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A spillway is a structure used to provide the controlled release of water downstream from a
dam A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use ...
or
levee A levee (), dike (American English), dyke (English in the Commonwealth of Nations, Commonwealth English), embankment, floodbank, or stop bank is a structure that is usually soil, earthen and that often runs parallel (geometry), parallel to ...
, typically into the riverbed of the dammed river itself. In the United Kingdom, they may be known as overflow channels. Spillways ensure that water does not damage parts of the structure not designed to convey water. Spillways can include
floodgate Floodgates, also called stop gates, 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 a ...
s and
fuse plug A 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 condition ...
s to regulate water flow and reservoir level. Such features enable a spillway to regulate downstream flow—by releasing water in a controlled manner before the reservoir is full, operators can prevent an unacceptably large release later. Other uses of the term "spillway" include bypasses of dams and outlets of channels used during high water, and outlet channels carved through
natural dams A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, tap water, human consumption, Industri ...
such as
moraine A moraine is any accumulation of unconsolidated debris (regolith and rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice shee ...
s. Water normally flows over a spillway only during flood periods, when the reservoir has reached its capacity and water continues entering faster than it can be released. In contrast, an
intake tower An intake tower or outlet tower is a vertical tubular structure with one or more openings used for capturing water from reservoirs and conveying it further to a hydroelectric or water-treatment plant. Unlike spillways, intake towers are intended ...
is a structure used to control water release on a routine basis for purposes such as water supply and
hydroelectricity Hydroelectricity, or hydroelectric power, is Electricity generation, electricity generated from hydropower (water power). Hydropower supplies one sixth of the world's electricity, almost 4500 TWh in 2020, which is more than all other Renewabl ...
generation.


Types

A spillway is located at the top of the
reservoir A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contro ...
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. The two main types of spillways are 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 height of water above the reservoir's spillway. The fraction of storage volume in the reservoir above the spillway crest can only be used for the temporary storage of floodwater; it cannot be used as water supply storage because it sits higher than the dam can retain it. In an intermediate type, normal level regulation of the reservoir is controlled by the mechanical gates. In this case, the dam is not designed to function with water flowing over the top if it, either due to the materials used for its construction or conditions directly downstream. If inflow to the reservoir exceeds the gate's capacity, an artificial channel called an auxiliary or emergency spillway will convey water. Often, that is intentionally blocked by a
fuse plug A 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 condition ...
. If present, the fuse plug is designed to wash out in case of a large flood, greater than the discharge capacity of the spillway gates. Although many months may be needed for construction crews 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 building a spillway with the required capacity would be costly.


Open channel spillway


Chute spillway

A chute spillway is a common and basic design that transfers excess water from behind the dam down a smooth decline into the river below. These are usually designed following an
ogee curve An ogee ( ) is the name given to objects, elements, and curves—often seen in architecture and building trades—that have been variously described as serpentine-, extended S-, or sigmoid-shaped. Ogees consist of a "double curve", the combinatio ...
. 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 a
hydraulic jump A 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 ...
, protecting the toe of the dam from erosion.


Stepped spillway

Stepped channels and spillways have been used for over 3,000 years. Despite being superseded by more modern engineering techniques such as hydraulic jumps in the mid twentieth century, since around 1985 interest in stepped spillways and chutes has been renewed, partly due to the use of new construction materials (e.g. roller-compacted concrete,
gabion A gabion (from Italian ''gabbione'' meaning "big cage"; from Italian ''gabbia'' and Latin ''cavea'' meaning "cage") is a cage, cylinder or box filled with rocks, concrete, or sometimes sand and soil for use in civil engineering, road buildin ...
s) and design techniques (e.g. embankment overtopping protection). 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 spillway

A bell-mouth spillway is designed like an
inverted bell The inverted bell is a metaphorical name for geometric shape that resembles a bell upside-down. By context In architecture, the term is applied to describe the shape of the capitals of Corinthian columns. The inverted bell is used in shape classi ...
, where water can enter around the entire perimeter. These uncontrolled spillways are also called morning glory, (after the
flower A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechani ...
) or glory hole spillways. In areas where the surface of the reservoir may freeze, this type of spillway is normally fitted with ice-breaking arrangements to prevent the spillway from becoming ice-bound. In some cases, bell-mouth spillways are gate-controlled. The spillway at
Hungry Horse Dam Hungry Horse Dam is an arch dam in the Western United States, western United States, on the South Fork Flathead River in the Rocky Mountains of northwest Montana. It is located in Flathead National Forest in Flathead County, Montana, Flathead Cou ...
(''pictured''), in Montana, U.S., the highest morning glory structure in the world, is controlled by a ring gate. One of the most well-known of these spillways is the one in
Covão dos Conchos Covão dos Conchos is an artificial lake in the Serra da Estrela mountains in Portugal that is famous for its Bell-mouth spillway. The spillway was built in 1955 with the aim of diverting water from Ribeira das Naves to Lagoa Comprida. It is a p ...
reservoir lake, in Portugal, which is constructed to look like a natural formation; a video of this went viral on the Internet in early 2016. The largest bell-mouth spillway is in
Geehi Dam Geehi Dam is a major ungated rockfill dam, rockfill embankment dam, embankment dam across the Geehi River in the Snowy Mountains of New South Wales, Australia. The reservoir impounded by the dam is known as Geehi Reservoir. History The structure ...
, in New South Wales, Australia, measuring in diameter at the lake's surface.


