Marine aggregates

Authors : Claude AUGRIS and Laure Simplet (Ifremer-REM/Geo-Ocean/ODYSC)

ZI Pointe du Diable,
CS 10070 - 29280 Plouzané

Télécharger la version .pdf d'application/pdf Article paru dans la revue Géosciences n°17 du BRGM, octobre 2013

 Siliceous and calcareous sand extraction

 Source of materials

The main marine materials currently extracted in France are siliceous sand used in the construction and public works sectors and calcareous sand and seaweed used in agriculture. The deposits worked today extend from Dieppe to the Gironde Department.

These materials have accumulated through continental and marine processes.

  1. Siliceous sands are commonly alluvium resulting from the weathering and erosion of rocks and then from their transport and deposition in the valleys of old river systems (Figure 3a), incised during the regression phases (glacial periods) of the Quaternary, when the continental shelf was emerging. This phenomenon led to the accumulation of sediments from all sources, and heavy minerals such as cassiterite (Sn), rutile (Ti), platinum, gold, diamonds…
  2. Calcareous sands are hydraulic dunes or banks formed under the effect of currents (coastal drift, swell, tide) which have redistributed part of their sediment. (Figure 3b).

Exploration techniques

The techniques used to identify and classify marine aggregates are either direct or indirect.

  • The direct method is coring
  • The indirect method involves geophysical tools (seismic reflection) to determine the thickness and structure of the sediment infill and the morphology of underlying bedrock (Figure 4).
  • However, this technique does not provide information on the type of sediments, so the seismic data is calibrated with ground-truthing using direct investigation methods such as coring (Figure 5).

Resources and Reserves

Studies undertaken within the National Inventory of Marine Materials (CNEXO, 1969-1980) estimated the volume of sediment at approximately 33 billion cubic metres. The work covered only part of the coastal areas between 10 and 50 m deep, within territorial waters (12 nautical miles).

In 2005, the Ministry in charge of Ecology asked for a reassessment of the resource potential in light of studies carried out in particular by Ifremer and some universities (Bretagne Occidentale, Bretagne Sud, La Rochelle, Bordeaux).

Consequently, resources on the eastern Channel and Loire-Gironde coasts and up to the limits of the French Exclusive Economic Zone (about 100 m deep) were estimated at 170 billion cubic metres (Figure 6).

This review, for the Channel and Atlantic coastlines, has re-evaluated the potential for soft sediments at 540 billion cubic metres.

Even if resources are abundant, the exploitable reserves are much lower and are estimated considering the following parameters:

  • The water depth necessary for dredging vessels, currently approximately 50 m;
  • The presence of human activities (fishing, cables, shipping routes, national defence areas);
  • Area providing key ecosystem functions and processes, essential to the ecological balance of the marine environment (spawning grounds, benthic flora and fauna ensuring the sustenance of commercial species, nurseries);
  • Areas protected under European and national provisions (ZNIEFF, Natura 2000, Marine Protected Areas);
  • The nature of sediment which must be directly usable with no discharge

These constantly evolving constraints limit the reserves to about 2% of the inventoried volume of resources

Mined deposits

Sea sand extraction is a relatively old and traditional activity since there are myths involving the barges used to load and transport sand in most French ports.

The deposits mined today are located between Dieppe and the Gironde (Figure 7) and present varying features:

  • depths between 5 and 50 metres,
  • areas between 0.2 and 50 km²,
  • distances from the coast between 0.3 and 38 kilometres.

Extraction sites occupy 128 m² of the seabed (0.03% of the surface area of the metropolitan EEZ). While for a long time the Atlantic coast provided two thirds of the siliceous aggregates extracted in France, the opening of two mining sites and the ongoing appraisal of three concessions in the English Channel will soon make the latter the most productive region. Brittany remains the only region exploiting limestone materials (shellfish sands). There is also a coral-sand operation in Guadeloupe, as well as a small silica-sand extraction activity in Saint-Pierre-et-Miquelon.

Almost all extraction operations use the trailing suction dredging technique (Figure 8); a traditional sand-dredging vessel is still in operation in North Brittany for maërl and shell sand extraction.

The annual quantity of materials authorised for exploitation, determined on the basis of the quotas issued by prefectural decree in 2013, is 10.5 million cubic metres (of which 2.5% for calcareous sediment).

