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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SECTIONTHREE

Project DescrlpUon

3.1 SITE LOCATION AND CONDITIONS

The Black Hills Reservoir and Dam are located approximately 10 miles northeast of Trinidad
near the town of Hoehne, Colorado, as shown on Figure 3-1. The dam is located in Sections 19
and 30, Township 31 South, Range 62 West, ofthe 6th Prime Meridian, in Las Animas County,
Colorado.

The Black Hills Reservoir Dam was designed in 1909 and constructed soon thereafter.
Modifications to the dam were performed in about 1921 and again in 1986. The crest elevation
of the dam is elevation 5762, approximately gage height 32. Currently, the dam has a maximum
height of about 39 feet, a crest length of approximately 6,840 feet, a maximum storage capacity
of20,300 acre-feet, and a drainage area of about 32 square miles. The upstream and downstream
slopes were designed as 1.5H:IV (horizontal to vertical) and 2H: IV slopes, respectively. The
original outlet works consists of a reinforced concrete conduit with two, 36-inch-square channels
(vertically sided with an arch roof) with concrete cutoff collars along the outside of the conduit,
and a concrete discharge structure. Releases are controlled by a 48-inch sluice gate located in an
intake structure at the upstream toe of the dam. Slope protection on the upstream slope of the
dam consists of reinforced concrete in the central portion ofthe dam and extending to the left
(north) abutment. Rock riprap covers the upstream slope of the right (south) portion of the dam.

Modifications to the dam in about 1921 consisted ofan approximate 6-foot-high raise to the crest
of the dam. The raise in the left portion of the dam was a downstream raise over a length of
about 4,000 feet. There is a 6-foot-high vertical concrete parapet wall for the northern most
1,500 feet of the dam. Slope protection in the area of the 6-foot high crest wall consists of
reinforced concrete. The crest of the dam segment with the concrete parapet wall does not line
up with the crest of the earth dam only segment. The original intake structure had two gates at a
height of about 30 feet below the top of the concrete crest wall. Subsequently the existing 48
inch gate, located at about gage height 10.9 (approximately 21 feet below the crest of the dam)
was added. Releases are controlled by operation of this gate.

The modifications performed in 1986 limited the storage capacity of the dam with construction
of a 20 to 25-foot-wide unlined spillway channel around the left abutment of the dam. The invert
ofthe spillway is estimated to be at about reservoir gage height 20 (Elevation 5750.2).

In 1998, a portion of the outlet works was exposed as part of an investigation of the condition of
the outlet works. The embankment and upstream facing was left in place to provide temporary
diversion of floods up to a 5 year recurrence interval, until the outlet works replacement was
designed and constructed. In May 1999, a flood exceeding the five-year-event overtopped the
temporary diversion structure causing significant damage at the dam site and to some land
owners and structures downstream.

The Black Hills Dam and Reservoir is an important facility for the Model Land and Irrigation
Company and for the residents and land owners living in the vicinity of the structure.

The dam and reservoir provides the safe and efficient transportation of water from the Model
Intake Canal to the Model Ditch, located at the toe of the embankment.

The dam and reservoir also serves as a regulating reservoir for the efficient distribution of water
diverted from the Purgatoire River over 20 miles away. Without the ability to store water,
changes in the way water is distributed for irrigation would take approximately three days to

URS Breiner Woodward Clyde

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