A tidal barrage is a dam-like structure used to capture the

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...

from masses of water moving in and out of a

bay A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay. A large bay is usually called a gulf, sea, sound, or bight. A cove is a small, circular bay with a na ...

river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of ...

due to tidal forces. Instead of damming water on one side like a conventional dam, a tidal barrage allows water to flow into a bay or river during high tide, and releases the water during

low tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables c ...

. This is done by measuring the tidal flow and controlling the

sluice gate 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 ...

s at key times of the tidal cycle. Turbines are placed at these sluices to capture the energy as the water flows in and out. Tidal barrages are among the oldest methods of

tidal power Tidal power or tidal energy is harnessed by converting energy from tides into useful forms of power, mainly electricity using various methods. Although not yet widely used, tidal energy has the potential for future electricity generation. ...

generation, with

tide mill A tide mill is a water mill driven by tidal rise and fall. A dam with a sluice is created across a suitable tidal inlet, or a section of river estuary is made into a reservoir. As the tide comes in, it enters the mill pond through a one-way ga ...

s being developed as early as the sixth century. In the 1960s the

Kislaya Guba Tidal Power Station The Kislaya Guba Tidal Power Station is an experimental project in Kislaya Guba, Russia. The station is the world's 4th largest tidal power plant in operation since the Annapolis Royal Generating Station ceased operation. With an output capacity ...

Kislaya Guba Kislaya Guba (meaning ''sour bay'' in Russian) is a fjord on the Kola Peninsula near Murmansk, Russia. The fjord is connected to the Barents Sea to the north and is primarily known as the site of the experimental tidal power project, Kislaya Gub ...

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...

was built.

Generating methods

The barrage method of extracting tidal energy involves building a barrage across a bay or river that is subject to tidal flow. Turbines installed in the barrage wall generate power as water flows in and out of the estuary basin, bay, or river. These systems are similar to a hydro dam that produces static head or pressure head (a height of water pressure). When the water level outside of the basin or lagoon changes relative to the water level inside, the turbines are able to produce power. The basic elements of a barrage are caissons, embankments,

sluice 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 ...

s, turbines, and

ship locks A lock is a device used for raising and lowering boats, ships and other watercraft between stretches of water of different levels on river and canal waterways. The distinguishing feature of a lock is a fixed chamber in which the water lev ...

. Sluices, turbines, and ship locks are housed in caissons (very large concrete blocks). Embankments seal a basin where it is not sealed by caissons. The sluice gates applicable to tidal power are the flap gate, vertical rising gate, radial gate, and rising sector. Only a few such plants exist. The first was the

Rance Tidal Power Station The Rance Tidal Power Station is a tidal power station located on the estuary of the Rance River in Brittany, France. Opened in 1966 as the world's first tidal power station, it is currently operated by Électricité de France and was for 45 y ...

, on the Rance river, in France, which has been operating since 1966 and generates 240MW. A larger 254MW plant began operation at Sihwa Lake, Korea, in 2011. Smaller plants include the Annapolis Royal Generating Station on the

Bay of Fundy The Bay of Fundy (french: Baie de Fundy) is a bay between the Canadian provinces of New Brunswick and Nova Scotia, with a small portion touching the U.S. state of Maine. It is an arm of the Gulf of Maine. Its extremely high tidal range is t ...

, and another across a tiny inlet in

. A number of proposals have been considered for a barrage across the

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, from

Brean Down Brean Down is a promontory off the coast of Somerset, England, standing high and extending into the Bristol Channel at the eastern end of Bridgwater Bay between Weston-super-Mare and Burnham-on-Sea. Made of Carboniferous Limestone, it is a ...

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe ...

Lavernock Point Lavernock ( cy, Larnog) is a hamlet in the Vale of Glamorgan in Wales, lying on the coast south of Cardiff between Penarth and Sully, and overlooking the Bristol Channel. Marconi and the first radio messages across open sea Following over ...

near

Cardiff Cardiff (; cy, Caerdydd ) is the capital city, capital and List of urban areas in the United Kingdom, largest city of Wales. It forms a Principal areas of Wales, principal area, officially known as the City and County of Cardiff ( cy, Dinas a ...

Wales Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...

. Barrage systems are dependent on high civil infrastructure costs associated with what is in effect a dam being placed across estuarine systems. As people have become more aware of environmental issues, they have opposed barrages because of the adverse effects associated with changing a large ecosystem that is habitat for many varieties of species.

Ebb generation

The basin is filled through the sluices until high tide. Then the sluice gates are closed. (At this stage there may be "Pumping" to raise the level further). The turbine gates are kept closed until the sea level falls, in order to create sufficient head across the barrage. The gates are opened so that the turbines generate until the head is again low. Then the sluices are opened, turbines disconnected and the basin is filled again. The cycle repeats with the tides. Ebb generation (also known as outflow generation) takes its name because generation occurs as the tide changes tidal direction.

