The effects of urbanization on flood characteristics of streams in the Dallas metropolitan area were studied by use of a digital model of the hydrologic system. The model was calibrated by using observed rainfall and runoff data from 19 storms in six basins having various degrees of urbanization. The calibrated models were used with a 57- year rainfall record to simulate 57-year records of annual peak discharges in 14 basins. The flood-frequency characteristics were defined by fitting the simulated 57-year records to log-Pearson Type III distributions.
Regional peak-discharge equations, which can be used to determine the maximum rates of discharge that could be expected to be equaled or exceeded on the average of once in 1.25, 2, 5, 10, 25, and 100 years, were derived from multiple-regression analyses. The relationships among flood frequency, drainage area, and a coefficient of impervious area are given in a nomograph.
The analyses indicate that in a fully-developed residential area, the flood peaks will be 1.2 to 1.4 times those from an undeveloped area; and the annual direct runoff will be about double that from an undeveloped area. Data were not sufficient to determine the increase in runoff from a highly industrialized area where the effective imperviousness approaches 100 percent.
