C150161 Feasibility Study

• Contract/PO #: C150161
• Contractor Name: Greeley and Loveland Irrigation Company
• Contract Type: Loan
• Water District: 4
• County: Larimer
• Loan Projects - Doc Type: Feasibility Study

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BOYLE ENGINEERING

[4]002

STATE OF COLORl\DO

OFFICE OF THE STATE ENGINEER
Division of Water Resources
Departme'nL of Natural Resources

1313 Sherman SO'eet, Room 818
Denver, Colorado 80203
Phone [phone]
FAX [phone]

October 24, 2003

80YlEENGlNEERlNG DENVER
RECEIVED
OCT 3 :'_ 2~n3

Mr. Ed A. Toms, P.E.
Boyle Engineering Corporation
215 Union Boulevard, Suite 500
Lakewood, CO 80228

Bill Owens
Governor

Creg E. \N:llrher
Executive Olrector

H~\ D. Simpson, P.E.
Stolte Engineer

www.wal:er.state.co.Us

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Subject: Review of Site Specific Probable MaXirum Precipitation (PMP) Study

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Dear Mr. Tl)ms: :

Thank you for submitting for the review lnd approval of the State Engineer the Site Specific
Probable Maximum Precipitation (PMP) Study fbr the contributing drainage basins to the Boyd and
Horseshoe Lakes in Larimer County. The study and report, dated August 2003, ~ere prepared by
Applied Weather Associates and reviewed by 1\11'- George H. Taylor, the Oregon State Climatologist.
We have completed our review of the study and report and our review comments are as outlined
below. Before we can consider the report to b~ acceptable for this project, the following comments
need to' be fully and satisfactorily'addressed. After you have had an opportunity to review the
comments, we will be available to meet with yoL to discuss the results of the review, if you so desire.
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Our primary comment on the report is focused on how the study treated the "E" center event of
the 1935 Cherry Creek storm event. We recoghize that the "E" center event has the characteristics of
a small, intense local storm embedded in a muth larger storm system that had the characteristics of a
general storm. The aforementioned study stat~s that although the "E~ center event exhibits local
storm characteristics, the storm center was analyzed as a general storm. It is our understanding,
hoy;~ver, that to account for the small area! 'extent of this stprm center,' an areal reduction factor was. -
applied to the rainfall depths to alloV'.the center to be used for the much larger drainage basins of this
study area. The areal reduction factors for the "E" center storm were, however, taken from the HMR
55A report for a local storm PMP. This! procedu~e appears to contradict the statement that the center
was analyzed as a general storm and does not appear to be consistent with how the areal reduction
factor was applied to the analyzed Hale and Cherry Creek storm centers of the same 1935 Cherry
Creek general storm event. Therefore, it is our mpinion that the local storm areal reduction procedures
should not be applied to the "E" center storm and any areal reduction factors applied to the storm.
center should be consistent with ,the treatment of the other storm centers of the 1935 Cherry Creek
storm event. Nevertheless, the procedures for the transpositioning and maximization of the "EH center
storm' event appear to b~ consi~tent with pUblis~ed literature and other. Site Specific PMP studies.

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Th~ depth-duratiorr relationship of the ge~eral storm PMP, as presented.'in tti~ Site'Spe~ifiG PMP
Study report, should be r:~-analyzed to account fcpr the effects of removing ,theJocal storm areal ..'
reduction factors from the rainfall depths of the "E" center storm event, as discussed in the above text.
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The study should then be re-submitted to this office for the approval of the State Engineer.
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'When replying, p1ease.refer to:
BOYD LAKE DAM
Water Division 1, DAMID: 040105