A FEMA Floodway is defined as the channel of the river or stream and any adjacent land area that will allow floodwaters to pass without increasing the water surface elevation by more than one foot. This is the national minimum standard that begins to put limits on floodplain development. Pierce County takes a no adverse impact approach to floodplain development that builds on this initial FEMA floodway concept.
Washington State (WAC 173-158) does not allow structures or development to occur within a regulated floodway. Additionally, because of the severity of the danger during a flood, as homes become substantially damaged for any reason they are not allowed to redevelop.
The FEMA Floodway is calculated by modeling the area inundated by the base flood and then mathematically squeezing the floodplain with an encroachment. With less room in the floodplain the water surface will rise as it is squeezed. Once the water surface has risen one foot the mathematical squeezing is stopped and at that point the FEMA Floodway is drawn. This can be seen in the graphic below.
Base Flood Elevation
The base flood is synonymous with the one-percent-annual-chance flood or what used to be called the 100-Year flood since it had a 1-in-100 chance of occurring any given year. The base flood elevation (BFE) is the height that the water is calculated to reach in the base flood. An example of this can be seen in the graphic below.