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1. What is a floodplain?

The following terms are used to describe the 100-year floodplain.

(1) “Floodplain” means the areas subject to FEMA regulations, generally adjoining a stream or river that would be covered by floodwater of a base flood except for sheetflood areas that receive less than one foot of water per occurrence. The floodplain consists of a floodway and floodway fringe.
(2) “Floodway” means the channel of a stream or river and the adjacent overbank areas that must be reserved in order to discharge a base flood without cumulatively increasing the water surface elevation more than one-half foot.
(3) “Floodway fringe” means that portion of the floodplain outside the limits of the floodway.

The 100-year floodplain designation does not mean that a certain property will be flooded once every 100-hundred years. The 100-year floodplain designation actually means there is a one (1%) percent chance per year that a property will be flooded. In addition, a 100-year flood could take place more than once during a 100-year time frame or in consecutive years.

2. What are official floodplain maps called?

Federal Emergency Management Agency (FEMA) floodplain maps are known as Flood Insurance Rate Maps (FIRMs). Butte-Silver Bow has forty seven (47) FIRMs, including FIRMs for the Big Hole River. As each FIRM covers a large geographical area one FIRM may include the floodplain boundaries for more than one stream or river.

3. What is the purpose of floodplain mapping?

FIRMs show a visual depiction of those areas of B-SB that FEMA has determined to be at risk for what is commonly referred to as a 100-year flood. Those areas of the community that have been determined to be at risk of a 100-year flood are depicted on the floodplain maps as Special Flood Hazard Areas (SFHAs). The recently adopted FIRMs now visually depict the SFHA boundaries digitized over aerial photographs of B-SB.

4. How are floodplains mapped?

Local governments that participate in the National Flood Insurance Program are issued FIRMs developed in cooperation with FEMA. In most instances, the floodplain profiles for individual streams or rivers are developed utilizing hydrologic and hydraulic studies and on-the-ground surveys. These three tools help determine the boundaries of the floodway and floodway fringe.

However, there may be floodplain profiles for other streams on a FIRM that were not developed utilizing hydrologic and hydraulic studies and on-the-ground surveys. As such, these floodplain profiles are not as accurate as floodplain profiles developed utilizing hydrologic and hydraulic studies and on-the-ground surveys. There are several streams in the Butte-Silver Bow jurisdictional area where the floodplain profiles were developed without FEMA utilizing these three tools.

5. How are floodplains (SFHA) illustrated on FIRMS?

The SFHA of each stream or river is visually depicted on a FIRM within two blue lines. There are three (3) categories of floodplain designation which may be included on each FIRM. Those three categories are:

Zone AE – This SFHA zone includes a visual portrayal of the floodway boundaries and the floodway fringe boundaries. Zone AE also includes the Base Flood Elevation (BFE) for different reaches of the stream or river. The locations of the BFE are illustrated with a wavy black line that runs perpendicular to the stream channel. The BFE is also shown near the wavy line. Zone AE boundaries were developed utilizing the three tools described above and are therefore the most accurate boundaries. In some cases, the floodway is the same width of the floodway fringe.

Zone A – The visual portrayal of Zone A boundaries does not include the boundary of a floodway as the floodway boundaries have not been determined within the visually depicted floodplain. In addition, the BFEs for the stream have not been calculated. As such, SFHA boundaries that are visually illustrated as Zone A are not as accurate as the Zone AE boundaries.

Zone AH – Zone AH is not regulated by B-SB as these areas are only susceptible to sheet flooding of less than one foot in depth.

6. How much of Butte-Silver Bow is regulated by FIRMS?

FIRMs have been issued for the entire jurisdictional area of the City and County of Butte-Silver Bow, other than the Town of Walkerville. Consequently, FEMA has issued FIRMs for the Big Hole River and all of the perennial streams, such as, Silver Bow Creek, Blacktail Creek, Basin Creek, Grove Gulch Creek, Little Basin Creek, Columbia Gulch and Brown’s Gulch. In addition, several intermittent streams that flow off of the East Ridge are also mapped.

7. Did the recently adopted FIRMs change the SHFA boundaries if the 1980s FIRMs?

In a majority of cases, the recently adopted FIRMs did not alter SFHA boundaries for individual streams within Butte-Silver Bow. As such, if a new FIRM shows a property or structure within the SFHA it is very likely that the property or structure was in the SFHA on the FIRM developed in the 1980’s.

