Highest tides in world caused by erosion of bay floor
© Dave Mathieson - The Citizen-Record
Charles O’Riley, who has spent much of his life studying tides gave a talk at the log cabin in Advocate on Sunday.
ADVOCATE – Predicting future ocean levels and how they will affect coastal communities, such as those in Advocate, is a mugs game.
That was the basic message given during a talk at the log building in advocate on Sunday.
Charles O’Riley, who is recently retired from the Canadian Hydrographic Service at the Bedford Institute of Oceanography in Halifax, talked about the difficulty of making predictions about future ocean levels.
“When I hear people make predictions of future water level averages to within one or two decimals, it is to laugh, said O’Riley. “The data does not exist.”
O’Riley said high tides in the Bay of Fundy are caused by the amount of water in the bay.
“5,000 years ago, when they were building the pyramids, the tides here were no different than Halifax,” said O’Riley. “The high tide has nothing to do with the smaller shape of the Bay of Fundy. It’s the fact that the Bay of Fundy is a pan of water that sloshes."
Adding or subtracting water to the system changes the tides and, thanks to erosion, much water has been added to the Bay of Fundy since the time the pyramids were built. O’Riley showed a map of the floor of the bay and pointed to an eroded hole at the base of the bay and said a city the size of Halifax could fit in it.
“Every pan of water has a natural oscillation and the Bay of Fundy oscillates at almost the same frequency as the main lunar forces,” added O’Riley. “It’s like pushing a child on a swing. As you put water in or take it away it’s like changing the length of the rope on a swing and you have to push slower or faster.
“So it’s the length and the depth of the Bay of Fundy that creates the high tides,” said O’Riley. “If you add water or take water away it will stop sloshing because the natural frequency is being changed.”
O’Riley said ocean levels could rise one to three feet in the next century.
He said there is a debate as to whether or not the change is man-made or not.
"Is there an anthropogenic effect, are humans causing this?” asked O’Riley. “If water levels are rising one foot a century or three feet as century, whether it’s man-made or not, it’s still going to effect you.”
Another factor that affects ocean levels is the rising and falling of the earths crust.
“The land is sinking and rising and that needs to be taken into account in order to determine what is going to happen with sea levels,” said O’Riley “You can’t just look at tide levels. Most of Canada is sinking, including Nova Scotia. The sinking will stop in three or four thousand years.”
We might experience a new ice age by then.
“It’s conceivable that we could see another ice age in within the next few thousand years,” said O’Riley. “In the last million years we’re probably living in the best climate available to mammals.”
Another factor making predictions difficult is the lack of data.
One problem is different countries use different reference points to determine the height of tides.
“When you get measurements from different countries it’s like comparing apples and oranges,” he said. “Everybody is using different measuring sticks.”
What makes matters even worse is that, thanks to erosion, reference points are always changing.
“You’re chasing the wind across the field, because the zero reference is always changing.” said O’Riley.
He said some reference points being used today are almost one hundred years old.
“It would take hundreds of millions of dollars to get, and keep, everything up to date,” said O’Riley.
And how does all this affect Advocate and other low-lying areas in Nova Scotia?
O’Riley was asked if it would be better to build dikes to protect Advocate or to move the community.
A wait-and-see approach might be the best bet.
“But if the problem gets worse then what you endured in the past might cost you a lot more money than what you endured in the future,” he said
There have been two major flood events in the last 200 years, the Saxby flood in 1869, and there was one in 1759.
O’Riley said with higher sea levels the frequency of an opportunity for that kind of flooding becomes higher.
“A storm has to hit within an hour of the high tide, so there’s a very short window of opportunity for flooding,” said O’Riley. “But the number of opportunities will increase with higher sea levels.”