Siphon spillway

A
siphon A siphon (from grc, σίφων, síphōn, "pipe, tube", also spelled nonetymologically syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in a ...
makes use of the difference in the height between the intake and the outlet to create a pressure difference needed to remove excess water. Siphons, however, require priming or the removal of air in the bend for them to function, and most siphon spillways are designed with a system that makes use of water to remove the air and automatically prime the siphon. One such design is the volute siphon, which makes use of water forced into a spiral vortex by volutes or fins on a funnel that draw air out of the system. The priming happens automatically when the water level rises above the inlets used to drive the priming process.


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. Also, a drop inlet, which resembles an intake for a hydroelectric power plant, transfers water from behind the dam directly through tunnels to the river downstream.


Design considerations

One parameter of spillway design is the largest flood it is designed to handle. The structures must safely withstand the appropriate spillway design flood (SDF), sometimes called the inflow design flood (IDF). The magnitude of the SDF may be set by dam safety guidelines, based on the size of the structure and the potential loss of human life or property downstream. The magnitude of the flood is sometimes expressed as a
return period A return period, also known as a recurrence interval or repeat interval, is an average time or an estimated average time between events such as earthquakes, floods, landslides, or river discharge flows to occur. It is a statistical measurement typ ...
. A 100-year recurrence interval is the flood magnitude expected to be exceeded on the average of once in 100 years. This parameter may also be expressed as an exceedance frequency with a 1% chance of being exceeded in any given year. The volume of water expected during the design flood is obtained by hydrologic calculations of the upstream watershed. The return period is set by dam safety guidelines, based on the size of the structure and the potential loss of human life or property downstream. The
United States Army Corps of Engineers , colors = , anniversaries = 16 June (Organization Day) , battles = , battles_label = Wars , website = , commander1 = ...
bases their requirements on the probable maximum flood (PMF) and the probable maximum precipitation (PMP). The PMP is the largest precipitation thought to be physically possible in the upstream watershed. Dams of lower hazard may be allowed to have an IDF less than the PMF.


Energy dissipation

As water passes over a spillway and down the chute,
potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors. Common types of potential energy include the gravitational potentia ...
converts into increasing
kinetic energy In physics, the kinetic energy of an object is the energy that 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 accele ...
. 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 Dam Tarbela Dam (, ) is an earth-filled dam along the Indus River in Pakistan's Khyber Pakhtunkhwa province. Located mainly in the Swabi district of the province, The dam is about from the city of Swabi, northwest of Islamabad, and east of Peshawa ...
could, at full capacity, produce 40,000 MW; about 10 times the capacity of its power plant. The energy can be dissipated by addressing one or more parts of a spillway's design. ;Steps First, on the spillway surface itself by a series of steps along the spillway (see
stepped spillway A stepped spillway is a spillway with steps on the spillway chute to assist in the dissipation of the kinetic energy of the descending water. This eliminates or reduces the need for an additional energy dissipator, such as a body of water, at the ...
). ;Flip bucket Second, at the base of a spillway, a flip bucket can create a
hydraulic jump A 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 ...
and deflect water upwards. ;Ski jump 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. ;Stilling basin 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 their design to include chute blocks, baffle blocks, wing walls, surface boils, or end sills.


Safety

Spillway gates may operate suddenly without warning, under remote control. Trespassers within the spillway are at 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.


