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INTRODUCTION Purpose and Scope of the Urban Study A program to define the effects of urban development on flood char- acteristics of streams in the Dallas metropolitan area was begun in 1962 by the U.S. Geological Survey in cooperation with the city of Dallas. The purpose of this study was to determine the effects of urbanization on the magnitude and frequency of floods, to establish a regional flood- frequency relationship of sufficient accuracy to be used for the design of drainage systems, and to determine the relative importance of the physical characteristics that influence the hydrology of an urban area. The analytical methods used encompass general statistical concepts. A digital model of the hydrologic system, which was calibrated by using data from 19 storms in each of six basins, was used to simulate 57-year records of annual peak discharges for 14 drainage basins. The simulated 57-year records were used to define flood-frequency relations by fitting to the log-Pearson Type III distribution (U.S. Water Resources Council, 1967, p. 7-9). Regional peak-discharge equa- tions, which can be used to determine the maximum rates of discharge that could be expected to be equaled or exceeded at average intervals of 1.25, 2, 5, la, 25, 50, and 100 years, were derived from multiple- regression analyses. The general relationships of flood-peak characteristics, drainage area, and degree of urbanization can be used to estimate the effects of varying degrees of urbanization and to estimate flood-peak character- istics at ungaged sites. Acknowledgments The author acknowledges the assistance of Mr. Harry H. Stirman, Director of the Department of Public Works of the city of Dallas, and Mr. Donald H. Deere, Drainage Engineer, Dallas Department of Public . Works. Mr. Robert W. Lichty, Research Hydrologist, U.S. Geological Survey, assisted with the development of computer applications and mod- eling techniques. -2-