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ship for an equivalent rural basin. For each station, peak discharge was estimated for the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year recurrence intervals. For the urbanized basin the flood-frequency esti- mates were derived either from actual peak discharge data or from synthesized data using a calibrated rainfall- runoff model. When both types of data were available, a weighted estimate was computed. Log-Pearson Type III procedures. as recommended by the Water Resources Council (1977), were used to fit each frequency curve to the data. Estimation of the skew coefficient of the annual peak data for urban basins was given considerable atten- tion because there are no recommended or generally accepted procedures available for estimating skew coef- ficients for urban areas. The regional skew map provided by the Water Resources Council (1977) was developed from rural data and does not necessarily represent urban conditions. Therefore, this map was not used directly for estimates of skew in the urban basins. Skew is possi- bly related to basin characteristics, including urban fac- tors which probably affect the magnitude of the skew coefficient. With these considerations in mind, attempts were made to relate station skew values to various basin and urban parameters. Many parameters were tried, and the only one that sbowed a relation to skew was a soils index, SCSS. SCSS is computed from equation I: 1000 SCSS=--IO CN where CN is the soil-cover-complex curve number as described by the Soil Conservation Service (1975). This parameter is an index of potential infiltration that could be related to the skew coefficient. The relationship defined by regression was: Gs =0.15(SCSS)-0.45 where Gs is tbe skew coefficient computed from the urban peak flow data. Even though the equation is sta- tistically significant, the standard error of regres,sion is approximately equal to the standard deviation of tbe skew values, so the equation offers little practical im- provement over the use of a mean skew and consequently the relationship was not used in this study. Stations with synthesized data were also studied, and it was found that the skew coefficient computed from these data related to an infiltration index defined from the cali- brated model parameters. Again. the relationship was poor and was not used to estimate the skew coefficients for this study, To provide regional skew estimates for this study it was decided that the most practical approach would (I) be to define an average skew value for eacb city or met- ropolitan area. For cities having three or more gaging sites, skew coefficients computed from the gaged flood records were averaged and then compared for consis- tency to (I) the mean skews from nearby cities, (2) the regional skew given by the Water Resources Council (1977), and (3) the mean skew defined by synthesized data if available. A skew coefficient was assigned to each metropolitan area on the basis of the computed mean and tbe above comparisons. These assigned city skew coefficients (see table I) were weighted with skew coefficients computed from actual flood-peak records according to the Water Resources Council (1977). For those stations having long-term (50- to 100-year) syn- thetic peaks based on rainfall-runoff modeling, the skew coefficients used were computed directly from the synthesized data because these data were considered more reliable than the city average skew values. Flood-frequency data for equivalent rural condi- tions at each study basin were estimated from the appli- cable Geological Survey flood-frequency reports. The specific report used for each city is referenced in table 1 by the author's name and date of the publication. Com- plete bibliographic references are given in the "Refer- ences" section of this report. Appendix II provides a listing of the most recent (1981) flood-frequency reports for all 50 States. These reports can be used to estimate the equivalent rural dis- charge at most sites in the United States. As future reports become available tbey should be used in place of the reports in this list. In addition to the two sets of flood-frequency data thus far described, the data base also includes f1ood- frequency estimates based on skew computed from the actual peak record, and flood-frequency estimates com- puted from model-synthesized data. Related information includes log-Pearson Type III mean and standard devia- tion, periods of record. Water Resources Council (1977) regional skew, average city skew. and weighted station skews. (2) ESTIMATING PROCEDURES FOR UNGAGED URBAN SITES The third phase of this project was to relate urban flood magnitude and frequency to watershed character- istics so that flood magnitude and frequency could be estimated for ungaged watersheds. Many attempts to derive a practical, easy-to-use method were made, most of which involved linear multiple regression of several dependent and many independent variables. This sec- tion of the report describes the more significant results. The three sets of estimating equations will be referred to as the seven-parameter equations, the three-parameter Estimating Procedures for Ungaged Urban Sites 9