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<br />... 1. Historic Flood Data--When information is available that indicates <br />a flood peak(s) which occurred either before, during or after the systematic <br />record is (are) the maximum within an extended period, it (they) should <br />b~ used to adjust the frequency computations. The recommended analytical <br />t~chnique for including this historic information in the flood series <br />is described in Appendix 6. A routine for managing historic information <br />should be contained in any computer program. ... <br />The results of the analysis should be shown graphically to permit an <br />evaluation of the effect on the analysis of including historic data. To <br />accomplish this, the largest flood event and all other events known to make <br />a complete series should be assigned a plotting position based on the entire <br />p~riod of known history. <br />Most applications require inclusion of historic information in the <br />flood analysis. Where such data indicate that an adjustment in the frequency <br />curve is required, the basis for the adopted adjustment should be clearly <br />stated. <br />2. Comparisons with Similar Watersheds--A comparison between flood <br />and storm records (see, e.g., (12)) and flood flow frequency analyses at <br />nearby hydrologically similar watersheds will often aid in evaluating and <br />interpreting both unusual flood experience and the flood frequency analysis <br />of a given watershed. The shorter the flood record and the more unusual <br />a given flood event, the greater will be the need for such comparisons. <br />Use of the generalized skew coefficient recommended by this guide is <br />one form of regional comparison. Additional comparisons may be helpful <br />and are described in the following paragraphs. <br />Several mathematical procedures have been proposed for adjusting a <br />short record to reflect experience at a nearby long-term station. Such <br />procedures usually yield useful results only when the gaging stations are <br />on the same stream or in watersheds with centers not more than 50 miles apart. <br />The recommended procedure for making such adjustments is given in Appendix 7. <br />Th~ use~f such adjustments is confined to those situations where records <br />ar~ short and an improvement in accuracy of at least 10 percent can be <br />demonstrated. <br /> <br />18 <br />