SYNTHESIZED FLOOD FREQUENCY FOR SMALL URBAN STREAMS IN TENNESSEE

Engineers involved in bridge, culvert, and highway design often need to know the magnitude and frequency of flood discharge from small streams where the drainage basin is urbanized. The results of a C-year study by the U.S. Geological Survey provide methods for estimating flood magnitudes for selected frequencies on small streams draining urban areas in Tennessee. A total of 22 rainfall-runoff sites located in basins with drainage areas of 0.21 to 24.3 square miles in size and in municipalities with populations between 5,000 and 100,000 were used to derive regionalized flood-frequency equations. Impervious area, measured from recent aerial photographs, ranged between 4.7 percent and 74.0 percent of the basin. The equations were derived by multiple regression analyses of synthetic floodfrequency estimates, derived from a rainfallrunoff modeling procedure, versus physical basin characteristics and a precipitation factor. These equations can be used to estimate the magnitude of future floods with recurrence intervals of 2 to 100 years on ungaged urbanized streams in Tennessee. One equation for each recurrence interval applies statewide. Flood-frequency estimates for stations used in the analyses and example computations demonstrating application of the regression equations to urban streams in Tennessee are given in the report. INTRODUCTION Engineers involved in bridge, culvert, and highway design often need to know the magnitude and frequency of annual peak discharge from small streams draining urban areas. City planners also need this information for flood insurance studies and for proper flood-plain management and development. The purpose of this report is to provide equations for estimating the magnitude and frequency of annual floods along urban streams in Tennessee with drainage areas from 0.21 to 24.3 mi2. However, these equations do not apply to streams where the magnitude of peak flow is affected significantly by temporary in-channel storage or overbank detention storage. The results presented in this report consist of equations derived by regression analysis of synthetic estimates of T-year (annual) floods versus physical basin characteristics and a precipitation factor. Prior to this statewide urban hydrology study, methods of estimating the magnitude and frequency of floods in the metropolitan areas of Nashville and Memphis were derived by Wibben (1976) and Neely (1984), respectively. Estimating methods for rural basins statewide were derived by Randolph and Gamble (1976). This study extends the previous urban studies and provides methods of estimating flood magnitudes and frequencies for urban areas statewide. The above methods for Memphis and Nashville should be used for those cities. Wibben (1976) indicated that the T-year floods from the gaged urban basins in Nashville were not significantly larger than those from rural basins. Consequently, regional equations for estimating peak runoff from rural basins (Randolph and Gamble, 1976) should be reliable estimators of T-year floods from urban basins in Nashville within the size and development range of his study. The data for this study were collected under a cooperative program with the Tennessee Department of Transportation and the Federal Highway Administration. Appreciation is expressed to the Tennessee Department of Transportation for providing aerial photographs of the urban basins in this study. The relation of flood-peak magnitude to the probability of occurrence, or recurrence interval, is referred to in this report as a flood-frequency relation. As applied to annual floods, recurrence interval is the average interval of time between exceedances of the indicated flood magnitude. For example, a flood with a lo-year recurrence interval may be expected to be equaled or exceeded on the average of once in lo-years or, stated another way, a flood that has a 1 in 10 chance of occurring in any given year. However, the fact that a flood of this magnitude occurs in any given year does not reduce the probability of a flood of equal or greater magnitude occurring within the same year, or in consecutive years.