A weir dam is described as any control or barrier placed in
an open channel to permit measurement of water discharge. The
latter may be computed from a formula expressing the discharge in
terms of crest length of the weir, depth of flow above the weir,
weir geometry, and other factors. A variety of weirs have been used
in streams, the so-called sharp-crested and trapezoidal forms being
relatively common; but broad-crested, triangular, and contracted
weirs are also favoured in certain circumstances. Spillways,
controls, and embankments designed to permit discharge measurements
are simply different kinds of broad-crested weirs.Water flows over
the top of a weir, although some weirs have sluice gates which
release water at a level below the top of the weir. The crest of an
overflow spillway on a large dam is often called a weir.
Weirs allow hydrologists and engineers a simple method of
measuring the rate of fluid flow in small to medium-sized streams,
or in industrial discharge locations. Since the geometry of the top
of the weir is known, and all water flows over the weir, the depth
of water behind the weir can be converted to a rate of flow. The
calculation relies on the fact that fluid will pass through the
critical depth of the flow regime in the vicinity of the crest of
the weir. If water is not carried away from the weir, it can make
flow measurement complicated or even impossible.
A weir may be used to maintain the vertical profile of a stream
or channel, and is then commonly referred to as a grade
stabilizer.
Because a weir will typically increase the oxygen content of the
water as it passes over the crest, a weir can have a detrimental
effect on the local ecology of a river system. A weir will
artificially reduce the upstream water velocity, which can lead to
an increase in siltation. The weir may pose a barrier to migrating
fish. Fish ladders provide a way for fish to get between the water
levels. Mill ponds provide a water mill with the power it requires,
using the difference in water level above and below the weir to
provide the necessary energy.
Weir dams are also used to divert water for other purposes as in
power production as is the case of the Ocoee Dam #2 on the Ocoee
River. Ocoee Dam Number 2 is a hydroelectric dam on the Ocoee River
in Polk County in the U.S. state of Tennessee. The dam impounds the
Ocoee No. 2 Reservoir and is one of four dams on the Toccoa/Ocoee
River owned and operated by the Tennessee Valley Authority. Ocoee
Dam No. 2 which was completed in 1913 is perhaps most notable for
its design, which utilizes a wooden flume that carries water from
the reservoir down the side of the Ocoee Gorge to the dam's
powerhouse 5 miles (8.0 km) downstream. Ocoee No. 2 is also
situated at the center of one of the nation's top whitewater
rafting locations, and the dam's releases help to maintain
consistent rapids on the river during warmer months. Ocoee Dam No.
2 is a rock-filled crib-type dam 30 feet (9.1 m) high and 450 feet
(140 m) long. The two units at the powerhouse downstream from the
dam have a generating capacity of 23,100 kilowatts. The dam
typically schedules major recreational releases on weekends in
Spring and Fall and five days per week in Summer months.
The dam's flume consists of a wooden trough situated upon a
shelf carved out of the cliffside. The trough carries the water to
a point just above the powerhouse where it drops the water 250 feet
(76 m) through two large steel pipes to the powerhouse below.
Without this flume system, the dam (at just 30 feet high) would be
practically useless as a power plant.
The growth of industry in Chattanooga some 30 miles (48 km) west
of the Ocoee River in the latter part of the 19th and early 20th
centuries required large amounts of electric power. Several private
entities attempted to meet this demand by building dams that could
produce hydroelectricity. In 1910, a group of financiers formed the
Eastern Tennessee Power Company to exploit the Ocoee's hydro-power
potential. ETPC completed Ocoee Dam No. 1 in late 1911, and work
began on Ocoee Dam No. 2 the following year.
To build the dam, ETPC constructed a wooden "crib" made of
10-foot (3.0 m) by 10-foot (3.0 m) timbers, and filled the crib
with stone. ETPC engineers realized that if the dam were built at
the ideal powerhouse site, it would be unable to utilize the
potential energy from the five-mile stretch immediately upstream in
which the river loses 250 feet (76 m) in elevation. Engineers
solved this dilemma by constructing the 5-mile (8.0 km) flume on
the cliffs above the river gorge, which allows just a 19-foot (5.8
m) drop in the water level from the point at which it exits the
reservoir to the point at which it spills through the pipes into
the powerhouse below. At 14 feet (4.3 m) by 11 feet (3.4 m), the
flume was unusually large for its day. The dam was completed in
1913, and its two generators went online in October of that year.In
1922, Eastern Tennessee Power merged with several other entities to
form the Tennessee Electric Power Company (TEPCO). TEPCO maintained
Ocoee Dam No. 2 until 1939, when a U.S. Supreme Court decision
forced the company to sell its assets to the Tennessee Valley
Authority, which had been established to oversee flood control and
development in the greater Tennessee River valley. The $78 million
TVA paid for TEPCO included $2.59 million for Ocoee Dam No. 2.
TVA made several improvements to the dam in the 1940s which
increased the dam's generating capacity by 15%. The dam's basic
diversion design was used by TVA in the construction of Ocoee Dam
No. 3 several miles upstream in 1942. By 1976, Ocoee No. 2's flume
had deteriorated, and TVA decided to shut it down. To prevent it
from being dismantled, several preservationist groups had the dam
and flume placed on the National Register of Historic Places in
1979. TVA renovated the flume with treated wood, and placed it back
in operation in 1983. A propane-powered tram was built above the
flume to allow authorities to inspect it.