Overview
At the Chili Bar Dam, the Low-Level Outlet (LLO) original gate was operated by an antiquated hydraulic system subject to substantial leaking and would often fail without warning. This posed significant safety concerns, as the primary function of the LLO was to provide rapid drawdown of the upstream portion of the American River. A second area of concern was the potential failure of the upstream trash rack due to age and excessive corrosion. Ballard was retained to rehabilitate the LLO and trash rack gate systems. 

Scope of Work
Ballard fabricated and installed secondary containment and collection devices to minimize leakage while abandoning the original hydraulic system. The original hydraulic lines were purged, flushed, and removed along with the existing trash rack. This operation required extensive diver-assisted dredging, removal of the original gate cylinder, yoke, and the supporting braces. Reinstallation included drilling and setting underwater mechanical and epoxy anchors through each of the 5,000-pound modular sections. Internal containment devices were installed inside and on top of the trash rack to prevent future hydraulic leaks. With the high risk of oil leakage during this operation, Ballard designed an oil collection hood fitted with a standpipe and pump system to evacuate any oil collected to the surface for final oil collection and disposal.

Innovative Solutions
During a post-commissioning dive inspection for the initial phase of work, Ballard discovered partial damage to the existing gate thimble. Ballard installed a temporary bulkhead on the downstream LLO outlet to eliminate the effects of differential pressure prior to replacing the dam components. The bulkhead was mounted on an adapter plate that was anchored to the downstream concrete. Once the bulkhead was fully cured and tested, Ballard used a modular crane barge on the upstream side of the dam to close, flush, and remove the lower hydraulic system.

Results
After all new components were installed, the gate was dry tested to ensure it functioned properly. The temporary bulkhead was removed and the LLO gate and remaining hydraulic systems were returned to service. The risk of catastrophic failures of the LLO was drastically decreased via system updates and the replacement of hardware. Ballard was able to mobilize to the PG&E site on a short notice basis and act in an efficient manner to remedy the situation prior to a system critical failure.