AN ALTERNATIVE BASIN CHARACTERISTIC FOR ESTIMATING IMPERVIOUS AREA AND URBAN FLOOD FREQUENCY AND ITS POTENTIAL APPLICATION IN MISSISSIPPI; PROCEEDINGS OF THE 17TH MISSISSIPPI WATER RESOURCES CONFERENCE JACKSON, MISSISSIPPI by R.E. Southard Prepared by the U.S. Geological Survey in cooperation with the Mississippi Water Resources Research Institute Mississippi Bureau of Land & Water Resources TABLE OF CONTENTS Abstract Urban basin characteristics Estimating percentage of impervious area Percentage of developed area Potential application in Mississippi     Estimating impervious area     Flood-frequency relations Conclusions References FIGURES 1. Location of the U.S. Geological Survey streamflow- gaging stations in Missouri 2. Location of the U.S. Geological Survey streamflow- gaging stations used in this study, in Mississippi 3. Comparisons of estimated impervious area equations(region D) TABLES 1. Basin Characteristics and flood discharges for selected U.S. Geological Survey streamflow-gaging stations ABSTRACT Information on the magnitude and frequency of floods is essential in the planning and design of bridges, culverts, dams, levees, and buildings in or near floodplains, and in establishing flood-insurance rates. This information commonly is developed using a statistical analysis of streamflow data for sites where streamflow records are available. Flood-frequency information commonly is needed for areas where streamflow records are unavailable. In ungaged basins, flood magnitudes of given frequencies commonly are estimated from regional relations based on gaged-site data and are a function of drainage area and other physical features or basin characteristics. In rural basins, flood peaks for a given frequency of occurrence are related to natural features of a basin such as drainage area, channel slope, and channel length. However, in urban basins, the existence of increased impervious area (for example, streets, parking lots, and driveways) can result in increased volumes and rates of runoff to the streams. Channel modifications, storm drains, curb and gutter street drainage, and other drainage features can also increase the magnitude of floods in urban areas. To account for the higher rates of runoff in urban areas, several basin characteristics have been developed for use in flood-estimating relations. This paper will briefly discuss methods of computing the urban basin characteristics, percentage of impervious area. An alternative basin characteristic termed percentage of developed area, which can be used to compute percentage of impervious area, is described and applied to basins in Mississippi. Statistical analyses indicate that percentage of developed area is applicable for use in estimating impervious area and is potentially useful in estimating the magnitude and frequency of floods in urban areas in Mississippi. LizardTech's Djvu plug-in is needed to view these reports. CLICK HERE TO VIEW THE ENTIRE REPORT For questions or comments, contact K. Van Wilson.