Sea defense – Cotentin

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Sea defense – Cotentin

Assessment and propositions for strenthening sea defense structures in the Cotentin and Bessin Natural Park territory.

Projet : 

The land in the Cotentin and Bessin Regional Natural Marsh Park includes marsh areas protectected by dikes and dune ridges. The project consists of reinforcing sea defense structures, taking into account environmental and socio-economic issues and the financial means of the owners. A site survey and the resulting propositions for strenthening the site also provide the necessary technical basis for carrying out hazard assessments.

 

The Cotentin and Bessin Regional Natural Marsh Park (PNRMCB) covers a coastal zone from Grandcamp-Maisy to Morsalines. In response to a request from the Local Water Commission from SAGE DOUVE-TAUTE at the end of 2008, the PNRMCB accepted to lead the project management of the assessment of sea defense structures which consists of:

  • 21 kms of brick-work rip rap and 14 kms of dune ridges in the maritime area
  • 45 kms of earth dikes in the waterway/maritime area (the Veys bay)

 

From an assessement of risks, hazards and issues, work priorities were established and three scenarios were compared:

  • Scenario 1: Do nothing, repair the damage as and when necessary, continue routine maintenance
  • Scenario 2: Treat the problems identified  when there is a high risk, repair the damage as and when necessary, continue routine maintenance in other areas
  • Scenarion 3: Treat all of the medium to high hazard or high risk dikes

 

For every problem identified, several techniques to repair it  were identified and then compared using five criteria: effectiveness, impact on the environment, implementing the work, cost of investment and running costs. Scenarios 2 and 3 represent the sums of 3.6 million € and 8 million € respectively for the works. A summary cost/benefit analysis does not exclude scenario 2 which only provides for interventions in sectors where there is a significant probability of damage to human life. An estimation of the economic damage shows that damage to agriculture often comes close to damage to buildings. However, it is difficult to measure damage to agriculture in a waterway/maritime area. The experience of Xynthia was valuable but strong doubts remain.

 

Today, we are left with the questions of project management and pooling of efforts; whether it be to comply with regulations or to reduce risk. In the areas under the spectrum of the Authorised Syndical Associations (ASA), investigations and measures to take in order to comply with regulations represent a higher cost compared to the investment for authorised  maintenance,further limiting the capacity for investment. Decisions about the works must therefore be taken on the basis of the socio-economic analysis provided.  The ASAs, whose parameter covers mainly agricultural land, cannot directly benefit from State aid, and with the Region having withdrawn, the question of financing the works and their scope arises. Discussion with the municipalties and inter-municipality associations concerned is therefore necessary.

Mission : 
  • Assessment of sea defense structures
  • A stress estimation: propagation modelling of the swells, set-up and run-up and constraints on the structures
  • Computational Fluid Dynamics (CFX)
  • Estimation of the risk of rupture
  • Risk assessment
  • Simulations of breaches in high-risk areas and affected flooded areas
  • Proposition of strenthening scenarios and related costs
  • Cost benefit analysis
Le projet
Pays/région : 
France
Client: 
Parc Naturel Régional des Marais du Cotentin et du Bessin
Période : 
2010-2011