PROJ00170

• Contract/PO #: C153481
• Contractor Name: Colorado River Water Conservation District
• Water District: 0
• Bill Number: XB 99-999
• Loan Projects - Doc Type: Feasibility Study

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3.0 HYDROLOGY AND HYDRAULICS

3.1 Probable Maximum Flood
The CDOW developed the probable maximum flood (PMFl for Elkhead Lake
Reservoir. The CDOW report to the State Engineer's Office entitled "Hydrologic
Analysi s of E1 khead Lake Reservoi r", dated May 4, 1984, detai 1 s the generati on
of the PMF. This report is included in Appendix C and summarized below.

El khead Lake Reservoi r 1 i es about ni ne mil es northeast of Cra i gin
northwestern Colorado. The drainage area, located in Moffat and Routt
Counties, covers 205 square miles. Approximately 50 percent of the drainage
area is covered by sagebrush and grass correspondi ng to a Soi 1 Conserv~ti on
Service (SCSI hydrologic soil Group C. The remaining 50 percent is covered by
forests of hydrologic soil Group B. This results in a basin curve number of
54 for an antecedent moisture condition of II. This value was later modified
to 65 for the fi na 1 PI~F determi nati on. The stream is 34 mil es long upstream
of the dam and drops a total of 3,607 feet over that distance. The time of
concentration for the storm event was calculated by a variety of methods. A
value of 20 hours, computed using the velocity method, was selected and later
modified to 16.2 hours.

The probable maximum precipitation (PMPl was derived using Hydrometeorological
Report No. 49, which applies to the area west of the continental divide in the
Colorado River Basin. The design precipitation was computed for June, which
is the month with the highest average rainfall. The final computed cumulative
"ainfall amounts are 4.7, 7.5, and 9.5 inches for the 6, 12, and 18-hour
periods, respecti vely. Abstractions are made from the gross precipitati on
using the SCS curve number method in combination with the Bureau of
Reclamation guidelines for minimum retention losses. A minimum retention rate
of 0.12 inch per hour was used.

The unit hydrograph for Elkhead Lake Reservoir was computed using the
procedure contained in the USBR publication "Design of Small Dams". The final
runoff hydrograph was computed usi ng a computer program developed by the
CDOW. The PMF hydrograph has a peak discharge of 37,387 cfs. It was approved
for design purposes by the State Engineer's Office on April 8, 1985.

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