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, the Federal Emergency Management Agency (FEMA) identifies floodplains on special maps and the types of activities and land uses in the floodplain areas 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 prepared by 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."
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. Botetourt County 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 Article III. Division 1. Flood Hazard Overlay District of the Botetourt County Zoning Ordinance.
Floodplain Overlay District
The Floodplain Overlay District is a mapped zoning area composed of the following subdistricts:
- Floodway: Area of which the floodplain must be capable of carrying the waters of a 100-year flood without increasing the water surface elevation of that flood more than one foot at any point.
- Flood fringe: The area of the 100-year floodplain not included in the floodway subdistrict. Such area is shown as zone AE on FEMA maps.
- Approximate floodplain: The floodplain area for which no detailed flood profiles or elevations are provided, but where a 100-year floodplain boundary has been approximated. Such area is shown on zone A on FEMA maps.
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.
Residential and non-residential structures with any portion of their footprint in the floodplain must have flood insurance if the mortgage on the structure is a federally-backed loan. Because the county participates in and follows regulations of the National Flood Insurance Program, residents can purchase flood insurance through the NFIP for their residential and non-residential 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 non-residential properties that are not in floodplain, but are close to it, also consider purchasing flood insurance.