FLOOD00079 - Laserfiche WebLink

Home Browse Search

1. Historic Flood Data--When prerecord flood information is available, it should be used to adjust the frequency computations. The recommended analytical technique for including historic data in the flood series is described in Appendix 6. A routine for managing prerecord information is contained in the computer program described in Appendix 13. The results of the analysis should be shown graphically to permit an evaluation of the effect on the analysis of including historic data. To accomplish this, the largest flood event and all other events known to make a complete series should be assigned a plotting position based on the entire period of known history. Most applications require inclusion of historic information in the flood analysis. Where such data indicate that an adjustment in the frequency curve is required, the basis for the adopted adjustment should be clearly stated. 2. Comparisons with Similar Watersheds--A comparison between flood and storm records (see, e.g.,(12)) and flood flow frequency analyses at nearby hydrologically similar watersheds will often aid in evaluating and interpreting both unusual flood experience and the flood frequency analysis of a given watershed. The shorter the flood record and the more unusual a given flood event, the greater will be the need for such comparisons. Use of the generalized skew coefficient recommended by this guide is one form of regional comparison. Additional comparisons may be helpful and are described in the following paragraphs. Several mathematical procedures have been proposed for adjusting a short record to reflect experience at a nearby long-term station. Such procedures usually yield useful results only when the gaging stations are on the same stream or in watersheds with centers not more than 50 miles apart. The recommended procedure for making such adjustments is given in Appendix 7. The use of such adjustments is confined to those situations where records are short and an improvement in accuracy of at least 10 percent can be demonstrated. 18