Marine Geosciences

 

 

Marine mining sites in mainland France

Siliceous material building material (1 % of French national use, compared to 15 % in the UK) Calcareous material soil fertilizing water treatment pharmaceuticals

 

Context

French mining areas range from 1 to 8 m in depth and cover 1 to 8 km2. Mining consist in digging holes (called sand pits) in a flat sea bottom, or in exploiting sandbanks (cutting off of the crest).
	The mining may modify water motions (tide, currents, waves) and then the natural sediment transport induced by water motions. It may then modify the surrounding sea bottom and the neighbouring coast.

 

Function of Ifremer

Statutory context

Any mining must be authorised by the local controlling authority. The operator must provide an environmental impact assessment including possible effects on shore stability and a monitoring schedule for the mining area and its surroundings.

Ifremer has been delegated to appraise these preliminary and follow-up studies.

Means, method and aim

Inventory of the state of the art : theoretical knowledge, observations - natural sites or scale models-(Armède, 1999) and existing simulation software.

Calibration and validation of a numerical model using Delft3D software (from Delft Hydraulics) and observations (in progress).

Predictive simulations of the influence of a mining area on the shore, leading to the definition of size and position criteria according to the morphodynamic context.

Contribution, with european co-workers, to a methodological guide concerning preliminary and follow-up studies (within a few years).

Known effects of a mining area on the shore

Locally, lowering of the sea floor modifies waves and currents. For the common case of a sandpit of average size located offshore the surf zone, waves and currents are decreased in intensity. If the sandpit is located in an active transport zone (daily (tide), seasonal or continuous), it may trap some sediments. Downstream, digging modifies the waves by refraction : waves are reduced behind the digging (in the so-called "shadow"), waves are increased on both fringes of the digging shadow.

Studies in progress

• Improvement of the numerical simulation of Migniot and Viguier (1979, 1983) laboratory experiments through :
  • introduction of the directional spreading of the incident wave spectrum in order to numerically represent the observed reduced refraction of the waves,
  • long term simulations, (with Delft3D software and in co-operation with the research team of Delft Hydraulics), with unsteady forcing conditions (variable waves should smooth morphologic change in the surf zone).
• Validation of the numerical simulations for regularly monitored field sites (Glénan, Loire estuary), in collaboration with CETMEF (Center for Maritime and Fluvial Technical Studies) which will use in-house numerical tools.

Goals

Numerical simulations of bathymetric evolution in the presence of a mining area for various schematic configurations at true scale.

Contribution to a methodological guide concerning the effects of a mining area on the shore :

• mining area positioning and size criteria in a given range of conditions,
• minimal requirements for a numerical simulation that should be implemented in more complex cases,
• specification of the frequency and extent of site monitoring to establish.