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Last modified
11/23/2009 10:50:51 AM
Creation date
10/4/2006 9:15:44 PM
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Floodplain Documents
County
Statewide
Community
State of Colorado
Title
Users Manual for Program PEAKFQ, Annual Flood Frequency Analysis Using Bulletin 17B Guidelines
Date
1/30/1998
Prepared By
USGS
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />The systematic record frequency curve is an initial pstimate of the Bulletin 17B frequency curve, <br />which is recomputed as the systematic statistics are modified for outlier and historic data adjustments. <br /> <br />Outlier and Historic Record Tests <br /> <br />The next step in the analysis is to detect and make ~ppropriate adjustments for low outliers, high <br />outliers, and historic peaks. The sequence of these tests and adjustments depend on the station skew <br />coefficient, G, computed in the first step. Because a relatively large skew coefficient of either sign (G > 0.4 <br />or G < -0.4) is likely to be caused by an outlier on the corresponding end of the sample, this possibility is <br />checked first and any necessary adjustment is applied befpre checking for outliers on the other end. If the <br />skew coefficient is of relative moderate size (-0.4 < G < 0.4), the existenge of both high and low outliers can <br />be checked before applying any adjustments. Program PE!AKFQ tests for low outliers using the following <br />equation: <br /> <br />XL = X - KN S <br /> <br />(2) <br /> <br />where XL = low outlier threshold in log units, and <br />KN = 10 percent significance levels values for n9rmal distribution for sample size N (see table 2). <br /> <br />The frequency curve is automatically adjusted for the eff~ct of low outliers using the conditional <br />probability adjustment described later. <br /> <br />High outliers also are tested for but no adjustment can be applied unless the user supplies necessary <br />information about the length of the historic period and the high outlier threshold. The equation for detecting <br />high outliers is as follows: <br /> <br />XH = X + K~ S (3) <br />where XH = high outlier threshold in log units. Prograrh PEAKFQ does not automatically use the high <br />outlier threshold (XH) in the analysis. <br />If an adjustment for historic data has previously been made, then the following equation is used to <br />detect low outliers: <br /> <br />XL = M - KJ S <br />where XL = low outlier threshold in log units, <br />M = historically-weighted logarithmic mean, and <br />S = historically-weighted logarithmic standard deviation. <br /> <br />(4) <br /> <br />- - <br />The computation of M and S is described in the next section. <br /> <br />PEAKFQ <br /> <br />5 <br /> <br />DRAFT. t/30/96 <br />
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