Tacoma Power’s Lake Cushman Floating Surface Collector (FSC) is a breakthrough project that helps fish migrate downstream over a high dam. Its design is the third generation of FSCs in the Pacific Northwest and incorporates design improvements that make it half the cost of similar structures, yet with better operation performance. Schnabel was responsible for the mechanical and hydraulic design of the entire facility.

The 210-ft-long floating vessel includes 10 primary pumps and two submersible dewatering pumps that create a current to attract fish, while guide nets extend out to each side of the lake to direct migrating salmonids into the facility. Once fish enter the net transition structure (NTS), velocity gradually increases from 0.2 fps to a maximum of 8 fps at the capture module. From there, dewatering begins and fish are sorted before being sent downstream. The 103-ft-long NTS was constructed upside down, launched into the lake, and then up-righted by filling selected pontoons on one side of the structure to perform a controlled rollover.

To complete the rollover without damaging empty pontoons, we designed temporary bracing to withstand upwards of 60 feet of hydrostatic pressure and drag forces that would occur during the event. We designed the FSC’s 215,000-lb floating steel space-frame structure, provided engineering services during construction, and performed testing for startup, commissioning assistance, and hydraulic performance. We also provided detailed weight estimates and buoyant force modeling to determine accurate ballast placement aboard the vessel.

The facility’s 10 individual pump bays contain a low head, high capacity, vertical lineshaft pump; together, they remove 244 cfs of water. Each bay uses our custom-designed rotating screens on a cassette to direct fish and handle debris while saving precious deck space on the vessel. Along with the pumps and traveling water screens, the completed vessel includes extensive metal fabrications, coating systems, three buildings, a utility water system, an air vacuum and compressor, over 320 hp of mechanical equipment, lighting, and a complete SCADA system onboard.

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