Exposure of coastal land owned by the French Coastal Conservation Agency to sea-level rise until 2150.

Without adaptation, coastal erosion and flooding are projected to significantly increase during the 21^st century due to sea-level rise (SLR). Yet, many important coastal practitioners have limited knowledge of their exposure to future sea-level rise, which prevents them from taking informed adaptation decisions.  Here, we quantify the exposure of the coastal land heritage owned by the French Conservatoire du Littoral (CdL), the French coastal conservation agency, over the metropolitan France, which protects 1,450km (13%) of the coastline in France. This updates a previous assessment performed in 2006, since when both sea-level projections and land area of the CdL have changed (Clus-Auby et al., 2006). Ultimately, this assessment informs the land acquisition strategy of the CdL, in order to review and adapt its long term strategy by aiming to acquire in the next decades the “shores of tomorrow” and ensure its primary mission: the conservation of coastal natural shorelines and emblematic landscapes for the benefit of future generations.

We focus on three types of land areas:

• The protected area, which is the land and the real estate already owned by the CdL;
• The authorized perimeter, which is the area that the CdL is currently already authorized to acquire;
• The 2015-2050 Strategy, which identifies areas that are considered for future acquisitions, in order to meet the strategic objective of protecting 33% of the coastline up to 2050.

Change in exposure of these lands are quantified by a cross-analysis between these CdL land assets on the one hand, and flooding and erosion prone areas under various SLR scenarios and horizons on the other hand. More specifically, the flood prone areas are calculated as the land area below the highest astronomical tide level plus a SLR elevation scenario (ranging from 0 to 4 m) and rely on high resolution (1 m) LIDAR data (bathtub approach). Erosion projections make use of empirical erosion models constrained by historical records of shoreline position and consider SLR using the Bruun rule.       

At the scale of metropolitan France, we find that the degree and evolution of the land exposure strongly varies with regions. For instance, in French Brittany, for present conditions (i.e. SLR at 0 m), flood prone area cover less than 20% of the protected area and increase linearly of ~5% per m of SLR. Hence under the highest SLR estimate of the AR6 in 2100 (SSP5-8.5), exposure of protected area only increase by a few % for this region. Conversely, for the same time horizon, the Western French Mediterranean coast shows a much larger increase of ~20% (SSP1-2.6) to ~50% (SSP5-8.5) of the protected area exposure. Overall, our results reveal that land area exposure change is more sensitive to SLR increases in the range 0-1 m than SLR increases beyond 2 m.

Clus-Auby, C., Paskoff, R. and Verger, F., 2006. Le patrimoine foncier du Conservatoire du littoral et le changement climatique: scénarios d'évolution par érosion et submersion. In Annales de géographie (No. 2, pp. 115-132). Armand Colin.