| Hurricane
Hortense:
A fortuitous warning
By Alan Ruffman
Edited by Peter Zimmer. Design by Tony Seed
SHUNPIKING MAGAZINE, VOLUME 2, NUMBER 15 / SEPTEMBER, 1997
Hortense Hammered Halifax
Deaths in Lunenburg Co. and on Eastern Shore
$30 million in losses. Yacht clubs in chaos.
HALIFAX (CP) -- Hurricane Hortense slammed ashore
early last evening just west of Lunenburg and left Nova Scotia reeling
in its aftermath. The RCMP report at least ...
P. 1 article, Sunday, September 15, 1996, The
Daily Star-Chronicle, Halifax
Of course it did not happen that way. A year ago there were no deaths,
and property damage was barely $3 million, as Hurricane Hortense struck
Nova Scotia a glancing blow as it went by, dying at the cold northerly
end of its ten-day trek from the Caribbean to its end a day later over
the Grand Banks south of Newfoundland.
Hortense was the first tropical cyclone whose winds were still at hurricane
strength (that is, with sustained winds of more than 118 km/hr) to touch
Nova Scotia since Blanche hit in 1975. Hurricane Beth brushed by Nova
Scotia in 1971, and Hurricane Edna sideswiped us in 1954. And the event
seems essentially forgotten with the short-term memory loss we might
generally attribute to the modern media amnesia. Hortense was last year's
disaster. It might be another twenty-two years before another real hurricane
tests our preparedness. Or it could be this month. Hurricanes are very
unpredictable, even in areas where they are much more common.
And the event seems essentially forgotten with the short-term memory
loss we might generally attribute to the modern media amnesia. Hortense
was last year's disaster. It might be another twenty-two years before
another real hurricane tests our preparedness. Or it could be this month.
Hurricanes are very unpredictable, even in areas where they are much
more common.
Our Nova Scotian police forces and Emergency Measures Organization (EMO)
officials did not appear to anticipate the hurricane-specific dangers
Hortense posed certain coastal regions: certainly no plans for evacuations
from storm-surge vulnerable areas (Lawrencetown, Green Bay) were instituted
in advance of the storm's arrival, as they should have been.
Repeated inquiries the author sent to all the municipal emos along our
Atlantic Coast brought little response: not one indicted that they had
hurricane evacuation schemes in place. Or that they've made such plans
since Hortense. No municipality in Nova Scotia stated they have prepared
maps to show areas susceptible to hurricane storm surge. Had a decision
been reached to move families back from susceptible coastline areas,
it seems the responsible Nova Scotian authorities would have had to
improvise. Maps probably do not exist that define the first priority
hazard areas for such an evacuation.
It is now clear, with the benefit of hindsight, that we were very lucky
in Nova Scotia vis-a-vis Hurricane Hortense. Had this storm come ashore
in the southwest part of the Province as a full-fledged Category 1 hurricane
(perhaps with winds at 140 km/hr), rather than just barely brushing
the northeast corner in its final dying stages, the damage costs could
have easily been an order of magnitude higher (i.e. $30 million). And
we likely would have lost lives.
Perhaps as often as once a century Category 3 hurricanes have come ashore
in Atlantic Canada, with winds between 177 and 209 km/hr, winds which
pack two to three times the force of Hortense's winds (and do on average
almost four hundred times the property damage we experienced last September).
As far as historical information is available no Category 4 or 5 hurricanes
have been this far north. Already in 1997 the Maritimes have experienced
the spent remnants of tropical storm Anna in early July, and most of
us were hoping for some rain from the remnants of a decaying Hurricane
Danny late in July (to no avail).
During 1995 Nova Scotia only saw the direct effects of two hurricanes:
the thundering storm swells from Hurricane Felix twice pounded our Atlantic
shores in August and in September the same from Hurricane Luis, passing
well offshore.
In 1996 there was the approach and offshore passage of Hurricane Bertha
and the heavy rains of Edouard as it headed for us, but then veered
offshore. Our media gave considerable coverage of Hurricane Fran's transit
across the southern United States coast and on into southwest Ontario.
And in September we all saw the news reports of the impact of Hurricane
Hortense as it tracked across northeastern Nova Scotia and then turned
eastward to pass off the south coast of Newfoundland as a dying extratropical
storm.
It's not just the winds
As it approaches Atlantic Canada from the open ocean, several aspects
determine the power of a hurricane to affect our lives and property
on shore: there are the hurricane-defining winds tearing at everything
standing in their way; the wind drives the ocean onto (and sometimes
over) our shorelines, in the form of towering storm waves and the less
obvious storm surges that add to our normal tides; there are torrential
rains, driven horizontally by the winds. And there is the storm's track,
the path of the eye of the hurricane, and whether it passes you to it's
left or right, and how far away it passes, and how quickly it travels.
