Foredune blowout formation and subsequent evolution along a chronically eroding high-energy coast

Abstract

Coastal dune systems provide important ecosystem services, while being vulnerable to marine erosion. In these environments, blowouts can develop and promote sand transport from the beach to the back of the dune, but are generally fought by coastal dune managers. There are only few quantitative studies on the 3D evolution of blowouts and how they can develop into parabolic dune. We investigate the morphological evolution of a 2-km long freely evolving dune system in southwest France from 1947 to 2021 using historical aerial photos and digital surface models from Lidar and UAV photogrammetry. The combination of these remote sensing methods shows an alongshore non-uniform erosion with a mean of 1.26 m/year, and with erosion rates in the north of the study area four times larger than in the south. Over the study period, three large blowouts developed in the northern, (more rapidly eroding) sector and subsequently evolved into parabolic dunes, with a depositional lobe migrating landward into the forest. Two parabolic dunes naturally stabilized by vegetation colonization, without any reactivation phase, with the third one still migrating landward with an average migration of 7.2 m/year. A high-frequency and high-resolution analysis of the active blowout was performed between 2014 and 2021. Compared to the adjacent areas, this blowout promoted dune landward migration. Since 2014, the high erosion scarp in the adjacent southern sector prevented the transport of sand resulting in a loss of dune volume due to marine erosion. In contrast, in the adjacent northern sector and in front of the blowout, the presence of vegetation and paleosols at the dune toe favored sand deposition and limited marine erosion. In chronically eroding sectors, promoting blowouts and thus landward dune migration may be considered as an efficient management approach to maintain the dune system.