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<br />,'I:h' -.: - f/,,, " "",J~- ':J::Js pea~s estimated for small mountain watersheds - from HUrricane Camille In Vlrglllla, 1969 <br /> <br />=-J"-al~,o';jE <br /> <br />Drainage area, <br />mi2 <br /> <br />Estimated peak flow, <br />ft3/s <br /> <br />Ginseng Hollow <br />Polly Wright Cove <br />Wills Cove <br />FreshVl/ai:er Cove <br /> <br />0,667 <br />0,953 <br />1,577 <br />2.333 <br /> <br />9,100 <br />12,200 <br />18,200 <br />23,100 <br /> <br />The peak es,lmates In table 3 were not based on hydraulic calculations, but on a rational formula <br />approach developed after Hurricane Camile by the USGS investigators. These estimated peaks <br />were developed from the physical evidence of the rainfall intensities and the scour from the floods. <br />They represe:lt flooding intensity measured in ft3/s per mi2, far greater than that for any reported <br />floods from other small drainages in the conterminous United States, However, because of the <br />lack of supporting hydraulic computations, no attempt was made to compare these peaks with <br />PMF estimates for the small basins involved. Their inclusion in this section of the report is intended <br />to point out that many events even larger than those studied have probably occurred in the <br />conterminous United States, <br /> <br />For the 61 flood peaks selected for comparison with PMF peaks in this study, all of the recorded <br />peaks had some field measurement and hydraulic computation to support their recorded value. <br />rJiost of these peaks are discussed in various water supply papers and professional papers prepared <br />by the USGS, It is not the intent of this study to question the accuracy of any published report <br />of a historic flood peak; however, significant inaccuracies in some reported peak flows pointed <br />out by USGS hydrologists and researchers have lead to the exclusion of these data. <br /> <br />Methodology and Accuracy <br /> <br />The techniques used to define PMP and rainflood PMF hydrographs for the selected study sites <br />are identical to those commonly used in recent USBR and Department of the Interior SEED Program <br />studies. Because of the limited time available for each study, no field investigations and little detailed <br />analysis of nearby gauge records were performed for any site in this study, C values (hydraulic <br />efficiency coefficients) for lag~time computations, flood wave travel times, loss rates, and appro~ <br />priate dimenSionless graphs were selected on the basis of the author's relevant experiences <br />throughout the United States, consultation with other USBR hydrologists, and comparisons with <br />nearby sites for which recent PMF studies had been prepared, All of the hydrologic parameters <br />selected for each site studied are reasonable However, additional refinements should be made <br />before any design studies based on the PMF are started for these sites, The accuracy of the PMF <br />peaks computed for this study is as good or better than the accuracy of the recorded peak flow <br />values at each site, <br /> <br />Geographic Considerations <br /> <br />Plate 1 shows the locations of the 61 sites selected, The top number at each location represents <br />the percent of the PMF peak computed, and the bottom number refers to the data entry in table <br /> <br />9 <br /> <br /> <br />