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FLOOD05215
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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 />Findings and Recommendations <br /> <br />A W A Team PMP Determination <br /> <br />The average rainfall depths for storm events transpositioned and clipped to the Cherry Creek <br />basin boundary have been computed. The largest Colorado rainfall storm centers that have <br />occurred east of the foothills were identified. These storm centers were maximized, <br />transpositioned and barrier moisture adjusted for the Cherry Creek location on the north side of <br />the Palmer Divide. The rainfall patterns of each of these storm centers were rotated and <br />critically centered over the basin for maximum rainfall depths within the Cherry Creek Basin <br />before being clipped to the basin boundary. The basin average rainfall amounts that were <br />produced by applying these procedures are presented on the next page. The Depth-Duration <br />curves for these storm were then plotted and an smoothed envelopment curve was drawn using <br />the most critical rainfall rates. Additionally, the Depth-Area curves were plotted for each storm <br />and a spatially enveloped and smoothed curve was drawn. Both of the envelopment curves are <br />presented following the storm rainfall table. <br /> <br />Because the storm isohyetal patterns were clipped to the explicit shape of the Cherry Creek <br />drainage basin, values for the standard area sizes included in Depth-Area-Duration analyses were <br />not computed. In order to include spatial enveloping in the site-specific PMP analysis, the <br />Depth-Area curves for each of the total storm durations were plotted and spatially enveloped. <br />The storm curves along with the envelopment curve are presented in Figure 7.3. The largest <br />rainfall value from the storm curves for the 386 square mile area size of the Cherry Creek basin <br />is 15.0 inches. The envelopment curve for that area size provides a 16.9 inch value resulting in a <br />spatial envelopment factor of 1.13. The result is a 13% increase in the rainfall over the 386 <br />square mile area size and this adjustment is applied to the enveloped rainfall amount from the <br />temporal envelopment curve. <br /> <br />A summary ofthe maximum rainfall values for the Cherry Creek basin is provided below: <br /> <br />Maximum 24-hour rainfall from adjusted storm rainfall <br />over the Cherry Creek basin from Table 7.1: <br />Temporally enveloped 24-hour rainfall over the <br />Cherry Creek basin from Figure 7.1 : <br />Spatially enveloped 24-hour rainfall over the Cherry Creek <br />basin using the spatial envelopment adjustment <br />factor of 1.13 from Figure 7.3: <br /> <br />13 .54 inches <br /> <br />14.0 inches <br /> <br />15.8 inches <br /> <br />As discussed in Section 5, storm durations for the 10 storm centers used were effectively <br />limited to 24-hours or less. In order to provide PMP values for the 72-hour duration, rainfall <br />amounts were computed using a 10% increase of the 24-hour value for the additional 48-hour <br />duration. This is a conservative measure to extend the storm duration to 72-hours since no <br />storms identified in the storm analyses lasted 72-hours (see Section 5). <br /> <br />Maximum 72-hour rainfall after temporal and spatial envelopment: <br /> <br />17.4 inches <br /> <br />2 <br />
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