Estimating flood-peak discharge magnitudes and frequencies for rural streams in Illinois

Flood-peak discharge magnitudes and frequencies at streamflow-gaging sites were developed with the annual
maximum series (AMS) and the partial duration series (PDS) in this study. Regional equations for both flood series
were developed for estimating flood-peak discharge magnitudes at specified recurrence intervals of rural Illinois
streams. The regional equations are techniques for estimating flood quantiles at ungaged sites or for improving
estimated flood quantiles at gaged sites with short records or unrepresentative data. Besides updating at-site floodfrequency
estimates using flood data up to water year 1999, this study updated the generalized skew coefficients
for Illinois to be used with the Log-Pearson III probability distribution for analyzing the AMS, developed a program
for analyzing the partial duration series with the Generalized Pareto probability distribution, and applied the
BASINSOFT program with digital datasets in soil, topography, land cover, and precipitation to develop a set of basin
characteristics. The multiple regression analysis was used to develop the regional equations with subsets of the basin
characteristics and the updated at-site flood frequencies. Seven hydrologic regions were delineated using physiographic
and hydrologic characteristics of drainage basins of Illinois. The seven hydrologic regions were used for
both the AMS and PDS analyses.
Examples are presented to illustrate the use of the AMS regional equations to estimate flood quantiles at an
ungaged site and to improve flood-quantile estimates at and near a gaged site. Flood-quantile estimates in four
regulated channel reaches of Illinois also are approximated by linear interpolation. Documentation of the flood data
preparation and evaluation, procedures for determining the flood quantiles, basin characteristics, generalized skew
coefficients, hydrologic region delineations, and the multiple regression analyses used to determine the regional
equations are presented in the main text and appendixes.