Estimation of flood-frequency characteristics and the effects of urbanization for streams in the Philadelphia, Pennsylvania area

This report provides a method for estimating the magnitude and frequency of floods for small streams in the Philadelphia, Pennsylvania area. Data collected at 21 streamflow gaging stations were used in a multiple-regression analysis to develop equations for computation of peak-flow characteristics. The flood equations were determined by relating flood-frequency characteristics computed using observed flow data from 13 stations and synthetically derived flow data from 8 stations to measurable basin characteristics. Significant characteristics in the equations are drainage area and impervious cover. Standard errors of estimate for the regression equations ranged from 38 to 43 percent. The equations can be used to determine peak-flow characteristics and to estimate the effect of urbanization on small streams with drainage areas from 1.1 to 64 square miles. The analyses indicate that increasing impervious area can significaantly increase peak flows. Examples are given for computing flood frequency for a site on an ungaged stream for an ungaged site on a gaged stream.