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THE QUEBEC REGION BULLETIN
FEBRUARY - MARCH 2011/VOLUME 14/NUMBER 1
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Flood waters and breaking waves
The case of December 6, 2010
December 6, 2010, in Sainte-Luce  December 6, 2010, in Sainte-Luce
M. Desrosiers

The exceptionally high water levels predicted by the Canadian Hydrographic Service (CHS) were confirmed last December 6 in the estuary and northwestern Gulf of St. Lawrence. At Rimouski, the tide reached a record 5.54 metres, a height never yet seen in the last 110 years, eclipsing the record of 5.44 metres recorded in 1914. An exceptional level was also observed at Québec City where water overflowed the river banks in several places.

Low atmospheric pressure is the primary culprit responsible for variations in water levels. Low pressures cause winds to whip up waves. During the storm last December 6, three factors came together:

  • There was a new moon, one of the times of the month when the tides are highest due to the alignment of the sun, moon and Earth.
  • An exceptionally low pressure area—971 mb—passed over Rimouski. The low atmospheric pressure causes the water level to rise since there is less weight bearing down on the surface of the water.
  • Winds from the north created a current driving towards the south shore, which pushed the water level higher yet and generated high waves.

Day-to-day team work

CHS provides a service forecasting high water and waves likely to affect coastlines in Quebec for the next 48 hours. In collaboration with the Canadian Meteorology Service, which produces atmospheric forecasts and wave forecasts, CHS contributes its knowledge about tides so that notices can be sent to Québec’s Ministère de la Sécurité publique (MSP). In turn, MSP informs the relevant governmental bodies, notably Transports Québec and coastal municipalities. CHS analyses water levels on a daily basis and compares them to the forecast to evaluate the influence of atmospheric conditions.

It is the waves that cause damage by eroding the coast. Normally, they break harmlessly on the beach. However, the higher the water level, the greater their influence. When water levels are high, the waves encounter unusual obstacles—either vegetation and loose soil or a protective wall and sometimes even a home.

 

Water levels and atmospheric pressure at Rimouski on December 6, 2010. The green line represents the water level ultimately reached and the gray line, the forecast transmitted to Quebec’s Ministère de Sécurité publique. The orange line corresponds to the predicted level without the atmospheric influence. The superimposed line shows the variation in atmospheric pressure.

Denis Lefaivre
Science
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