PROJ00116

• Contract/PO #: C153477
• Contractor Name: Victor, City of
• Water District: 0
• County: Teller
• Bill Number: XB 99-999
• Loan Projects - Doc Type: Feasibility Study

I
il
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

storm (thunderstorm) event. The general-type PMP storm event evaluated for a 24-
hour duration, was found to be the most severe. The total rainfall depth for
this event was estimated to be 26.0 inches.

Rainfall data for the 100-year storm was determined from the Precipitation-
Frequency Atlas of the Western United States, Volume III, Colorado (Reference 3).
A 24-hour storm was evaluated for the 100-year event. The total rainfall depth
for this storm was estimated to be 4.95 inches.

Incremental rainfall amounts for the PMF were arranged in a critical time sequence
in accordance with the U.S. Bureau of Reclamation procedures for evaluating PMP
storm events. This critical distribution sets the largest rainfall increments at
two-thirds of the storm duration with the next highest increments placed in
succession on either side of the critical peak.

Incremental rainfall amounts for the 100-year storm were arranged in accordance
with the Colorado Soils Conservation Service's (SCS) type IIA storm. According
to SCS this type of rainfall distribution should produce the critical peak runoff
over Western Colorado.

The HEC-1 model util izes the SCS runoff curve number (CN) to determine runoff
hydr09raphs. Based on information from the SCS office in Cripple Creek, Colorado,
a curve number of 70 was utilized for the East Fork Basin. This value is based on
soils and land use information for the area. A minimum retention loss rate of
0.05 inches per hour (in/hr) was used for the design storm sequence for the
assumed saturated soil moisture conditions of the basin.

A one (1) hour unit hydrograph for the East Fork Basin was determined based upon
basin characteristics. The computed tp (time to peak discharge for the beginning
of storm rainfall and qp (peak discharge) for the East Fork Basin unit hydrograph
are 2.01 hours and 1,196 cfs, respectively.

Hydrologic reservoir routing was utilized to evaluate the operation and storage
capacity of the structure. Reservoir flood routing was completed by the Modified
Puls Method utilized in the HEC-1 model. Several assumptions were made in modeling

32