And then your particular location on the land, where you stand to meet
the storm (just back from beach-front, on a cliff-top, in a river valley
inland, ...) will help determine what the storm's wind, rain and waves
do to you and your property.
The Wind first
Winds of a tropical cyclone spiral counter-clockwise in the northern
hemisphere in towards the eye of the storm (as seen from above, as in
satellite images), getting stronger as they close in on the centre (or
"eye"). When they are circling the eye with sustained speeds above 118
km/hr (74 mph) the storm is ranked as a hurricane.. A Category 5 hurricane
(seldom seen in the Caribbean, probably never seen in these parts) has
sustained windspeeds in excess of 247 km/hr (155 mph).
It's not just the hurricane winds whirling around the eye
that affects the wind force you'll feel on the ground (or in a ship
at sea): the storm as a whole is moving along the storm track, adding
(or subtracting) its overall speed over the surface to the speed of
the wind around the eye. If one stands on the track of a fast-moving
hurricane and looks in the direction of travel, the winds on the surface
are much stronger on the right-hand side of the track than on the left
side. The last place one wishes to be during a hurricane is on the right
side, or windy side, of a hurricane's track.
For Hortense (a dying hurricane with winds just above the minimum 118
km/hr and a forward velocity of about 34 km/hr) coming ashore on the
Eastern Shore, the sustained wind speeds on the surface just to the
right of the eye would have been about 118 plus 34 km/hr, or
152 km/hr (95 mph) while the winds to the left of the eye would be only
84 km/hr (118 minus 34). The wind to the right is 80 per cent
faster than to on the left side.
But the force of the wind varies as the square of the velocity of the
wind. For Hortense the destructive force of the wind was about three
times greater on the right side (seaward side) than it was on the landward
left side as she passed Canso.
The Ocean meets the shore: storm waves
The hurricane wind drives the ocean into big waves. In 1995 in the open
ocean well off from Nova Scotia, Hurricane Luis created huge sustained
30 metre (100 foot) waves that severely hampered the Cunard luxury liner
Queen Elizabeth II for several hours and provided spectacular
wave data on Canada's offshore anchored wave-rider buoys. More normal
storm wave heights would average in the 10 to 20 m range. Towards land,
the bottom's profile can modify and limit and direct the waves' amplitudes
and forces.
As with wind, the force of these waves, the energy they carry, increases
as the square of their size and speed. Double the wave, and you've got
four times the force. As the winds come ashore, so do the waves, transferring
their colossal energies to the cliffs, boulders, beaches and dunes,
and whatever we've built in these areas.
These waves pound cliffs into boulders, throw boulders onto cliffs,
break the granite boulders into rocks, wharves into matchsticks, ships
into scrap-metal, reshape beaches....
Off the harbour of Halifax the waves of Hortense peaked at 8.7 metres
-- only three storeys tall.
And tides...
The storm waves ride on top of the ocean's surface. Here
in Atlantic Canada we're very aware of the tides as the ocean itself
rises and falls twice daily. And if we've paid attention at all, we
know that during each month some tides, called spring tides, have a
bigger range (rise higher, ebb lower) than the smaller neap tides. At
the Halifax tide gauge the height of the biggest normal tides are about
a half-metre higher than the month's lowest high tides.
Hortense went past Halifax close to high tide on the 15th of September.
The highest tides of September occurred on the 1st and 28th. The lowest
high tides were on the 7th and 21st.
and a Storm Surge...
But the astronomical tides are not the only forces that raise the level
of the ocean; hurricanes (and other windstorms) can also produce a local
increase in the height of the ocean's surface, and this is the storm
surge.
The storm surge has two components; the low atmospheric pressure in
the storm that "sucks" the ocean upwards (at the centre of the storm
the pressure is lowest and the rise greatest), and the push of hurricane
winds that piles up water before them (as well as churning the surface
into the storm waves).
In mid-ocean the absolute height of the ocean doesn't really matter
during a hurricane: at a coastline the story is much different. Strong
onshore winds can rapidly pile up the nearshore waters onto the coast
adding to the sea level rise from the low pressure. In fact the low
pressure component turns out to be only 10 to 20 per cent of a typical
hurricane's storm surge in the right quadrant of the storm: there most
of the extra sea level is result of the wind's push.
The duration of the storm surge depends on the rate at which the storm
is moving, hence for how long the strong winds act to pile up the coastal
waters.
