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FAQs: Floodplains

What is a floodplain?

Are all floodplains the same?

Where can I find information about floodplain locations or floodplain studies?

Who is responsible for regulating development in floodplains in Fairfax County?

How do I obtain permission to work in a floodplain?

How close can I build my house/addition to a floodplain?

How can I determine my property's flood zone designation?


What is a floodplain?
In general, a floodplain is the flat area located adjacent to the main stream channel. When stream banks overflow during or after a storm, the floodplain provides natural storage for the excess water. In Fairfax County, the 100-year frequency storm is used to determine the limits of the floodplain. The technical definition of a floodplain as it pertains to the development process can be found in the Zoning Ordinance, Section 20-300.
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Are all floodplains the same?
No. There are three distinct kinds or categories of floodplains in Fairfax County, each of which carries different development restrictions. These three categories are:

  1. Minor Floodplain: drainage area greater than 70 acres but less than 360 acres (Zoning Ordinance, Section 2-903).
  2. Major Floodplain: drainage area 360 acres or greater.
  3. Those which are designated as a floodplain by the Federal Insurance Administration. These floodplain areas are delineated on the Federal Emergency Management Agency's Flood Insurance Rate Maps which break the county into one of four different "flood zones."

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Where can I find information about floodplain locations or floodplain studies?
The only way to definitively determine if a creek or stream is large enough to have regulatory floodplain adjacent to it is to have an engineer or surveyor compute the drainage area of that creek or stream at the point where it crosses the downstream property line. There are roughly 900 stream miles of floodplain in the county. About 500 miles have been studied and are designated on county tax maps. There is currently no information available from the county on the roughly 400 miles of floodplain that have not been studied. Copies of county tax maps are available through the Maps & Publications Center located on the first floor of the Government Center.

If the study was along a major stream that has a name (e.g., Bear Branch), it was probably done for the county by either the U. S. Geological Survey or by Massey Engineers. The U.S. Geological Survey study information as well as the Massey studies are available at GIS Mapping in Suite 117 of the Fairfax County Government Center, 12000 Government Center Parkway (703-324-2712, TTY 711). For technical information about those studies, contact the Stormwater Planning Division at 703-324-5500, TTY 711.

If a study was done by a developer (usually involving creeks or streams without a name), a microfilm copy should be available through Plan & Document Control at 12055 Government Center Parkway, Room 111 (703-324-1730, TTY 711).
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Who is responsible for regulating development in floodplains in Fairfax County?
The Environmental and Site Review Division (703-324-1720, TTY 711) is responsible for all the regulatory aspects of development in and adjacent to floodplains. This includes granting the written permission to work in a floodplain (which is required prior to anyone disturbing a floodplain) and approving the record plats which establish floodplain easements.

The Stormwater Planning Division (703-324-5500, TTY 711) reviews and approves the floodplain studies required to be submitted as part of the development process. Stormwater Planning also manages the Community Rating System, provides Federal Emergency Management Agency flood zone determinations for the public, and applies to the Federal Emergency Management Agency for corrections to the Flood Insurance Rate Maps where discrepancies are found.
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How do I obtain permission to work in a floodplain?
The Director of Land Development Services (703-324-1780, TTY 711) has the responsibility for determining whether a given use is permitted within minor floodplains, and for assuring that any use proposed in a major floodplain falls into one of the predetermined permitted uses in the Zoning Ordinance, Section 2-903. The Zoning Ordinance also requires that the Director's approval for any permitted use in a floodplain be in writing. If the response from Land Development Services indicates the work may be done, that letter must be made a part of any construction plan.

Requests for permission to work in a floodplain should be submitted in writing to:

Director, Land Development Services
Department of Public Works and Environmental Services
12055 Government Center Parkway, Suite 444
Fairfax, VA 22035-5504

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How close can I build my house/addition to a floodplain?
For additions to residential structures that existed prior to August 14, 1978, the answer to that question is detailed and is best answered by looking in the Zoning Ordinance, Section 2-903.8.

For new home construction or additions to houses built after August 14, 1978, there is both a horizontal and a vertical setback requirement. The horizontal setback required is 15 feet from the edge of the 100-year water surface. The vertical setback requirement is 18 inches between the computed 100-year water surface elevation and the lowest part of the lowest floor (including basements).

For further information, please contact Zoning Administration at 703-222-1082, TTY 711.
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How can I determine my property's flood zone designation?
This information is often required by lenders when obtaining a home mortgage. Contact the Stormwater Planning Division at 703-324-5500, TTY 711. This is the only office in the county which has both the effective Flood Insurance Rate Maps and all the amendments and revisions made to those maps since they became effective in 1990.
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