Flood-plain delineation for Difficult Run Basin, Fairfax County, Virginia

Water-surface profiles of the 25-year and 100-year floods and maps on which the 25-, 50-, and 100-year flood boundaries are delineated for streams in the Difficult Run basin in Fairfax County, Virginia. The techniques used in the computation of the flood profiles and delineation of flood boundaries are presented. Difficult Run heads at about 500 ft. elevation near the city of Fairfax and discharges into the Potomac River at about 70 feet above mean sea level. Stream channel slopes are fairly steep, the main channel of Difficult Run has an average fall of about 25 feet per mile. Stream channels are well defined with established flood plains covered in most cases with trees and dense brush. Development within the basin has been gradual and mostly residential. In 1965 most of the development was in the area of Fairfax City and the town of Vienna and imperviousness for the basin at that time was computed to be less than 1 percent. Since 1965 considerable additional residential development has taken place within the basin in the Vienna and Reston areas and ultimate development with an overall imperviousness of 30 percent is anticipated with higher percentages of imperviousness near centers of anticipated development. (Woodard-USGS)