Geomorphologic characteristics and evolution of managed dunes on the South West Coast of France

Abstract

The calibrated foredunes of the Aquitaine coast (SW France) constitute an example of secular extensive application of nature-based solution. Being one of the longest and largest dune systems in Europe, this iconic landscape offers considerable services to coastal societies. Since the mid-half of the 18th century, human work on sediment flows has resulted in the emergence of large sets of foredunes, currently constituting an almost 230 km-long managed barrier against inland sand invasion, coastline erosion, coastal flooding and also a major reserve of biodiversity. Last massive mechanic shaping of the foredunes following a so-called “ideal profile” date back to the 1970s–80s and aimed to limit erosion dynamics and sediment mobility. Since, they have been facing multiples disruptions over the years which have considerably modified their morphology, especially during the last decades. Calibrated foredunes are currently exhibiting impressive dimensions with a mean dune height of about 16.6 m (peaking at >30 m in some locations) and a dune width mean along the coast of 187 m. Today, 55.5% of them meet the “ideal profile” shape standards or surpass the in terms of dimensions whereas 21,5% of the calibrated foredunes are considered “narrowed” and need specific attention. This regional analysis, based on extensive airborne LiDAR datasets from 2011 to 2018, includes (i) a dune morphology classification based on several basic geomorphological indicators (dune width, height, crest position, back dune elevation) compared to a calibrated “idealised” shaped profile, (ii) a quantification of marine beach-dune and aeolian sediments flows (in m/year and m/m2/year) and forms on dunes (scarps, blowouts, depositional lobes), as an indicator of the recent dynamics affecting dune morphology and (iii) a study of management control of calibrated foredunes (4613 interventions) including actions taken after the outstanding winter of 2013–2014. As expected, intensive dune management are mainly focused where assets are directly threatened. The study highlights the importance and the efficiency of human management on dune evolutions (long term dune form stability, control of gullies and incipient frontal blowout after marine erosion, protection of human issues). However, today management actions (wind fence, branch cover, planting and reshaping) are not conditioned by foredune morphology or last decade's beach-dune dynamic (sediment availability, chronic coastline erosion). Considering long-term erosion projections and expected impact of climate change, the analysis shows the need of more global and extensive strategies in order to preserve the protection role of foredunes in the coming decades.