MIDDLETON, NS - A student moves a large three-dimensional map of Nova Scotia from a lab to the boardroom at the NSCC in Middleton.
It’s a unique model of a coastal province. Sea level in many communities is scant metres from roads, back doors, and critical infrastructure. Dikes have held the water out for centuries in some places, but things may change if scientists locally and around the world are correct in their sea level rise projections.
Dr. Tim Webster takes those projections, plus historical and current data, and makes maps that can show you how tides, sea level rise, and storm surges are going to affect your community. Or all three at once.
His Applied Geomatics Research Group printed off that 3-D model of Nova Scotia that’s based on elevation mapping the AGRG has been doing for the past 15 years.
Webster looks at places like Amherst, Halifax, Bridgewater, Wolfville, and Annapolis Royal where under the right weather conditions it won’t take much of a rise in the level of the ocean to inundate downtowns with seawater.
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Coastal Mapping
The science is complicated. It’s not just sea levels rising. It’s the land subsiding. It’s crustal dynamics. It’s the lunar harmonics of the Bay of Fundy. It’s super storms and nor’easters. It’s the gravitational pull of glaciers.
In the end, scientists from around the world seem to agree that sea levels are rising – whatever the contributing factors.
Webster has been at the NSCC for more than 25 years, teaching for 10 years at Lawrencetown’s Centre of Geographic Sciences in remote sensing and GIS, before the research group was established in 2000 through some initial funding from the Canada Foundation for Innovation.
“Even back then the focus of my research was on high resolution mapping of the coastal zone,” he said. “In about 2003 we began using these high resolution elevation models derived from LiDAR (Light Detection and Ranging) from an aircraft – laser based imaging – using those to map out areas of flood risk from both storm surges today and sea level rise into the future.”
They did the mapping across the Maritimes for years, and then in 2014 obtained another CFI grant to obtain a new LiDAR system called a topobathymetric LiDAR that not only has the ability to survey the elevations on the land, but can survey underwater elevations. This newest technology gives them the ability to study waves and their impacts as they roll up the near-shore and dissipate their massive energy on the land.
Fluctuations
Webster said fluctuations of sea levels have been going on for millennia and the geological record reflects that. In Charlottetown the tide gauge record has measured the water level relative to the wharf going back to 1911.
“So that’s a really great times series, or really great record of sea level history,” said Webster. “The tide gauge at Saint John, at Halifax, and at Charlottetown are all very similar, showing about a 32-centimetre a century increase in relative sea level. I use that word ‘relative’ sea level because that is essentially the sea level change relative to where the gauge is -- relative to the Earth.”
Webster said sea level rise predictions from groups like the Intergovernmental Panel on Climate Change represent global sea level estimates.
“When we get to the local scale we have to combine not only what’s going on with the sea level on a global scale but what’s going on with our land surface in a regional scale,” he said.
He explained that the Maritimes was on the edge of the continental glacial ice mass called the Laurentide Ice Sheet that was centered over Hudson Bay where the Earth’s crust was depressed the most by its weight going back two million years.
“We were actually raised a little bit – it’s called the forebulge,” he said, “and as that ice melted Hudson Bay is rebounding back quite rapidly, so it’s actually still uplifting. We on the other hand are subsiding. Based on using global positioning systems and other methods it looks like we’re subsiding at around 15 centimetres a century. That is reflected when we look at that relative sea level rise calculation.”
Water Expands
While subsidence of land has an impact on local sea levels, greenhouse gases like carbon dioxide trap the sun’s heat in the atmosphere. Much of that heat is absorbed by the oceans of the world causing what Webster calls the steric effect – heat causing atoms to push away from each other.
“Water expands as it heats up,” he said, noting that it’s not necessary to have new contributions of water to have sea level rise. “If you simply heat that existing water up it’s going to take up more volume, therefore rise higher. With global warming we’re seeing less ice formation in the winters, glaciers melting, ice sheets melting, etc. and those are all contributing to the sea level rise as well.”
Webster has been making high-resolution elevation maps called digital elevation models.
“We basically get a laser shot – a couple of shots a metre – so we can produce a continuous surface representing the land at every metre,” he said. “It’s good to about 15 centimetres absolute accuracy on the vertical. With that level of detail we can then use the computer geographic information system to simulate sea level rise either from a storm surge or thinking about projecting 50 or 100 years down the road to see where that water is going to inundate.”
The maps are useful to people like municipal planners, those looking at critical infrastructure, utility owners like Nova Scotia Power, and the Department of Transportation.
