WSP02349

• File Number: 8449
• Description: Platte River Studies
• State: CO
• Basin: South Platte
• Date: 3/1/1993
• Author: City of Greeley
• Title: Peterson Reservoir Dam Hydrology and Hydraulics
• Water Supply Pro - Doc Type: Publication

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HYDROLOGY AND HYDRAULICS

Probable Maximum Flood
The drainage basin for Peterson Reservoir extends south from the reservoir and encompasses an area of
2.7 square miles. The reservoir water surface area is approximately 0.09 square miles. The distance
from the basin divide to the reservoir is 2.06 miles and has an average slope of 659 feet per mile.

The U.S. Army Corps of Engineers HEC-l computer program was used to compute the Probable
Maximum Flood (PMF) and route it through the reservoir and across the spillway. The HEC-l computer
printout for the analysis is included in the Appendix.

The soils in the basin are a typical mountainous and forested environment consisting of shallow to deep
sand and gravels with some rock outcroppings. The composite "CN" number used by SCS methods for
these soils is estimated to be 70.

Precipitation losses from infiltration and interception are accounted for in order to compute the excess
rainfall which produces runoff. For a mountainous area, typical loss rates can be represented as a
uniform loss of 0.1 inches per hour,' which is in the slow category. Uniform infiltration loss analysis
has shown, with experience, to be slightly more accurate than the SCS option and was, therefore, used
for the analysis. Basin imperviousness is estimated at a conservative 45 percent to account for rock
outcroppings.

Different coefficients of precipitation losses do not vary the results in the PMF significantly due to the
large amount of rainfall resulting in direct runoff. For PMF calculations, the minimum loss is carried
throughout the stonn. Factors which have a greater effect on the PMF are the procedures for the
determination of the unit hydrograpb and basin geometry.

The USBR Dimensionless Unit Hydrograpb Method was used for determination of synthetic hydrographs.
Time to peak cr,) values were determined from the Snyder lag equation, T,= c,( L*LjS".')N. For
bydrograph calculations, the drainage basin was divided into two areas. The first basin is a 2.6 square
mile basin that includes the overland flow area above the reservoir. A dimensionless unit hydrograpb
was developed for that area in accordance with the USBR procedures. The second basin is the area of
the lake surface (which is 0.09 square miles) for which a unit hydrograph was determined that bas no

'. Flood Hydrology Manual, USBR, 1989 page 112