PROJ00444

• Contract/PO #: C150012
• Contractor Name: Model Land and Irrigation Company, The
• Contract Type: Loan
• Water District: 19
• County: Las Animas
• Bill Number: EIA
• Loan Projects - Doc Type: Feasibility Study

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SECTIONFIVE

Preliminary Hvdrologv and Hvdraulics

The drainage basin watershed to the Black Hills Reservoir consists mostly offair to good cover
grassland on a fine to coarse grained sandy loam soil. The surface area of the watershed is
estimated to be 32 square miles with the ground elevation varying between about 5,725 and
6,450 msl with an average slope of approximately 90 feet per mile.

We developed an estimated inflow storm event for a 100-year frequency, 24-hour duration
rainfall using the National Weather Service's NOAA Atlas 2: Precipitation-Frequency Atlas of
the Western United States, Vol. III - Colorado. The storm event was routed through the reservoir
using the Army Corps of Engineers Hydrologic Engineering Center's HEC-l model. It was
assumed that the reservoir level was at the spillway elevation at the time the storm occurred, and
that the outlet gate was in the closed position.

As a Class III dam, the dam is required to safely pass a 100-year frequency, 24-hour duration
rainfall storm event. The flood routing of the 100-year frequency, 24-hour duration rainfall
storm event through the existing spillway resulted in a peak discharge of approximately 1,100 cfs
through the spillway and a peak stage elevation of5756.2 msl (26.0 gage). The peak stage is
approximately 6 feet below the top ofthe concrete parapet wall. Upon completion of the
modifications to the embankment and replacement of the outlet works, the ability of the dam to
safely pass a 100-year frequency, 24-hour duration rainfall storm event will be restored to the
previous capacity established when spillway modifications were made in 1986.

We also evaluated flood routing through a temporary diversion pipe, which could be installed
upstream of the planned construction area at the intake elevation, to the downstream side ofthe
embankment during construction. The combination of a 36-inch diameter, temporary HOPE
diversion pipe and the reservoir storage capacity would convey a 5-year frequency storm event
with 1 foot of freeboard. This capacity would be considered the minimum capacity suitable for
the construction period to replace the outlet works and rehabilitate the dam.

The new outlet works will consist of a four-foot-diameter reinforced concrete pipe, cast into a
reinforced concrete cradle. The outlet works was designed to provide the minimum flow
requirement of250 cfs, as agreed to with the Colorado State Engineer's office. Additional
details of the new outlet works can be found in Section Six.

URS Breiner WolHIWanl Clyde

68OOCl44333.00IR2.doc 1131JOC1(3:18PM)IPROJECTS 5-1