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

HMS has conducted extensive research on both local and general flood in!)
events in the HMR 55A area of influence, This work includes studies performed
for the Colorado Water Conservation Board (Henz ,md Kelly, 1989) on heavy
rainfall events at elevations above 7,500 feet in the Colorado Front Ran!)e; for
the Wyoming State Engineer's Office on western states heavy rainfall events
relative to PMP calculations in Wyoming; for the Urban Drainage I~ Flood Control
District of Denver, Colorado on the transposition of lhe 1976 Big Thompson
Canyon and 1985 Cheyenne flash floods 10 the Boulder Creek basin; and for the
United States Geological Survey in re-constituting rainfall associated with
Colorado flash flooding events in Virginia Canyon near Idaho Springs. Rifle and
Government Creeks near Rifle, and Robideau Creek near Delta, Colorado,
Additionally HMS completed a site-specific PMP study for the Lincoln/Grizzly
Creek Reservoir in the high mountains about 12 miles southeast of Aspen. This
study has been accepted by the Colorado State Engineer's Office.. Using this
research as a basis, HMS has identified in Table 1 the key local and general
storms and their peak rainfall which were Gonsidered for PMP determination in
Mason Reservoir,

Table 1: Key local storms affecting ttle Mason Reservoir site-speGific PMF',
Detailed meteorological data was obtained for all storms,

Date
5e tember 10,1938
June 6-7, 194!!.____.
Au ust 1, 19!15
Au ust 14, Hl90

Amount of rainfall
8.1 inches /1 hour
6 inches /2 hours,
5.8 inches / 90 minutes
5-8 inches / 2 hours

All of these, except the Opal storm, were strong super-cell thunderstorm
complexes associated with orographiC precipitation enhancement which
occurred along the Continental Divide from Wyomin!J to New Mexico, HMS was
able to obtain detailed meteorological data to Gonstruct composite atmospheres
for each of the storms in Table 1 including the Masonville, Colorado storm of
1938,

The local storm list includes two recent storms which are not discussed in
HMR 55A: the Cheyenne Flash Flood storm of 198b and the Opal, Wyoming
storm of 1990, The Opal, Wyoming storm which occurred west of the
Continental Divide in southwestern Wyoming was investigated in detail as was
the Cheyenne, Wyoming storm. Both of these storms were investigated for their
timelintensity precipitation distribution and the structure of sub-cloud and cloud
layer weather factors affecting the rainfall. As can be seen in Table 1, the
amount of total rainfall in each of these storms and the Golden, Colorado storm
is comparable to the Masonville storm. For this reason these storms were

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