Flood generation

The basin is filled through the turbines, which generate at tide flood. This is generally much less efficient than ebb generation, because the volume contained in the upper half of the basin (which is where ebb generation operates) is greater than the volume of the lower half (filled first during flood generation). Therefore, the available level difference – important for the turbine power produced – between the basin side and the sea side of the barrage, reduces more quickly than it would in ebb generation. Rivers flowing into the basin may further reduce the energy potential, instead of enhancing it as in ebb generation. Of course this is not a problem with the "lagoon" model, without river inflow..

Pumping

Turbines are able to be powered in reverse by excess energy in the grid to increase the water level in the basin at high tide (for ebb generation). Much of this energy is returned during generation, because power output is strongly related to the head. If water is raised 2 ft (61 cm) by pumping on a high tide of 10 ft (3 m), this will have been raised by 12 ft (3.7 m) at low tide.

Two-basin schemes

Another form of energy barrage configuration is that of the dual basin type. With two basins, one is filled at high tide and the other is emptied at low tide. Turbines are placed between the basins. Two-basin schemes offer advantages over normal schemes in that generation time can be adjusted with high flexibility and it is also possible to generate almost continuously. In normal estuarine situations, however, two-basin schemes are very expensive to construct due to the cost of the extra length of barrage. There are some favourable geographies, however, which are well suited to this type of scheme.

Tidal lagoon power

Tidal pools are independent enclosing barrages built on high level tidal estuary land that trap the high water and release it to generate power, single pool, around 3.3W/m². Two lagoons operating at different time intervals can guarantee continuous power output, around 4.5W/m². Enhanced pumped storage tidal series of lagoons raises the water level higher than the high tide, and uses intermittent renewables for pumping, around 7.5W/m². i.e. 10 × 10 km² delivers 750MW constant output 24/7. These independent barrages do not block the flow of the river.

Environmental impact

The placement of a barrage into an estuary has a considerable effect on the water inside the basin and on the ecosystem. Many governments have been reluctant in recent times to grant approval for tidal barrages. Through research conducted on tidal plants, it has been found that tidal barrages constructed at the mouths of estuaries pose similar environmental threats as large dams. The construction of large tidal plants alters the flow of saltwater in and out of estuaries, which changes the hydrology and salinity and could possibly harm marine mammals that use the estuaries as their habitat. The La Rance plant, off the Brittany coast of northern France, was the first and largest tidal barrage plant in the world. It is also the only site where a full-scale evaluation of the ecological impact of a tidal power system, operating for 20 years, has been made. French researchers found that the isolation of the estuary during the construction phases of the tidal barrage was detrimental to flora and fauna, however; after ten years, there has been a "variable degree of biological adjustment to the new environmental conditions." Some species lost their habitat due to La Rance's construction, but other species colonized the abandoned space, which caused a shift in diversity. Also as a result of the construction, sandbanks disappeared, the beach of St. Servan was badly damaged and high-speed currents have developed near sluices, which are water channels controlled by gates.

Turbidity

Turbidity (the amount of matter in suspension in the water) decreases as a result of smaller volume of water being exchanged between the basin and the sea. This lets light from the Sun penetrate the water further, improving conditions for the

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...

. The changes propagate up the food chain, causing a general change in the

ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

Tidal fences and turbines

Tidal fences and turbines, if constructed properly, pose less environmental threats than tidal barrages. Tidal fences and turbines, like

tidal stream generator A tidal stream generator, often referred to as a tidal energy converter (TEC), is a machine that extracts energy from moving masses of water, in particular tides, although the term is often used in reference to machines designed to extract energy ...

s, rely entirely on the kinetic motion of the tidal currents and do not use dams or barrages to block channels or

estuarine An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environmen ...

mouths. Unlike barrages, tidal fences do not interrupt

fish migration Fish migration is mass relocation by fish from one area or body of water to another. Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousa ...

or alter

hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is call ...

, thus these options offer energy generating capacity without dire environmental impacts. Tidal fences and turbines can have varying environmental impacts depending on whether or not fences and turbines are constructed with regard to the environment. The main environmental impact of turbines is their impact on fish. If the turbines are moving slowly enough, such as low velocities of 25-50 rpm,

fish kill The term fish kill, known also as fish die-off, refers to a localized die-off of fish populations which may also be associated with more generalized mortality of aquatic life.University of Florida. Gainesville, FL (2005) ''Plant Management in Fl ...

is minimalized and

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel ...

and other nutrients are able to flow through the structures. For example, a 20 kW tidal turbine prototype built in the St. Lawrence Seaway in 1983 reported no fish kills. Tidal fences block off channels, which makes it difficult for fish and wildlife to migrate through those channels. In order to reduce fish kill, fences could be engineered so that the spaces between the caisson wall and the rotor foil are large enough to allow fish to pass through. Larger marine mammals such as seals or dolphins can be protected from the turbines by fences or a sonar sensor auto-braking system that automatically shuts the turbines down when marine mammals are detected.

