“Every second these devices measure the variations in water level”, explains Nicolas Pouvreau, tide specialist at the Hydrographic and Oceanographic Service of the Navy (Shom).
Heir to the first hydrographic service in the world (1720), the national establishment manages 50 tide gauges: 40 in mainland France, 8 overseas, one in Monaco and one in Madagascar.
In the entrance to the establishment located in Brest, a long panel presents the curve of sea level for more than 300 years.
The first observations were made there in 1679 on behalf of Louis XIV.
From 1711, tide scales were installed on the banks of the Penfeld, on which operators relied to record and record the water levels daily.
It was in 1846 that a tide gauge was installed, one of the first in the world with automatic and continuous recording of sea level.
Destroyed by the bombings in 1944, it was reinstalled in 1949 in a simple concrete structure placed on the side of the quay within the naval base.
Inside, a so-called calming well, communicating with the outside sea, filters out level fluctuations linked to choppy water.
In 1992, the mechanical tide gauge was replaced by a digital device and then in 2004 by a sensor.
Every second, it sends a radar wave which, reflected by the surface of the water, makes it possible to calculate the height of the water.
“Observations have continued in Brest from decades to centuries and today show that the sea level has risen by around 35 cm in 300 years”, notes Nicolas Pouvreau, underlining an acceleration of this evolution for a century.
– three millimeters more per year –
Between 1700 and 1900, the sea level rose in Brest by 5 to 10 cm, while since 1900 the increase has exceeded 25 cm, with an average rise in recent years of more than three millimeters per year.
“This is one of the longest and therefore the most important water height observation series in the world”, underlines the geophysicist, assuring that the measurements collected in Brest correspond “quite well” to what is happening. goes global.
“One of the characteristics of tide gauges is that the data they provide is available in real time. This is fundamental for anticipating the risk linked to wave-submersion phenomena”, notes Matthieu Chevallier, head of the marine forecasting department. from Meteo France.
“If we want to know the sea level before 1993, we only have data from the tide gauges”, underlines Gonéri Le Cozannet, specialist in coastal risks and climate change at the French national geological service and contributor to the IPCC reports.
The mean sea level, one of the most important indicators of climate change, has also been calculated since 1993 using data provided by altimetry satellites.
“To be able to assess the risks in relation to extreme events, and therefore rare, it is necessary to have very long data”, explains the specialist, regretting the low number of tide gauges providing data over more than fifty years.
These devices also made it possible to determine the hydrographic datum, namely the common reference level for nautical charts and tide directories.
That of Brest, and it alone, is used to calculate the tidal coefficients, while that of Marseille, classified as a historical monument since 2002, has been used to calculate altitudes in France, as well as in Switzerland, a country without sea.