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<br />6,2 Non-standard t.pproaches. Non-standard approaches to hydrology 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 e)[amination of the largest floods that have historically occurred in the <br />nearby area or region. The usual method is to develop an envelope curve 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 within similar hydrologic basins. <br />Envelope curves arEI useful in the development of Probable Maximum Flood (PMF) <br />estimates because ttley provide a basis to evaluate the accuracy of the PMF estimate. <br />Envelope curves willllventually increase in magnitude over time as more floods will occur. <br />Eventually, the envelope curve will approach the PMF as an upper limit. (Bureau of Reo, <br />1989). The envelopE' curve is not quantified by a probability, so consequently, it 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 />Disadvantaaes <br /> <br />1. Based on Observlld flood data. <br /> <br />1. Does not assign a probability to given <br />flow. <br /> <br />2. Provides convenilmt method of <br />summarizing extreme floods. <br /> <br />2. Sensitive to measurement errors <br />affecting extreme floods. <br /> <br />2. Gives accuracy estimates of PMF. <br /> <br />6.2.2 Paleohvdroloov: Paleohydrology is a method of evaluating flood events by some <br />methods other than clirect flow measurement. Paleofloods are major floods that have <br />occurred before the historical record, but are determined by geomorphological 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 data 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 />awr <br />