Presentation of changes in sea level at the port of Brest since the beginning of the 18^th century at the REFMAR Days

The sea level is an important parameter used in studies on climate change. Brest is proud to have been one of the first sites in the world where systematic observations of sea level were conducted in 1679 by the French astronomers Picard and La Hire. Subsequently, sea level measurements continued, first under the leadership of the French Royal Academy of Sciences, then the Bureau des Longitudes and finally since 1846 by SHOM, which makes this port unique in the world. The observations show that the sea level has risen about 30 centimeters in 300 years. The mean sea level curve will be presented at the REFMAR Days.

 

Changes in sea level at Brest since 1711 from the work of Nicolas Pouvreau (SHOM, 2013)

 

Brest, a city open to the ocean, ideal for sea level observation

In his "Mémoire sur le flux et reflux de la mer" printed in 1789, scientist Pierre-Simon Laplace (1749-1827) presented the port of Brest "As one of the most favourable for tide observations [...] This port probably owes this advantage to its advanced position in the sea, and especially because its harbour has a very narrow entrance in relation to its size, and therefore the irregular oscillations of sea waters are greatly weakened". The geographical, geomorphological, bathymetric, anthropogenic, hydrological, meteorological and hydrodynamic characteristics explain why the city of Brest is an ideal natural observatory for observing the sea level.

 

Why was the sea level measured in Brest in the past?

Understanding the laws of gravitation and trying to predict the tide

Until the mid-19^th century, sea level measurements were commissioned by astronomers for two reasons: to understand the Newton's law of universal gravitation (1687) and to try to determine a method for predicting the tide. Great scientists like La Hire, Picard, Cassini II, Lalande, Laplace, Lévêque and Rochon all asked for sea level measurements to be taken in Brest during this period. And in fact, it was thanks to the sea level observations made between 1711 and 1716 that Laplace developed his Tidal Equations in 1799, which remained in use at SHOM until the early 1990s.

Creating nautical charts

In the early 19^th century, Beautemps-Beaupré, Director of the Dépôt des Cartes et Plans (the current SHOM) invented "modern hydrography" in order to make navigation safe by providing accurate charts on which all the soundings were defined according to the lowest astronomical tide (standard still used today). To develop bathymetric charts, the hydrographic soundings must be corrected for the tide. This operation is called reduction of soundings. The nautical chart for Brest meeting these standards was issued in 1822.

Tide forecasts

In the late 1830s, the industrial revolution swept across France. The Dépôt des Cartes et Plans, future Service Hydrographique de la Marine, was a key element in this modernization. The advent of steamers greatly reduced the delivery time for goods. Predicting the tide at ports became a priority to determine the best times to load / unload merchandise. To meet this need, a network of ten tide gauges managed by the Ministry of Defense was set up starting in the 1840s, and the Dépôt de la Marine published the first tide table in 1839. It includes predictions for the port of Brest and a dozen other ports.

 

Why observe the sea level today?

Measuring water levels today is important for new reasons, including:

• safe navigation at port entrances,
• creating nautical charts,
• improving tidal predictions and hydrodynamic models,
• monitoring the rise in mean sea level to support the French plan for adapting to climate change (PNACC),
• calibrating satellite altimetry measurements (Jason 1, 2 and 3, Cryosat 2, Sentinel 3 satellites),
• providing valuable information for integrated coastal management (coastline, habitats,),
• statistical and historical studies of extreme levels and storm surges,
• forecasting information for coastal oceanography (Prévimer EuroGOOS),
• support for the national tsunami warning center in the Mediterranean and North-East Atlantic (CENALT) operated by the CEA in partnership with SHOM and CNRS,
• support to the storm surge and flood warning system operated by Météo-France in partnership with SHOM,
• etc.

How are changes in water level measured?

Originally sea level measurements were recorded by observers in registers. Water levels were read on tide staffs. In Brest, the tide staffs were installed at the exit of the dry docks.

In 1846, a tide gauge, a mechanical device for continuously measuring the sea level, was installed at the Brest naval base. It operated continuously until it was destroyed by Allied bombing in 1944. Reinstalled in 1952, this technology was used in Brest until the early 1990s.

Since then, the Brest tide gauge observatory has been equipped with digital gauges (first ultrasonic sensors and now radar sensors) as part of SHOM's RONIM network.

 

Presentation of the timeline at the REFMAR Days

A 4.40-meter poster will be on exhibit at the REFMAR Days (blue timeline above). SHOM experts will be on hand to answer any questions you may have.

 

Reference:

• Pouvreau N. (2008). Trois cents ans de mesures marégraphiques en France : outils, méthodes et tendances des composantes du niveau de la mer au port de Brest. Doctoral thesis, University of la Rochelle, defended on September 26, 2008, 466p.

 

To fin out more:

• Brest tide gauge observatory and water level measurements displayed in near real time
• Gauges and applications
• Construction of sea level records
• Tide Gauge Instrumentation (tide staff, analogue tide gauge, digital tide gauge)
• Presentation of the RONIM Network
• SONEL - Observation System for Long term Variations in Sea level