Saint-Laurent: is it possible to predict the arrival of high tides?
Local residents of Eastern Quebec probably remember 6 December 2010, when exceptional weather conditions, combined with very high tides, caused flooding along the St. Lawrence shoreline. These tides caused multi-million dollar damage to public and private infrastructure.
ANALYSIS – That day, the water level recorded by the tide gauge (device used to measure sea level) in Rimouski reached 5.5 meters, exceeding the previous record dating from 1914 by 20 'having reached a historic high on December 6, 2010, this high tide phenomenon occurs more often than once every 100 years, as evidenced by the event of January 11, 2016, when strong winds and high tides again caused flooding along the coast of Eastern Quebec.
So, when could the next high tides of the St. Lawrence occur? Cédric Chavanne, Daniel Bourgault and Dany Dumont, researchers in physical oceanography at the Institut des sciences de la mer de Rimouski (ISMER), examined this question in an article published in 2016 in the scientific journal The Canadian Naturalist. Here are the highlights.
What causes high tides?
First you have to understand what ingredients are needed to produce a big tide. The tide is a daily oscillation of the sea whose level rises and falls alternately. This oscillation is mainly due to the gravitational attraction of the moon and the sun exerted on the masses of water contained in the oceans. We then speak of an astronomical tide.
But sea level oscillations can also be caused by variations in atmospheric pressure and winds. In this case, we speak of a meteorological tide. To obtain exceptional high tides, it is necessary to obtain high astronomical and meteorological tides at the same time. Astronomical high tides are predictable decades, even hundreds of years in advance, because the movements of the stars are well known. On the other hand, high meteorological tides are associated with storms, and are only predictable a few days in advance, with great uncertainty about their amplitude.
It is therefore impossible to predict long in advance when the next coincidences of high astronomical and meteorological tides in the St. Lawrence will occur. However, it is possible to predict the times of the year favorable to the phenomenon of high tides. To do this, we must understand the workings of the astronomical tide.
Tides according to the stars
Let's start by looking at the tides caused by the gravitational pull exerted by the moon, which is stronger than that exerted by the sun. Indeed, although the moon is much smaller than the sun, it is also much closer to the earth so that its force of attraction outweighs that of the sun. For reasons explained elsewhere, the moon will generate two high tides and two low tides per day. We then speak of semi-diurnal tide (twice a day).
It is characterized by the tidal range, which is the difference between a successive high tide and low tide. In Rimouski, the tidal range of the semi-diurnal tide caused by the moon is 2.54 m.
Let us now examine the tides caused by the gravitational attraction exerted by the sun. Just like the moon, it will generate two high tides and two low tides each day, but with a lower tidal range: 0.81 m in Rimouski.
When the sun and the moon are du same side of the Earth (new moon) or each on an opposite side (full moon), their effects on the tides are additive, that is, the high tide caused by the moon comes at the same time as high tide caused by the sun. This generates high tides called spring tides, with a tidal range of 3.35 m in Rimouski.
Solstices and equinoxes
But to obtain very high astronomical tides, it is necessary to consider other even longer cycles, some of which are less well known to the public. You may have heard of the equinox tides, which happen twice a year around March 20 and September 22 (more or less a week due to the spring tide/neap tide cycle).
At the equinoxes, the sun is above the equator and the semidiurnal tides are at their maximum. On the other hand, at the solstices (around June 21 and December 21), the semi-diurnal tides are minimal. However in Rimouski, the spring tides at the equinoxes have only 5 centimeters more tidal range than those at the solstices.
If we are interested in the maximum level of high tides, and not in the tidal range, we must also consider the average water level over a tidal cycle, which also has a semi-annual cycle: it is almost 10 ;centimeters higher at the solstices than at the equinoxes in Rimouski, and therefore prevails over the semi-annual semi-diurnal tide cycle.
The highest astronomical tides therefore occur around the solstices in Rimouski , and not around the equinoxes, but the difference on this semi-annual cycle is only a few centimeters, which does not explain the occurrence of exceptional high tides.
The role of atmospheric pressure
So back to our second ingredient needed to produce an exceptional high tide: the atmospheric tide. During the passage of a storm, the atmospheric pressure decreases. The atmosphere then exerts less weight on the sea and the level then tends to rise locally. In addition, winds blowing in the direction of the coast will push the water towards the coast and raise the sea level at the coast. The strong waves generated by the wind will thus be able to break higher on the shore.
During the storm of December 6, 2010, the maximum water level recorded at the Rimouski tide gauge exceeded approximately 1 meter the maximum level predicted by the astronomical tide. This is much higher than the semi-annual variations of the astronomical tides, but it is similar to the variations of the spring tide/neap tide cycle.