Marrying coastal safety objectives with natural development of sand dunes – SONO

Sandy coasts are complex and poorly understood environments that are under unprecedented threat posed by anthropogenic pressures and climate change. Most beaches worldwide are backed by coastal dunes, with the composite beach-dune system both buffering the hinterlands from severe storm waves and providing outstanding ecosystem services. However, most dunes on developed shores worldwide have been managed intensively for decades to minimize marine and aeolian erosion, which sometimes reduced vegetation dynamics and natural community diversity. Coastal dune management doctrine is progressively switching to a softer approach reinstating natural dynamic processes. Many coastal dune managers nowadays suggest that in certain coastal dune environments dunes maintained as dynamic systems are more resistant to marine erosion and more resilient than fixed dunes, although this has never been demonstrated nor even studied quantitatively. The major advances in unmanned vehicles technologies, satellite remote sensing of the Earth and in the modelling of complex systems suggest that we are at a tipping point. At the crossroad of Oceanography, Geomorphology and Ecology, the challenging objective of SONO is to make fundamental progress in the understanding of the interactions between aeolian, marine and biotic processes driving coastal dune evolution using systematic innovative field and remotely sensed measurements and well-adapted mesoscale modelling techniques. SONO is designed to provide quantitative estimates of the beach-dune system behaviour, including resilience to extreme events and ecosystem shifts in a context of climate change and increasing anthropogenic pressure, ultimately to designing optimal management strategies. Field work will be carried out at 2 primary sites in SW France (Truc Vert and Hourtin), which are under contrasting chronic erosion and free evolution states, with adjacent sections of managed dune providing accurate comparison with fixed dunes. Measurements will involve the collection of quarterly and pre/post-storm UAV-photogrammetry-derived digital elevation models over 4 km of dunes completed by long-term sediment transport measurements using a dense array of acoustic sensors; seasonal vegetation composition surveys at the most dynamic ecotones; regular beach topographic surveys using DGPS and nearshore bathymetries collected by a very first single-beam-sounding-equipped USV. Directional wave data, tidal elevation, wind speed and direction will be collected throughout the project at nearby stations. Using relevant physical and biological indicators, the influence of coastal dune management strategies on coastal safety and ecosystem services will be addressed. In addition, quarterly UAV-based topographic surveys will be collected at the 2 secondary sites of Anse du Gurp and Trencat, which are at a late stage of free behaviour. Large-scale (> 10 km) digital elevation models of the beach dune system will also be generated through 0.5-m resolution Pléiades tri-stereo satellite images in SW France, and along different coastal dunes systems worldwide in Australia, the US, the Netherlands and UK, covering transgressive barrier islands, coastal transgressive dunefields including those with star dunes, strongly managed sectors or highly-disturbed systems through notches excavated to excite free behaviour. This outstanding dataset will be used to develop and further test a new class of cellular automaton model for the composite beach-dune system, which will be used to quantify feedbacks and interaction patterns between biotic and abiotic processes in order to provide generic solutions applicable on wave-dominated beach-dune systems worldwide. Overall, in close collaboration with coastal dune stakeholders SONO will provide new fundamental and practical insights into coastal resilience to storms covering geomorphological and ecosystem aspects in a worldwide context of climate change and increase in anthropogenic pressure.