FLOOD04822

• County: El Paso
• Community: Colorado Springs
• Stream Name: Plum Creek, Mason Reservoir
• Basin: Arkansas
• Title: Site-Specific Probable Maximum Precipitation for Mason Reservoir
• Date: 1/1/1997
• Prepared For: Black and Veatch
• Prepared By: Henz Meteorological Services
• Floodplain - Doc Type: Floodplain Report/Masterplan

The Big Elk Meadow and Broomfield storms occurrE~d in early May and were
dominated by snowfall at elevations above 10,000 feet. HMS dismissed the
Broomfield storm from further analysis since its ston'n total rainfall was much less
than other general storms being evaluated. HMS adjusted the time of
occurrence two weeks into the warm season before maximization for the May
general storms. Standard storm maximization is given in HMR 55A (Table 13.1)
for each of the historic storms for 10 square mile arElas. Since this area closely
approximates the size of the Mason Reservoir drainage basin these values were
accepted for use in the present study. They were checked and the warm season
adjustment was made before elevation adjustments were applied.

Table 7 shows the maximized, seasonally adjusted and elevation adjusted
values of peak storm rainfall for each of the historic storms. As indicated earlier,
the small size of the Mason Reservoir basin allows for easy application of the 10
square mile values in HMR 55A (Table 13.1) to the adjustment calculations. No
area PMP maps were drawn for the basin for this reason. However, Figure 8
shows the complex topography surrounding this basin and is included to assist in
orographic discussions. Note that elevation adjusted rainfalls were calculated
using both HMR 55A and HMR 57 methodologies and the HMR 55A values are
listed in Table 7..

HMR 57 closely mirrors HMR 55A in its elevation adjustment methodol09Y for
local storms and convective storms. For elevations above 6,000 feet msl, a
decrease in local storm PMP of 9 percent per thousand feet of elevation gain
was utilized, approximating a pseudo-adiabatic decrease in moisture available
for convective activity. HMR 57 cites a study carril~d out by Hentz and Kelly
(1989) as support for its elevation adjustment mE!thodology. This study
investigated heavy rainfall events and their characteristics at elevations above
7,500 feet along the Colorado Front Rangl~. This citation provides a pc)int of
validation for the HMS precipitation elevaticlO adjustment methodolc)91Y.

Table 8 shows the elevation adjustment of the historic general storm maximized
rainfalls from Table 13.1 and their incremental extension to 72-hour general
storm convention. Information on both the Cherry Creek and the Gibson dam
storms are included for review since both are close to the Mason Reservoir
transposition envelope. Please note that the values in this table for the Frijole
Creek storm were obtained from information on the storm in the Colorado
Climate Center's Extreme Precipitation Event files and personal communication
with Hal Bishop of Bishop-Brogden Associates, a Denver engineering firm which
completed a hydrologic study of this storm (BisllOp, '1982).

The selection of a controlling general storm PMP from Table 7 shows the
infiuence of both the complex convective and general precipitation event storms
at different time intervals. 1"01' time periods of G hours and less the complex
convective general storm dominates and exceeds even the local storm PMP

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