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The Storm Surge and Waves at Halifax with Hurricane Juan

Prepared by Peter Bowyer, October 17, 2003

Storm Surge in General

A storm surge, as the name implies, is a surge of water at a coastline as a direct result of a storm. A storm surge at any coastline is the onshore pileup of ocean water (or lake water) caused by a combination of high winds and low pressure with a cyclone (a low pressure system). Storm surges occur both with tropical cyclones (like hurricanes) or extratropical cyclones (like winter storms). The lower central pressure of the storm causes the ocean to rise up, much like the way a carpet bulges underneath a vacuum moving across it. It is sometimes assumed that this effect is the dominant factor in storm surges associated with hurricanes. In fact, it is the wind ahead of the hurricane that plays the key role in setting up large surges at the coastline; sometimes, 75% or more of the surge is the result of the wind, not the low pressure.

A simple and general rule-of-thumb for estimating a storm's low pressure contribution to the surge is as follows: 1 cm of rise above predicted tidal values for each 1 hPa below the standard atmospheric pressure of 1013 hPa. For example, Hurricane Juan had a central pressure of 974 hPa when it made landfall just west of Halifax Harbour. The pressure effect alone would result in a surge of approximately 39 cm, however, the surge measured in Halifax Harbour was about 150 cm. Clearly, a more important mechanism was at work.

The wind is the key. As a hurricane approaches a coastline, the very strong pressure gradient (a large difference in atmospheric pressure over a small area of geography) pushes the ocean surface ahead of the storm and predominantly to the right side of the track. Other factors such as the speed of the storm's motion, the angle at which the storm makes landfall with the coast, and the coastal bathymetry (the shape of the seabed) all help determine the specific storm surge experienced at each point along the coast but the wind is the most important factor.

The Surge with Juan-the Big Picture

Four independent events have to happen for a "worst case scenario" surge event to occur in the Maritimes: a spring tide; a lunar perigee; a daily high tide; a powerful storm. How did these line up with Hurricane Juan?

Firstly, the new moon on September 25 meant that the biweekly run of spring tides-one of the two monthly peaks in the tidal cycle-would give large tides on September 28. Secondly, the occurrence of the September 28 lunar perigee-the point in the moon's monthly orbit when it is closest to the earth-meant that the spring tides were higher than normal. Thirdly, the Atlantic coast of Nova Scotia experiences two daily high tides, opening two windows of flooding-opportunity each day during a run of perigean spring tides. Finally, the arrival of Juan--a category 2 hurricane--near the time of high tide lit the fuse leading to a damaging surge event.

Hurricane Juan came straight north for 24 hours, taking about the worst possible track for setting up high water levels along the Atlantic coast of Nova Scotia. A straight track sets up the worst waves while the northward track sets up a bad surge. The highest surges occurred east of the point of landfall, which was near Prospect, Nova Scotia.

The Surge at Halifax-a Detailed Picture

The accompanying graphs of data collected by the Halifax Harbour tide gauge (operated by the Canadian Hydrographic Service) show: the predicted water level based on astronomical tides (green line); the actual water level recorded by the tide gauge (orange line); the difference between these two values as a result of the storm-the storm surge (red line); the old record water level (light blue line); the new record water level established by Hurricane Juan (dark blue line). The three graphs show the tide gauge data surrounding the arrival of Juan during successive zooms in time: 4 days, 1 day, and 8 hours, respectively.

Storm surge graph, shows the tide gauge data surrounding the arrival of Juan during successive zooms in time
Storm surge graph, shows the tide gauge data surrounding the arrival of Juan during successive zooms in time
Storm surge graph, shows the tide gauge data surrounding the arrival of Juan during successive zooms in time
Storm surge graph, shows the tide gauge data surrounding the arrival of Juan during successive zooms in time
Storm surge graph, shows the tide gauge data surrounding the arrival of Juan during successive zooms in time
Storm surge graph, shows the tide gauge data surrounding the arrival of Juan during successive zooms in time

The surge began its rapid rise at 9:45 p.m. ADT, September 28, reaching a peak of about 150 cm (5 feet) at 1:00 a.m. ADT, and establishing a new record storm surge at Halifax (since at least 1940). Together, with the already high tidal levels, a record water level of 290 cm (above the tidal benchmark known as, "chart datum") was recoreded in Halifax. This value is greater than all Halifax water level data since 1961, although the new record has yet to be officially confirmed by the Canadian Hydrographic Service. Unfortunately, with this record came widespread flooding along the waterfronts of both Halifax and Dartmouth.

