The Ecological Ramifications of the Annapolis Tidal Station
The Annapolis Tidal Station was built in 1984 on the Annapolis River in Nova Scotia, Canada, and to this day, it is one of only a few tidal generating stations of its kind in the world. The purpose of this webpage is offer a brief introduction to the Annapolis Tidal Station and outline one of the main ecological ramifications resulting from its construction, maintenance, and continued presence …So let’s get started!!!
What is the Annapolis Tidal Generating Station, and How Does it Work?
The Annapolis Tidal Generating Station is equipped with a Straight Flow or STRAFLO turbine. The station allows water from the sea to enter a head pond through a set of sluice gates with the incoming tide. Once the head pond is filled to its maximum capacity, the sluice gates close, trapping water in the head pond. When the tide falls, and the water level is 1.6 meters or more below the level of the head pond, a set of 18 wicket gates are opened like Venetian blinds, allowing water to flow from the head pond to the sea through the STRAFLO turbine. As the water rushes through the turbine at 400 cubic meters per second, a four-blade runner is turned, generating electricity. The Annapolis Tidal Station generates power twice a day, producing 80-100 megawatt hours daily…That’s enough electricity to power roughly 4500 homes per year!
Still unsure about how it all works? Check out this video which summarizes what the Annapolis Tidal Station is, and how it operates!
What are the Ecological Impacts of the Annapolis Tidal Station?
There are many ecological consequences that the Annapolis River has faced as a result of the construction, maintenance, and presence of the Annapolis Tidal Station. The following represent a small sample of such consequences:
- Fish Mortality/Shortened Lifespans
- Water Stratification/Salinity Changes
So, why do these consequences matter? Let’s find out by taking a closer look at fish mortality and shortened lifespans.
Anadromous fish in the Annapolis River, such as American shad, striped bass, and Atlantic salmon are negatively impacted by the presence of the turbine and the barrage in which it is located. The barrage serves as an obstacle to such fish when they migrate to their spawning grounds upriver as well as when they leave the estuary to return to the sea. The only ways for fish to get past the barrage are by travelling through the fish passes, through open sluice-ways, or through the turbine itself. Unfortunately, since the turbine is located along the thalweg, which is the historical migration route for many fish, particularly those travelling to the sea, many fish often pass through the turbine, rather than utilizing the fish passes. Fish can be maimed, fatally injured, or killed as they pass through the turbine primarily in three ways:
- Cavitation: This involves deadly bubbles of dissolved gasses exploding in the turbine. Such explosions have the capacity to pulp the muscles of nearby fish.
- Pressure Changes: As fish move through the turbine, they experience a deadly and instantaneous drop in pressure.
- Physical Contact with Turbine Blades: Although this is not very common, it is still possible for fish passing through the turbine to be injured or killed when struck by one of the turbine’s four blades.
Although declines in fish stocks in the Annapolis River following the installment of the tidal station have not been recorded, the life spans of fish in the Annapolis River are notably shorter. For example, the American shad, which were once recorded to be as old as 13 years of age, are now found to be no older than the age of 7. While researchers initially predicted a fish mortality rate from travelling through the turbine itself of 5%, new research has emphasized that true fish mortality rates are significantly higher. In fact, one study found that the fish mortality rate from passing through the turbine ranges from 20-80%, depending on the species.
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