Cerro Reservoir Dam

Challenges

Cerro Reservoir consists of three high-hazard earthen embankment dams situated across separate drainages located in Montrose County, Colorado. The upstream slide gate at the western dam’s outlet works had been inoperable for more than 20 years.  As a result, flows through the conduit were controlled with downstream gate valves, resulting in the majority of the outlet lines remaining pressurized.

Previous dam safety inspections performed by the Colorado Office of the State Engineer (SEO) also identified uncontrolled seepage exiting the ground surface downstream of the western and eastern dams.

Solution

RJH oversaw a diving inspection of the submerged outlet works, which identified severe deterioration of the cast-iron conduit, structural deficiencies at the low- and mid-level intake structures, and significant deterioration of a concrete manhole.  RJH identified several failure modes associated with the outlet works and worked with the owner to develop a series of repair alternatives.

A key component of the rehabilitation required increasing the diameter of the outlet works from 8 inches to 18 inches in order to conform to SEO requirements of draining the top five feet of reservoir storage within five days.  RJH led an alternatives’ workshop with the owner and the SEO to identify the preferred alternative.

Final design included breaching the western dam and replacing a portion of the embankment; replacing the existing outlet works with an 18-inch-diameter conduit encased in concrete, an intake structure with a hydraulic-operated slide gate, and a baffled impact basin with a grouted riprap plunge pool; and adding a filter and toe drain to address uncontrolled seepage at the eastern and western dams.  RJH provided construction administration and resident engineering services during construction.