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

STATE OF COLORADO

OFFICE OF THE SlATE ENGINEER
Division of Water Resources
Department of Natural Resources

1313 Sherman Street, Room 818
Denver, Colorado 80203
Phone [phone]
FAX [phone]

November 6, 2003

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

Bill Owens
Governor

Greg E. Walcher
Executive Director

Hal D. Simpson, P.E.
State En gineer

www.water.state.co.us

When replying, please refer to:
BOYD LAKE AND HORSESHOE LAKE DAMS
Water Division 1, DAMID: 040105 and 0401213

Subject: Approval of Site-Specific PMP Study

Dear Mr. Toms:

Thank you for submitting the report for the Site-Specific Probable Maximum Precipitation
(PMP) Study for the drainage basins above the Boyd Lake and Horseshoe Lake Oams in Larimer
. County for the review and approval of the State Engineer. The results of the study are presented in
the report titled "Site-Specific Probable Maximum Precipitation (PMP) Study for the Boyd and
Horseshoe lakes Drainage Basins" and dated August, 2003. Applied Weather Associates prepar,ed
the study report and a third party review of the study was performed by Mr. George H. Taylor. Mr.
Taylor, the Oregon State Climalogist, by a letter dated September 4, 2003, has recommended that
the Site-Specific PMP study for these project sites be accepted.

Although Mr. Taylor recommended the acceptance of the study, modifications to the study
were made based on our independent review. The modifications resulted from our concerns of the
treatment of the "E" center event of the 1935 Cherry Creek storm event. Our concerns with the
treatment of the "E" center event are outlined in our letter dated October 24, 2003. The re-evaluation
of the general storm PMP based on our concerns resulted in a change in the rainfall depths for the!
one-hour and three-hour storm durations. As presented in your memorandum dated October 20,
2003, the new rainfall depth for the one-hour storm duration is 12.8 inches and is 15.1 inches for the
three-hour storm duration. These changes should be made to the general storm PMP results as
presented on Page 42 of the aforementioned study report. The study report should be revised and
submitted to this office for our review and files.

Our acceptance and approval of the study results are primarily based on Mr. Taylor's
recommendation and are effective as of the date of this letter. The depth/duration relationship of the
PMP general and local storms as presented in the tables on Pages 42 and 35, respectively, and
modified as described in the above text are considered to be acceptable. These rainfall depths
should be used in the determination of the characteristics of the inflow design flood for the purposes
of designing the enlargement of the spillways for these dams.