BOARD02224

• Board Meeting Date: 9/27/1999
• Description: Flood Section - Platte River Basin Issues - Report Regarding the Cherry Creek Dam Safety Study
• Board Meetings - Doc Type: Memo

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CWCB Construction Bill 99-173 with Section 16 which includes the language"... the Board is
hereby authorized to expend up to seventy-five thousand dollars ($75,000) to corporate with
NWS and Corps, and other state agencies and agencies of local governments for a technical
evaluation and public review of the probable maximum precipitation and probable maximum
flood values associated with the Cherry Creek Reservoir."

For the July 21, 1999 formal technical meeting, the CWCB and the Corps co-hosted a discussion
with the National Weather Service (NWS) regarding their PMP study for the Cherry Creek basin.
The purpose of the technical discussion was for the NWS to address the PMP methodologies,
analyses, assumptions, and results, and to clarify issues that have surfaced at the public meetings
and other discussions. The July 21st meeting, which was a round table format, was chaired by
Bill Miller, the Corps' Project Manager for the Cherry Creek Darn Safety Study.

THE JULY 21,1999 TECHNICAL DISCUSSION

As mentioned previously, the purpose of the July 21 st technical discussion was for the NWS to
address the Cherry Creek Dam Safety issues for the PMP and to clarify questions and concerns
which have surfaced at recent public meetings and discussion groups. The technical discussion
group was made up of professionals who have a working knowledge of the PMP process,
procedures and best professional judgements that must be considered for a PMP analysis (see
attached group list).

The NWS, in cooperation with the Corps, conducted the July 21 st PMP presentations. The NWS
presented the following technical discussions:

· NWS Probable Maximum Precipitation (PMP) methodologies under NOAA
Hydrometeorological Report No. 52 (HMR 52) and NOAA Hydrometeorological Report
No. 55A (HMR 55A);
· "Cherry Creek Site Specific PMP Study" by Douglas R. Kluck and Douglas D. Fenn, dated
July 13, 1995; and
· "Antecedent Precipitation Study Approach," dated July 1997.

The Corps' Cherry Creek Darn Safety Study has three major study activities which are the basis
for the determination of the level of risk that exists for the Cherry Creek Darn and Reservoir as
the public and government officials view the system today. These three major study activities
are:

· Site Specific PMP study by the NWS;
· Probable Maximum Flood (PMF) event and it's resultant reservoir inflow hydro graph; and
· Reservoir routing procedures and the project operation, which determines reservoir pool
levels and the maximum discharge capacities for the existing spillway and outlet, works.

It should be understood that each one of these major study activities requires a high level of "best
professional judgement", "an expectation of a reasonable degree of correlation for historical and
regional conditions" and "a recognition of climatological and hydrological pararneters".

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