Floodplains and Floodplain Management
After a large rainstorm, the amount of water flowing in a stream may exceed the capacity of the channel to convey the water and the excess will overflow and flood the adjacent land. The resulting area covered by water is called a floodplain. Generally, the size of the floodplain varies based on the topography of the land and the magnitude and duration of the rain event.
Floodplains that have not been significantly altered and are covered by vegetation can help filter pollution from water, reduce down-stream flooding by detaining some of the floodwater, and provide habitat for wildlife.
To help preserve floodplains and to protect life and property from flood risks, floodplains are identified on special maps and the types of activities and land uses in floodplain are restricted or prohibited by federal and local regulations.
The most common mapped floodplain for regulatory purposes is established from a rainfall event that has one percent annual chance of occurring in any year. Although this rain event is often called the “100-year storm,” the magnitude and duration of the rain event is based on statistical rainfall probabilities and is not limited to a single occurrence over a 100-year period. The area flooded by the one percent annual chance rainstorm is called the “Base Flood” and is identified on Flood Insurance Rate Maps created by the Federal Emergency Management Agency (FEMA) as the “Special Flood Hazard Area”. Like the one percent annual chance storm, the floodplain resulting from the Base Flood is often called the “100-year floodplain.” Information about a countywide update of the Flood Insurance Rate Map for Loudoun County is available at the county’s Risk MAP web page here
FEMA is responsible for floodplain mapping and regulation throughout the country. FEMA, through the National Flood Insurance Program (NFIP), works through local governments and their regulatory processes to manage floodplains at the local level. Loudoun County Government is a participant in the NFIP and has a floodplain management program that is compliant with NFIP requirements. The County manages floodplains in accordance with the Revised 1993 Loudoun County Zoning Ordinance, Section 4-1500, Floodplain Overlay District. Additional information about the NFIP and Loudoun County is available here.
The Floodplain Overlay District is a mapped zoning area composed of Major Floodplain and Minor Floodplain. The county’s Major Floodplain is based on the “Special Flood Hazard Area” shown on FEMA’s Flood Insurance Rate Map for Loudoun County. The map also shows the county’s Minor Floodplain, which continues upstream from the Major Floodplain. Major Floodplain typically has a minimum drainage area of one square mile and Minor Floodplain typically has a drainage area from 100 acres up to one square mile (640 acres) or where it intersects Major Floodplain.
A residential or commercial structure with any portion of its footprint in the floodplain must have flood insurance if the mortgage on the structure is from a federally-insured financial institution. Because the county participates in and follows regulations of the National Flood Insurance Program, residents can purchase federally-backed flood insurance for residential and commercial structures and the contents of those structures. Floods greater than the Base Flood (“100-year flood”) can and do occur, so FEMA recommends owners or renters of residential or commercial properties that are not in floodplain, but are close to it, also consider purchasing flood insurance.
Updates to FEMA's Flood Insurance Rate Map and the county's Floodplain Overlay District are made when new and more accurate data and analyses are available and approved after undergoing technical reviews.
If you have questions or would like more information about floodplains and floodplain management in Loudoun County, please contact the Department of Building and Development Floodplain Help Line at 703-737-8746 or by email.