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<br />the 1 OO-year total, but signilicantly less than H'e 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 Ille <br />same order of magnitude as a 500-year eve1t. fIn 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 ,"vents lor watersheds downwind from <br />major urban areas (population greater than one miliion). Studies of downwind rainfall tor <br />Chicago, St. Louis, Detroit, Washington, Houston, New Orleans, and ClevelancJ Ilave <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 weattler <br />patterns found that the number of heavy ra.instorms 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 wllere the study watershed falls within <br />the sphere of urban influenGe. Measured rainfall data cocld be used to supplem"nl the <br />generalized data which, along with conservative selection of loss rates, could account tor <br />some of the urbanization effects on rainfall. Sensitivity tests using increased ra,nlall <br />amounts could be performed to evaluate this approach <br /> <br />6.3. HYPOTHETICAL STORM CA.l.CULI\TION WITHliN HEC-1. Once ttle <br />individual is familiar with tile derivation of Ilypothetical storms, the potential exists to <br />perform many of the calcliations with HEC-1. Tile current version of the computer <br />program Can develop the SPS and PMS for areas east of the 1 (15th meridian, with ttle <br />user-supplied point rainfall index and shape faclor. The latest version of HE:C;'-I <br />incorporates procedures for the development of annual. series hypothetical-frequency <br />storms for the user-supplied point rainfall depth-duration array. i\djustments for depth, <br />area, and annual series are made automatically, incremental depttls are determined, and <br />the entire storm pattem is arranged within tile program. <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DR4.Vr <br /> <br />7.14 <br />