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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The Cherry Creek Darn and Reservoir beneficiaries and users must appreciate that the existing .-
darn and reservoir was originally designed and constructed under federal guidelines and financial _
resources; therefore, it must be operated under certain federal requirements and guidelines.
However, the Colorado interests do have an expectation that the NWS and Corps provide
technical data that is responsible and provide a project that is reasonably safe to Colorado.

THE DAM SAFETY DETERMINATION

The Corps' darn safety standards require that the NWS PMP methods and procedures must be
used to compute the rainfall values for all their darn safety hydrologic studies. The PMP
definition is "the greatest depth of precipitation for a given duration that is physically possible
over a given size storm area at a particular geographical location at a certain time of the year".
The Corps has a cooperative agreement with the NWS, in which the NWS develops the PMP
values that are used in the spillway design flood on Corps projects.

The NWS has two basic procedures for their PMP studies: generalized regional - HMR 52 and
HMR 55A. These procedures are adjusted through a site-specific study. These studies involve
the maximization, transposition, and envelopment relationships and analyses using historical data
for large, extreme events.

The design storm event for the original Cherry Creek Darn and Reservoir project used a
procedure, which increased the 1935 flood event values at Elbert, Colorado and Hale, Colorado .-
for the reservoir design. In 1995, the Corps proposed to replace their original reservoir design _
hydrology with the July 12, 1995 site-specific PMP study. The 1995 analysis yields a need for
an additional 100,000 ac-ft of storage or detention in the Cherry Creek Basin forPMF event
under the existing Corps reservoir design criteria.

What is the technical issues with the July 13, 1995 Cherry Creek Site Specific Study
prepared by the NWS and Corps?

These technical issues are a general consensus of what the Board staff has received during the
1999 Corps' public hearings, the July 21, 1999 technical discussions, and July 22, 1999 public
hearing.

I)

Issues regarding the development ofHMR 52 and 55A:
· The development ofHMR 52 used only one Colorado storm event -1935 Hale Storm;
· It is questionable if the maximization method for the selected 29 storm events which
were used in the development of the HMR 52 is regionally acceptable for Colorado's
elevation
. and topographic effects;
· For HMR 55A, 16 Colorado major storm events were used in its development; and
· The site specific studies for the Cherry Creek Basin, which a mix of HMR 52 and
55A does not consider Colorado data from the past 25 years. The state has had a
number of large flood events that includes accurate regional data for the Cherry Creek
Basin.

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