A
spillway is a structure used to provide the controlled release of flows from a
damA dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...
or
leveeA levee, levée, dike , embankment, floodbank or stopbank is an elongated naturally occurring ridge or artificially constructed fill or wall, which regulates water levels...
into a downstream area, typically being the river that was dammed. In the UK they may be known as
overflow channels. Spillways release floods so that the water does not overtop and damage or even destroy the dam. Except during flood periods, water does not normally flow over a spillway. In contrast, an
intake is a structure used to release water on a regular basis for water supply,
hydroelectricityHydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy...
generation, etc.
FloodgateFloodgates are adjustable gates used to control water flow in flood barriers, reservoir, river, stream, or levee systems. They may be designed to set spillway crest heights in dams, to adjust flow rates in sluices and canals, or they may be designed to stop water flow entirely as part of a levee or...
s and
fuse plugA fuse plug is a collapsible dam installed on spillways in dams to increase the dam's capacity.The principle behind the fuse plug is that the majority of water that overflows a dam's spillway can be safely dammed except in high flood conditions. The fuse plug may be a sand-filled container, a...
s may be designed into spillways to regulate water flow and dam height. Other uses of the term "spillway" include bypasses of dams or outlets of a channels used during highwater, and outlet channels carved through natural dams such as
moraineA moraine is any glacially formed accumulation of unconsolidated glacial debris which can occur in currently glaciated and formerly glaciated regions, such as those areas acted upon by a past glacial maximum. This debris may have been plucked off a valley floor as a glacier advanced or it may have...
s.
Types
A spillway is located at the top of the reservoir pool. Dams may also have bottom outlets with valves or gates which may be operated to release flood flow, and a few dams lack overflow spillways and rely entirely on bottom outlets.
There are two main types of spillways: controlled and uncontrolled.
A
controlled spillway has mechanical structures or
gates to regulate the rate of flow. This design allows nearly the full height of the dam to be used for water storage year-round, and flood waters can be released as required by opening one or more gates.
An
uncontrolled spillway, in contrast, does not have gates; when the water rises above the lip or
crest of the spillway it begins to be released from the reservoir. The rate of discharge is controlled only by the depth of water within the reservoir. All of the storage volume in the reservoir above the spillway crest can be used only for the temporary storage of floodwater, and cannot be used as water supply storage because it is normally empty.
In an intermediate type, normal level regulation of the reservoir is controlled by the mechanical gates. If inflow to the reservoir exceeds the gate's capacity, an artificial channel called either an
auxiliary or
emergency spillway that is blocked by a
fuse plugA fuse plug is a collapsible dam installed on spillways in dams to increase the dam's capacity.The principle behind the fuse plug is that the majority of water that overflows a dam's spillway can be safely dammed except in high flood conditions. The fuse plug may be a sand-filled container, a...
dike will operate. The fuse plug is designed to over-top and wash out in case of a large flood, greater than the discharge capacity of the spillway gates. Although it may take many months to restore the fuse plug and channel after such an operation, the total damage and cost to repair is less than if the main water-retaining structures had been overtopped. The fuse plug concept is used where it would be very costly to build a spillway with capacity for the probable maximum flood.
Chute spillways
Chute spillways are common and basic in design as they transfer excess water from behind the dam down a smooth decline into the river below. Most often, they are lined on the bottom and sides with concrete to protect the dam and topography. They may have a controlling device and some are thinner and multiply lined if space and funding are tight. In addition, they are not always intended to dissipate energy like stepped spillways. Chute spillways can be ingrained with a baffle of concrete blocks but usually have a 'flip lip' and/or dissipator basin which creates
hydraulic jumpA hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface...
, protecting the toe of the dam from erosion.
Stepped spillways
Stepped channels and spillwaysRecent advances have permitted the construction of large dams and reservoirs and channels. These have required the development of new spillway designs. A key feature of a spillway system is the safe dissipation of a major component of the kinetic energy of the waters to avoid damage and failure...
have been used for over 3,000 years. Recently, new construction materials (e.g. RCC, gabions) and design techniques (e.g. embankment overtopping protection) have increased the interest in stepped spillways and chutes. The steps produce considerable energy dissipation along the chute and reduce the size of the required downstream energy dissipation basin.
Research is still active on the topic, with newer developments on embankment dam overflow protection systems, converging spillways and small weir design.
