Storm surge blasted weather stations meant to study post-tropical storm Fiona

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Red Head Harbour, on Prince Edward Island’s north shore, is under most conditions a well-protected location. Boats are moored snugly inside a “bullpen” with a slender entrance into St. Peters Bay – which escapes into the Gulf of St. Lawrence through its own narrow channel.

Last Saturday, as post-tropical storm Fiona dealt PEI a ferocious backhand of northerly winds, that protection was nowhere near sufficient. A tidal gauge recorded as the water crossed two metres above the benchmark local sea level, overtopping the wharf. At 5:55 a.m. on Sept. 24, water levels peaked at 2.7 metres. After they began receding, at 10:10 a.m., the station stopped transmitting.

Don Jardine tends tidal gauges around PEI on behalf of the University of Prince Edward Island’s Climate Lab and the Mi’kmaq Confederacy. On Tuesday, he visited to find out what happened. Parts of the wharf deck were torn off. Planks floated in the harbour. And the station had been knocked flat.

“We had bolted it into the concrete,” Mr. Jardine said. “But just one of the four bolts was still holding. The plate bent so that the station came down.”

At Cap-aux-Meules, on the Magdalen Islands, readings from a station spiked above 2.1 metres before it, too, was slain by the beast it had been studying. Seawater “may have flooded one of our junction boxes, which is supposed to be waterproof,” said Phillip MacAulay, physical scientist with the Canadian Hydrographic Service. In addition to Cap-aux-Meules, new record water levels were recorded at three of the service’s other stations: Port aux Basques in Newfoundland, and at Escuminac and Shediac in New Brunswick.

How storm surges work

Storm surge is produced by water being pushed toward the shore by the force of the winds moving cyclonically around the storm. The low pressure of the storm has far less impact than the water being forced toward the shore by the wind

1

The wind circulation around the eye of a hurricane blows on the ocean surface and produces a vertical circulation in the ocean

As the hurricane moves towards land, it brings the wind-driven surge with it

2

Storm motion

Eye of the hurricane

2

1

Water on ocean-side flows away without raising sea level much

Wind-driven surge

Once the hurricane reaches shallower waters near the coast, the vertical circulation in the ocean becomes disrupted by the ocean bottom

3

3

The water can no longer go down, so it has nowhere else to go but up and inland

4

4

murat yükselir and JOHN SOPINSKI /

THE GLOBE AND MAIL, SOURCEs: national

oceanic and atmospheric administration

How storm surges work

Storm surge is produced by water being pushed toward the shore by the force of the winds moving cyclonically around the storm. The low pressure of the storm has far less impact than the water being forced toward the shore by the wind

1

The wind circulation around the eye of a hurricane blows on the ocean surface and produces a vertical circulation in the ocean

As the hurricane moves towards land, it brings the wind-driven surge with it

2

Storm motion

Eye of the hurricane

2

1

Water on ocean-side flows away without raising sea level much

Wind-driven surge

Once the hurricane reaches shallower waters near the coast, the vertical circulation in the ocean becomes disrupted by the ocean bottom

3

3

The water can no longer go down, so it has nowhere else to go but up and inland

4

4

murat yükselir and JOHN SOPINSKI /

THE GLOBE AND MAIL, SOURCEs: national

oceanic and atmospheric administration

How storm surges work

Storm surge is produced by water being pushed toward the shore by the force of the winds moving cyclonically around the storm. The low pressure of the storm has far less impact than the water being forced toward the shore by the wind

The wind circulation around the eye of a hurricane blows on the ocean surface and produces a vertical circulation in the ocean

1

As the hurricane moves towards land, it brings the wind-driven surge with it

2

Eye of the hurricane

Storm motion

2

Wind-driven surge

1

Once the hurricane reaches shallower waters near the coast, the vertical circulation in the ocean becomes disrupted by the ocean bottom

3

Water on ocean-side flows away without raising sea level much

The water can no longer go down, so it has nowhere else to go but up and inland

4

4

3

murat yükselir and JOHN SOPINSKI / THE GLOBE AND MAIL, SOURCEs: national oceanic

and atmospheric administration

Storm surge, simply put, is a rise in sea level due to the combination of low atmospheric pressure and strong winds. Low pressure at the storm’s eye pulls the water level up, much like a vacuum cleaner lifts a carpet. In extreme storms, this can raise sea levels by half a metre. The role of wind is typically more significant: A hurricane or tropical storm can push large amounts of seawater, piling it up against the coast.

