate risk of flooding.


The interactive map below shows the 100-year and 500-year flood zones in Hamilton County as well as the floodways. The floodway is the channel of a river or stream as well as the land next to the channel that has to stay open to carry the deeper, faster moving water during a flood. In the map below, the floodway is blue, the 100-year flood zone is red, and the 500-year flood zone is pink.

Flood plains protect the health and quality of our rivers, lakes, and streams and the water that comes through our faucets. They help to control the flow of water and can absorb pollutants and sediments, such as gravel and clay, before they drain into streams and rivers. Flood plains are home to many types of plants and wildlife, including threatened and endangered species. It is important for communities to protect our flood plains both to keep our rivers, lakes, and streams healthy and to keep our homes and businesses from being damaged by floods.

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