Additionally, around one million cubic meters are imported annually through the ports of the eastern Channel. Around ten companies, including large groups, are involved in this activity, which employs around 6,500 people (mariners, administrative staff, commercial and technical positions) (source UNPG).

 

The exploitation of marine aggregates is governed by the Mining Code, and requires three key documents:

  • A ministerial decree for granting the concession, the duration of which may be requested for up to 50 years;
  • A prefectoral decree to start production, which lays down operating conditions, authorised quotas and environmental monitoring procedures;
  • A prefectoral decree for temporary occupation of the maritime public domain, which defines the “public land” fee (fixed per cubic metre extracted and by type of material) for extractions within territorial waters (12 nautical miles).

Since 2006, applications for concessions and mining inauguration can be filed conjointly (Decree 2006-798 of 6 July 2006).

 Environmental impacts

Any type of extraction from the seabed immediately affects the marine environment (reduction in the abundance and variety of benthic populations, water pollution due to turbidity, changes to seabed morphology) and is liable to induce longer-term repercussions, which are less readily detectable, as they are complicated to discriminate from natural seasonal or annual variations. Consequently, the Mining Code that regulates the extractive offshore industry requires a detailed environment impact assessment for any mining licence application (figure 9).

Water is the first medium altered by aggregate extraction, by creating turbidity, at depth by the passage of the sling spout, and at the surface through the discharge of fine particles.

However, even slight turbidity cannot be neglected because of its impact on benthic flora and fauna. The fine particles form a plume which, driven by the currents, is redeposited either at sea, possibly remobilized, or on the coast. Following extraction, a change in the morphology of the exploited site may modify the seabed current regime. By altering the equilibrium of surface sediments, these extractions could thus influence coastal erosion, particularly in the case of near-shore operations at shallow depths. In addition, excavations can make these areas temporarily unsuitable for trawling.

The effects of exploitation on biological resources are both immediate, and therefore obvious, and long-term, in which case only a serious follow-up would allow to measure the impact.

Destruction of the benthic community in the extraction area is undeniable. This destructive and immediate impact mainly affects invertebrates. It is also important to mention the risk of destroying spawning grounds for species laying on the bottom (herring in the Eastern Channel and the North Sea), which are of significant commercial interest, and nurseries where young individuals are concentrated.

Longer-term effects are not as evident. They are difficult to differentiate from natural seasonal or annual variations.

As a consequence, the Mining Code, which governs offshore extractive activity, requires the provision of a detailed impact study (the content of which is defined by the Environmental Code) when applying for a research permit, a concession and production. This impact study focuses on bibliographic census and document analysis, the acquisition of new data in the fields of geology, physics, benthic biology and fisheries, and their interpretation.

In the geological domain, one of the main tools for characterizing the marine environment is side-scan sonar. This tool maps the seabed, on a horizontal plane, by providing an «acoustic» image (sonogram), comparable to an aerial photograph, useful for seafloor classification. Assembling sonograms creates a mosaic of images that indicates the distribution of the different seabed substrates and the detailed morphology of the seabed (Figure 10). To verify the hypotheses of identification by this technique and to specify the nature of the constituents, it is imperative to calibrate the data by spot through extensive ground-truthing (e.g. taking samples and/or underwater videos).

The data acquired by the side scan sonar used for site surveys provides:

  • a reference state of the seabed directly concerned by the extraction project, but also of the surrounding environment;
  • a precise delineation of the different types of loose and unconsolidated grounds, useful for defining sediment-fauna (habitat) relationships in the context of the impact study;
  • knowledge of hydrodynamic conditions (currents, swells) and sedimentary transport directions to assess the risk of extraction on coastal stability;
  • the provision of practical information for subsequent dredging: seabed morphology and currents, identification of exploitable areas by differentiating sand groups from other sedimentological facies and rock outcrops.

Based on the guidance that Ifremer provides to the State authorities on the formal appraisal of applications, periods of up to five years of environmental monitoring are recommended

Conclusion

With the current pace of extraction, the marine material reserves known today represent approximately one century of production

As this reserve is in increasing demand, the government has initiated a “national strategy for the sustainable management and development of continental and marine aggregates and materials and quarry materials and substances".