A floodway is a specific, regulated area of a riverine floodplain that is designated for the conveyance, or movement, of floodwaters during a major event. This designated channel is intentionally kept clear of obstructions to ensure that high-velocity water can flow through without causing widespread damage elsewhere. The strict management of floodways is a primary component of flood control and risk reduction for communities located in flood-prone areas.
Defining the Floodway’s Critical Function
The floodway’s existence is defined by a specific hydraulic necessity: it must be wide and deep enough to pass the flow of the “base flood” without raising the water level more than a minimal amount. The base flood, commonly referred to as the 100-year flood, is the flood event that has a one percent chance of being equaled or exceeded in any given year. This area is the most dangerous part of the floodplain, characterized by the deepest water and the fastest current velocities.
This designated zone is the channel of the river or watercourse and the adjacent land area that must be reserved to discharge the base flood without causing a significant increase in the water surface elevation (WSE). Federal regulation mandates that any encroachment must not result in an increase of the WSE exceeding a specific height, which is often set by local communities but must not exceed one foot. Many communities and states have adopted a stricter “zero rise” policy, meaning the increase must be zero feet.
If obstructions, such as buildings, fill material, or debris, are placed within the floodway, they disrupt the flow, causing the water to back up. This backup, known as a surcharge, increases the flood elevation upstream, potentially causing flooding on properties that would otherwise remain dry.
Distinguishing the Floodway from the Floodplain
The floodway is not the entire floodplain; rather, it is a sub-area within the larger Special Flood Hazard Area (SFHA), which is the land inundated by the base flood. The SFHA, often mapped by the Federal Emergency Management Agency (FEMA) as Zone A or AE, is composed of two primary parts: the floodway and the flood fringe. The regulatory requirements differ significantly between the two zones.
The flood fringe is the remainder of the floodplain that lies outside the floodway boundary. This area is where floodwaters are stored, but they do not contribute significantly to the high-velocity movement of water downstream. Flood fringe areas generally have shallower, slower-moving floodwaters compared to the floodway.
FEMA’s Flood Insurance Rate Maps (FIRMs) are the official documents that delineate these boundaries. While development in the flood fringe is still regulated, it is typically permitted if structures are elevated or floodproofed to or above the Base Flood Elevation. The floodway, conversely, is the area reserved for high-velocity flow and is subject to far more stringent regulations.
Land Use Regulations and Restrictions
Property located within a designated regulatory floodway faces the most restrictive land use requirements due to the need to maintain unobstructed water flow. The overarching standard is the “zero rise” requirement, which means any proposed development cannot cause any measurable increase in the Base Flood Elevation. This rule effectively prohibits the placement of fill material or the construction of new structures in most circumstances, as these activities invariably displace floodwaters.
For any activity in the floodway—including new construction, substantial improvements to existing structures, or placing fill—a development permit is required from the local jurisdiction. To obtain this permit, the applicant must provide a detailed hydrologic and hydraulic analysis, often called a “no-rise” certification. This certification must be signed and sealed by a registered professional engineer, demonstrating through complex modeling that the project will not increase flood levels anywhere in the community.
If the engineering analysis shows any increase, the project will not be permitted unless a formal map revision is pursued through FEMA. Local governments are responsible for enforcing these standards, which are mandated by federal law through the National Flood Insurance Program, to ensure the community’s continued eligibility for federal flood insurance and disaster assistance.