A tidal barrage is a dam-like structure that is built across the mouth of a river or bay in order to harvest energy from the ebb and flow of tides in the location. A tidal barrage is referred to as dam-like because even though it looks like a dam, it doesn’t function like a conventional dam.
How a Tidal Barrage Works
A tidal barrage is usually made of reinforced concrete and spans an estuary, bay, river, or other ocean inlet. The barrier contains gates called sluices that can open and close. At low tide, the gates open and allow the tide to flow into the river as normal as the tide rises. When high tide is reached, the gates are closed and prevent the water from retreating back to the ocean as it normally would. Instead, the water is forced through specific channels that direct it through turbines in order to produce electricity. The diagrams below illustrate.
As you can see, the concept of a tidal barrage is actually quite simple and the most similar to other forms of hydropower that involve damming a river. Such systems have been put in place in France and Russia in the 1960’s, so the concept is not new.
The turbine is what generates the electricity as a result of water moving through it. In the diagram above, the turbine is positioned only in the outflow stream of the system. In general, the outflow is more efficient at producing energy that then inflow and many systems avoid blocking the inflow so as to ensure as much water is possible is captured as the tide comes in. However, some systems do employ turbines on the inflow stream as well. Turbines in the inflow stream (high tide diagram) are referred to as flood generators while those on the outflow stream (low tide diagram) are called ebb generators.
The turbines depicted are sometimes called Kaplan turbines and have an appearance similar to that of windmill. There are other types of turbines including the Francis Turbine and the Pelton Wheel, which have slightly different shapes and either more or less blades. The type of turbine used will be based on water head, which is just another way of saying water pressure as a result of its height behind the dam. Kaplan turbines are well suited to low head sites because they have low resistance to turning.
Advantages of Tidal Barrages
Tidal barrages benefit from their simplicity and long history. They are easy to build, relatively easy to maintain, and have a proven track record of reliability. Given their similarity to traditional hydroelectric power plants found on rivers, the expertise to run tidal barrages efficiently is easily found.
Disadvantages of Tidal Barrages
Unfortunately, there are a number of disadvantages to tidal barrages that have kept them from becoming more popular. The first disadvantage is cost. Building a large dam across the mouth of a river or the inlet of a bay is more complicated and costly than building a dam on a river. While water on a river is easily diverted around the dam site, water in a bay or river mouth is not so easily diverted. The cost per kilowatt of energy generated is substantially higher for tidal barrages than most other forms of energy generation. As the price of fossil fuels climbs, however, the cost of tidal barrages is becoming less of a hindrance to their construction.
More than the economic impact, the environmental impact of tidal barrages is a serious consideration in their construction. Estuaries, river mouths, and bays are sensitive ecosystems that often represent the spawning grounds for species that may not breed anywhere else. Altering the flow of sea water negatively affects a number of marine mammals as well as fish, crustacean, and plant life. The French La Rance Tidal Barrage has resulted in the loss of several different species of flora and fauna, though over a decade most species have returned or adapted. Beaches, dunes and other nearby land habitat are also negatively affected by the change in tide due to building a barrage. Impacts must be carefully studied before any project is started, which substantially raises the cost of constructing a barrage. A list of some of the major impacts is found here:
- Turbidity – Barrages increase the amount of particulate matter floating in water, which decreases penetration of sunlight and negatively affects plants, algae, and fish.
- Salinity – The decreased exchange of water as a result of the dam decreases salinity, which can have a profound effect on wildlife.
- Sediment damming – Building a barrage prevents the normal movement of sediment into the ocean. This decrease nutrients and is also a major problem for barrage operation.
- Turbine death – Fish and other aquatic animals are killed by the blades of turbines. No good solution to the problem has yet be developed.