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Toronto Islands slipping into Lake Ontario through erosion

Edward Skira

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Perhaps they need to cut a channel in the spit to allow erosion from the bluffs to again feed the islands.


http://www.theglobeandmail.com/servlet/story/RTGAM.20070808.werosion08/BNStory/National/home

Toronto Islands slipping into Lake Ontario through erosion
How the Islands are being eaten by the Spit

MARTIN MITTELSTAEDT
From Wednesday's Globe and Mail
August 8, 2007 at 5:04 AM EDT

The islands are one of Toronto's most bucolic places - a beautiful waterfront park in Lake Ontario formed out of an archipelago of sandbars.

But here at their southwestern tip, the scene is anything but breathtaking. Erosion caused by the pounding action of waves from Lake Ontario is chewing away at the islands. It's not a pretty sight.

Mature cottonwood trees are tumbling into the lake, as are the islands' dunes, considered an ecological rarity because there aren't any others in this area of Canada for more than 100 kilometres. So much of the islands has been washed away that old water and gas mains buried decades ago, thought to be safely away from the then-existing shoreline, now lie exposed in open water. Even the city's popular nude beach at Hanlan's Point, once a wide expanse of buff-coloured sand, is beginning to be stripped away.

"It's sad. It's valuable real estate washing away," observed Warren Hoselton, Toronto Islands park supervisor, who has been watching with increasing alarm as the lake claims parts of his park. He is concerned that the erosion is set to worsen. "We're at the tipping point now."

The problem of disappearing real estate at one of Toronto's local landmarks has an unusual cause. In Toronto parlance, the islands are being eaten by the spit.

For decades, Toronto has been dumping construction waste from downtown building sites into Lake Ontario.

The rubble has been shaped into a five-kilometre-long peninsula jutting into the lake near the islands and known locally as the Leslie Street Spit.

The spit is blocking the lake currents that, in the past, built up the islands from sand washed off the Scarborough Bluffs.

Now, whenever storms blow across the lake from the south, their large, rolling waves cut away at the exposed southwesterly tip of the islands. With no fresh sediment from the bluffs to replenish what is washed away, the islands are slowly losing ground to the lake.

The rate of erosion has been estimated at about eight metres a year, a relatively sedate pace but one that is still worrisome, given that the islands are only a few hundred metres wide at many points and, in some areas, trees that previously resisted the waves have toppled over. Once tree cover is lost, exposed sand is no match for waves, and is quickly washed away.

According to an estimate by the Toronto and Region Conservation Authority, up to about a sixth of the park could be lost over the next century if no solutions are found. But the problem could easily worsen. A pickup in the pace of erosion would occur should the high water levels existing in the mid-1990s return and coincide with big winter storms.

"All we need is one high-water year and we're really in trouble," said Joanna Kidd, a spokeswoman for the Toronto Bay Initiative, an organization that tried to preserve the islands' rare dunes but was running a losing battle with the erosion.

The worst damage on the islands is around Gibraltar Point, where the entire near-shore area is suffering.

The point lies more than a kilometre to the west of the park's most popular picnic area at Centre Island, and about 3½ kilometres from the residential community at the eastern tip of the islands.

No buildings have been washed away, but the point's washroom nearly fell into the lake after a particularly bad winter storm a few years ago, according to Mr. Hoselton. The building was only saved by placing giant limestone boulders between it and the waves, although this is providing only a temporary respite.

Moranne McDonnell, an erosion specialist at the Toronto and Region Conservation Authority, says that sand on the lakebed offshore from the islands is also washing away, from the same forces causing erosion on land.

This makes it difficult to protect the shoreline with giant rocks, because they'll tumble into the lake. "Anything you place along the shoreline will eventually become undermined and will fail," Ms. McDonnell said.

Next in the path of the waves is an arts centre that overlooks the lake, followed by one of Toronto's four drinking-water filtration plants, which would eventually become vulnerable if nothing is done.

The islands are considered dear to many Torontonians, who view them as the city's version of New York's Central Park. More than 1.2 million people visit them every year, brought across Toronto inner harbour from the city's downtown by a fleet of municipal ferry boats.

The city has already protected much of the islands behind bulkheads, cement sandbags and breakwalls, but plans to save the vulnerable southwestern areas have been stalled since the late 1980s for budget reasons.

Proposals for halting the erosion are under study by the conservation authority. It has looked at ringing the shoreline with a massive, offshore breakwall nearly a kilometre long that would block storm waves.

