A no-rise certification is a licensed engineer’s finding that a project in a regulatory floodway will not increase the base flood elevation. The number that matters is 0.00 feet. It is not a FEMA map change. It is not a floodplain development permit. It is the technical showing the community usually needs before it will let you put fill, a culvert, a bridge, or a pad in the floodway.
The rule sits in 44 CFR 60.3(d)(3). NFIP communities that have a mapped floodway have to prohibit encroachments that raise the base flood, unless the applicant first gets a Conditional Letter of Map Revision from FEMA.
Floodway, not just “in the floodplain”
The floodway is the channel plus the overbank FEMA reserved to carry the 1-percent-annual-chance flood. The rest of the Special Flood Hazard Area is the flood fringe. A no-rise is a floodway problem. Work only in the fringe can still need a local floodplain permit, elevation, and floodproofing. It does not automatically need this certificate.
If the community has AE zones and no adopted floodway, the test is different (often a one-foot surcharge cap, plus a CLOMR if you would exceed it). Do not treat an unnumbered A zone like a studied floodway.
What the engineer actually models
You compare an existing-conditions hydraulic model to a proposed-conditions model of the same reach. Same flows, same method, same downstream boundary — only the project changes. If water-surface elevations do not go up (0.00 feet, or they drop), the engineer can certify no-rise.
That is hydrology and hydraulics, not a letterhead. See hydrologic and hydraulic modeling. On the Gulf that is often a bayou or canal with a flat slope, tide at the downstream end, and a lot of overbank storage. 1D HEC-RAS is common. 2D is worth it when the floodway is a network of ditches or the overbank is the real conveyance.
A no-rise does not mean the project is dry. It means you did not push the base flood onto someone else.
If you cannot show 0.00
The community can still allow the work, but only after FEMA issues a CLOMR for the rise and the floodway revision. After construction you follow with a LOMR so the FIRM matches what you built. That is the LOMR / LOMA / CLOMR path, not a no-rise.
Do not start dirt work in the floodway on a promise that “we will get the CLOMR later.” The local floodplain administrator’s NFIP standing is what you are asking them to risk.
What it is not
- Not a LOMA. A LOMA takes a structure or lot out of the SFHA because the natural ground was mapped wrong. It does not authorize floodway fill.
- Not an elevation certificate. An EC records elevations. It does not prove surcharge.
- Not a 404 permit. Floodway and waters of the United States are different maps.
- Not a promise about insurance.
How we use it
We model the reach, say whether no-rise is honest, and write the certification when the numbers support it. If they do not, we say so and talk about a CLOMR — or a smaller footprint — before you spend a year on a letter FEMA will not give you.
Cypress Environment & Infrastructure works from Biloxi, Mobile, and Pensacola, throughout the Southeast. Office hours are Monday–Friday, 9 a.m.–5 p.m.
See flood zones and base flood elevation and What is a floodplain?.