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

Table 9 below summarizes the comparison between the standard HMR local
storm PMP and the slightly modified form supported by HMS based on
experience in storm QPF applications of the CSM and it's reconstitution of
extreme precipitation events demonstrated earlier.

Table 9: HMS site-specific local storm PMP rainfall for 10 square miles.

Method 1"hr 2-hr 3"hr
HMS 3.92" 5.55" 5.95"
PMP

4-~
:_ 6':~~J.

5-hr
6.50"

6'~ Total
6'~J 6.50"

Clearly these HMS assumptions set the sta(le for a reduced local storm F'MP
which is based on realistic physical assumptions about how the atmosphere
produces thunderstorm rainfall. The total local storm PMP rainf~11I is 6.5
inches in 6 hours. This value is consistent with the findings of Stanton
(1990) once elevation and orographic impaGt corrections for the Mason
Reservoir are made. He performed a volumetric analysis of rainfall at venous
elevations during the Big Thompson Canyon Flash Flood of 1976 and found
that less than 0.5 per cent of the volumetric rainfall of 6-8 inches fell at
elevations above 10,500 feet. The results of the HI\11S local storm PMP are
consistent with these findings. Note that the heaviest rainfall was Galculated by
Stanton to have fallen in the 7,500 feet to 8,500 feet elevation ran~les of the Little
Thompson River, Big Thompson River and Buckhorn Creek basins.

These observations are consistent with the elevation zones of primary
orographic lift and storm formation found in the region surrounding the Mason
Reservoir basin. While the HMS local storm PMP of 6.5 inches is Gonservative
for the 11 ,600 feet average elevation of the Mason Fteservoir drainage basin, it
consistently reflects the orographic affects on precipitation observed during the
Big Thompson Canyon Flash Flood. The local PMP storm presents a significant
design challenge to the reservoir. However, as shown in the nexit Section of
this report, the general storm appears to present a greater ch~lllengEl.

5.2 HMS site-specific general storm PMP

The calculation of the site-specific general storm PMP follows the methodology
of the local storm PMP but focuses on the addition of historical general slmm
atmospheric characteristics. The calculation of the HMS site-specific general
storm PMP utilizes HMS Convective Stoml Methodology (CSM) with
components of the historical Extreme P'mcipitation event Atl'ntDsphere
(EPA) and climatic extremes. In effect the EPA emulates the HMR's historical
storm review by examining conditions throu(lh a greater depth of the
atmosphere. The EPA was described earlier.

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