Marine sedimentology
Mis à jour le 19/07/2024
De l’acquisition des données à l’étude de leur répartition et leur dynamique
The nature of sediments is an important factor influencing hydrodynamics or wave propagation in coastal areas. Sediments are also useful for nautical charts, where they are used to define the safest route, areas of potential risk and ideal anchorage points. Knowledge of the nature of the seabed is even more important for the installation and monitoring of submarine pipelines and cables, the extraction of marine aggregates, the development of marine renewable energies and the search for old wrecks or partially or completely buried explosive devices.
For all these needs, detailed and precise information is necessary concerning the nature, thickness, extent and dynamics of the sediments. The burial and reappearance of World War II bombs comes from the impact of currents, swells, weight and shape of the object; but the factor determining the possibility or impossibility of burial is the nature of the seabed. The need of fishermen and biologists is also important because the life of benthic organisms closely depends on the nature of the sediments and their dynamics. Sediments are thus a factor impacting biology and a marker of storms and past and present changes in sea level.
The study of marine sediments is a very old activity of the Shom, with the first acquisitions of the nature of sediments in the 1800s. At the end of the 19th century the hydrographic service published a first map of the sediments of the Iroise Sea from of sediment samples taken in large numbers with lead sulphate but also, from that time onwards, with grabs and corers. Like other sciences, sedimentology is subject to growth and diversification in the techniques used, increased interaction with other sciences and fragmentation of information. The successive arrival of new sampling systems, acoustic imaging systems and increasingly accurate data on seabed morphology has led to improvements in the accuracy of data and our knowledge of processes.
Each year, the areas studied with the nautical means currently used cover less than 1% of the continental shelves. It is therefore impossible to have global coverage of homogeneous quality studied with new systems. The solution implemented therefore consists of searching for all the data, integrating them into the Shom sedimentological database and then producing sediment maps at different scales ranging from 1/10,000 for impact studies to 1/500,000.
It is therefore increasingly necessary to have very precise sediment maps near the coasts and medium-scale seabed maps on the continental shelf and the deep ocean. However, it will take more than a century before we have a map based on data acquired with modern means. The solution adopted by the Shom here is to start from an approximate knowledge established with the means of the past and to gradually complete it with the most recent data in order to have up-to-date local and global sedimentary maps of the knowledge.
Sediment dynamics
The nature of the seabed can be summarised in 4 main classes: rock, pebbles and gravel, sand and mud.
The rock is the preferred place for the development of marine organisms and can present significant reliefs making its location important for biology and navigation safety. It is also the domain of stability, the only changes observed corresponding to erosion phenomena around their perimeter or covering phenomena of varying duration by sediments.
Pebbles and gravel create flat and stable bottoms with the exception of very energetic areas such as the Raz Blanchard where these elements can be moved by currents and form megarides.
On the other hand, the finest sediments: the mud generates flat bottoms where the sedimentary structures are depressions, due to the evacuation of interstitial gas, which can form craters of a few hundred meters in diameter in certain environments outside the metropolis. Muds are important environments for biology and the trapping of pollutants that can be resuspended by currents. These muds are the main source of turbidity.
Sands are particles varying from 50 microns to 2 mm whose particularity is to easily move and form dunes. Underwater dunes are comparable to those found on certain planets, in deserts and in rivers. Their only differences are that they are made up of heterogeneous sediments which can include gravel and pebbles and that their height is less than that of aeolian dunes since in the marine domain the highest dunes are around 30 meters at most while they can be 10 times higher in deserts. The Shom has been carrying out studies on these dunes for more than 20 years because these structures move on the bottom at speeds most often of a few meters per year but can sometimes exceed 50 meters per year. Such dunes cover the entire seabed to the north and west of Brittany at depths of between 70 and 220m. There are also hundreds of them in the navigation channel of the North Sea generating reliefs of 19 to 25m deep where boats have drafts of more than 20 meters. Finally, their movement is the cause of the masking of wrecks and explosive devices for periods which can exceed a century. The activity carried out by Shom on these dunes includes recurring surveys, studies aimed at developing models which should ultimately enable the optimization of the surveys and the organization of international conferences (Marine and River Dune Dynamics) since 2000.