Gallery

File:Lake Berryessa overflowing into Glory Hole spillway at Monticello dam.jpg, Lake Berryessa overflowing into the glory hole spillway at
Monticello Dam Monticello Dam is a high concrete arch dam in Napa County, California, United States, constructed between 1953 and 1957. The dam impounded Putah Creek to create Lake Berryessa in the Vaca Mountains. Lake Berryessa is currently the seventh-large ...
. File:Hope Mills Dam and Spillway high water.jpg, A labyrinth spillway and a fish ladder (left) of the
Hope Mills Dam The Hope Mills Dam, also known as Hope Mills Dam #1, is a concrete gravity dam on Little Rockfish Creek in Hope Mills, North Carolina, USA, which created Hope Mills Lake. Four different dams were built on the site including the current one. The fi ...
in
North Carolina North Carolina () is a state in the Southeastern region of the United States. The state is the 28th largest and 9th-most populous of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, Georgia and So ...
File:Burdekin Dam.jpg, Spillway with flip bucket at
Burdekin Dam The Burdekin Dam, officially the Burdekin Falls Dam, is a concrete gravity dam with an uncontrolled spillway across the Burdekin River, located south west of Ayr, and Home Hill in North Queensland, Australia. Built for the purpose of irrigatio ...
File:Hoover Dam and Arizona Spillway, 1983.jpg, Water enters
Hoover Dam Hoover Dam is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the border between the U.S. states of Nevada and Arizona. It was constructed between 1931 and 1936 during the Great Depression and was dedicated on Se ...
's Arizona side channel drum-gate spillway (left) during the 1983 floods File:Utedamspillway.jpg, A labyrinth spillway entrance (bottom) at the
Ute Dam Ute Dam (National ID # NM00293) is a dam at Logan, New Mexico in Quay County, about west of the Texas state line. The earthen dam was completed in 1963 by the New Mexico Interstate Stream Commission, without federal funding, and with a height o ...
in
New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ker ...
File:Crystaldamogeespillway.jpg, An ogee-type spillway at the Crystal Dam in
Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...
File:Newwaddelldamspillway.jpg, An emergency spillway with
fuse plug A 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 condition ...
(bottom) and an auxiliary ogee spillway (top) at
New Waddell Dam The New Waddell Dam is an embankment dam on the Agua Fria River in Maricopa County, Arizona, northwest of Phoenix. It serves as part of the Central Arizona Project (CAP) while also providing water for the Maricopa Water District. The dam create ...
File:Ohzuchi dam.jpg, Semicircular spillways of Ohzuchi Dam ( Shiga Pref., Japan) File:Spillway, Bonneville Dam-2.jpg, Low-height spillway of
Bonneville Dam Bonneville Lock and Dam consists of several run-of-the-river dam structures that together complete a span of the Columbia River between the U.S. states of Oregon and Washington at River Mile 146.1. The dam is located east of Portland, Oregon ...
with
sluice gates Sluice ( ) is a word for a channel controlled at its head by a movable gate which is called a sluice gate. A sluice gate is traditionally a wood or metal barrier sliding in grooves that are set in the sides of the waterway and can be considered ...
File:HungryHorseDamSpillway.jpg, upBell-mouth spillway of Hungry Horse Dam in operation. File:Horse Mesa Dam Spillway.jpg, A drop inlet in use at
Horse Mesa Dam The Horse Mesa Dam is a concrete thin arch dam located in the Superstition Mountains, northeast of Phoenix in Maricopa County, Arizona. The dam is long, high and was built between 1924–27. The dam includes three conventional hydroelectric ge ...
in Arizona, ''circa'' 1940 File:Glory Hole, Lake Berryessa.jpg, The spillway at Monticello Dam, Lake Berryessa, in operation. February 19, 2017 File:Geehi Dam Spillway.jpg, The
Geehi Dam Geehi Dam is a major ungated rockfill dam, rockfill embankment dam, embankment dam across the Geehi River in the Snowy Mountains of New South Wales, Australia. The reservoir impounded by the dam is known as Geehi Reservoir. History The structure ...
spillway has never seen water as of March 2019


See also

*
Dam safety system Dam safety systems are used to monitor the state of dams, including external physical threats to the dams, and issuing emergency warnings at various degrees of automation. This includes the use of differential GPS and SAR remote sensing to monit ...
*
Reservoir A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contro ...
*
Stepped spillway A stepped spillway is a spillway with steps on the spillway chute to assist in the dissipation of the kinetic energy of the descending water. This eliminates or reduces the need for an additional energy dissipator, such as a body of water, at the ...
*
Fish ladder A fish ladder, also known as a fishway, fish pass, fish steps, or fish cannon is a structure on or around artificial and natural barriers (such as dams, locks and waterfalls) to facilitate diadromous fishes' natural migration as well as move ...
* Tailrace fishing *
Toddbrook Reservoir Toddbrook Reservoir, a feeder for the Peak Forest Canal, opened in 1838. It is sited above the town of Whaley Bridge in the Derbyshire High Peak area of England. The reservoir is a Site of Special Scientific Interest (SSSI) providing habitats f ...
*
Oroville Dam crisis In February 2017, heavy rainfall damaged Oroville Dam's main and emergency spillways, prompting the evacuation of more than 180,000 people living downstream along the Feather River and the relocation of a fish hatchery. Heavy rainfall during ...


References


External links

*{{cite web , last=Chris , first=Fish , title=The Glory Hole , url=http://www.trophybassonly.com/id56.htm , url-status=dead , archive-url=https://web.archive.org/web/20110601110020/http://www.trophybassonly.com/id56.htm , archive-date=2011-06-01 - information, images, and construction information about the
Lake Berryessa Lake Berryessa is the largest lake in Napa County, California. This reservoir in the Vaca Mountains was formed following the construction of the Monticello Dam on Putah Creek in the 1950s. Since the early 1960s, this reservoir has provided wate ...
glory hole. * Hydraulic structures Dams Flood control