BOARD00961

• Board Meeting Date: 9/22/2003
• Description: Flood Section - Probable Maximum Precipitation Site-Specific Study for the Cherry Creek Reservoir-Study Findings and Recommendations
• Board Meetings - Doc Type: Memo

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Comparison of the A W A 2003 Study Team & the NWS 1995 Study Team Results
Topic A W A 2003 Study Team NWS 1995 Study Team
Basin Maximize, transposition, and apply barrier Use generalized PMP values from HMR
approach and elevation adjustments to Colorado 55A with procedures from HMR 52, modify
storms for the Cherry Creek Basin the rainfall with orographic factor
Source of Storm analyses of Colorado storms, most HMR 55A (most recent storm 1978), HMR
rainfall data recent storm 1997 52 (storms east of 102.2 degrees west
longitude)
Orographic Evaluate terrain slopes within and upwind Computer orographic adjustment factors for
influences of the Cherry Creek drainage basin to locations within the basin using ratios of
identify upslope, level and downslope 100-year point rainfall values
regions
Storm Recomputed maximization factors for 1935 HMR 55A computed storm maximization
Maximization and 1965 storms, used published factors of factors
other storms
Storm Only selected Colorado storms were trans- Storms used to establish HMR 55A PMP
Transposition positioned to the Cherry Creek basin values for the Cherry Creek basin were not
explicitly identified
Storm No constraints on storm centering within No constraints on storm centering within the
Centering the Cherry Creek basin boundaries Cherry Creek basin boundaries
Storm HMR 55A orientation limits accepted HMR ~ orientation limits accepted
Orientation
Within Storm Critically centered and oriented observed Within / without rainfall distributions in
Rainfall Colorado storm rainfall patterns used for HMR 52 used. These were calculated from
Distributions distributions within the Cherry Creek mostly Midwest rainfall and east coast
(storm size) drainage basin boundaries storms (onlv 1 Colorado storm used)

Applied Weather Associates - Technical Review - Final Results

AWA
Neutral = 0%
N/A
-15%
64%
15.8 inches

NWS
N/A
K-factor + 10%
?
81%
21.1 inches

. miles basin

Recommendations
The CWCB staffhas worked in close cooperation with all interested parties throughout this project
to come up with the best scientific answer that is conservative and defensible but representative of
the Cherry Creek Basin and Colorado climatolo,giG~ Jlqtl hydrological characteristics. The CWCB
staff recommends that the CWCB Members~ Probable MaximUm Precipitation (pMP)
Site-Specific Study for Cherry Creek Reservoir that was completed by the Applied Weather
Associates in August 2003. The CWCB staff further recommends that the CWCB Director present
e Bo ~_~Qhe Technical Review to the Cherry Creek Basin interests, sponsoring
c . e~r~~ongressional Delegation, the US Army Corps of Engineers - Omaha
District and the Colorado General Assembly in response to Senate Joint Resolution 99-023.

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Flood Protection . Water Project Planning and Finance. Stream and Lake Protection
Water Supply Protection. Conservation Planning