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Table la. Correspondence between qualification codes in the peak-flow file and in the peakfq program and how peakfq handles the associated peaks . p..kfq program Peak.flow fn. D 3 x 3and8 K 6crC H 7 1,2,4, 5.9.A, B,orE Description dam failure Peak excluded from analy~is. dam failure and discharge g~eater than stated Peak excluded from analy~is. known effect of regulation ~r urbanization By default, peakfq exclud~s peaks with qualification codes of 6 or C. The user may include these peaks by sp~cifying YES under the "Include urban-regulated peaks" column on the Modify/Options menu. historic peak By default, peakfq will include or exclude peaks with qualification code of 7 based on the Bnlletin 17-B computed high-outlier threshold and the length of the historic period. The user can modify these criteria by specifying the "Historic return period" and "Discharge threshold": on the Modify/Historic menu and the "Low outlier criteria" on the Modify/Low menu! maximum daily average, estPnate, less than indicated value, unknown regulation or diversion, snowmeltlhurricanelice.jamldebris dam breakup. year unknown or not exact, month or day unknown ' Peak always included in aflalysis. PRINCIPLES OF COMPUTATION The Bulletin 17B computational analysis is illustrated in figure 1. The following sections provide an overview of the major computational steps. FIGURE 1 NEAR HERE Systematic Record Analysis The systematic record analysis involves the computation of the mean, standard deviation and coefficient of skewness (X, S, and G, respectively) of the common logarithms of the annual peak floWs in the systematic record. At some sites, annual peaks of magnitude zero can occur; more generally, the annual peak may occasionally fall below or be equal to some lo~er limit of measurement called the gage base (which may be zero). To account for this possibility, the number of peaks below the gage base is computed I in addition to the mean, standard deviation, and skewnes~ of the logarithms of the above-base systematic peaks. The statistics of the systematic peaks and the number of peaks below the gage base are used to compute the systematic record frequency curve as follows: log Qs, p = X + S kG, p (I) . where Qs, p = systematic frequency curve at exceedance 'probability p, and kG, p = the Pearson Type III standardized ordinat~s for station skew (G) and exceedance probability p. PEAKFQ 3 DRAFT -1/30/98