FLOOD02838

• County: Douglas
• County: Arapahoe
• Community: Greenwood Village, Aurora
• Stream Name: Cherry Creek
• Basin: South Platte
• Title: Probable Maximum Precipitation Study for Cherry Creek Reservoir - Final Report
• Date: 8/1/2003
• Prepared For: CWCB
• Prepared By: CWCB
• Floodplain - Doc Type: Project

3. Sensitivity of Variables

A sensitivity analysis of various parameters used in HMRs 52 and 55A and the parameters in
the 1995 NWSSSS has been completed, This analysis is used to determine the appropriate
precision for some parameters and the sensitivity of PMP values to variations of parameters. The
results from these sensitivity analyses were used in the storm adjustments.

3.1 Dewpoint temperatures

The standard assumptions of saturated atmospheric profiles and maximum storm
efficiency are used in this study. Since storm maximization factors are very sensitive to
dewpoint temperature variations (about 5% for each degree F dewpoint change), new storm
analyses completed in this study evaluated dewpoint temperatures to the nearest half degree F
when dewpoint data from multiple locations were used. This provides a 2,5% change in storm
maximization and transposition adjustments for each increment change (1/2 degree F) in
dewpoint values.

3.2 PMP Storm Duration and Time Distribution

This study uses the standard total storm duration used in PMF analyses, i.e. a total rainfall
period of 72 hours. To provide the most critical conditions for the maximum flood, the
hydrologist computing the PMF should apply the most stressing time distribution of the rainfall
within the 72 hours consistent with Colorado extreme rainfall storms. The study provides time
distributions derived from historic Colorado Front Range storms for use in the PMF
computations.

3.3 Storm Centering, Storm Orientation and Storm Shape Ratios

The location of the heaviest rainfall within the Cherry Creek basin is determined by the
topography of the basin and both surface winds and winds in the storm cloud layer over the
basin. Storm orientation is determined primarily by winds in the storm cloud layer (See Section
5).

Sensitivity analyses were completed using two Colorado storms; Plum Creek, June 16,
1965 and Holly, June 17-18, 1965. These two storms were chosen because the Plum Creek
storm is a multi-centered storm similar to other Front Range storms that occur close to the
foothills and Holly is a single center storm similar to other eastern Colorado storms that occur
over the plains. Results of the analyses are shown in Table 3.1 for storm centering and Table 3.2
for storm orientation, Figures 3.1 - 3.16 show the translated and rotated storm isohyetal
patterns over the Cherry Creek basin. Additionally, hydrologic modeling runs were made with
the NWS storm center moved from the middle of the basin 10 miles southward toward the
Palmer Divide. The results of the hydrologic modeling are presented in Table 3.3.

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