Overview
The Diablo Dam is an arch dam with vertical walls rising 160 feet from the bed of the Skagit River. In February 2022, one of the power generation turbines went offline. After multiple investigations, a large piece of concrete dislodged from the intake structure was discovered and found wedged in between the turbine and the penstock walls. Divers also found that a large debris mat had built up on the outside of the trash racks from 125 feet to 60 feet of depth, and four feet to 60 feet thick from top to bottom.

Scope of Work
Divers began by removing the debris mat from the face of the intake structure. Once this had been documented, production clamshell debris removal operations commenced removing 700 yards of woody debris, large rock, and muck from the forebay of the intake structure. Temporary fixes were developed to stabilize the intake structure and prevent further concrete spalling that could damage the turbines. Two of the existing trash rack sections were removed to provide diver access to the interior of the intake structure, as the upland access point required 100 feet of penetration. To mitigate the risk of demoed debris entering the penstocks and turbines, steel debris guards were welded to the trash rack walers. Ballard completed the project by reinstalling the trash racks and installing nine sensors to monitor vibrations at certain flow rates.

Innovative Solutions
As an emergency project, there was no clear scope established. As work progressed and Ballard discovered the extent of the damage, the scope continually evolved, requiring Ballard to adapt to ever changing conditions and operations in short notice at a very remote location. Challenges included an increase in debris from an estimated thirty yards to 700 yards, as well as crane accessibility and stability in the remote location. Ballard worked with SCL to navigate this roadblock and maintain an aggressive mobilization effort and begin the emergency work on time with no delays.

Results
With 145 dives in approximately 60 days, this was a successful project in terms of schedule, budget, safety, and quality. Ballard demobilized from Diablo Dam with the hydro project at the same operational capacity as it was prior to the damage.