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<br />several gaging stations and the drainage area 01 tile !Jaging stations, The gaging stations should be <br />wIThin similar hydrologic basins, Envelope curves am uselul in the development of Probable Maximum <br />Flood (PMF) estimates because they provide a basis to evaluate the accuracy of the PMF estimate, <br />Envelope curves will eventually increase in magnITude over time as more floods will occur, Eventually, <br />the envelope curve will approach the PMF as an upper limit (Bureau of Rec, 1989), The envelope <br />curve is not quantified by a probability, so consequently, it should only be used as a convenient way <br />for displaying and summarizing data on actual occurrences of extreme floocls, (Hydrology <br />Subcommittee, 1986), <br /> <br />Advantages <br /> <br />,pisadvantages <br /> <br />1, Based on observed flood data, Does <br />not require rainfall-runoff transformation. <br /> <br />1, Does not assign a probability to giv,m <br />flow, <br /> <br />2, Provides convenient method of <br />summarizing extreme floods. <br /> <br />~~. Sensitive to measurement errors <br />affecting extreme floods, <br /> <br />2, Gives general accuracy estimates of <br />PMF, <br /> <br /><I. Usually considerable scatter of data <br />below envelope curve, <br /> <br />4, Cannot be applied to basins with major <br />watershed or stream changes, <br /> <br />6,2,2 Paleohydroloqy- Paleotlydrology is a method of evaluating flood events by some methods other <br />than direct flow measurement Paleofloods are major floods that have occurred before the historical <br />record, but are determined by geomorphological or biological information, Paieoflood hydrology is the <br />scientific study of past or ancient floods that have occurred prior to the tome of a,) Direct measurement <br />by modern hydrologic procedures, or b,) Documentation by other human records, Paleohydrologic <br />information enhances short-term streamflow records by decreasing the uncertainty of fiood-frequency <br />estimates, Geomorphologicai evidence of paleofloods include: slackwater deposits or scour lines, <br />Biological evidence includes: fiipped trees, scars, tree ring anaiysis, radiocarbon dating of organic <br />material in flood deposits, etc, Once paleofloods have been identified, there are several statistical <br />methods to incorporate the data in discharge-frequency analyses, including plotting positions or <br />historically weighted moments, <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />(l,!5 <br /> <br />DRAFf <br />