For most of the Canada's Atlantic coastal areas and for most hurricanes
the counter-clockwise winds push a surge ashore as the storm tracks
in from the south or southwest. Strong offshore winds can actually remove
water from the coast, giving a negative storm surge.
and waves and tides and surge combined...
If the storm surge arrives along with, or persists through a normal
high astronomic tide, then the hurricane damage will be compounded.
The high tide and the storm surge add together to move anchored boats,
float wharves, and shift shore properties to record heights. Further,
the very high sea level surface serves to carry the extra high hurricane-whipped
storm waves well inland to areas normally immune from any wave action.
For Hortense the surge was a metre or a bit more on the Eastern Shore.
Measured off Halifax harbour, Hortense's storm surge combined with the
day's high tide came within inches of matching the highest instantaneous
water level recorded in the last thirty-five years. It was the highest
since 1980. Had it come at a Spring high tide it would have been nearly
a third of a metre (or about one foot) higher.
There are old reports of a storm surge in Newfoundland in 1775 that
would have been about twenty feet. Some reports had thirty feet. (In
your mind's eye, stand where you can see the ocean and raise its level
as far as you can see by the height of a two-storey house, and then
put big waves on it, waves the height of a three-storey building or
more.)
The Saxby Gale of 1869 added its storm surge to the year's highest spring
tides of the Bay of Fundy to flood Moncton, other towns along the Fundy
shore and all the Fundy dykelands.
During Hurricane Andrew (Category 4, Florida), waves on top of the record
storm surge rolled right through the second story corporate headquarters
of Burger King in the Miami area. It could have been much worse. Though
ocean levels rose in excess of 5.2 metres (17 ft.) over the normal astronomic
tide during the storm surge along a portion of the Florida coast south
of Miami, Hurricane Andrew's storm surge fortunately came at about the
time of the normal astronomic low tide. Six hours earlier or later the
waves would have had another metre or so of ocean to ride on.
On a rugged rocky, or cliff-like, coast, the storm surge and hurricane
waves are of little consequence. However, on shallow, shelving shorelines,
edged by sand beaches, low estuaries, marshes, or erodible soft rocks
such as the red sandstone of Prince Edward Island, the extra rise of
sea level can be dramatic and dangerous. In Atlantic Canada a storm
surge of sixty centimetres (just two feet) is considered significant
and the Atmospheric Environment Service and Canadian Hydrographic Service
try to give the public warning of such events.
The Rain
A tropical cyclone draws up immense quantities of moisture from the
warm sea offshore, then drops it as torrential rain, generally in the
‘northwest quadrant', i.e. on the forward left side of the hurricane.
US meteorologists have developed a rule-of-thumb to predict the amount
of rainfall (in inches) you'd expect to get if you are in this quadrant:
divide 100 by the forward speed of the storm in knots. Shunpiking's
research staff converted this to metric units: 5,000 divided by the
forward speed in km/hr gives the approximate rainfall in millimetres.
Hortense's northwest quadrant passed over much of Nova Scotia and gave
us some heavy rains that resulted in some local flooding. Hortense's
rain fit the rule-of-thumb: its speed was about 18.2 knots or 34 km/hr
and areas of Nova Scotia to the left of Hortense's track received 5.5
inches (140 mm) of rain in a matter of hours -- rather more than would
fall here in the course of an average month.
If a hurricane stalls in its forward progress, then some areas can receive
truly horrendous amounts of rain. Last month (July 1997) Hurricane Danny
took almost four days to get from the Alabama coast across Georgia to
the Atlantic coast. During that time some areas received a metre (40
inches) or more of rain. There was extensive flooding and lives were
lost.
The streams, rivers, swamps, the natural (and man-made) drainage systems
often cannot handle the deluge of hurricane rains. Flooding can be almost
instant and catastrophic. Steeper slopes can become water-saturated
and develop mudslides and slumps in the highly lubricated soils.
In 1954 it was the rain of Hurricane Hazel that caused the deaths of
81 persons in the Toronto area. Here in 1971 it was the rain on the
left side of Hurricane Beth that tore out all the undersized culverts
and undermined houses in Dartmouth as the swollen Dartmouth lakes sought
their natural exit to the sea in downtown.
Our historical experience with hurricanes
While hurricanes are rare in our area, and Category Two or Three events
even rarer, we have experienced them. And we will again, sometime, somewhere
in Atlantic Canada.
Hortense should be taken as a fortuitous reminder.
There's lots of historical experience with both extratropical storms
(hurricane remnants) and, less often, with full-blown hurricanes in
the Atlantic Provinces. Some have been very serious and have been accompanied
by large hurricane storm surges (see Timeline, below).
