Buffalo Bill Tailrace Repair

Written by: Jay Dallman, WYAO

An Advanced American Construction Diver checks the hot water flow to his wet suit before entering the water.

Buffalo Bill Power Plant on the Shoshone River near Cody, Wyo., was first brought on-line in 1992 as part of the Buffalo Bill Dam Modification which included raising the dam by 25 feet and adding 25.5 Megawatts of generating capacity.

In 2004, diving inspections of the power plant tailrace area revealed holes and other damage to the concrete.

The tailrace is the rock and concrete lined channel that carries water away from the power plant after it powers the turbines and passes through the draft tubes.

The tailrace is part of the river channel adjacent to the power plant. Subsequent inspections of the tailrace indicated that the damage was becoming progressively worse over time. The damage was projected to become so severe that it would eventually limit the ability to generate power or perform maintenance activities.

When Buffalo Bill Power Plant was constructed, the tailrace area was constructed at an elevation that was below the Shoshone River bed, similar to a bathtub. This area was armored with large granite rip-rap. Over time, power plant operations combined with high flows from Buffalo Bill Dam spillway gate and outlet works dislodged some of the rocks and they migrated to the lowest level of the tailrace near the stop log sills. Due to constant water flow, the rocks ball milled or rolled around resulting in significant damage to the concrete.

In attempting to obtain a clear definition of the damage, the Wyoming Area Office sought the help of Technical Service Center engineers to create a digital map of the tailrace area and pinpoint areas of damage. The digital map was created using sonar scanning and sophisticated computer software to create a Point Cloud Survey.

Based on the information from the point cloud surveys, TSC Engineers were able to design the concrete repairs, stainless steel armoring plates to prevent further concrete damage, and a barrier fence to prevent the migration of the rocks down into the draft tube area of the tail race.

The length and height of an upstream deflection dike was also increased as part of the design repairs in an effort to deflect high upstream river flows away from the tail race.

A Technical Proposal Evaluation Committee (TPEC) was used to review the contractor proposals that were submitted under the TPEC Process used for this project, potential bidders were supplied with the Design Package and Specifications. How they accomplished the work was up to each bidder.

There were several options that could potentially be used to make the repairs, including performing all of the work under­water using divers; constructing a coffer dam to divert the Shoshone River, using pumps to de-water the entire tailrace area and completing the work under dry conditions; or a combination of the two.

After the TPEC completed a thorough review of the bids that were received, the committee made a recommendation to the Contracting Officer and the contract was awarded to Advanced American Construction (AAC) from Portland, Oregon.

AAC elected to complete the work underwater using divers. Construction started in November, 2014. AAC's first action was to install a floating barge/work platform which was put together using modular sections, pinned together into one large unit.

Once the platform was complete, AAC placed numerous pieces of equipment on the barge in order to support their work activities including air compressors, hot water heaters for divers, diving control building, and grout pump.

Over the course of the next five months, AAC Divers installed the stainless steel armoring plates adjacent to the draft tube area, grout in areas of damaged concrete, and install a stainless steel barrier fence. The size of the upstream deflection dike was also increased with the installation of very large granite rock.

This project involved a large number of high consequence crane lifts and countless underwater dives.

AAC was one of those contractors that truly operating with safety as their number one priority.

They worked very closely with Buffalo Bill Operations and Maintenance Personnel as well as the on-site Reclamation Construction Inspector to plan and coordinate the work to ensure that it was completed safely and as planned. This coordinated effort resulted in a project that was completed on-scope, on-schedule, and on-budget, and without any safety incidents.

Point Cloud or 3D view of BB Unit 3 Draft Tube and Tailrace Bathymetry. Damage Area 5 is shown with two rocks lodged in the hole.

An excavator is lowered to the work platform for transport to the upstream deflection dike.

Buffalo Bill Power Plant during construction. The large amount of rock placed in the tailrace area can be clearly seen.

A work platform module is lowered into place in the Buffalo Bill Power Plant Tailrace Area.

Published on May 25, 2015