Elsevier

Coastal Engineering

Volume 99, May 2015, Pages 148-166
Coastal Engineering

Land–sea interaction and morphogenesis of coastal foredunes — A modeling case study from the southern Baltic Sea coast

https://doi.org/10.1016/j.coastaleng.2015.03.005Get rights and content

Highlights

  • Medium-to-long term coastal foredune morphogenesis is studied in an integrated model.

  • Successful field application demonstrates the model robustness and potential.

  • Coastal foredune morphogenesis is highly sensitive to environmental conditions.

  • The subaerial model can be easily incorporated in other subaqueous transport models.

Abstract

Coastal foredunes are developed as a result of the interplay of multi-scale land–sea processes. Basic driving mechanisms 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 conditions and various processes acting on multi-temporal and spatial scales.

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 triggers a settling of the moving sediment. However, although the mechanism 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 patterns (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 comprehensive 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 aerodynamic 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 supply, the vegetation coverage and the growth rate. On a longer time scale their morphology is affected by climate change such like sea level oscillations (Tamura, 2012). Morphological development of a coastal foredune can be generally divided into three phases: incipient (or embryo) period, established period, and relict period (Hesp, 2002). However, as there are many environmental factors (e.g., wind strength and direction, storm frequency, beach width and migration, vegetation growth, sea level change) involved in the evolution of foredunes, some natural coastal foredunes may not go through all these three phases. For example, the foredune plains developed on a barrier coast (Swina Gate) at the southern Baltic Sea (as shown in Fig. 1) are characterized by well-preserved established sequences with a relatively stable accretion rate during the last several millenniums (Reimann et al., 2011).

Foredune dynamics is investigated usually within a framework of beach–dune interaction (e.g., Psuty, 1988, Hesp, 2002, Saye et al., 2005, Ollerhead et al., 2012). Cycles of sediment exchange between the foredune system and the beach, and between the beach and the nearshore submarine zone are recognized in this framework, within which processes involved in the beach–dune interaction and foredune evolution are studied at a range of spatial and temporal scales. On a short-term scale characterized by a temporal scale of seconds to days and a spatial scale of tens of meters, considerable efforts and progress have been made by field experiment (e.g., Hesp, 1983, Arens, 1994, Davidson-Arnott et al., 2005, Bauer et al., 2009) and modeling (e.g., Kriebel and Dean, 1985, Arens et al., 1995, Bauer and Davidson-Arnott, 2003, Jackson et al., 2011) to improve the knowledge about mechanisms that control the morphological development of coastal dunes. For the medium-term (temporal scale of months to decades and spatial scale of hundreds to thousands of meters) and long-term (temporal scale of centuries to millennia and spatial scale of kilometers to tens of kilometers) morphological evolution of coastal dune fields most of the existing studies are conceptual and descriptive (e.g., Hesp, 1988, Hesp, 2002, Sherman and Bauer, 1993, Orford et al., 2000, Aagaard et al., 2007, Anthony et al., 2010, Reimann et al., 2011, Ollerhead et al., 2012, Tamura, 2012, de Vries et al., 2012). Only recently numerical modeling became a tool for investigation of medium-to-long term coastal dune morphogenesis (e.g., Baas, 2002, Nield and Baas, 2008, Luna et al., 2011). Morphogenesis of some coastal dune types such as parabolic, nebkha and transgressive dunes has been studied numerically. However, there seems to be still a lack of a numerical model which is able to simulate a complete morphogenesis and evolution of coastal foredunes and foredune sequences on a medium-to-long term, probably due to the multi-scale characteristics of the physical and biological processes acting on a beach system and technical challenges in accurately describing the morphological response of the system to these processes.

In fact, although a coastal system is composed of two parts, i.e., subaqueous and subaerial, little effort has been done to combine these two parts into one integrated numerical model. Existing modeling studies on medium-to-long term coastal morphological evolution may underestimate the contribution of the subaerial coastal part to the whole system, especially the role of fordunes played in the transition between land and sea (Zhang et al., 2012). Thus, the main target of this study is to develop an integral model that is able to resolve the morphogenesis of coastal dune fields (especially the foredunes) on a medium-to-long term scale and on the other hand provide a reasonable estimate of the sediment budget for the subaqueous zone of the beach system.

The rest of the paper is organized as follows. After a brief explanation of the modeling principles on medium-to-long term coastal dune morphogenesis, details of a cellular automata approach for resolving subaerial processes, a process-based model for calculating alongshore and cross-shore subaqueous sediment budget and the coupling of these two models are described in Section 2. Application of this integrated model to a 1 km-long coastal section in the southern Baltic Sea is presented in Section 3. Discussion of the model with respect to its potentials as well as its shortcomings is given in Section 4, followed by conclusions in Section 5.

