Land–sea interaction and morphogenesis of coastal foredunes — A modeling case study from the southern Baltic Sea coast Wenyan Zhang a, ⁎, Ralf Schneider b , Jakob Kolb b , Tim Teichmann b , Joanna Dudzinska-Nowak c , Jan Harff c , Till J.J. Hanebuth a a MARUM — Center for Marine Environmental Sciences, University of Bremen, Leobener Str. 1, 28359 Bremen, Germany b Institute of Physics, Ernst-Moritz-Arndt-University of Greifswald, Felix-Hausdorff-Str. 6, 17489 Greifswald, Germany c Institute of Marine and Coastal Sciences, University of Szczecin, Mickiewicza 18, 70-383 Szczecin, Poland abstract article info Article history: Received 8 November 2014 Received in revised form 13 March 2015 Accepted 16 March 2015 Available online 28 March 2015 Keywords: Coastal dunes Aeolian transport Cellular automata Extreme wind event Vegetation Coastal foredunes are developed as a result of the interplay of multi-scale land–sea processes. Basic driving mech- anisms of coastal foredune morphogenesis as well as natural processes and factors involved in shaping the foredune geometry are quantitatively studied in this paper by a numerical model. Aeolian sediment transport and vegetation growth on the subaerial part of a beach is simulated by a cellular automata (CA) approach, while the sediment budget in the subaqueous zone, serving as a sediment source/sink for the foredune ridges, is estimated in a process-based model. The coupled model is applied to a 1 km-long section of a barrier coast (Swina Gate) in the southern Baltic Sea for a 61-year (1951–2012) hindcast of its foredune development. General consistency is shown between the observational data and simulation results, indicating that the formation of an established coastal foredune results from a balance between wind-wave impacts and vegetation growth. Driven by an effective onshore wind and a boundary sediment supply, small-scale dunes develop on the backshore and migrate landward. They are then trapped in a narrow strip characterized by a large density gradient of vegetation cover which separates the hydrodynamically-active zone and the vegetated zone. Continuous accumulation of sediment in this strip induces the development of a foredune ridge. According to the simulations, the formation of an established coastal foredune has to meet three preconditions: 1. an effective onshore aeolian transport; 2. a net onshore or lateral sediment supply; and 3. a climate favoring vegetation growth. The formation of a new foredune ridge in front of an already existing foredune is determined by a combination of the sediment supply rate, the extreme wind-wave event frequency and the vegetation growth rate. Simulation results demonstrate a remarkable variability in foredune development even along a small (1 km) coast section, implying that the medium-to-long term land–sea interaction and foredune morphogenesis is quite sensitive to boundary condi- tions and various processes acting on multi-temporal and spatial scales. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Dunes are a common morphological feature in many coastal and arid environments. The basic factors involved in the formation of a dune are a certain amount of movable sediment on the surface, a flow (of e.g., water or air) acting on the bed surface which is strong enough to transport the sediment and an obstacle or perturbation which trig- gers a settling of the moving sediment. However, although the mecha- nism for the formation of a dune is clear, combinations of different flow strength and directions, sediment properties (e.g., grain size and composition), constraints of local topography and boundary conditions (e.g., source supply) can lead to quite different and complex dune pat- terns (Werner, 1999; Kocurek and Ewing, 2005). The interplay among aeolian transport, vegetation cover and hydrodynamic forces (e.g., storms) makes the morphological development of coastal dunes even more variable compared to dunes in an arid environment (e.g., desert) and imposes challenges to researchers for a comprehen- sive study of the dune morphogenesis (Hesp, 2002). Among various dune patterns developed at the backshore, foredunes are most vulnerable as they stand at the foremost seaward line on the edge of the backshore, persistently reshaped by hydrodynamic and aero- dynamic forces. Foredunes are able to develop where winds are effective in moving sediment onshore and a trapping of the moving sediment by a line of shore-parallel obstacles exists. This trapping of sediment is usually caused by vegetation (e.g., pioneer grasses and shrubs). Foredunes can range from relatively flat terraces to markedly convex ridges (Hesp, 2002) due to a variation of the driving wind spectrum, the sediment sup- ply, the vegetation coverage and the growth rate. On a longer time scale their morphology is affected by climate change such like sea level oscil- lations (Tamura, 2012). Morphological development of a coastal foredune can be generally divided into three phases: incipient (or Coastal Engineering 99 (2015) 148–166 ⁎ Corresponding author. Tel.: +49 421 218 65642. E-mail address: wzhang@marum.de (W. Zhang). http://dx.doi.org/10.1016/j.coastaleng.2015.03.005 0378-3839/© 2015 Elsevier B.V. All rights reserved. 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