Coastal Community
“We are such a costal community that much of our infrastructure is along the coast,” Webster said. “So these high resolution elevation maps really provide us the precision we need to be able to map out the projected inundation of a storm surge that may be anywhere from half a metre on top of the high tide to maybe a metre-and-a-half. The most famous storm surge is the Saxby Gale (1869) which was thought to be about two metres.”
The storm surge for super storm Sandy was four metres.
“If we have a storm surge of four metres on top of high tide, that would inundate many of our areas in Nova Scotia,” he said. “It would inundate major sections of downtown Halifax – lower Water Street, the rotary – many other coastal communities like Bridgewater, Yarmouth, Amherst along the Tantramar Marsh area, places even inland like Oxford would probably be affected.”
In the Annapolis Valley he points to the Wolfville area with its low-lying marsh area protected by dikes. Grand Pré.
Annapolis Royal would be vulnerable to such events. Coastal communities along the North Mountain right on Bay of Fundy could be severely damaged from that type of event.
Those would be the effects right now without taking into consideration any future rise in sea levels.
“Probably our most critical transportation network is the Atlantic Gateway through the Tantramar Marsh – the Trans Canada Highway,” he said. “If that were ever inundated that would severely affect the movement of goods in the order of millions of dollars a day.”
New Phenomenon
Webster said there’s a new phenomenon out there being described as ‘fingerprinting’ which basically says, for instance, if we lose all of the ice on Greenland then the gravitational effect that that mass of ice would have had to pull water towards it is no longer going to be there, and in fact the water will actually fall away from that from a gravitational standpoint.
“Which is somewhat counter-intuitive when you think about the contribution of the water coming from the melting ice,” he said. “But interestingly enough, when we think of the north and northern Canada we think okay, it’s showing that it’s warming up the fastest, there’s obviously ice there to melt and so forth. You would think inherently you would be facing a condition of inundation with sea level rise, but in fact this (Hydrographic Survey of Canada) report demonstrates that there will be some communities in the north, because of this gravitational effect, where sea level is actually going to fall moving into the future. Of course the big danger there would be how that would affect navigation. What could be safe to navigate today, in the future may not be safe. That somewhat leads us to our new research looking at submerged bathymetry and near-shore bathymetry – safe harbours, safe passage of vessels, etc.”
But that melting northern ice would still contribute to global sea level rise.
Climate Change Inertia
Webster said sea level rise predictions can be based on curbing greenhouse gas emissions – or not curbing them.
“But even if we stop them today there’s such an inertia with the climate that we’re still going to be seeing melting into the future until that comes to an equilibrium,” he said. “I don’t think anybody in the scientific community would argue one little bit about a projection of about a one-metre global sea level rise by 2100. There are projections for example in from the United States from NOAA that say if this part of the ice shelf collapses, and this part of Antarctica melts we could have a worst case scenario of a 2.5-metre sea level rise by 2100.”
There are other projections going all the way up to five metres.
“So there are these extreme numbers that are out there. When we generate these maps using the high-resolution elevation models we don’t just try to predict one inundation level,” he said. “We typically flood the elevation model at every 10 centimetres and we flood it into a sort of extreme-maxima case so that as the science changes, and there’s new predictions, the municipality, or whoever we’ve done the research for, they can just go to their GIS system and pull that appropriate flood level out. We’ve not just given them the flood level for what we thought back in 2010 to be the expected worst case number.”
Mitigation
What can we do?
“The government is contributing funding to researchers and other organizations to look at mitigation,” Webster said. “There’s sort of the approach of ‘defend’ – reinforcing the dikes is an example. Building protective structures, and we see this around the world. In London the big flood gates, obviously in Holland they’re the experts at protecting their land.”
A second option is what Webster calls ‘accommodation’ -- let some areas go.
“In many cases if we were to remove some of the dikes, as an example, and allow some of that area to return to salt marsh, that has a buffering effect for storms and so forth,” he said. “It dissipates the energy, therefore we’re not going to be as impacted where we are trying to protect our infrastructure.”
The final option?
“Of course the last one is to retreat and move our infrastructure back,” he said. “I don’t think anybody really wants to hear that, but I think at some point in time that should be the answer to the Trans Canada Highway at the Tantramar Marsh. We could keep on building up those dikes, but eventually I think there would come a point where you’re just putting you finger in the dike to hold everything back.”
Webster said things aren’t going to stop at 2100.
“They’re going to keep going,” he said. “So any kind of really long-term infrastructure you’re thinking of building you may even want to think beyond that 80-year horizon.”
