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<br />6.2 Non-standard Approaches. Non-standard approaches to hydrolog~' have not been <br />used on a regular basis to solve hydrologic problems, but show potential for increasing the <br />accuracy of flood estimates, These methods may become more increasingly used in the <br />future for solving hydrologic problems, Several non-standard approaches to flood <br />hydrology are illustrated below, <br /> <br />6,2,1 Envelooe Curves- Another means of estimation of flood potential for a particular <br />region is based on examination of the largest floods that have historically occurred in the <br />nearby area or region, The usual method is to develop an envelope CUrvll or relationship <br />between the maximum historic flood at several gaging stations and the drainage area of <br />the gaging stations, The gaging stations should be wnhin similar hydrologic basins: iuc <br />Envelope curves are useful in the development of Probable Maximum Flood (PMF~ <br />estimates because they provide a basis to evaluate the accuracy of the PMF estimate, <br />Envelope curves will eventually increase in magnnude over time as more floods will occur. <br />Eventually, the envelope curve will approach the PMF as an upper limit. (Bureau of Rec, <br />1989), The envelope curve is not quantnied by a probabilny, so consequently, n should <br />only be used as a convenient way for displaying and summarizing data on actual <br />occurrences of extreme floods, (Hydrology Subcommittee, 1986), <br /> <br />Advantaaes <br /> <br />Disaclvantaaes <br /> <br />1. Based on observed flood data, <br /> <br />1, Does not assign a probabilny to given <br />flow, <br /> <br />2, Provides convenient method of <br />summarizing extreme floods, <br /> <br />2, Sensnive to measurement errors <br />affecting extreme floods, <br /> <br />2, Gives accuracy estimates of PMF, <br /> <br />6.2,2 Paleohvdroloav- Paleohydrology is a method of evaluating flood events by some <br />methods other than direct flow measurement, Paleofloods are major floods that have <br />occurred before the historical record, but are determined by geomorpholo~Jical or biological <br />information, Geomorphological evidence of paleofloods include: slackwater deposits or <br />scour lines, Biological evidence includes: flipped trees, scars, tree ring analysis, <br />radiocarbon dating of organic material in flood deposits, etc, Once paleofloods have been <br />identified, there are several statistical methods to incorporate the dala in discharge- <br />frequency analyses, including plotting positions or historically weighted moments, <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />6.4 <br /> <br />fRIJFT' <br />