The drawback is that it would recreate a stagnant pool of water in the lake, so an alternative approach being considered is to dot the offshore with a series of smaller breakwalls that would allow some water circulation and still dissipate much of the power contained in storm waves.

Some consideration is even being given to trying to mimic nature by replenishing the sand that once came from the bluffs, using material dredged from elsewhere in the harbour area.

Stopping the erosion won't be cheap and it won't happen any time soon. Ms. McDonnell says breakwalls could cost anywhere from $10-million to $13-million, and would take years to complete.
 

Prometheus The Supremo

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Even the city's popular nude beach at Hanlan's Point, once a wide expanse of buff-coloured sand, is beginning to be stripped away.
the "nude" beach is being "stripped" away.

i hope they figure something out soon. that is alot of erosion.
 

Admiral Beez

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The islands are a natural sandbar, they're supposed to erode and change. I'd rather see a openly accessible and changing coastline than a fortified wall of concrete baricades keeping the lake at bay.
 

Prometheus The Supremo

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you'd think that all the silt from the west end rivers would somehow at least have some positive effect?
 

299 bloor call control.

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The islands are a natural sandbar, they're supposed to erode and change. I'd rather see a openly accessible and changing coastline than a fortified wall of concrete baricades keeping the lake at bay.
But the issue is that the erosion is being caused by something manmade in the first place. It's like saying that we should let an animal mutate because that's what it naturally did after we dumped pollution all over its habitat.
 

Edward Skira

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And since they are deprived of silt they will eventually disappear altogether unless something is done.

you'd think that all the silt from the west end rivers would somehow at least have some positive effect?
Obviously the currents are going in another direction or they are not providing as much silt as the bluffs do.
 

299 bloor call control.

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.... and i love spending a day of my summer weekends at Hanlans... that beach really isn't that wide to begin with.. if it's eroding at 8 metres a year... there won't be a beach left... :(
 

Prometheus The Supremo

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is all the sand from the bluffs ending up at the east shores of the spit? has there been growth?

it's time to create more islands on the west side or a western spit, but then you have the problem of poor water circulation.
 

adma

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Isn't the heart of the Hanlan's nude beach rather further north than the part being most eroded?
 

3Dementia

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That "8 metres" per year must be a misprint... the spit has been around long enough to have reduced the islands to a strip of lawn, 2 trees and a Venice-style RCYC clubhouse.
 

jeicow

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When did they stop dumping dirt on the spit/Has it stopped? I don't remember seeing any new dirt or any signs of it occuring last time I was there, but that said, I've likely been around there half a dozen times in my life.
 

Mongo

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8 metres per year? That can't be right. The Leslie Street Spit has extended roughly as far as it currently does since the mid-1960s -- it's become wider but not really much longer. This would imply 320m of erosion in that time, where most of the island is less than 250m wide, and the area around Gibraltar Point (where the erosion is worst, according to the article), is mostly under 150m in width.

I took a look at erosion maps of the area around Gibraltar Point from 1999, and compared them to current maps.

1999 erosion map: http://tntmen.abuzar.net/tnt/beach/fig1.gif

At that time, the area around Gibraltar Point had already suffered about 80m of erosion in thirty-odd years (from when the Leslie Street Spit reached close to its current length to 1999), or about 2.5m per year. Since then, a span of 8 years, it looks like an additional 20m or so has been eroded away -- again about 2.5m per year.

2.5m of erosion per year is a lot. At that rate, the island would be cut in half in about 32 more years (it's about 80m wide at Gibraltar point). However, looking at the current map, I am not sure how quickly the remainder of the neck of land will erode. It started out as a protruding point of land, exposed to the worst of Lake Ontario, and has since been eroded to where the area is now concave. I would think that the erosion rate would slow down, although it has apparently not yet done so.

Even so, the neck of land is now so narrow that almost any erosion rate would be too high, so I agree that if we decide to keep Centre Island from splitting in two, action does need to be taken within the next few decades.

Bill
 

Admiral Beez

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But the issue is that the erosion is being caused by something manmade in the first place. It's like saying that we should let an animal mutate because that's what it naturally did after we dumped pollution all over its habitat.
Well then, the obvious solution is to take the fill that was being dumped on the Spit, and instead dump it on the island to stop erosion.
 

Chuck

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We could also cut the Leslie Street Spit off of the mainland, creating a second set of "Toronto Islands".
You'd have to remove a fairly large swath of land for that to be effective. I suspect that a lot of sediment is also trapped by the vastly expanded Ashbridge's Bay area.

Perhaps a narrow band of wetlands could be added along the south shore of the islands to add a buffer to oncoming waves?
 

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