What is a Deep and/or Fast Flowing Water Floodway?
Pierce County set the minimum threshold for the deep and/or fast flowing floodway for life safety. The curve below comes from a US Bureau of Reclamation study for any size child to be in danger.
The Channel Migration Zone page described the ability of a river to erode a bank but water has a strong carrying capacity. It only takes 12 inches of water to float cars and small SUV’s off a road. Learn more about flood safety.
An example of this type of floodway is located in the Riverside/Clear Creek area. You can find more information in the fact sheet located to the right.
Deep and/or fast flowing (DFF) floodways occur where the topography increases either the depth of the floodwaters and/or the speed in which the floodwater moves through an area. The DFF floodway is anywhere the combination of flood depths and velocity exceed the area shown on the graph below. Simply stated it is where flood waters are three feet deep or moving at three feet per second, or a combination of the two.
These areas are regulated as floodways and are identified based on analysis and field observations following a flood event. In a deep and/or fast flowing water area, like other floodways, development and repairs are severely limited to ensure people are moved out of danger.