Mechanism of Coastal Foredune Formation and Evolution at Multi-Decadal Scale

Montreuil, A-L.; Robin, N.; Raynal, O.; Houthuys, R.; and Chen, M., 2024. Mechanisms of coastal foredune formation and evolution at multi-decadal scale. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 478-482. Charlotte (North Carolina), ISSN 0749-0208. Coastal dunes are a substantial part of the natural sea defence, characterized by a strong interaction between geomorphological, biological and ecological processes. Foredune morphodynamics is controlled by a broad spectrum of forcing processes acting at different spatial and temporal scales. The process-response of foredunes suffering from long-term erosion is reasonably well understood, while much less is known on the evolution of prograding foredunes. This study documents the evolutionary processes of a prograding foredune system on a multi-decadal scale (75 years) at an unmanaged macrotidal beach along the Belgian coast. The methodology is based on an holistic approach combining several methods. A Ground Penetrating Radar (GPR) campaign was performed to reveal internal structures of the foredune field. In addition, airborne LiDAR surveys (2000-2023) and 9 aerial photographs (1948-2023) were used to estimate the geomorphological evolution and vegetation density of the site. Results indicate a seaward migration of the system by 77 m over the last 75 years (1 m/year) with the formation of five consecutive well-developed foredune crests through distinct morphological phases. Two periods of constant incipient dune progradation and vegetal colonization (1971-1989 and 2005-2023, around 3m/year) are identified. This pattern is interrupted by two stagnation periods in 1948-1971 and 1989-2000 when aggradation of the developed foredune is observed. Once this latter is sufficiently high and colonized by vegetation, sand deposition on the dune crest by aeolian processes is less effective inducing a triggering of the progradation for the system. The general evolutionary pattern of the accretional foredunes reflects the signature of natural processes including aeolian sand transport and vegetation growth which are effective dune generators on a naturally prograding sandy coast. This study further shows that the use of GPR combined with chronological topographic data is an efficient tool to investigate foredune evolution over the last decades and can be applied for similar coastal settings.