Salinity

As a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem. "Tidal Lagoons" do not suffer from this problem.

Sediment movements

Estuaries often have high volume of sediments moving through them, from the rivers to the sea. The introduction of a barrage into an estuary may result in sediment accumulation within the barrage, affecting the ecosystem and also the operation of the barrage.

Fish

Fish may move through sluices safely, but when these are closed, fish will seek out turbines and attempt to swim through them. Also, some fish will be unable to escape the water speed near a turbine and will be sucked through. Even with the most fish-friendly turbine design, fish mortality per pass is approximately 15% (from pressure drop, contact with blades,

cavitation Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, ca ...

, etc.). Alternative passage technologies ( fish ladders, fish lifts, fish escalators etc.) have so far failed to solve this problem for tidal barrages, either offering extremely expensive solutions, or ones which are used by a small fraction of fish only. Research in sonic guidance of fish is ongoing. The Open-Centre turbine reduces this problem allowing fish to pass through the open centre of the turbine. Recently a run of the river type turbine has been developed in France. This is a very large slow rotating Kaplan-type turbine mounted on an angle. Testing for fish mortality has indicated fish mortality figures to be less than 5%. This concept also seems very suitable for adaption to marine current/tidal turbines.

Energy calculations

The energy available from a barrage is dependent on the volume of water. The

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 potenti ...

contained in a volume of water is: §174, p. 260. :

E\, =\, \tfrac12\, A\, \rho\, g\, h^2

where: *''h'' is the vertical tidal range, *''A'' is the horizontal area of the barrage basin, *''ρ'' is the

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

of water = 1025 kg per cubic meter (seawater varies between 1021 and 1030 kg per cubic meter) and *''g'' is the acceleration due to the

Earth's gravity The gravity of Earth, denoted by , is the net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation). It is a vector qua ...

= 9.81 meters per second squared. The factor half is due to the fact, that as the basin flows empty through the turbines, the

hydraulic head Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22. It is usually measured as a liquid surface elevation, expressed in units of length, ...

over the dam reduces. The maximum head is only available at the moment of low water, assuming the high water level is still present in the basin.

Example calculation of tidal power generation

''Assumptions:'' * The tidal range of tide at a particular place is 32 feet = 10 m (approx) * The surface of the tidal energy harnessing plant is 9 km² (3 km × 3 km)= 3000 m × 3000 m = 9 × 10⁶ m² * Density of sea water = 1025.18 kg/m³ Mass of the sea water = volume of sea water × density of sea water ::= (area × tidal range) of water × mass density ::= (9 × 10⁶ m² × 10 m) × 1025.18 kg/m³ ::= 92 × 10⁹ kg (approx) Potential energy content of the water in the basin at high tide = ½ × area × density × gravitational acceleration × tidal range squared ::= ½ × 9 × 10⁶ m² × 1025 kg/m³ × 9.81 m/s² × (10 m)² ::=4.5 × 10¹² J (approx) Now we have 2 high tides and 2 low tides every day. At low tide the potential energy is zero.
Therefore, the total energy potential per day = Energy for a single high tide × 2 ::= 4.5 × 10¹² J × 2 ::= 9 × 10¹² J Therefore, the mean power generation potential = Energy generation potential / time in 1 day ::= 9 × 10¹² J / 86400 s ::= 104 MW Assuming the power conversion efficiency to be 30%: The daily-average power generated = 104 MW * 30% ::= 31 MW (approx) Because the available power varies with the square of the tidal range, a barrage is best placed in a location with very high-amplitude tides. Suitable locations are found in Russia, USA, Canada, Australia, Korea, the UK. Amplitudes of up to 17 m (56 ft) occur for example in the

, where tidal resonance amplifies the tidal range.

Economics

Tidal barrage power schemes have a high capital cost and a very low running cost. As a result, a tidal power scheme may not produce returns for many years, and investors may be reluctant to participate in such projects. Governments may be able to finance tidal barrage power, but many are unwilling to do so also due to the lag time before investment return and the high irreversible commitment. For example, the

energy policy of the United Kingdom The energy policy of the United Kingdom refers to the United Kingdom's efforts towards reducing energy intensity, reducing energy poverty, and maintaining energy supply reliability. The United Kingdom has had success in this, though energy in ...

recognizes the role of tidal energy and expresses the need for local councils to understand the broader national goals of renewable energy in approving tidal projects. The UK government itself appreciates the technical viability and siting options available, but has failed to provide meaningful incentives to move these goals forward.

References

{{Reflist, 2 Tidal power