floodway

The floodway is the portion of a floodplain that carries the main flow of floodwaters during a base flood, typically the 1-percent-annual-chance flood (the 100-year flood). In many regulatory contexts, the term is used to describe the channel and adjacent floodplain area necessary to convey the base flood without causing a significant rise in water surface elevation. The floodway is determined through hydraulic modeling using river cross-sections, topography, and flood-flow data, and is shown on flood hazard maps such as flood insurance rate maps in the United States.

The primary purpose of the floodway is to preserve the river’s capacity to convey floodwaters and to limit increases in downstream flood levels. This makes the floodway a key element in flood risk management and land-use planning. It is distinguished from the surrounding floodplain area known as the flood fringe, where development can occur with appropriate flood-proofing or elevation measures but where encroachment into the floodway is generally restricted because it can raise water levels.

Regulatory practice typically requires permits for any development within the floodway. Proposals for fills, new structures, or substantial alterations must demonstrate that they will not cause a rise in the base flood elevation. In some jurisdictions, only limited or nonstructural uses are permitted, or activities may be allowed if they do not impede the conveyance of floodwaters.