Two nearby communities might experience very different storm surges during the same maelstrom. One reason is that a cyclone’s winds move in a circular motion, so a community east of the eye can experience winds in the opposite direction to a community west of the eye.

In some cases those winds actually pull water away from shore, lowering sea levels. That’s called “reverse” or “negative” storm surge, and it happened in Bedford Basin during Fiona, lowering water levels there by about 30 centimetres.

Tides are another factor. Port Hawkesbury, N.S., got lucky: Fiona’s worst arrived at low tide. The record-breaking levels at Port aux Basques, on the other hand, coincided with high tide.

On top of all that, there are the waves – something tidal gauges don’t fully capture. According to Environment Canada, during Fiona, a buoy over Banquereau Bank (east of Cape Breton) recorded waves averaging 12 to 15 metres, with peak waves reaching an astonishing 30 metres.

Even bathymetry – basically underwater topography – can strongly influence storm surge. Get unlucky on just two or three of these factors, and the result can be devastating. In the most extreme cases, such as 2005′s Hurricane Katrina, storm surges reached eight metres. In Canada, the worst recorded surges ranged between two and three metres, according to Natural Resources Canada. Surges greater than one metre have been observed on all of Canada’s coastlines: the Pacific, the Atlantic and the Arctic.

A few metres is more than enough.

David Sansom, president of the Red Head Harbour Authority, was checking on eight boats tied up at the wharf during Fiona. In just half an hour, he estimated, the storm surge rose between five and seven feet, reaching the bumper of his pickup truck, forcing him to retreat. Ten minutes later, the wharf gave way.

“It came up that fast, and with that much force, and that’s what started all the damage,” he said. “We lost about 70 per cent of our wharf system.”

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Jason Simpson owns Simpson Aqua Ventures Ltd., an aquaculture company that operates out of Savage Harbour, about 10 kilometres west of Red Head Harbour. After Fiona, he discovered storm surge swept two of his five boats – which had been resting on trailers on what he’d assumed would be safe ground – into the harbour. One he found in a marsh many hundreds of metres away. “The other is upside down in the middle of the bay,” he said.

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Future storm surges will be worse in most coastal communities. Mainly, that’s because of sea-level rise. A report published earlier this year by the U.S. National Oceanic and Atmospheric Administration found that a height difference of between 30 cm and 70 cm separated nuisance flood events from highly destructive ones. Just in the next few decades, it warned, all manner of high-water events, from minor to severe, will become more frequent.

“In other words, much of the coastline is already close to a flood regime shift with respect to flood frequency and, consequently, damages,” noted the report.

Beyond peak hurricane season, another factor is diminishing ice cover in the Gulf of St. Lawrence.

Like a heavy carpet, the presence of ice acts to suppress waves and can protect the shoreline from storms. As climate change reduces the amount and duration of ice cover each year, the coast is more exposed and therefore more subject to erosion. In addition, when ice occurs in broken chunks rather than a solid mass, it can amplify the wind’s ability to stir up damaging waves.

To better predict what such changes will mean in the future, a team based at the National Research Council in Ottawa recently undertook a detailed analysis of storm surges along New Brunswick’s Acadian Peninsula.

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The work, published earlier this year, began by identifying the 50 worst storm surges to hit the area during the past half century. To establish their ranking, the team used data from the Escuminac station – coincidentally, one where Fiona set a record last week. In total, 19 of those 50 events have occurred since 2010.

The team then used the real data to improve a computer model that can show the risk levels to various communities in the area for storm surges of different magnitudes. Compared with the detailed flood maps that the U.S. government has prepared for hurricane-prone regions along the Gulf and Atlantic coasts, the effort is still in its early stages. But the ultimate aim is to help guide communities on deciding how best to allocate resources to protect themselves.

“You can essentially quantify the costs from these types of events under a do-nothing scenario and it gives you a means to compare and choose from alternative actions to reduce risk,” said Enda Murphy, a coastal engineering researcher and co-author on the study.