Gulf Voices: Tidal power - A green dream?
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By Stephen Hawboldt
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Stephen Hawboldt in Annapolis Royal, Nova |
FOR MANY DECADES, harnessing the mighty tides of the Bay of Fundy has been a dream of engineers, investors and politicians. This is not surprising. The Bedford Institute of Oceanography in Halifax has estimated that more water moves in and out of the Bay of Fundy daily than all of the rivers of the world combined.
First envisaged in the 1950s, the world's second tidal generating station was finally built in Annapolis Royal in the 1980s. The developers saw this as a pilot project to prove the economic and ecological integrity of their dream of using Fundy tides to generate electricity. They choose Annapolis Royal because a causeway was built there in the early 1960s. The causeway was originally designed as a transportation link and a water control structure to prevent flooding of former salt marshes located upstream.
The Annapolis Royal Tidal Generating Station works on a very simple principle. The incoming tide fills the lower reaches of the Annapolis River upstream of the causeway creating a head pond. Just as the tide turns, the gates are closed. When the elevation differential between the falling tide and the head pond reaches a certain point, the gates are opened and the rushing water spins the turbine. The Annapolis station only works on the falling tide.
The plant continues to generate electricity into the now rising tide until the elevation differential between the head pond and the incoming tide is too small. The turbine goes off-line and the rising tide again fills the head pond to restart the process when the tide drops.
Because the tidal generator in Annapolis Royal was placed in the existing Annapolis Causeway, it is not easy to ascertain the project's real impacts.
Those opposed to the Annapolis project have blamed the tidal station for excessive fish kills. Fish passage studies using tagged fish have shown a mortality rate of 40 percent, plus or minus 30 per cent. It is difficult to know what impact, if any, handling the fish to tag them had on their ability to survive passage through the turbine system. While pressure differentials are killing fish, it is unclear if the mortality rate is greater or less than would be expected in a traditional hydro installation. Nova Scotia Power has made improvements to the original fish way designed by Fisheries and Oceans Canada. As the turbine operates on the ebb tide, the fish-way is only needed during the generation cycle.
Built in 1960, the Annapolis Causeway had, and continues to have, a profound impact on the dynamics of the Annapolis estuary. When the electrical generator was inserted into the existing causeway, 20 years later, the ecological impacts of the original causeway were still being felt but not necessarily measured. For example, slow but steady shoreline erosion at Fort Anne National Historic Site of Canada, just down stream of the Annapolis Causeway, was not discovered until the mid 90s. After extensive research, it was found that this particular shoreline was unable to regain sediment lost to erosion due to the causeway. Sediment picked up on the incoming tide was deposited above the causeway and the sediment in the spring freshet from the uplands was unable to rebuild the eroded shoreline due to the causeway. Subsequently, the original hypothesis that the erosion was somehow related to the tidal power plant was disproved.
The original dream was for the construction of a giant tidal generating facility with barrages built across the water course that would have controlled the water levels in the upper Bay of Fundy including the Minas Basin and Cumberland Basin. This would have shut off part of the Bay of Fundy to the incoming tide. According to oceanographic modeling done at the time, the backed-up tide would have caused mean sea level in Boston to increase by half a foot.
Furthermore, the mud flats in the Upper Bay of Fundy are essential to millions of Arctic nesting shorebirds. These birds feed on Corophium, a type of mud shrimp that lives in the tidal flats at the head of the Bay. In a two-week period, they double their weight and migrate over the ocean to South America. A tidal barrage would compromise this critical habitat and potentially cause the collapse of this global population.
The ecological, economic and social disruptions of such designs are simply too great. Inserting generators into a barrage such as a causeway is not the only way to harness the tides. Several companies are now promoting turbines that would sit on the sea floor. They are somewhat like wind generators but using water pushed by the tides rather than air blown by the wind. Some designs look a little like giant, several stories high, eggbeaters while others have propellers. As there are very strong currents in the Gulf of Maine and Bay of Fundy, it is not surprising that this region is being viewed as a prime development site.
The builders of these new turbines and are advertising this type of tidal power as being environmentally friendly and an alternative to fossil fuels. Governments in Nova Scotia and New Brunswick appear poised to again exploit the tides of Fundy. The energy strategy recently announced for Nova Scotia has tidal power as one of its foundation alternatives to fossil fuel generated electricity.
The claims that this new generation of tidal power is environmentally friendly are not unlike those made in the 1950s for barrage-type tidal power generation.
How ecologically friendly these new structures might be could depend upon many factors. If they were located in the upper Bay of Fundy where sediment transport is the key ecological process, the impact upon millions of shorebirds, nutrient cycling and many other functions could be profound. Further down the Bay, there might be concerns about possible impacts on migrating fish and whales. Just as some sites are not suitable for wind turbines due to their effect on migrating birds, it logically follows that some Fundy sites may interfere with fish movements. Relatively narrow channels with high velocity water flows may be highly desirable turbine locations. These channels are often routes for migrating fish.
So far, tidal power has not been the green dream that its developers believed. The new generation of turbines may show more promise but they will need to be tested first. We should not automatically assume that they are the great green hope.
Stephen Hawboldt is the executive director of the Clean Annapolis River Project in Annapolis Royal, Nova Scotia.
© 2006 The Gulf of Maine Times