Effects of urban development on floods in northern Virginia

Graphical and mathematical relations are presented to estimate the flood-peak magnitudes having recurrence intervals ranging up to 100 years for drainage basins with various degrees of urban or suburban development. Five independent variables are required for use of the relations. They are the size, length, and slope of the basin, which may be measured from maps, and the percentage of impervious surface and type of drainage system, which may be evaluated by a basin inspection but in actual practice will usually be estimated for future developed conditions. Based upon analysis of flood information for 81 sites, 59 of which are in the Washington, D.C., metropolitan area, the relations should be useful for design of drainage systems and for definition of good limits. The relations presented are applicable only to the Washington, D.C., area, but the method of analysis is general and may be used for any .area where the major floods result from rainfall.

Urban and suburban development are shown .to affect floodflows to a significant degree. Improvements of the drainage system may reduce the lag time to one-eighth that of the natural channels. This lag-time reduction, combined with an increased storm runoff resulting from impervious surfaces, increases the flood peaks by a factor that ranges from two to nearly eight. The flood-peak increase depends upon the drainage-basin characteristics and the flood recurrence interval.