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
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.
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 1û2 in. to determine if additional
ballast is needed to adjust the resisting moment to overturning,
says Michael E. Ruthford Jr., the Corps project structural