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PROJECT DESCRIPTION
Black Rock Dam is located in a narrow canyon on the Zuni Indian Reservation on the Zuni
River in west central New Mexico immediately upstream from the town of Black Rock. The
dam was initially constructed in 1908 and has had several repairs and modifications since then.
The original reservoir capacity was 15,000 acre-ft; however, sedimentation has reduced the
present reservoir capacity to about 2,300 acre-ft. The dam height above the original streambed
is about 80 feet and the structural height is 110 feet. The drainage basin contributing to the
reservoir is about 633 mi². The dam is classified as a high hazard dam.
The dam was constructed using a combination of hydraulic fill (estimated to be 60,000 yd³), on
the upstream side, and rockfill (estimated to be 40,000 yd³), on the downstream side. The
crest length is 780 feet and the crest width is about 20 feet at an elevation of 6446 feet. The
downstream face consists of hand-placed stepped masonry blocks, and the upstream face is
protected by layers of gravel and hand-placed riprap. The spillway is an ungated ogee section
with a crest length of 171 feet at a crest elevation of 6436 feet. The spillway is located on the
left abutment, has a concrete- lined floor with masonry walls, and has a maximum capacity of
6,000 ft3/s (a 20-year recurrence interval storm). The outlet works is on the right abutment
and primarily diverts irrigation releases. It also serves for the reservoir's evacuation
requirements. The outlet works is comprised of a 6-foot diameter tunnel with a 2-foot
manually-operated slide gate enclosed within a gate tower. There is no power at the dam;
however, power is available nearby.
The foundation of the dam generally consists of alluvial material and terrace deposits. The
abutments consist of alluvial and eolian soil deposits overlaid by intensely stratified basaltic
and alluvial deposits. The basaltic rock is relatively strong and the soil deposits are composed
primarily of sand, silt, and clay.
The dam has several identified dam safety deficiencies that require corrective action:
overtopping due to inadequate spillway capacity, uncontrolled foundation seepage, and piping
through cracks in the outlet works tunnel lining. Foundation seepage has been exacerbated by
flooding difficulties during project construction and three partial failures of the structure.
Due to the effective height of the dam, the probable failure of the outlet tower during the
maximum credible earthquake (MCE) is not considered to be a dam safety issue; however, as it
is expected to fail during the MCE, has deteriorated, and affects several project requirements,
replacement of the outlet tower is desired.
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