Regulatory Floodway Administration & No-Rise Certifications

Regulatory Floodway Administration & No-Rise Certifications

Understanding the Regulatory Floodway

In riverine floodplains, the Special Flood Hazard Area (SFHA) is conceptually divided into two distinct parts: the regulatory floodway and the floodway fringe.

The regulatory floodway includes 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 (usually one foot federally, though some states have stricter standards, such as a 0.1 foot rise or no allowable rise during mapping).

The floodway is the most hazardous part of the riverine floodplain. It carries the deepest, fastest-moving water and the bulk of the flood debris during an event. Development in the floodway fringe generally causes floodwaters to spread out and rise only slightly, but development in the actual floodway acts as a direct obstruction, causing significant upstream backing up of water. Therefore, the NFIP imposes severe restrictions on any encroachment into the regulatory floodway to protect upstream and adjacent properties from artificially increased flooding.

The "No-Rise" Certification Requirement

The most stringent regulation regarding the floodway is found in 44 CFR 60.3(d)(3). It mandates that a community must "prohibit encroachments, including fill, new construction, substantial improvements, and other development within the adopted regulatory floodway unless it has been demonstrated through hydrologic and hydraulic analyses performed in accordance with standard engineering practice that the proposed encroachment would not result in any increase in flood levels within the community during the occurrence of the base flood discharge."

In practical terms, "any increase" means exactly that: a 0.00 foot rise. A developer cannot assume that a "small" piece of fill, a narrow bridge pier, or a seemingly insignificant fence is negligible. To obtain a local floodplain development permit for any activity in the floodway, the applicant must provide a "no-rise" certification.

The Step-by-Step Engineering Review

A no-rise certification is not a simple letter stating an opinion. It is a formal document backed by rigorous mathematical modeling. The process typically involves a complex step-by-step engineering review:

1. Hydrologic and Hydraulic (H&H) Analysis: A registered professional engineer must perform an H&H analysis. This involves using approved hydraulic modeling software (such as HEC-RAS, developed by the U.S. Army Corps of Engineers) to simulate the river's flow and water surface elevations.
2. Duplicate Effective Model: The engineer starts by obtaining the original hydraulic model used by FEMA to create the effective FIRM. They run this model on their own computers to ensure they can replicate the published BFE data exactly.
3. Corrected Effective / Existing Conditions Model: The engineer updates the duplicate effective model with any physical changes to the river channel or floodplain that have occurred since the original map was produced (e.g., natural erosion, previously permitted bridges). This establishes the true, current baseline.
4. Proposed Conditions Model: The engineer then adds the physical dimensions of the proposed development (the bridge pier, the fill volume, the building footprint) into the model and runs the simulation again.
5. Comparison and Certification: The engineer compares the results of the Proposed Conditions Model against the Existing Conditions Model. The water surface profiles must be compared at every cross-section in the model, upstream and downstream. If the difference is 0.00 feet (no rise) or negative (a decrease in water elevation), the engineer can sign and seal the no-rise certification.

The local Floodplain Administrator is responsible for reviewing this submittal, often relying on a community engineer to verify the technical accuracy of the modeling before issuing a permit.

The CLOMR Requirement Prior to Encroachment

What happens if the proposed project does cause a rise in the BFE? Does that mean the project is absolutely impossible to build?

Not necessarily, but it requires a significantly more complex and public process. If a proposed project in the floodway will cause a rise in the Base Flood Elevation, the community cannot issue a permit based on their local authority alone. The applicant must first apply to FEMA for a Conditional Letter of Map Revision (CLOMR).

A CLOMR is FEMA's formal comment on a proposed project that would, upon construction, affect the hydrologic or hydraulic characteristics of a flooding source and thus result in the modification of the existing regulatory floodway, the effective BFEs, or the SFHA boundaries.

Crucially, the CLOMR must be obtained prior to any encroachment occurring. To get a CLOMR for a project causing a rise in the floodway, the applicant must meet strict conditions outlined in 44 CFR 65.12, which include:

• Evaluating alternatives that would not cause a rise.
• Notifying all affected property owners that their flood elevations will increase.
• Legally securing the right to flood the newly impacted properties (often through purchasing flowage easements or buying the properties outright).
• Certifying that no existing structures will be impacted by the increased flood heights.

Only after FEMA reviews the data and issues the CLOMR can the local community issue the floodplain development permit. Once the project is complete, the applicant must then follow up with a Final Letter of Map Revision (LOMR) within six months to officially change the FIRM to reflect the new, higher BFEs.