Sea level
Mis à jour le 19/07/2024Sea level observation
Sea level observation is one of Shom's historical activities, and is still an area of technical and scientific expertise.
Historique
Les premières observations de marée de l’époque moderne ont été commanditées en France à la fin du règne de Louis XIV et s’inscrivaient dans le contexte des travaux de Newton sur la gravitation ayant fourni une base scientifique au phénomène de marée.
Un des plus anciens marégraphes du monde fut installé dans le port militaire de Brest en 1846 par Chazallon. Cet observatoire est depuis lors régulièrement entretenu et modernisé, ne s’interrompant temporairement qu’à la suite des bombardements de 1944.
Entre temps, de nombreux autres marégraphes avaient vu le jour partout dans le monde à mesure que progressait le besoin de connaitre et de comprendre l’environnement marin.
What is a tide gauge?
A tide gauge is an instrument used to measure variations in sea level.
The first tide gauges were simple graduated scales fixed along the quays, and on which an operator regularly recorded the water level.
In the middle of the 19th century, the first automatic tide gauges appeared. The latter consist of a float in a well, whose variations with the tides are transcribed, via a system of pulleys and gears, on a sheet of paper wrapped around a drum.
Since the end of the 20th century, tide gauges have done away with the need for mechanical parts, and most of those installed today use radar sensors to measure variations in water level.
The Shom network
Through the RONIM network (sea level observation network), Shom operates and maintains fifty tide gauges on the coasts of mainland France and overseas. Data are transmitted in real time to the data.shom.fr portal.
Why observe sea level?
Observation of sea level is involved in the design of several Shom products: tide predictions and navigation maps.
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Predict the tide
The Shom provides tide predictions for most ports in France as well as many ports around the world. These predictions require observations: sea variations observed by tide gauges over at least one year are analyzed to extract elementary signals called “harmonic components”.
These harmonic components have an amplitude and phase specific to each port, and are used to make predictions.
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Probe the seabed
One of the first steps in establishing a nautical chart consists of carrying out a bathymetric survey: a ship equipped with a depth sounder measures the depths. For the same point, the depth under the ship varies depending on the tide. It is essential to measure the latter concomitantly with the survey in order to remove this variable at the depth measured by the ship.
In addition to these aspects directly linked to hydrography, sea level observation is also used to monitor and manage maritime and coastal risks.
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Knowledge of extreme levels
Tide gauge observation over a sufficiently long period makes it possible to calculate probabilities associated with storm events causing an abnormal increase in sea level (surge). On this basis, extreme level statistics are established which are used in particular to size coastal structures (dykes, quays, pontoons).
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Wave-submersion vigilance
In collaboration with Météo-France, Shom operates digital simulations to anticipate storm events and their effects on the coast. The results of these calculations are the basis of the Wave-Submersion Vigilance intended to warn populations of the dangers of marine submersion.
Tide gauges make it possible to feed these simulations and correct the parameters if necessary.
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Tsunamis
The Shom tide gauges are included in several tsunami monitoring networks in the different oceans and seas around the world.
A sudden and unpredictable event, a tsunami requires a rapid response from the authorities. While simulations can be used to predict its arrival on a coastline, tide gauges are the only devices capable of confirming the existence of a tsunami, measuring its intensity and raising the alert.
In France, the CEA is responsible for the Centre national d'alerte tsunami (Cenalt).
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Average level monitoring
Observing variations in sea level since the beginning of the 19th century, tide gauges are unique witnesses to the evolution of the oceans.
Recordings are filtered and transformed into annual mean sea levels, enabling us to calculate sea level rise rates.
Since the variations involved are very small (a few millimeters per year), it is necessary to measure any vertical movements of the observatory which could distort the measurement. This work is carried out by Shom, in cooperation with the IGN and the University of La Rochelle, through the SONEL group.