Floodplain Management


Determining Base Flood Elevations

Base flood elevations (BFEs) are also known as the 1% annual change flood elevation or 100-year flood elevation. These elevations can be determined in different ways depending on the source of the floodplain map and zone of the site.

How to Determine BFEs

If the site is in a Zone AE, Zone A1-30, Zone AO or Zone AH on the current effective Flood Insurance Rate Map (FIRM), then the BFE can be determined (to the nearest tenth of a foot for Zone AE or Zone A1-30) by using the current effective FIRM and Flood Insurance Study (FIS).

Straight River in Steele County (AE Zone)

In this example, we are looking at a site located just downstream of cross-section “AF”, as identified on the FIRM. We can find the Base Flood Elevation by referencing this cross-section using the Floodway Data tables located in Steele County’s Flood Insurance Study.

site located downstream of cross-section AF, as identified on the FIRM

Regulatory Flood Protection Elevation

To determine the Regulatory Flood
Protection Elevation (RFPE), add in the stage
increase, as well as the 1 foot of freeboard
required by Minnesota Rules.

 

100-year flood elevation
+ stage increase
+ 1 foot freeboard
_____________________

RFPE

 

1091.0
0.3
1.0
_______________

1092.3

 

chart to determine Regulatory Flood Protection Elevation

For the most accurate BFE determination, scale the distance along the stream reach from a given landmark, and evaluate the flood profile accordingly.

graph to determine Regulatory Flood Protection Elevation

Since Steele County has had their maps updated since 2003, that same location shown above can also be found through the National Flood Hazard Layer (NFHL). Through this Digital Flood Insurance Rate Map (or DFIRM), BFEs are shown on the map at their respective cross-section. To find the RFPE, you would cross-reference the Flood Insurance Rate Map to identify the appropriate stage increase as shown above. Know whether you are referencing 1929 Datum (NGVD 29) or the current national standard, 1988 (NAVD88), and know how to convert it.

Digital Flood Insurance Rate Map (or DFIRM)

Developments Proposed in a Zone A

Any developments proposed in Zone A (where base flood elevations are not identified) would require applicants to reasonably utilize other BFE and floodway data as a basis for floodproofing or elevating to or above the RFPE. This would typically require running a floodplain model consistent with hydrological and hydraulic engineering standards. In many areas, models have already been created, and this model-based elevation data is available through Minnesota Geospatial Commons. There are a number of models available for A-Zones and other areas that haven’t been mapped at all. In other instances, the local watershed district or the DNR may be able to provide an estimated BFE.

Availability of Preliminary Maps

Some areas may have better data because they are in the process of getting new floodplain maps. This preliminary elevation data can be accessed from our FTP site, and be used as “best available data.”

Resources