Eagle County, CO, is home to the scenic Sylvan Lake State Park. Located in the Rocky Mountains about 30 mi. west of Vail, Sylvan Lake is a popular spot for outdoor enthusiasts, with camping, fishing, hiking, wildlife observation, and photography among the many activities people enjoy there.

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The 44-surface-ac. Sylvan Lake is held by a dam that was installed by a mink farmer in the early 1940s. In 1960, the Colorado Division of Wildlife acquired the property, turning it over to the Colorado State Parks system a few years later.

The Sylvan Lake dam was considered a Class I, high-hazard dam because of the campsites located downstream from the outlet. During a 1998 inspection, a Colorado Division of Water Resources dam safety engineer discovered pinholes and a 4-in. deflection in the 30-in. corrugated metal dam outlet pipe.

Colorado State Parks hired a private consultant, Christopher Manera, P.E., president of Colorado River Engineering, to advise on the best way to fix the problem. Manera's first task was to conduct a feasibility study. Initial inspections showed that although the pipe was deteriorated, there was no immediate threat of collapse. That gave the team plenty of time to research replacement and rehabilitation alternatives.

Over the course of further evaluations, Manera found that - other than replacement of the outlet structure, which would require a complete breach of the dam - the only viable option was trenchless rehabilitation.

Trenchless rehabilitation was a less expensive option than replacement. In addition, because Sylvan Lake is a high-elevation reservoir, the construction season lasts a mere four months. "Replacement of the dam would have taken at least two summer seasons," says Manera.

Bypass pumps set up at the inlet to allow for flow diversion during installation

A concrete gatehouse was built at the outlet to control spillway erosion.

The team decided the best method of rehabilitating the outlet was to use cured-in-place pipe. Other trenchless methods, such as sliplining or fold-and-form lining, would have reduced hydraulic capacity of the pipe to unsatisfactory levels and would not have met the state's water-level drawdown requirements for dam safety. "We ran the hydraulic calculations, and that immediately removed any other rehabilitation alternatives," notes Manera.

The lining portion of the project was awarded to Breckenridge, CO–based Western Slope Utilities (WSU), a full-service utility contractor and licensee of Inliner Technologies' method of cured-in-place pipe. Clarke & Co. Inc. of Grand Junction, CO, was the general contractor for the dam rehabilitation project.

With the lake being such a popular recreational destination, the state didn't want to consider scheduling the project during summer months. Springtime wasn't an option either: Because the dam was built over an original stream system, the creek flow was heaviest in the spring due to mountain runoff from the long winter snow. Work began in September 2002.

The lake hadn't been drained in more than 50 years, and the team didn't know what it was going to find. "The dam was originally built with onsite materials," says David Fox, project engineer for Colorado State Parks. "We didn't know what to expect and how much it was going to cost."

The lake is home to a large number of freshwater shrimp. The Colorado Division of Wildlife lifted fishing restrictions because it wasn't sure how well the fish would winter. The team elected to build a cofferdam at the far end of the lake to retain 4 - 6 ft. of water, which would help ensure that the remaining fish survived.

Workers guide a winch operator as the Inliner tube is pulled through the pipe from upstream the dam outfall.

WSU workers prepare to install the calibration hose inside the lining tube.

Although the fall season was determined as the best time to schedule the project, early high-mountain snows and warm temperatures brought an inflow into the lake at 4 - 6 cfs during what is traditionally a dry period. A temporary weir was constructed to allow for bypassing the inflow.

Original plans called for replacement of the entire inlet structure; however, after the lake was drained, the concrete inlet box was discovered to be in excellent condition, and only the trash rack, slide gate, and lift mechanism needed to be replaced.

The project team had originally contemplated building a road through the reservoir to transport equipment needed to install the cured-in-place pipe from the upstream end. The ground was very soft and boggy, however, and the cost for road building would have been significant.

As an alternative, the team elected to insert the liner from the downstream end of the 160-ft. pipe. "Because the lake sits at 8,500 feet above sea level, we anticipated a significant decrease in equipment efficiency," relates Dan Cohen, WSU's director of operations. "We have extensive experience with high-altitude construction, and we were able to accurately compensate for the decrease."

The wall thickness of the liner was determined by measuring the hydrostatic buckling load along the length of the pipeline as it traveled through the dam. The wall thickness varied between 17 and 27 mm. "We designed the liner to be 17 millimeters thick at the ends, where earth and hydrostatic loading were minimal," explains Cohen. "In the middle section, we transitioned up to 27 millimeters - not only to compensate for the higher loads but also to add structural integrity to the pipe, which was damaged and ovalized."

The smooth liner improved the flow rate by decreasing the effect of the ripples in the corrugated metal pipe (CMP), says Fox. "Although we elected to use 1-inch liner, which reduced the effective diameter of the pipe from 30 inches to 28 inches, friction was reduced after the pipe was rehabilitated."

The team also modified the design; where the water enters the new liner, there is a gap where a portion of the old CMP is still visible. Because the area was small, the contractor fabricated a 28-in. steel orifice plate to prevent water jetting between the old CMP and the new liner interface.

Work was completed in less than six weeks. Colorado State Parks and the state's dam safety personnel were very pleased with the outcome.

EC - January/February 2004