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Last modified
1/29/2010 10:11:56 AM
Creation date
10/5/2006 1:21:17 AM
Metadata
Fields
Template:
Floodplain Documents
County
Douglas
Arapahoe
Community
Greenwood Village, Aurora
Stream Name
Cherry Creek
Basin
South Platte
Title
Probable Maximum Precipitation Study for Cherry Creek Reservoir - Facts
Date
1/1/1997
Prepared For
CWCB
Prepared By
CWCB
Floodplain - Doc Type
Project
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<br />File: PMPtopics799summary2.doc <br /> <br />July 22, 1999 <br /> <br />volume of runoff in a PMP event. <br /> <br />20. The values associated with the maximum one hour rainfall should be re-evaluated. <br />Questions remain on the validity of the measurement of 11 inches attributed to the first hour <br />of the storm based on water measured in a horse trough, especially considering that it <br />appears that rainfall on the order of a couple of inches fell over the region during the <br />previous evening. 1935 Cherrv Creek (Hale) storm. (Tomlinson) <br /> <br />The group consensus was this is based on best available data. <br /> <br />21. The location of the storm center with respect to the Palmer Ridge, i.e. just downwind of <br />the ridge, is consistent with other extreme rainfall events (e.g. Plum Creek, 1965) with the <br />associated storm isohyetal patterns showing sharp decreases beyond about 15 miles <br />downwind, i.e. to the north and northwest 1/ the Palmer Ridge does provide a significant or <br />even a dominant influence on the position of the storm center, trans positioning constraints <br />may be appropriate in positioning the design storm over the watershed when computing the <br />PMF. (Tomlinson) <br /> <br />The group agrees with the NWS that there are topographic effects on the north and south <br />sides of the Palmer Ridge. <br /> <br />22. Other large rainfall storm centers along the Front Range, e.g. west of Denver, are located <br />over the foothills or even further up the east slope of the Rockies, suggesting a "rainshadow" <br />for extreme storm rainfall centers over portions of the region between the Palmer Ridge and <br />the foothills to the northwest (Tomlinson) <br /> <br />The group agrees with the NWS that there are topographic effects on the north and south <br />sides of the Palmer Ridge. <br /> <br />23. The maximization procedure used in HMR 55A used a storm representative dew point <br />value which was computed using observations taken 560 miles to the southeast Possibly re- <br />analysis could identifY observational data closer to the rainfall event which would more <br />accurately represent the moist mass which provided the available moisture for the storm <br />dynamics to convert to rain on the ground. (Tomlinson) <br /> <br />The NWS will examine the data and determine its likely effect on HMR 55A and on the <br />Cheny Creek site specific PMP. <br /> <br />24. The accepted depth-area-duration values used together with the maximization factor of <br />163% produced maximized rainfall values which were not consistent with and significantly <br />larger than maximized values derived from analyses of other large storm events along the <br />Front Range. To provide a more consistent analysis, the values were undercut (decreased) by <br />about 15% by the authors of HMR 55A. Current PMP evaluations impose an upper limit of <br />150% when applying maximization factors to storm D-A-D's. Possibly an updated dew point <br />analysis as discussed in 23 above would provide an improvement for the maximization factor. <br />(Tomlinson) <br /> <br />5 <br /> <br />Topics and summary of 21 July 1999 Technical Meeting <br />
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