Overview
The Tiger Creek Afterbay Dam is a 120-foot high, 448-foot-long variable radius concrete arch dam with a gate overpour spillway in Amador County, CA. The original construction was completed in 1931. The dam contains a low-level outlet (LLO) discharge pipe at the base of the dam that required inspection and repairs. The project required the removal of sediment and debris to create safe access for divers to inspect and repair the LLO gate with water depths as great as 85 feet, off-load the sediment into highway dump trucks, and dispose of the sediment as reclamation soil in a nearby quarry.

Scope of Work
The project began with the mobilization of all barges, excavators, and necessary equipment gathered to begin dredging and surveying the area. The Tiger Creek Afterbay Dam is a discontinuous body of water with limited access via highway, further prohibiting easy equipment mobilization from conventional means by water and, thereby, required unique equipment selection to perform deep water sediment removal. The team started off filling approximately two barges a day, excavating as deep as 92 feet. After the first week, however, production saw a significant increase in process efficiencies resulting in roughly three barges a day filled and ready to off-load. In-water dredging was completed using a fixed-arm excavator, mounted on top of an equipment barge, that was equipped with a 360-degree rotating enclosed environmental bucket to limit sediment loss. The fixed-arm excavator, equipped with a GPS system, permitted sub-foot accuracy for bucket positioning and improved bucket control on steeper slopes.

Innovative Solutions
Mechanical issues and turbidity spikes, as well as woody and rocky debris, created challenges, but communication with the owner and prompt action allowed for resolutions in a timely manner. Approximately 27,000 cubic yards of sediment and debris were removed from within the dredge prism, which exposed the LLO gate for inspection access.

Results
The crew was able to mitigate issues during this stage and continue to complete the dredging process ahead of schedule. Dive inspections concluded the project, providing a detailed overlay of the underwater dredged area as well as the LLO.