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<br />flood record. Use of this relationship is equivalent to rejection at <br />the 1 percent level of significance (one-sided). When one or more low <br />outliers are identified, they are deleted from the record and the remaining <br />record should be treated as an incomplete record, using the combination <br />of probabilities method discussed in Appendix 4. <br /> <br />C. Refinements to Frequency Curve <br />The accuracy of flood probability estimates based upon statistical <br />analysis of flood data deteriorates for probabilities more rare than <br />those directly defined by the period of systematic record. This is <br />partly because of the sampling error of.the statistics from the station <br />data and partly because the basic underlying distribution of flood data <br />is not known exactly. <br />Although additonal flood data can often be obtained and incorporated <br />in the analysis of flood levels at high flows and rare exceedance levels, <br />procedures for doing so cannot be standardized to the same extent as <br />those used for systematic records. The purpose for which the flood <br />frequency information is needed will determine the amount of time and <br />effort that can justifiably be spent to obtain and analyze historic <br />data, to make comparisons with other watersheds, and make and utilize <br />flood estimates from precipitation. The remainder of the recommendations <br />in this section are guides for use of these additional data to refine <br />the flood frequency analysis. <br />The analyses to include when determining the flood magnitudes 'with <br />0.01 exceedance probability vary with length of systematic record as <br />shown by an X in the following tabulation: <br />Length of Record Available <br />Analyses to Include 10 to 24 25 to 50 50 or more <br />Statistical Analysis X X X <br />Historic Data X X X <br />Camparisons With Similar Watersheds X X <br />Flood Estimates From Precipitation X <br />All types of analysis should be incorporated when defining flood <br />magnitudes for exceedance probabilities of less than 0.01. The following <br />sections describe how to include the various types of flood information <br />in the analysis. <br /> <br />17 <br />