Massive reinforced concrete structures designed to wash away during a major flood form the centerpiece of a $50-million flood control and storage capacity expansion nearing completion in central California. Only the third North American installation and the largest in the world, the 450-ton fusegates will increase flood protection and storage capacity, saving $4 million from conventional methods, project officials say.
The gates, designed by Hydroplus Inc., Falls Church, Va., are intended to enhance the capacity of the 42-year-old Terminus Dam, a 1,180-ft-long earthfill structure located 18 miles east of Visalia, Calif. In a major flood, the six 21-ft-tall concrete fusegates would use a system of pipes and wells to channel floodwaters down the spillway, easing forces that could threaten the dam.
The project will raise the dams design protection level from a 46-year to a 70-year flood and increase Lake Kaweahs 142,000 acre-ft capacity by 42,000 acre-ft, says Norbert F. Suter, senior project manager for the Army Corps of Engineers Sacramento district. Jointly funded by federal, state and local agencies, the project includes environmental mitigation and $5.5 million for road projects.
Fusegates won the nod over more conventional approaches because of perceived environmental and budget advantages. A curved ogee weir located 200 ft to 250 ft upstream of the existing spillway would require excavation of 150 ft of hillside, Suter says. A power failure could put the main dam at risk. "The state would not allow mechanical devices in the spillway that could fail," says Suter.
But the fusegates are designed to be sacrificed in a major flood. Gradually rising water levels would activate the gates through a system of pipes and wet wells. When the lakes level exceeded 21 ft over the top of the gates, the wet well would start filling 3-ft-dia pipes beneath the first gate. The water "fills up the chamber that energizes the pipe that creates uplift on the gate," Suter says. Each gate would be pushed out of the way in succession. "The uplift pushes them out of the way, and they just wash on down," Suter says. Once the water recedes, the owner could put new fusegates in place.
To prevent trees or other floating debris from jamming the chambers, the Terminus project will use a different intake design from the 40 existing fusegates in 14 countries. "The inlet well is normally attached to each fusegate," says Hasan Kocahan, business development manager for Hydroplus. Instead of the typical design, which calls for an intake pipe on each gate, Hydroplus designed a separate intake tower at the right abutment linked by a network of pipelines to each gate.
|Each fusegate displaces 450 tons and is placed atop concrete slabs that are up to 6.5 ft thick.|
The project began in fall 2001 with a $3-million contract that included excavation of 100,000 cu yd of rock. Whitaker Construction Co., Santa Maria, Calif., was awarded the $8.7-million contract for the second phase, currently valued at about $10 million, to build the fusegates.
Whitaker first poured a waste slab up to 8 ft thick to counteract highly fractured rock conditions with different elevations, says Matt Bousman, Whitakers project manager. The fusegates 24,000-sq-ft foundation includes 16 slabs up to 6.5 ft thick, Bousman adds. About 600 rock anchors tie the concrete to the rock face.
After completion, each gate is jacked up hydraulically 1�2 in. to determine if additional ballast is needed to adjust the resisting moment to overturning, says Michael E. Ruthford Jr., the Corps project structural engineer.