|Flood mitigation projects are the most common applications submitted for FEMA mitigation funding. Understanding basic flood concepts and terms will ensure that you collect the right information and documentation for FEMA’s cost-effectiveness requirements.|
These concepts include:
- Floor Elevation (FFE)
- Flood Elevations and Discharges, and
- Streambed Elevation.
Flood mitigation projects protect structures from flood damage.
In general, the first floor elevation, or FFE, is the lowest floor. For a one-story house without a basement, the FFE is where a person enters the front door.
Flood elevation is how high the water rises in a flood event. Flood events are often referred to as 10-year, 50-year, or 100-year flood events.
A 100-year flood is not a flood that happens once every hundred years. The number “100” is a measure of probability in any given year, in this case, the probability of the flood occurring is 1 in 100. Since 1 divided by 100 is .01, the 100-year flood has a .01, or 1%, chance of occurring in any given year.
Similarly, a 10-year flood has a 1 in 10 chance, or 10%, annual chance of occurring. A 50-year flood has a 1 in 50 chance or 2% annual chance. A 500-year flood has a one-fifth of one percent annual chance.
Larger floods will have higher flood elevations. Here’s a residential structure and the flood elevations for that structure. Here’s the flood source and the elevations for the channel and surrounding area.
The 100-year flood elevation is also known as the Base Flood Elevation or BFE. The BFE is the same as the 100-year or 1% annual chance flood elevation for the structure.
The community’s Flood Insurance Study (FIS) provides flood elevations information. Here’s a residential structure and the flood elevations for that structure. Here’s the flood elevations data, usually depicted in a graph in the FIS.
The same graph provides the streambed elevation—the elevation for the bottom of the flood source, and the baseline for flood elevations.
The FIS also provides peak discharge values for the four flood events. “Discharge” is the rate of flow for each flood event—the larger the flood event, the larger the discharge value.