When the winds shifted at about 1:00 a.m. the water in Halifax Harbour drained immediately back into the Atlantic. It took the harbour waters 5 hours to rise 2 metres ( 6.7 feet) with the incoming tide and surge. The hurricane force east-southeast winds acted to dam the water up in the harbour, preventing it from draining back into the Atlantic as it normally does with the ebb tide. When the winds at the mouth of the harbour shifted from southeast to south, that was enough to "lift the floodgate" and allow the 2 metres of excess water to drain back into the ocean. The harbour returned to its original level in an hour and a half.

The Waves at Halifax

A field of 9-13 -metre (30-43 feet) significant wave heights--an average of the highest one third of the waves--approached the coast of Nova Scotia with Hurricane Juan. The Environment Canada buoy 44258, just outside Halifax Harbour, recorded maximum significant wave heights of 9 metres at the same time that Juan was making landfall. The maximum waves around the same time were 19-20 metres. The accompanying graph shows that the largest of these waves occurred between 12:20 and 1:20 a.m. ADT.

storm surge graph, shows that the largest of these waves occurred between 12:20 and 1:20 a.m. ADT
Storm surge graph, shows that the largest of these waves occurred between 12:20 and 1:20 a.m. ADT

The high waves contributed to the pileup of water in the harbour and the more exposed locations near the mouth of the harbour felt the full brunt of the waves.

It Couldn't Have Been Much Worse

Besides Halifax experiencing the highest winds in Hurricane Juan, the highest storm surge and highest waves combined to arrive at the coastline at same time, making the Halifax waterfront flooding problem even worse. Sometimes with landfalling hurricanes in Canada, the storm surge peaks near the time of landfall, whereas, the highest waves can lag the storm by an hour or two. As well, the straight-line path of Juan in the 24 hours before landfall allowed for substantial wave growth with the storm.

In many ways, it couldn't have been much worse.

Actually, It Could Have Been Worse

When we look a little closer at the water levels in Halifax, it becomes apparent that the event had a silver lining; Juan missed the high tide! Had Juan arrived only 2 hours earlier, the peak surge in Halifax Harbour would have coincided exactly with the high tide, possibly resulting in an additional 45 cm of elevated water. Had it arrived 10.5 hours later, it would have coincided with the higher of the two daily high tides and instead, an additional 60 cm would have resulted. An additional 60 cm of water in the harbour could have had staggering consequences.

In addition to the timing being less than "perfect" for creating the worst possible conditions, the hurricane approached the coast at a slight angle. The coastline and its sea bed are oriented facing southeast, not south. A more damaging surge event would have occurred if Juan approached from the southeast rather than from the south.

When we look a little closer at the wave growth we see that Juan was moving just a little too quickly to allow for maximum wave growth. By the time it made landfall, Juan was already outstripping the waves that it was generating. Had the storm's acceleration been a bit slower the waves being generated would have moved in closer harmony to the storm's speed, thereby allowing for larger waves.

In many ways, Juan was no "perfect storm"; it could have been much worse.

Conclusion

In comparison to storms of memory, there is little doubt that Hurricane Juan was "the big one" for the Halifax Regional Municipality. However, many less-than-perfect factors tell us that "a bigger one" is still out there: a storm with higher winds, larger waves, greater storm surge, and a more sinister combination of all three.

Therefore, it's not a question of IF?, just WHEN? How long? What we know is that it has been 110 years since the last similar event. Statistics tell us that it will likely be another hundred years before Halifax sees another Juan. Statistics also tell us that a worse hurricane could hit Halifax next week.

It has been said that, "Climatology is what you expect, but weather is what you get!" Climatology tells us that we should expect to wait a very long time before we see another Juan. Meteorology teaches us to keep watching anyway.

Acknowledgements

I would like to thank the following people for helping with this report:

  • Lorne Ketch of the Atmospheric Science Division for preparing the map and graphs for this report and for providing data interpretation and critical review.
  • George Parkes of the Canadian Hurricane Centre for providing data interpretation and critical review.

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