Coastal erosion and marine aggregates: the UNPG publishes a scientific reference document to objectively assess potential impacts

A complex interaction between extraction and coastal dynamics

Le coastline designates the interface zone between sea and land. Contrary to a fixed representation, it evolves under the effect of natural processes: swell, currents, river inputs and extreme weather events.

Two phenomena structure this dynamic:

• theerosion, which corresponds to the retreat of the coastline;
• theaccretion, or the accumulation of sediments.

The reference document reminds us that these developments fall under a overall sedimentary balance, also influenced by human development. Coastal structures (dikes, groynes) or urbanization, for example, modify the flow of materials.

In this context, the extraction of marine aggregates must be analyzed at different spatial scales:

• la coastal area, very active;
• la offshore coastal area, where the concessions are located.

Direct impacts ruled out, but indirect effects studied.

The document specifies that current extractions no longer take place on beaches or in the immediate coastal zone. They are located on the continental shelf, which limits the direct impacts on the coastline.

"The ongoing aggregate extraction […] does not directly impact the coastline."

On the other hand, indirect effects remain possible. They include, in particular:

• the modification of ocean currents ;
• the disturbances of sedimentary transport.

These interactions are described as near fieldthat is to say, located around the extraction areas. Their intensity depends strongly on hydrodynamic conditions and the configuration of the seabed.

An important limitation of the reference framework must be highlighted: it does not allow us to predict the long-term evolution of the coastline, but only to assess trends related to a given project.

A regulated and targeted extractive activity

Marine aggregate extraction in France remains marginal at the national level, with 8,7 million tonnes produced in 2022that is about 2% of the total.

It is based on a structured system:

• 17 active dealershipscovering approximately 160 km²;
• an operation ensured by 18 specialized ships ;
• nearly 900 direct and indirect jobs.

These materials meet specific technical needs in construction projects:

• concrete for port infrastructure;
• embankments or subgrade layers for marine works.

In some coastal areas, they represent up to 80% of the needs, particularly due to logistical constraints. The transport of aggregates, a heavy material, remains a determining factor in construction costs.

A standardized methodology for impact studies

The framework introduces a common methodology intended for design offices, project managers and permitting authorities.

The analysis is based on a comparison between:

• un initial reference state ;
• a projected situation at the end of operations.

Four main models are used:

• sea ​​states;
• currentology;
• transport of coastal sediments;
• coastal transit.

Additional modules can be added depending on the context: estuaries, lagoons or specific areas.

"The guide proposes […] a unified method for analyzing the potential indirect effects of concessions."

In practical terms, a consulting firm can use these digital models to simulate the evolution of the seabed and identify any changes in sediment transport.

A decision-making tool for maritime projects

This framework aims to harmonize teaching practices offshore extractive projects. It is aimed at project developers, government agencies and stakeholders.

It falls within an existing regulatory framework, including:

• environmental impact studies;
• the authorization procedures related to the public maritime domain.

The involvement of specialist actors such as maritime engineering offices or scientific organizations (hydrodynamics, marine geology) remains essential for operational implementation.

An important nuance: the guide does not address the impacts related to other coastal developments, such as beach nourishment or protective structures.

A scientific basis for objectifying the debate

The result of three years of collective work, this document constitutes a shared technical basis between public and professional actors.

“A validated method […] allowing for the assessment of potential indirect effects”

It provides a rigorous analytical framework, without definitively ruling on all the impacts. Its main contribution lies in the knowledge structuring and the transparency of the methods used.

For professionals in building and coastal development, this reference framework becomes an operational tool for integrating sedimentary issues into project design, while taking into account the limitations of model interpretation.