Changing tides

Tidal speeds in Nova Scotia’s Bay of Fundy can reach a staggering five metres per second.  By comparison a very fast river flows at about two to three metres per second, “and you wouldn’t want to fall into a current like that,” says Queen’s University professor in coastal engineering Ryan Mulligan.

So it stands to reason that anything slowing that flow would have significant impact on the ecosystem. That includes tidal turbines affixed to the ocean floor, a form of renewable electricity under consideration by Nova Scotia’s department of energy.

The Bay of Fundy has the highest tides in the world. Queen's University coastal engineer Ryan Mulligan is studying the environmental impacts of harnessing that power with tidal turbines. (Andrea Schaffer flickr)

The Bay of Fundy has the highest tides in the world. Queen’s University coastal engineer Ryan Mulligan is studying the environmental impacts of harnessing that power with tidal turbines. (Andrea Schaffer flickr)

Mulligan’s research is providing important insights into how such a field of turbines might affect the bay’s powerful currents and its resulting impact on the environment.

Tidal turbines work much like wind turbines, the key difference being that tidal currents, not wind, drive the blades that generate electricity. Also, tides are reliable and predictable, unlike wind or solar power, making this form of renewal energy hugely appealing, says Mulligan.

Nova Scotia has been operating North America’s only tidal power generating station near the town of Annapolis Royal since 1984. But critics say its design is not as environmentally-friendly as it could be.

The system relies on tides to fill a human-made reservoir from which water is released as needed to generate electricity. The reservoir has caused river bank erosion and the dam has created a lethal trap for some marine wildlife.

Turbines affixed to the ocean floor are a more appealing option. They don’t involve flooding a basin and can be designed to allow marine life to swim past. Even so, they are not without environmental consequences.

Forecasting tidal flows

The difficulty is that there are only a few such installations in the world — most of which are in the United Kingdom — and research into their impact is still in its early stages.

This makes Mulligan’s work all the more timely. He has developed a sophisticated computer model using existing data on tidal speeds and suspended sediments in the Bay of Fundy. This gives him a baseline against which to measure the impact of a field of turbines on the bottom of the bay.

His computer model estimates the turbines’ impact by simulating a semi-permeable barrier across Minas Passage, an area of the bay where currents are particularly strong. At their maximum, the currents in the passage could generate about seven GW of power, enough to power roughly 2.5 million homes.

That would require a full-scale array of several hundred turbines, which Mulligan’s model estimates would result in a drop of nearly 30 per cent in the speed of tidal flows.

“This is not a negligible change,” he says.

The problem with settling sediments

Mulligan has calculated that the reduced flow is likely to lead to a decrease of about 70 per cent in suspended sediments in the water rushing through the passage. Those sediments will instead settle out, leading to silting in adjacent channels, shipping harbours and ecologically-productive tidal flats.

This is crucial information for other researchers — marine ecologists, harbour engineers and fisheries scientists — who can now build on Mulligan’s findings to fine tune predictions about local environmental impacts.

“Taking energy out of the system impacts the physical environment,” says Mulligan. “It will change the habitat and the species that can live in these places. It could also affect the socioeconomic environment — fishing, aquaculture, navigation. We need to know these things.”

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