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<br />Users Manual for Program PEAKFQ, Annual Flood <br />Frequency Analysis Using Bulletin 17B Guidelines <br /> <br />By Wilbert O. Thomas, Jr., Alan M. Lumb, Kathleen M. Flynn, and William H. Kirby <br /> <br />Abstract <br /> <br />Estimates of flood flows having given recurrence intervals or probabilities of exceedance are needed <br />for design of hydraulic structures and floodplain manageJjlent. Program PEAKFQ provides estimates of <br />instantaneous annual peak flows having recurrence interv~ls of 2,5, 10,25, 50, 100, 200, and 500 years <br />(exceedance probabilities of 0.50,0.20,0.10,0.04,0.02,0.01,0.005, and 0.002, respectively). As <br />implemented in program PEAKFQ, the Pearson Type III frequency distribution is fit to the logarithms of <br />instantaneous annual peak flows following Bulletin 17B guidelines of the Interagency Advisory Committee <br />on Water Data. The parameters of the Pearson Type III frequency curve are estimated by the logarithmic <br />sample moments (mean, standard deviation, and coefficient of skewness). This documentation provides an <br />overview of the computational procedures in program PEAKFQ, provides a description of the program <br />menus, and provides an example of the output from the program. <br /> <br />INTRODUCTION <br /> <br />Program PEAKFQ performs statistical flood-frequency analyses of annual-peak flows following <br />procedures recommended in Bulletin 17B of the Interagency Advisory Committee on Water Data (1982). <br />The Bulletin 17B guidelines contain a complete and definitive description of the recommended procedures. <br />The following sections document implementation of the Bulletin 17B guidelines in program PEAKFQ. This <br />information is intended to assist the user with options to the program. <br />The Bulletin 17B procedures characterize the magnitude and frequency of instantaneous annual peak <br />flows at gaging stations where these data are observed. The magnitudes of the annual events are assumed to <br />be independent random variables following a log-Pearson Type III probability distribution; that is, the <br />logarithms of the peak flows follow a Pearson Type III distribution. This distribution defines the probability <br />that any single annual peak will exceed a specified streaIIlfiow. Given this annual probability, other <br />probabilities, such as the probability that a future design period will be free of exceedances, can be <br />calculated by standard methods. By considering only annual events, the Bulletin 17B guidelines reduce the <br />peak-streamflow frequency problem to the problem of estimating the needed design floods using the record <br />of annual peak flows at the site. The parameters of the Pearson Type III frequency curve are estimated from <br />the logarithmic sample moments (mean, standard deviatiqn, and coefficient of skewness). The needed <br />design floods are estimated from this frequency curve cOlhputed by program PEAKFQ. <br /> <br />PEAK STREAMFLOW RECORDS <br /> <br />The peak data fall into two classes: systematic and historic. The systematic record includes all annual <br />peaks observed in the course of one or more systematic gaging programs at the site. In a systematic gaging <br />program, the annual peak is observed (or estimated) for each year of the program. Several systematic <br />records at one site can be combined, provided that the hy~rologic conditions during the periods of record <br />are comparable. The gaps between distinct systematic-record periods can be ignored, provided that the lack <br />of record in the interim was unrelated to the hydrologic conditions. Thus, if a flood record was interrupted <br />for lack of funds for data collection, the interruption could be ignored and the available data could be used <br />as if no interruption had occurred. On the other hand, if the record was interrupted because of prolonged <br /> <br />PEAKFQ <br /> <br />DRAFT. 1/30/98 <br />