Laserfiche WebLink
<br />3.4. AREAL ADJUSTMENT. Regardless of which publications were used. one <br />now has six rainfall-depth values that must be further mod~ied by one or more adjustment <br />factors, The first adjustmenf factor is applied to the rainfall data taken from the <br />publications, These depths are "point rainfall depths"; that is, as measured at a rain gage, <br />i.e., a single point. The hypothetical stann will be applied to a spec~ic watershed having <br />a defined drainage area, For example, the amount of rainfall from a particular return- <br />period event over a "say" 50-square-mile area would not be the same as that at a point, <br />but would be less, A storm cannot be as intense when spread over a large area as ~ can <br />be over a single point. Although the rainfall depth for any fin~e drainage area will be <br />smaller than the value at a point, the adjustment is often not made unless the study area <br />is more than 10 square miles. The application of the adjustment factor for small areas <br />resuns in rainfall values that are little different from the point values. When the drainage <br />area is larger than 10 square miles, the adjustment becomes significanf, particularly for the <br />30- and GO-minute durations, It should be noted that the shape of the storm may not <br />necessarily be the same as the shape of the drainage area of the basin. In the absence <br />of s~e-spec~ic information on the aerial extent of the storm, ~ is recommended that an <br />elliptical storm pattern w~h a major to minor axis ratio of 2,5 be used, This shape can be <br />cenfered over the basin and the aerial coverage determined as the length of thll major axis <br />squared and divided by 3,18, Since the NWS publications provide no guidance for <br />adjustments for durations of less than 30 minutes, extrapolation to shorter durations is <br />subject to questions, <br /> <br />The adjustment factor found for each du ration is used to modify the corresponding <br />rainfall depth for that duration by multiplying these two quant~ies, Once this step has been <br />completed, all rainfall values have been adjusted for the particular drainage area being <br />studied. <br /> <br />3.5. PARTIAL-TO'ANNUAL SEFlIES ADJUSTMENT. The previously described <br />rainfall amounts are for partial-duration series of rainfall values, Conversion of rainfall <br />values to an annual series may be needed, This adjustment is applicable only to return <br />periods more frequent than 10 years, The rainfall depth-frequency curve is converted from <br />one for which one or more events per Yllar were used (called a partial-duration series, <br />which is one in which all events abOve some selected base value are used) to one using <br />only a single event each year (called an annual series, in which the single hillhest event <br />each year is used, even though the second highest in some years may be greater than the <br />highest in other years), For economic analysis of agricultural areas, ~ is possible that <br />damages are sustained by flood events that have a probabil~y of occurrence of more than <br />once per year. Where several floods per year are causing sign~icant damage, it would be <br />desirable to use a partial-duration series and not make the adjustment. In many economic <br />analyses, especially in urbanizing areas, these multiple floods (once- or twiGe-per-year <br />events) do not cause sign~icant damagEl. Therefore, the adjustment factoI' would be <br />applied to prevent the use of biased (high) rainfall depths to determine the low end of the <br />frequency curve, The adjustment from pallial-to-annual series is performed by muniplying <br />the rainfall depths for each duration by the appropriate conversion factor. The conversion <br />factors are as follows: <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7,5 <br />