BOARD02358

• Board Meeting Date: 5/19/2003
• Description: Flood Section - Cherry Creek Probable Maximum Precipitation Technical Review
• Board Meetings - Doc Type: Memo

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15,2003 deadline. The Final comments and responses to the final comments will be included in the final ..
report. .,

After lunch there was approximately two and a half hours of discussion about the A W A PMP Study results,
reviewing the A W A Site-Specific PMP Study, and the Corps Cherry Creek Lake Project Operations and
Maintenance funds proposals for Federal Fiscal Year 04. The language in the House Committee Report
should include the latitude for the Corps' to conduct PMP and PMF Technical Reviews.

Presentation by Ed Tomlinson, Applied Weather Associates:
The Tomlinson and Henz PowerPoint presentations are attached but a presentation summary follows. Also
included are dialogue and comments from meeting attendees.

Bill Miller, USACE: asked about the congressional efforts taken by CWCB and others regarding funding.
Should language be added to the House report that can be implemented in the FT 2004 budget? Larry agreed
to discuss this in an hour or so.

Ed Tomlinson, A W A: stated that he is very proud of this study and made no assumptions without validating
them with data and analysis. He compared NWS results to the A W A results. Tomlinson showed a colorful
terrain map of the Palmer Divide, including the basin outline and the influential moisture inflow barrier
ridges. The Palmer Divide's highest elevation is 7,700 feet, and the lowest elevation is 5,500 feet on the north
side, and 5200 feet on the south side. There are multiple ridges on the north side of the Palmer Divide. The
combination of the Palmer and Kiowa barriers are the highest features. Tomlinson showed a 3-D map for the
Cherry Creek basin looking to the north with the radiating inflow lines. Tomlinson showed a 3-D map looking
to the south of the basin, including barriers and inflow wind lines.

Tomlinson gave the definition ofPMP with emphasis on the "physically possible" concept for the Cherry .-
Creek basin. ..

"Theoretically the greatest depth of precipitation for a given duration that is physically possihle
over a given size storm area at a particular geographic location at a certain time of year"

Major issues that A W A identified were: "physically possible" All assumptions must be meteorologically
consistent. Storm characteristics must be consistent with extreme rainfall storms in Colorado regarding
particular geographic locations. Topographic effects on rainfall production, storm rainfall pattern orientation,
storm centering, moisture depletion by the upwind barriers "given size storm area" and spatial characteristics
of Colorado storms. Tomlinson went over the major issues of the PMP study; assumptions must be
meteorologically consistent; storms used must be consistent with Colorado storms; topographic effects of the
Palmer Divide; storm rainfall pattern orientation; storm centering; moisture depletion by the upwind barriers;
spatial characteristics of Colorado storms. Project objectives were to review ofNWS site-specific study; have
the A W A team conduct a site-specific study and compare the results of the A W A team and NWS study from
1995.

In the review ofNWS study; HMR 52 used spatial rainfall patterns from storms not like Colorado storms;
HMR 55A PMP values could not be reproduced because of the lack of data. Ed Tomlinson also concluded
that K-factors that increase rainfall amounts should not be used for the down slope terrain into Cherry Creek.
Tomlinson showed the comparisons of the 1995 NWS study and the 2003 A W A study PMP values.

Three factors were found in the A W A team PMP study results; moisture depletion by the Palmer Divide and
Kiowa barriers; spatial distributions of extreme rainfall; orographic effects within the basin such as down
slope winds in the basin - affects on the k-factor.

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Flood Protection? Waw: Project Planning and Finance? Stream and Lake Protection
Water Supply Protection? Conservation Planning