However, the SFHA boundaries on the new FIRMs were digitized by FEMA and then layered over aerial photographs of Butte-Silver Bow. As a result, it is much easier to establish whether a property or building is located within, or in close to proximity, to the SFHA. The adoption of the new FIRMs has resulted mortgage companies reevaluating the floodplain status of many properties within Butte-Silver Bow.

However, the Sand Creek floodplain, which runs south to north just to the west of Harrison Avenue, was significantly reduced north of Four Mile Road. A new flood study was undertaken at that time which incorporated the impact of the Sand Creek diversion that routes a significant portion of the Sand Creek stream flow through Bert Mooney Airport property. The adoption of a new SFHA for the diversion channel significantly reduced the amount of flood waters that would flow north of Four Mile Road. Consequently, the Sand Creek SFHA boundaries were significantly reduced on the new floodplain maps north of Four Mile Road.

New FIRMs were adopted for those portions of the Big Hole River adjacent to Butte-Silver Bow that were not previously regulated by FEMA approved floodplain maps. Prior to the adoption of the new FIRMs the only reach of the Big Hole River with FIRMs was from Melrose to just west of Divide. The adoption of the new FIRM’s in 2012 includes the entire stretch of the Big Hole River flowing adjacent to Butte-Silver Bow.
The visually depicted SFHA boundaries within the town of Melrose were greatly reduced with the adoption of the new FIRMs.

8. Where can I review a paper copy of a FIRM?

The official repository for paper copies of FIRMs is the Butte-Silver Bow Planning Department office. However, FIRMs for Butte-Silver Bow can also be viewed at the FEMA website, specifically the Map Service Center webpage.

The FIRMs viewed at the FEMA website are PDFs, so you can zoom in on a particular area of the map.

9. How are floodplains regulated in Butte-Silver Bow?

Floodplains in Butte-Silver Bow (B-SB) are regulated by Title 18, Floodplain Ordinance, of the B-SB Municipal Code. Title 18 was updated in January of 2012 when new and/or updated floodplain maps were adopted by the Butte-Silver Bow Council of Commissioners. (link to Title 18 of the Municipal Code.) Official floodplain maps for B-SB are issued by the Federal Emergency Management Agency (FEMA).

The most recent FIRMs for Butte-Silver Bow were adopted in January of 2012 and replace the FIRMs issued in the 1980s.

10. What kind of development is permitted in with the SHFA?

Development is permitted within certain zones of the FIRM maps provided that a Floodplain Permit has been issued for the development.

Zone AE - Floodway

The Butte-Silver Bow Floodplain Ordinance does not permit the construction of new residential, commercial or industrial buildings within a depicted floodway. Alterations to buildings within the floodway that are valued at more than 50% of the buildings value are also not permitted.

Zone AE- Floodway Fringe

The floodway fringe does allow the construction of new residential, commercial and industrial buildings and alterations to the buildings. However, new construction is required to meet the following regulations:

Residential – the top of the lowest living floor must be elevated a minimum of two (2’) feet above the BFE for that particular location of a mapped stream or river. The residence can be elevated utilizing infill (soil pad), piers or stem walls. The soil infill (pad) must extend outward for a minimum of fifteen feet in every direction from the foundation of the residence. The fill soil must meet floodplain standards regarding soil materials and be compacted to floodplain regulations.

To ensure that the top of the lowest floor is elevated a minimum of two feet above the BFE a Floodplain Development Permit Application must be submitted to the Butte-Silver Bow Planning Department for review and approval prior to any construction taking place. It is important to know, an approved residence located on fill within the floodway fringe is still considered within the 100-year floodplain and therefore may be result in the landowner having to purchase flood insurance.

Zone A

Zone A also permits the construction of new residential, commercial and industrial buildings. However, since BFEs have not been determined by FEMA an engineering firm must be utilized to determine the BFE for the location. The residence must be constructed a minimum of two feet above the determined BFE. A Floodplain Development Permit is also required for new construction within this zone.

11. How do I apply for a floodplain development permit?

A copy of a Floodplain Development Permit Application can be downloaded and printed using the link below, or from the State of Montana Department of Natural Resources and Conservation (DNRC) Floodplain Management website.

Floodplain Development Permit Applications must be submitted to the Butte-Silver Bow Planning Department. There is a $35.00 application fee. For more information, the Planning Department phone number is 406-497-6250.