- The first British invasion of Quebec City was aborted
when the Boston-based armada of 1711 was driven ashore on the northern
shore of the Gulf of St. Lawrence by what was probably a hurricane;
2,000 lives were lost.
- The September 1775 hurricane that struck eastern
Newfoundland may yet prove to be Canada's most tragic disaster with
a reported 4,000 lives lost and a poorly documented, localized, sudden
storm surge reported to be twenty or even thirty feet (six to nine
metres!).
- The Saxby Gale of October 1869 was a tropical cyclone
that tracked into New Brunswick near the mouth of the Bay of Fundy
during the highest of the year's high astronomic spring tides. The
storm surge overtopped all the Bay of Fundy dykes, flooded the downtown
business district of Moncton, and flowed onto the streets of Great
Village and Maitland in Nova Scotia.
- The August 1873 hurricane flattened whole forests
in Cape Breton Island, drove anchored vessels far inland from the
shore, and sank or destroyed over 1,100 vessels, with deaths approaching
600 in the Gulf of St. Lawrence and among the fishing fleet of the
Magdalen Islands.
In Newfoundland the ‘August Gales' of 1927 and 1935 spawned
folksongs among outport fishers. In 1944 an unnamed serious hurricane
spent itself up the spine of Nova Scotia. Carol of 1953, Edna of 1954
and Beth of 1971 are still remembered clearly in some places.
Those who don't
learn from
history are doomed to repeat it
Information about such storms go back over the past two hundred
fifty years or so. Yet Canada does not have a systematically compiled
hurricane climatology nor do we detail the northern ends of the significant
hurricanes which we have suffered. There is not even a comprehensive
list of the most serious historical hurricanes experienced in Atlantic
Canada and no attempt has been made to ‘categorize' our most powerful
hurricanes. It is work that is only just beginning.
At one time in the eighteenth and nineteenth centuries,
hurricanes used to impose most of their tragedies offshore on vessel
crews as they suddenly engulfed unsuspecting and helpless sailing vessels.
This is no longer the case with modern radio and satellite warning systems
and much more substantial vessels not dependent on the wind for propulsion.
It is not the high winds and flying objects (or even the rain) that
cause most hurricane deaths, or even the property losses now in North
America. It is at the coastline that a hurricane now exercises its fury
on humans and their built environment.
In North America, and throughout the world, coastlines have been under
persistent development pressure with residential and recreational uses
dominating the growth along the eastern seaboard of the United States
and Canada. Where the developed coastlines are low and flat they are
susceptible to storm surge and wave damage during hurricanes. Eastern
Canada is not as guilty of this building trend since we have had lower
population pressures and because we are regularly disciplined by the
significant waves and winds of severe winter storms. We also tend to
have more rugged, higher coastlines, save for parts of the Gulf of St.
Lawrence; most structures are thus high enough to be safe from a hurricane's
waves.
However Hurricane Hortense reminded us that we are not at all immune
from the effects of hurricanes along our coastlines. Prevention and
planning are by far the easiest and cheapest response. The most susceptible
coastline areas can be defined and mapped, development controls for
future building should be put in place, just as we do within the floodplains
of rivers, and when the next tropical cyclone threatens, local police
and fire department officials should know where to evacuate families,
or to close roads, to prevent the loss of life. Weather forecasters
in the Atmospheric Environment Branch, storm surge modellers in the
Department of Fisheries and Oceans, Emergency Measures officials both
federally and provincially, police and emergency response organizations,
insurance companies, highway engineers, erosion specialists, municipal
planners and their Councils, and residents of the Maritimes would all
do well to take note of Hortense's warning. —Alan Ruffman
lan Ruffman is a marine geologist and geophysicist and president,
Geomarine Associates Ltd. in Halifax. He has become a student of historical
hurricanes of Atlantic Canada through his research on the major September
1775 hurricane. He has recently received a modest research contract
from the Atmospheric Environment Service to document the 1869 ‘Saxby
Gale' and is seeking all accounts of this hurricane. Alan was recently
awarded a fellowship at the American Antiquarian Society's Library in
Worcester, Massachusetts to further research the 1775 hurricane.
Information graphics:
Windspeed, Force and Damage by Peter Zimmer
Hurricane Gusts by Peter Zimmer
Timeline: Major hurricanes making landfall in Atlantic Canada, 1700-1996
by Peter Zimmer;
Hortense Storm Track, courtesy Geological Survey of Canada (Atlantic),
"Hurricane Hortense Strikes Atlantic Nova Scotia: An Examination of
Beach Response and Recovery", R.B. Taylor et al.
"Hortense
was last year's disaster. It might be another twenty-two years before
another real hurricane tests our preparedness. Or it could be this month.
Hurricanes are very unpredictable…"
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