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<br />areas of 10 square miles, The calculations were determined for a dense grid covering the <br />study region, and a smooth analysis of the data was produced, After meteorological and <br />topographical inconsistencies were addressed, a fianal map of a 24-hour, 1 a-square mile <br />PMP was obtained. Using smoothed regional analyses of 1-to 6-hour, 6- to 24-hour, and <br />72-hour to 24-hour precipitation ratios derived from observered storms of record, a 10- <br />square mile PMP wa:s derived for 1-, 6-, and 72-hour durations. Using relationaships from <br />severe storms of rE!Cord and data supplied form adjacent PMP studies, depth-area <br />relationships were ;assembled for various subregions based on topographical and <br />meteorological variations in such parameters. Nomographs were then constructed that <br />allowed the user to determine values of PMP for various size drainages w~hin the HMR <br />55A study region and within the areal and durational limitations of the report, (Bureau of <br />Reclamation, 1989) <br /> <br />6. SUPPLEMENTAL INFORMATION. <br /> <br />Additional material to consider in deriving these storm data which are also <br />applicable to hypothEltical storm development include: <br /> <br />6.1. EXTRAPOLATION OF FREQUENCY DATA. The technical Papers of the <br />NWS indicate that a limited extrapolation (to 200-year return periods) is appropriate based <br />on the available genE"alized data. However, extrapolation to a 500-year retum period, as <br />is olten required for the hydrology necessary in flood-insurance studies, is of questionable <br />validity. Rainfall for the extrapolated 500-year event may be only 20 to 30% greater than <br />the l00-year total, but significantly less than the SPS for the same storm duration. While <br />an SPS cannot be assigned a specific frequency of exceedance, it is likely that it is of the <br />same order of magnitude as a 500-year event An evaluation of the 500-year rainfall <br />should Include an examination of the SPS rainfall to assist in development of appropriate <br />estimates of the 500-year event. An adjustment of an extrapolated 500-year rainfall total <br />may be necessary to ensure reasonable compatibility with SPS. <br /> <br />6.2. URBANIZATION EFFECTS. Meteorological studies have shown an increase <br />in the number and intensity of thunderstorm rainfall events for watersheds downwind from <br />major urban areas (population greater than one million). Studies of downwind rainfall for <br />Chicago, St. Louis, Detroit, Washington, Houston, New Orleans, and Cleveland have <br />shown an increase in warm-season rainfall ranging from 10% at New Orleans, to 25% at <br />Chicago for areas up to 30 miles downwind from the city. A study of St. Louis weather <br />patterns found that the number of heavy rainstorms has increased dramatically since 1960, <br />with 5-minute rainfall rates increased by at least 50% over large downwind areas, While <br />there is currently no direct way of incorporating urbanization effects into rainfall estimates, <br />the existence of this increase should be recognized where the study watershed falls within <br />the sphere of urban influence. Measured rainfall data could be used to supplement the <br />generalized data whi,;h, along with conservative selection of loss rates, could account for <br />some of the urbanization effects on rainfall. Sensitivity tests using increased rainfall <br />amounts could be performed to evaluate this approach. <br /> <br />6.3. HYPOTHETICAL STORM CALCULATION WITHIN HEC-1. Once the <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7.10 <br />