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TABLE 3.2 Sample Log(10) Skews for West Slope Central Sierra Basins Basin Area Period I-DayQ 3-Day Q 7-DayQ Feather 3,624 1902-1997 -0.258 -0.230 -0.252 Yuba 1,339 1904-1997 -0.389 -0.332 -0.412 American 1,888 1905-1997 -0.187 -0.062 -0.159 Mokelumne 627 1905-1997 0.067 0.067 0.008 Stanislaus 904 1916-1997 -0.056 0.000 0.016 Toulumne 1,533 1897-1997 -0.190 -0.132 -0.180 Merced a 1,037 1902-1997 -0.086 0.014 0.Dl5 Mean -0.157 -0.096 -0.138 Std Dev. 0.148 0.144 0.163 Calculations by HEC-FFA v3.0 (1992) a low outlier (1977) removed according to Bulletin 17-B procedures. SOURCE: USACE, Sacramento District. 50 Improving American River Flood Frequency Analyses If either 0.0 or -0.1 is combined in a weighted average with the sample skew of -0.06 using the Bulletin l7-B weights, and the result is rounded to the nearest tenth, the result is -0.1. Unfortunately, the Bulletin 17-B weights are not optimal in this case because an unbiased estimate of the precision of the regional skewness estimators has not been employed (Tasker and Stedinger, 1986). That consideration would result in more weight on the regional estimate of -0.1. In addition, the error in the regional estimates is almost surely highly correlated with the error in the at-site estimator, and this would further change the optimal weights. The committee recommends that the regional skew value of -0.1 be adopted as the weighted skew coefficient for the logarithms of the three-day rain flood discharges for the American River. The choice of the skew coefficient can be considered a critical decision and Bulletin l7-B encourages hydrologists to perform site-specific studies to improve estimates of the skewness coefficient. The Bulletin 17-B skew map was developed almost 25 years ago and has a very steep gradient in the region of the Fair Oaks gage making its preeision questionable in this area. USACE (1998) incorrectly read the Bulletin 1 7- B skew map as +0.1 by using the centroid of the basin rather than the location of the gage, which yields 0.0 for a weighted skewness coefficient. When a map skew of 0.0 is combined with the station skew of -0.067 and rounded a value of -0.10 is obtained. Table 3.2 provides estimates of skewness coefficients for the American River and six other rivers for three durations: I-day, 3-day, and 7-day using the available records up through 1997. The skews of the American River for those three durations equal-0.187, -0.06, and -0.159, which average to -0.136. If one looks regionally over the seven sites in the table, then the computed skewness -