This is a phenomenon well known to the inhabitants of the coasts.
The sea rises higher and falls lower than usual.
The scenery changes every six hours as the water comes and goes, often offering breathtaking views.
This spectacle, we can start to observe it this weekend: the high tides - the first of 2023 - are indeed back.
The tidal coefficients will exceed 90 from Saturday January 21, to reach 105 on Monday January 23 and Tuesday January 24.
But how do we explain such a phenomenon?
What are the best dates to observe it?
Instructions with Nicolas Pouvreau, sea level expert in the tides and currents department of Shom (Hydrographic and Oceanographic Department of the Navy) and coordinator of the
Read alsoWhere to observe the high tides in 2023?
What is a high tide?
In the Bassin d'Arcachon, the cabins on stilts are surrounded by sand at low tide.
JEAN-PIERRE MULLER / AFP
The question may make you smile for someone who has been watching this show for years.
However, the answer is not so simple when you try to explain the ins and outs, warns Nicolas Pouvreau.
First there is the definition.
High tides are tides of great amplitude, which we will determine using a tool giving approximate information: the tidal coefficient, which varies from 20 to 120. When it exceeds 90, it indicates the imminent presence of this phenomenon. natural.
And the higher the coefficient, the greater the tidal range - the difference between high tide and low tide - will be.
Then there is the explanation - less obvious to the non-scientist.
With this question: how do these tides, which are “larger” than the others, form?
Originally, a tide is a variation in the amplitude of the water level on the coast.
However, the latter is not uniform, depending on the power of the forces of attraction of the Moon and the Sun on the Earth.
To understand it, we must refer to Newton's apple, symbol of the law of attraction of bodies.
“
The apple is attracted to the Earth and vice versa.
Considering the mass of the Earth and the mass of the apple, it is the apple which is attracted by the Earth.
The same mechanism works for the Earth, the Moon and the Sun”
, explains Nicolas Pouvreau.
"The effects of the Moon on the Earth are greater than the effects of the Sun on the Earth, because the mass of the Moon is less important than that of the Sun, but above all, because the distance between the Moon and the Earth is smaller than between the Sun and the Earth.
»
The ocean tides are therefore mainly due to the attraction of the Moon on the Earth and, to a lesser extent of the Sun.
Another element is added: the position of the three stars.
High tides occur when there is a more or less perfect alignment between the Sun, the Earth and the Moon, and the forces of attraction add up.
Result: we can witness an extraordinary spectacle, during which the sea will drop to a very low level, then reach remarkable heights, all this in the space of six hours.
Read alsoAlong the coast to Mont-Saint-Michel, where to observe the high tides in the English Channel
What are the favorable periods for high tides?
If the Moon plays an important role in the formation of these high tides, the Sun is not left out.
Because as Nicolas Pouvreau explains, the "most" high tides often occur during the spring equinoxes (between March 20 and 22, depending on the year) and autumn (between September 21 and 23).
Either when the distance of the Earth-Moon couple and the Sun is the closest.
During these periods, the tidal coefficients can exceed the bar of 110, like what will happen in 2023. According to the Shom (as a reminder, the Hydrographic and Oceanographic Service of the Navy), the coefficient will rise to 111 on March 22, while it will be 112 on September 30.
However, science can play tricks on us.
Because other dates are also to be taken into account, such as that of February 22, a day also classified at 112.
What about other, less impressive tides?
These can occur throughout the year.
As revealed by the site of the tourist office of the Channel, there are almost every month.
Even if in winter, they are often more impressive and dangerous, because of the risks of depression, associated with storms and strong winds.
"We can have a tidal coefficient of 112, but if there is a very big depression, the height of the water at low tide will be higher, just like at high tide"
, warns Nicolas Pouvreau.
With the key, risk of submersion.
Read alsoExceptional hike for high tides
What is a tide of the century?
In March 2015, Mont-Saint-Michel became an island again on the occasion of the tide of the century.
DAMIEN MEYER / AFP
In the concert of high tides, some are doing well. On March 21, 2015, a "tide of the century" took place.
Its coefficient reached 118, a threshold very close to the maximum.
A spectacular phenomenon, since Mont-Saint-Michel has once again become an island.
And contrary to what one might think, these century tides do not take place once every 100 years, but every 18 years, when the Earth, the Moon and the Sun are in perfect alignment (nodal cycle ).
The previous comparable tide took place in March 1997. What about the next ones?
They already have a date: March 3, 2033 and March 14, 2051.
But why such an exaggeration in the name?
To call for caution.
Because such a tide, even if it drops very low and allows fishermen on foot to indulge their passion, will also rise very quickly.
The risk of being caught in the waters is significant.
In 2015, this event was bereaved by the death of two people.
Where are the best places to see them?
If the high tides are observable all along the French Atlantic coast, some places offer more striking spectacles, like Saint-Malo and Mont-Saint-Michel.
"This is where we find the highest tidal ranges in continental Europe"
, observes Nicolas Pouvreau.
In the English Channel, tidal waves propagate from west to east.
What makes Saint-Malo and Mont-Saint-Michel unique is the existence of the Cotentin peninsula.
The latter will act as an obstacle, which will allow the tide waves to reflect and add up.
What about the Mediterranean?
"On the French coasts, the tidal range is less important because it is a semi-closed sea"
, explains Nicolas Pouvreau.
The absence of a continental shelf prevents an amplification of the tides.
“The waves are rising very slowly, and when they suddenly arrive on the Mediterranean coast, they have not had time to be increased”
, continues the expert.
Read alsoAlong the coast to Mont-Saint-Michel, where to observe the high tides in the English Channel
Can climate change affect high tides?
"The latest IPCC reports show that the sea level will continue to rise
," warns Nicolas Pouvreau.
A phenomenon accelerated by the melting of glaciers and polar ice caps, but also by the thermal expansion of water (when water heats up, its volume increases).
Result: the average level has increased by about 25 cm since 1880, and by about 7.5 cm over the past 25 years.
This is not without consequence on high tides, the level of which will always be higher.
This situation raises questions about coastal protection.
Because the phenomenon of sea level rise is often correlated with increasingly virulent storm surges.
In 2010, storm Xynthia, a combination of high winds and high tides, caused great damage in France and elsewhere due to flooding, with a particularly heavy human toll as a consequence.
Is it dangerous, a high tide?
At low tide, it is not uncommon to see walkers and shore fishermen venturing along the coast.
Damien MEYER / AFP
Apart from violent storms, high tides present many risks.
Because the beauty of the show can make some dizzy people lose their minds.
In case of low tide, some go fishing on foot, while others venture far out to sea.
“However, the water rises very quickly during high tides”
, warns Nicolas Pouveau.
“It happens every year that people get caught in the trap.”
The risks are all the greater in the event of fog, which causes all visibility on the coast to be lost.
Not to mention the low pressure phenomena which could lead to overflows on the open sea.
If you want to venture out to sea, a few precautions should therefore be taken into account.
"The first thing to do is to bring tide predictions, water heights, and times of low tide and high tide
," continues the expert.
As much data available on the Shom website (in the Tide section).
In addition, you should not go alone on the coast.
Beforehand, tell your loved ones and have a phone - charged - with you.
Useful information
The Shom, or the Hydrographic and Oceanographic Department of the Navy, remains the reference site for all those who would like to have information on high tides.
The public establishment provides all the necessary information on these natural spectacles (on data.shom.fr).
It is possible to follow live the evolution of the sea level, according to the different ports.
For those who would like more specific information on the tide, it is possible to write to the email address: tide@shom.fr.