If you are lucky enough to live next to the ocean, or been there on vacation, you probably noticed that the level of the ocean changes throughout the day: the tide. In Biarritz, and on the Atlantic coasts of Europe, it can reveal beautiful beaches (at low tide) and then hide them away (high tide). It is always a good thing to check before heading down to the coast!
Colin, our researcher, who also is a surfer, is there to explain a few things about this phenomena that you might not have known.
1) It all comes from the Sun and the Moon!
Where does the tide come from? It is a natural phenomenon due to the gravitational pull of our favorite celestial bodies: the sun and the moon. The moon, in relatively close orbit has a strong pull on the waters of the ocean and is often the first thing we associate with tides, but the sun also has a strong effect.
2) Tidal harmonies
The tide goes up, the tide goes down, easy right? It is a bit more complicated than that, tides are actually made up of 4 harmonic composants:
- Semi-diurnal: have a period (time between peaks or troughs) of around 12 hours
- Diurnal: once every 24 hours or so
- Low frequency: bimonthly, monthly, semi-annual, annual
- Overtides: periods shorter than 9 hours and caused by interactions of the other tidal composants and the bottom of the ocean in the area
The diurnal and semi-diurnal harmonies have the biggest influence on the tide in most places, and account for most of its variation. Across wide, relatively shallow areas, the other harmonies can become very influential however.
3) Same day, same tide?
The relative positions of the sun and the moon are constantly changing. In this way, the biggest and smallest tides are at specific times of the month and of the year. The largest tides in a given month will be around the syzygies: the New and the Full Moon. At that point in time the Sun, the Earth and the Moon will all be on the same axis and the gravitational forces will add up.
In the same way, when the Earth is at perihelion, i.e. as close as it will get to the sun in a given year (generally in the beginning of the month of January), tides will be affected. The same goes for when it is at is aphelion, the furthest it gets from the sun.
If a high tide is to occur at perihelion with a concommitent perigee, you can expect a king tide.
4) It has its upsides and its downsides
The tide is expressed differently in each part of the world, due to many additional effects: inertia of water movement, land deformations, propagation of different waves, induced by factors such as the Coriolis force, or the size and shape of basins (open or closed, deep or not).
5) The tidal coefficient? Made in France!
In France, a unique way of expressing the amplitude of the tide is to use an indicator called the tidal coefficient.
According to the SHOM definition (Service Hydrographique et Océanographique de la Marine), it is the quotient of the semi-diurnal tidal range of the harmonic formula by the average tidal range value for equinoxic spring tides, admitted at 6.1m at Brest, the reference port.
You did not understand that sentence? Do not worry, neither do we! What we need to remember is that the tidal coefficient is simply an indicator of the importance of the tide at a given time. It is a dimensionless number, usually between 20 and 120.
- Coefficient 120: Extraordinary spring tides of equinox spring water
- Coefficient 95: Mean spring tides
- Coefficient70 : Mean tides
- Coefficient45 : Average neap tides
- Coefficient20 : Lowest neap tides
In other countries, a different system is used, where water levels are calculated in relation to an average.
6) Strange waves: the mascarets
Mascarets are atypical waves. These are tidal waves that travel up rivers. At the time of the tidal shift, a wave is formed and then moves up the stream against the direction of the current. The tidal bore is a well-known phenomenon in Gironde, where it delights surfers, kayakers and other surfers during high tides. This phenomenon can generate impressive waves: up to 6 meters high in the Qiantang River, they travel at 40 km/h!
7) And in Biarritz?
In Biarritz, the tide is semi-diurnal, with a period of 12h25. From one day to the next, we have a time lag of about 50 minutes. Here, the tide is of the so-called “mesothelial” type, i.e. with a tidal range of between 2 and 4 meters.
As a bonus, we give you some vocabulary to win at Scrabble ?
Syzygie: Sun-Earth-Moon alignment. This phenomenon occurs during the New Moon and the Full Moon.
Marnage: difference between the highest and lowest level, in a given place.
Estran: tidal balancing zone.