In the practice of land development, it isn’t uncommon to encounter sites that are partially covered by a mapped 100-Year FEMA floodplain. That floodplain can have several implications for a project’s path forward depending on the situation, so it’s important to correctly interpret what the Flood Insurance Rate Map (FIRM) panel is saying. The FIRM panel provides a lot of information, and in many cases, it shows an area called the Regulatory Floodway inside of the 100-year floodplain. A common question we receive from clients (and from other engineers for that matter) is, “What is the Floodway?”
First, some quick context and a few acronyms before diving into that question. The Federal Emergency Management Agency (FEMA) administers the National Flood Insurance Program, or NFIP. As part of the NFIP, FEMA publishes Flood Insurance Rate Maps (FIRM panels) that show the floodplain in an area. The local jurisdiction such as a town, city, or county is responsible for overseeing floodplain management in that area and ensuring that the requirements of FEMA and NFIP are satisfied for a given development. FEMA empowers local jurisdictions to enforce floodplain development requirements over and above what is required by the NFIP, so always check with your local floodplain administrator at the start of a project to learn more about any floodplain requirements that may be specific to that jurisdiction.
There are several types of floodplains that may be shown on a FIRM panel, but the Floodway appears only in Zone AE. Zone AE is a studied reach of riverine (i.e. creeks, streams, rivers) flooding and shows how far flood waters will expand around a stream reach during the 100-year storm event. The 100-year storm has a 1% chance of occurring in any given year and is the basis for the majority of floodplain regulations. On most (though not all) recently studied, mapped streams, the 100-year Zone AE floodplain will be shown with two parts: the 100-year flood fringe and the Regulatory Floodway. The Floodway is usually shown as a hatched area around the centerline of the watercourse like in the example below.
By FEMA’s definition, the Regulatory Floodway is,
“… the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height.”
In simpler terms, the Floodway is the area around a watercourse that cannot be filled in or have obstructions placed in it without causing water surface elevations to increase over a certain amount upstream of the filled area or obstruction. That “certain amount” is commonly 1 foot, though again, requirements may vary in some jurisdictions. Since the Floodway is reserved to ensure floodwaters can safely pass downstream, it carries heavier regulatory requirements than the flood fringe. In general, any project that will fill inside the Floodway boundary, alter the location of the Floodway (i.e. rerouting a channel), or increase the extents of the Floodway will require that an application for a Conditional Letter of Map Revision or Letter of Map Revision be submitted to FEMA to evaluate, approve, and update the FIRM panel to reflect the change.
The flood fringe is simply the remainder of the 100-year floodplain that is outside of the Floodway. This area typically carries less stringent regulations, but requirements for developing within the flood fringe will vary by jurisdiction.
About D.J. Strickland, PE, CFM
D.J. Strickland is a Project Manager for the Land Development division in Birmingham, Alabama. He earned his B.S. of Civil Engineering from Auburn University, and has experience in hydraulic and hydrologic modeling, floodplain management, floodplain mapping, and advising colleagues on successfully navigating floodplain-related issues.