Bell-mouth spillways
Some spillways are designed like an
inverted bellThe inverted bell is a metaphorical name for geometric shape that resembles a bell upside down.In architecture, the term is applied, e.g., to describe the shape of the capitals of Corinthian columns....
so that water can enter all around the perimeter. These uncontrolled spillway devices are also called
morning glory,
plughole,
glory hole or
bell-mouth spillways. In areas where the surface of the reservoir may freeze, bell-mouth spillways are normally fitted with ice-breaking arrangements to prevent the spillway from becoming ice-bound.
Chaffey DamChaffey Dam is a dam 16 kilometres north of Nundle and 44 kilometres south of Tamworth in New South Wales. Its main purposes are regulating the flow of the Peel River and to augment the water supply of Tamworth....
, located near
Tamworth, New South WalesTamworth is a city in the New England region of New South Wales, Australia. Straddling the Peel River, Tamworth, which contains an estimated population of 47,595 people, is the major regional centre for southern New England and in the local government area of Tamworth Regional Council. The city...
in Australia has a classic example of an inverted-bell spillway. It was the first one of its kind in Australia.
In some cases bell-mouth spillways are gate controlled. The spillway at
Hungry Horse DamHungry Horse Dam is an arch dam on the South Fork Flathead River in the Rocky Mountains of the U.S. state of Montana. It is located in Flathead National Forest, in Flathead County, about south of the west entrance to Glacier National Park, southeast of Columbia Falls, and northeast of Kalispell...
(pictured right), the highest morning glory structure in the world, is controlled by a 64 by ring gate.
Other types
Other spillway types include an
ogee crest which over-tops a dam, a
side channel that wraps around the topography of a dam and a
labyrinth which uses a 'zig-zag' design to increase the sill length for a thinner design and increased discharge. There is also a
drop inlet which resembles an intake for a hydroelectric power plant but transfers water from behind the dam directly through tunnels to the river downstream.
Design considerations
The largest flood that needs be considered in the evaluation of a given project, regardless of whether a spillway is provided; i.e., a given project should have structures capable of safely passing the appropriate spillway design flood (SDF). A 100-year recurrence interval is the flood magnitude expected to be exceeded on the average of once in 100 years. It may also be expressed as an exceedance frequency with a one per cent chance of being exceeded in any given year.
Energy dissipation
As water passes over a spillway and down the chute,
potential energyIn physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...
converts into increasing
kinetic energyThe kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
. Failure to dissipate the water's energy can lead to scouring and erosion at the dam's toe (base). This can cause spillway damage and undermine the dam's stability. To put this energy in perspective, the spillways at
Tarbela DamTarbela Dam on the Indus River in Pakistan is the second largest dam in the world by structural volume. It is located in Haripur District, Khyber Pakhtunkhwa, about northwest of Islamabad.The dam is high above the riverbed. The dam forms the Tarbela Reservoir, with a surface area of approximately...
could, at full capacity, produce 40,000 MW; about ten times the capacity of its power plant. The energy can be dissipated by addressing one or more parts of a spillway's design. First, on the spillway surface itself by baffles and/or steps along the spillway. Second, at the base of a spillway, a
flip bucket can create a
hydraulic jumpA hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface...
and deflect water upwards. A
ski jump can also direct water horizontally and eventually down into a plunge pool or two ski jumps can direct their water discharges to collide with one another. Third, a
stilling basin at the terminus of a spillway serves to further dissipate energy and prevent erosion. They are usually filled with a relatively shallow depth of water and sometimes lined with concrete. A number of velocity-reducing components can be incorporated into the their design to include chute blocks, baffle blocks, wing walls, surface boils or an end sill.
Safety
Spillway gates may operate suddenly without warning, under remote control. Trespassers within the spillway run the risk of drowning. Spillways are usually fenced and equipped with locked gates to prevent casual trespassing within the structure. Warning signs, sirens, and other measures may be in place to warn users of the downstream area of sudden release of water. Operating protocols may require "cracking" a gate to release a small amount of water to warn persons downstream.
The sudden closure of a spillway gate can result in the stranding of fish, and this is also usually avoided.
External links
- information, images, and construction information about the
Lake BerryessaLake Berryessa is the largest lake in Napa County, California. This reservoir is formed by the Monticello Dam, which provides water and hydroelectricity to the North Bay region of the San Francisco Bay Area....
glory hole.