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<br />the 1 DO-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 raintall <br />should include an examination of the SPS raintall to assist in development of appropriate <br />estimates of the 500-year event. An adjustment of an extrapolated 500-year raintall 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 raintall ranging from 10% at New Orleans, to 25% at <br />Chicago for areas up to 30 miles downwind from the city. A study of SI. 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 raintall 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 which, 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 />individual is familiar with the derivation of hypothetical storms, the potential exists to <br />perform many of the calculations with HE:C-1. The current version of the computer <br />program can develop the SPS and PMS for areas east of the 105th meridian, with the <br />user-supplied point rainfall index and shape factor. The latest version of HEC-1 <br />incorporates procedures for the development of annual-series hypothetical-frequency <br />storms for the user-supplied point rainfall depth-duration array. Adjustments for depth, <br />area, and annual series are made automatically, incremental depths are determined, and <br />the entire storm pattern is arranged within the program. <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7.14 <br />