Reconstruction and analysis of historical water levels measured in Saint-Nazaire since 1863

The project for the reconstruction and verification of the analog records of water levels observed in Saint-Nazaire aims to provide additional information about the evolution of sea level constituents. This study, funded by the MEDDE and SHOM, will safeguard the historical heritage by digitizing and studying the sea level observations since the 19th century in Saint-Nazaire. To this end, the analog data stored at SHOM and at the Nantes Saint-Nazaire Grand Port Maritime were digitized.

 

Saint-Nazaire tide gauge observatory (Yann Ferret, photo credit SHOM, October 2013). Click on the photo to enlarge.Saint-Nazaire tide gauge observatory (Yann Ferret, photo credit SHOM, October 2013). Click on the photo to enlarge.

Saint-Nazaire tide gauge observatory (Yann Ferret, photo credit SHOM, October 2013). Click on the photo to enlarge.

 

General context

The study of global climate change and its impact on sea level rise over the long term is an important societal issue. In this respect, the analysis of centuries-old historical records of water levels is proving to be an ideal way to provide valuable arguments regarding the trends observed. Currently in France, only the records of Brest, Marseille and the Charente coast go back more than 100 years and are available in digital format. However, despite the large number of French tide gauge records, the majority are still in analog form and are therefore not taken into account in the studies. This study will help perpetuate the body of knowledge acquired at Saint-Nazaire since 1863.

 

Study objectives

  • Preserve all the sea level measurements taken in Saint-Nazaire since 1863 and constitute a consistent, high-quality record;
  • Understand the local influence of the Loire and/or anthropogenic changes on the evolution of sea level constituents;
  • Improve the understanding of sea level trends on the French Atlantic coast, then compare our results with other centuries-old sea level records on the European and worldwide coastlines;
  • Provide vital information for the secular study of extreme levels (better statistics for coastal and flood risk protection plans for example).

 

Geographic and historical contexts of the Saint-Nazaire tide gauge observatory

Saint-Nazaire is located at the mouth of the Loire River and is subject to a semi-diurnal macrotidal tide cycle. In 1863, a tide gauge observatory was installed there and began taking continuous measurements of water levels.

Over time, the tide gauge station has evolved, mainly as a result of anthropogenic changes in the geography of Saint-Nazaire:

  • Because of port development, large areas have been reclaimed from the Loire since the 19th century and the coastline has changed considerably;
  • Since 1863, the location of the tide gauge observatory has changed. Until 1950, it was located at the end of the Vieux Môle jetty, and in 1950-1951, it was relocated to the East jetty of what was then the new outer harbor. It is still located there today.
  • The tide gauges used to measure water levels have changed over time. Until recently, a series of float tide gauges were used. In 2007, the Saint-Nazaire station joined the RONIM measurement network and was upgraded with a radar tide gauge operated by the Nantes Saint-Nazaire Grand Port Maritime.

Evolution of the geographic context of Saint-Nazaire and its tide gauge observatory since the 19th century (Yann Ferret, photo credit SHOM, 2014). Click on the photo to enlarge.

Evolution of the geographic context of Saint-Nazaire and its tide gauge observatory since the 19th century (Yann Ferret, photo credit SHOM, 2014). Click on the photo to enlarge.

 

Successive steps required for the construction of the historical tide gauge record of Saint-Nazaire

Search for tide gauge archives:

Analog archives are composed mainly of:

  • marigrams regularly covering 15 days to 1 month of measurements;
  • paper records, corresponding to water levels recorded by the tide observer every hour (or every ¼ h) or bi-monthly tide logs indicating the times and water levels at HW and LW.

Most of the papers archives from the Saint-Nazaire observatory are kept at SHOM (1863-1920) and the Nantes Saint-Nazaire Grand Port Maritime (1936 - present).

Timeline of the Saint-Nazaire observatory to present day (Yann Ferret, photo credit SHOM, 2014). Click on the photo to enlarge.

Timeline of the Saint-Nazaire observatory to present day (Yann Ferret, photo credit SHOM, 2014). Click on the photo to enlarge.

 

Digitization of analog data (paper records)

Data digitization is a tedious but important task, because only once converted into digital format can the historical data be analyzed and used to answer scientific questions.

During digitization, it is also possible to identify outliers and/or potentially incorrect values (recording error, misuse of measurement equipment) and correct them when the metadata permit.

Diagram illustrating the steps involved in digitizing tidal data (Yann Ferret, photo credit SHOM, 2014). Click on the photo to enlarge.

Diagram illustrating the steps involved in digitizing tidal data (Yann Ferret, photo credit SHOM, 2014). Click on the photo to enlarge.

 

Standardization of height and time

The time system used is not the same depending on the period (True Solar Time, Solar Mean Time of a point, Universal Time) and the concept of vertical datum may also change over time. Has the tide gauge zero remained the same? Does the tide gauge zero correspond to the chart datum? Has the chart datum remained stable since the 19th century?

Thus, it is extremely important to standardize the data and rebuild a consistent time series in terms of height and time. This is done using the metadata associated with the measurements (marigram control records, levelling records, etc.).

 

Future actions

  • Further research in the archives (departmental, municipal, scholarly societies, DREAL) to complete the time series;
  • Continue digitizing - Provide answers to scientific questions;
  • Post the validated high frequency data from this work on the REFMAR website and the corresponding daily, monthly and annual mean levels on the Sonel website.

 

To find out more:

 

References

 

 

Last updated: May 13, 2014

Further reading about the reconstruction of tide gauge records:

List of programs, projects and reconstructions of tide gauge data