In 1968 the US Congress established the National Flood Insurance Program that included standards for delineating floodplains and (optionally) floodways along significant sources of flooding. This post explains how it works as administered by Maricopa County, Arizona in 2020.
Floodplain
The flood inundation limit (1% annual-chance or 100-yr in Maricopa County; sometimes also the 0.2% annual-chance or 500-yr in other jurisdictions) is estimated with approximate methods (usually for Zone A) or modeled with detailed methods like HEC-RAS (successor of HEC-2) or another detailed model (for Zone AE). This is called the floodplain.
FIRM and FIS Report
For detailed models (Zone AE), flood elevations are also published by FEMA. If the model was a one-dimensional stream water surface profile model like HEC-RAS (successor of HEC-2), FEMA publishes selected cross-sections on a Flood Insurance Rate Map (FIRM) and report, identifying them with letters.
Work Map
But the original Flood Insurance Study submitted to FEMA by the community may have typically identified cross sections by numbers (like 4.326 or 0.015) that represent river miles upstream from a confluence. And it may have used additional cross sections not shown on the FIRM. It also may have established floodway elevations (see below). So it's usually worth a bit of work to attempt to locate the topographic work map and model that FEMA requires to be submitted with all Flood Insurance Studies and revisions.
Floodway
Problem
Every building placed in a floodplain not only incurs risk to itself, but can increase flooding to its neighbors by obstructing and redirecting flow. The cumulative effect of development in the floodplain is to raise flood elevations.
So on one hand, it's desirable to prohibit development in the floodplain unless there are either 1) engineering calculations showing no elevation increase or 2) an official notification, approval, and map revision process to allow flood elevation increase. But this can be unfeasibly expensive for a small project like a house.
So on the other hand, it's desirable to allow some development in the floodplain as long as 1) buildings are high enough for the cumulative result and 2) the cumulative result is not too much.
Solution
The National Flood Insurance Program uses the concept of floodways to provide a way to establish how much of a floodplain can be developed without extensive additional studies for every new project. A flood study may provide two calculations: one of the base condition and the other with the river flow squeezed enough to raise the base water surface as much as 1 foot. The squeezed flow corridor is the floodway, and the resulting elevation is the floodway elevation. HEC-RAS (and its predecessor, HEC-2) makes it easy to do both calculations in the same model.
Once a floodway is adopted, floodplain development can be allowed outside the floodway as long as it is safe from (1 foot above) floodway (squeezed) flooding. In Maricopa County, this is called the regulatory flood elevation. This development can proceed without meeting any water surface change requirements.
Without Floodway
In Maricopa County, Arizona, building floors must be at or above the Regulatory Flood Elevation. The Regulatory Flood Elevation is two (2) feet above the Base Flood Elevation if no floodway
has been delineated. If a floodway has been delineated then the Regulatory Flood Elevation is
one (1) foot above the floodway elevation (or one (1) foot above the Base Flood Elevation,
whichever is higher, though the floodway elevation should always be higher).
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