Contrary to earlier predictions, sediment texture in the Bay of Fundy is unlikely to change if we introduce large-scale tidal power development, according to the head of Dalhousie University’s Oceanography Department.
Sediments in the Gulf of Maine are also unlikely to change much with the installation of tidal turbines in the Minas Passage, Paul Hill told a crowd of about 50 students and academics gathered at Dal Tuesday.
“The overarching question that motivated this research was: Will extraction of tidal power affect the seabed grain size in the Gulf of Maine and the Bay of Fundy?” said Hill, a geological oceanographer who studies fine sediment transport in the coastal ocean.
“If you take out a whole bunch of energy, and put it into electricity, is that going to affect the energy that impinges on the seabed, and thereby affect the grain size that’s on the seabed?”
Grain size, ranging from the particles in mud to stones the size of footballs, is important because it affects how much food is in the seabed, how it is used for shelter and reproduction, Hill said.
“Grain size is a fundamental structure of the benthic ecosystem. So if you change it, you’re going to change the ecosystem.”
One purely tidal model tried to predict what would happen if a two-gigawatt project made up of 1,000 turbines were placed in the Minas Passage, Hill said.
“This is estimated to be the maximum power that you could extract without having big changes in the amplitude of the tide, which would precipitate a whole bunch of problems and regulatory hurdles.”
That model didn’t take into account factors including waves, winds, density differences in the water due to temperature and salinity changes, and atmospheric pressure disturbances, he said.
It predicted grain sizes would halve in the inner Bay of Fundy with a two-gigawatt project. In Massachusetts Bay, near Boston, the grain size doubled under that old model, Hill said.
“Worrisome for some, there’s a predicted increase in the amplitude of the tidal currents down around Boston, and there’s also a predicted increase in some of the tidal heights down there.”
But questions remained about whether the “simple, hydrodynamic equilibrium model” worked at all, Hill said.
So researchers gathered as much archived US and Canadian data they could find about nearly 10,000 sediment samples collected over decades from the 1960s onward.
In basins, they found grains that were finer than expected. “There are also a few places where the grain sizes are coarser than we expect based on the (old) model.”
Scientists had also failed to take into account all the sediment being added to the mix by the constant erosion of cliffs surrounding the Bay of Fundy, Hill said.
“(In) this area of maximum impact around the Minas Passage, the grain size is determined by cliff erosion. It’s not determined by the flow. That cliff erosion is unlikely to be affected by tidal power development,” he said.
Using a 3-D tidal circulation model, Hill and his team of researchers determined that sediment texture probably would not change much if large-scale tidal power development were pursued in the Minas Passage.
“We argue that sediment size on the seabed is unlikely to be affected by tidal power development. This is controversial because there are a lot of people who would like tidal power to be affecting things that we care about. We’re saying — probably not in this case.”
Species at risk
In an interview, Hill said he doubts his research will quiet everyone’s fears about large-scale tidal power development.
“It will make some people more comfortable,” he said. “But … people are entrenched in their opposition or support for tidal power. There’s some people who are, like, full steam ahead, and others that are saying we shouldn’t do this — we don’t know enough. The thing is that this is a very complicated energetic system, so it’s really hard for us to know what all the consequences of tidal power development are going to be.”
There are concerns that turbines could hurt species at risk that frequent the inner Bay of Fundy, including Atlantic sturgeon, Atlantic salmon, American eel, and striped bass, he told the crowd at Dal.
“Having a major installation of turbines in the Minas Passage through which these species pass could have consequences for them,” Hill said.
“That’s where, if you hear about environmental concerns and you hear about the concerns of some of the inshore fishers, this is really where most of the concern lies. There’s also concern about marine mammals and how they would interact with the turbines.”
Hill cautioned that changing the flow of water over the ocean floor with man-made structures has had drastic affects before.
“We’ve made mistakes in the past in messing around in this incredibly dynamic and energetic system,” Hill said.
He pointed to the causeway built over the Petitcodiac River in the 1950s to join Moncton and Riverview as a “classic one.”
The Petitcodiac went from looking like a wide, tidal river to what amounts to a lake, Hill said.
“There was incredibly rapid sedimentation that happened downstream of the causeway.”
There is work going on to restore the river to something like its natural state, he said, but the causeway is still blocking much of that portion of the Petitcodiac.
Nova Scotia has focused most of its attention on tidal power development at the Fundy Ocean Research Centre for Energy, which has four test pads in the Minas Passage where companies can try out their devices.
“Nobody knows what’s going to work best for tidal power generation,” Hill said.
About 160 billion tonnes of water flows into the Minas Basin with each flood tide, he said.
“This flow is four times greater than all of the rivers of the world combined.”
That makes it an attractive place to try to harness the kinetic energy flowing in and out of that basin every day, Hill said.
Cape Sharp Tidal Ventures successfully deployed a 1,000-tonne turbine in the Minas Passage last fall.
“It’s still generating power and sending power back to shore,” Hill said.
“So it’s there doing its thing, so this is really getting revved up, finally.”
Nova Scotia Power and an Irish outfit named OpenHydro installed a test turbine in 2009 that didn’t work so well.
“It had its fins ripped off in a matter of a week,” Hill said.
Despite the risks, the lure of pulling energy out of the ocean is strong.
“Tidal power is really attractive because it’s sustainable,” Hill said.
“The tides are going to be running like they do for a heck of a long time, so if we can harness them for power then that would be useful.”
Tides are much more predictable than wind and sun, he said.
“So it’s easier to build your base power requirements around a source of energy that’s predictable, unlike the wind and the sun,” he said.
“And, obviously, tidal power has low carbon emissions. So there’s a fair bit of work going on around here about whether tidal power can and should be part of our future.”