Sand dune ecosystems as a form of defence against coastal hazards: assessment on the Noirmoutier coast (Vendée)

L’Atlas Bleu / Protecting

After storm Xynthia in February 2010, there was a growing and acute awareness that dune systems help protect against marine weather hazards, both in areas where the human stakes are high and elsewhere.  For this reason, they should be incorporated, in some way, into natural defence systems and monitored and maintained regularly. With their protective function becoming central to the social vision of their role, there has been an increasing interest in conserving sand dune ridges, primarily through the national strategy for coastline management that follows on from the Grenelle de la Mer oceans initiative. However, in spite of recommendations for relevant action, there are no operational tools in place for effective diagnosis. 

Through an initiative from LITEAU, an exploratory approach has been implemented and developed (Debaine & Robin, 2012).  The general principle is as follows. Effective coastline management in areas where there are sand dunes can be considered on three interconnected levels: a management intervention level in the field, where the problem is addressed directly; an intermediate level of detailed diagnosis, whereby decision-makers develop a targeted response strategy based on their overview of the condition of the dune system; and a more general level, whereby dune systems are listed and classified at an aggregate level to generate spatial statistics about their condition.  

To visualise the extent of protection afforded against marine weather hazards, a number of geoindicators for dune formations, plant cover, anthropization and dynamics were identified (see examples of geoindicators in the table on p.72) and spatialised using generic data (LIDAR, aerial photographs, satellite images). 

These geoindicators were inserted into specially constructed spatial units (boxes) located at zero metres NGF, with a width of five metres, and extending back a distance of three hundred metres from the coastline. By linking these geoindicators in the boxes, through spatial reasoning, it is possible to make a multi-criteria assessment of the extent of protection offered.

In this restricted example of a sand dune ridge in the southern part of Noirmoutier, the overall protective function of the dune system is determined by adding together the extent of protection afforded against flooding, deflation and marine erosion.

From red to green on the diagram, we move from a low overall level of protection (borne out in the past through ruptures in the dune system and flooding, thus validating the method) to a very reasonable level of protection.

The northern part of the sandbar is particularly vulnerable because it combines high human and social stakes close to the coastline, in an area where the natural environment does not afford a good level of protection.