Section snippets

The model

Numerical models for aeolian sediment transport fall into two categories: (1) conventional models based on the approximation of Navier–Stokes equations to solve the aerodynamics and the resulting sediment transport on the dune surface (e.g., Jackson and Hunt, 1975, Weng et al., 1991, Van Boxel et al., 1999, Van Dijk et al., 1999, Herrmann et al., 2005, Luna et al., 2011); and (2) probabilistic models such as cellular automata (e.g., Werner, 1995, Baas, 2002, Zhang et al., 2010). Although the

Model application and result analysis

The integrated model is applied to a 1 km-long section of a barrier coast (Swina Gate, see Fig. 1) in the southern Baltic Sea for a 61-year (1951–2012) hindcast of its foredune development. The reason for choosing this particular coastal section for a numerical study is that: (1) this section represents a naturally accretionary coast characterized by well-preserved established foredune ridges (Fig. 1). It is more than 5 km away from an artificial pier (constructed in the 18th century) at the

Discussion

Development of a model fulfilling two basic requirements: (1) affordable computational expenses and (2) capture of the external key driving mechanisms and the internal nonlinear responses of the system, is a pre-requisite for reliable numerical studies (Zhang et al., 2013). These requirements serve as a base for the principles we follow in constructing an integrated model for investigation of medium-to-long term coastal foredune morphogenesis. Although the subaerial model is based on a cellular

Conclusions

Three major conclusions are drawn from the study. First, a successful application of the integrated model to the southern Baltic coast demonstrates the robustness and potential of a coupling between cellular automata approach and process-based profile-resolving models for a practical study of medium-to-long term coastal foredune morphology. Simulation results indicate that the morphogenesis of coastal foredunes is a result from a competition among wind-wave impacts, external sediment supply and

Acknowledgments

The historical wind and precipitation data (1951–2012) of the southern Baltic Sea were kindly provided by Dr. R. Weisse from Helmholtz-Zentrum Geesthacht, Germany. The simulations were carried out at MPI-IPP (Max-Plank-Institute for Plasma Physics) in Greifswald and Garching, Germany. We thank the Polish Maritime Office (Szczecin) for providing the valuable source data of high-resolution DEM and annual profile measurement from 2005 to 2012. Constructive comments from two anonymous reviewers and

References (66)

  • M.A. Fonstad

    Cellular automata in geomorphology

  • H.J. Herrmann et al.

    Calculation of the separation streamlines of barchans and transverse dunes

    Phys. A

    (2005)
  • P.A. Hesp

    Morphodynamics of incipient foredunes in N.S.W., Australia

  • P.A. Hesp

    Morphology, dynamics and internal stratification of some established foredunes in southeast Australia

  • P.A. Hesp

    Foredunes and blowouts: initiation, geomorphology and dynamics

    Geomorphology

    (2002)
  • G. Kocurek et al.

    Aeolian dune field self-organization — implications for the formation of simple versus complex dune — field patterns

    Geomorphology

    (2005)
  • D.L. Kriebel et al.

    Numerical simulation of time-dependent beach and dune erosion

    Coast. Eng.

    (1985)
  • T.A. Łabuz

    Present-day dune environment dynamics on the coast of the Swina Gate Sandbar (Polish West coast)

    Estuar. Coast. Shelf Sci.

    (2005)
  • M. Larson et al.

    Prediction of cross-shore sediment transport at different spatial and temporal scales

    Mar. Geol.

    (1995)
  • T. Reimann et al.

    Reconstruction of Holocene coastal foredune progradation using luminescence dating — an example from the Świna barrier (southern Baltic Sea, NW Poland)

    Geomorphology

    (2011)
  • S. Saye et al.

    Beach–dune morphological relationships and erosion/accretion: an investigation at five sites in England and Wales using LiDAR data

    Geomorphology

    (2005)
  • J.S. Schoonees et al.

    Evaluation of 10 cross-shore sediment transport/morphological models

    Coast. Eng.

    (1995)
  • T. Tamura

    Beach ridges and prograded beach deposits as palaeoenvironment records

    Earth Sci. Rev.

    (2012)
  • W.Y. Zhang et al.

    A multi-scale hybrid long-term morphodynamic model for wave-dominated coasts

    Geomorphology

    (2012)
  • W.Y. Zhang et al.

    A coupled modeling scheme for longshore sediment transport of wave-dominated coasts — a case study from the southern Baltic Sea

    Coast. Eng.

    (2013)
  • J. Zheng et al.

    Numerical models and intercomparisons of beach profile evolution

    Coast. Eng.

    (1997)
  • S.M. Arens

    Aeolian Processes in the Dutch Foredunes

    (1994)
  • S.M. Arens

    Transport rates and volume changes in a coastal foredune on a Dutch Wadden island

    J. Coast. Conserv.

    (1997)
  • S.M. Arens et al.

    Air flow over foredunes and implications for sand transport

    Earth Surf. Process. Landf.

    (1995)
  • R.A. Bagnold

    The Physics of Blown Sand and Desert Dunes

    (1954)
  • J.A. Battjes

    Surf similarity

  • S.R. Bishop et al.

    Modelling desert dune fields based on discrete dynamics

    Discret. Dyn. Nat. Soc.

    (2002)
  • J.T. Carley

    Validation and application of beach storm erosion models in Australia

  • Cited by (27)

    • Numerical simulations of wave climate in the Baltic Sea: a review

      2023, Oceanologia
      Citation Excerpt :

      Owing to a relatively mild wave climate in several semi-sheltered regions of the Baltic Sea and especially owing to difference in the periods of some types of vessel waves, long ship waves may serve as a fundamentally new forcing component and a source of substantial danger to some sections of sedimentary coasts (Soomere et al., 2003). Simulation of wave extremes is also often a part of the analysis of land-sea interaction and morphogenesis of coastal foredunes (Zhang et al., 2015). Properties of extreme wave fields determine several core parameters of beaches such as the closure depth and are thus the starting point of, e.g., express techniques for estimates of sand loss or gain in specific types of beaches (Kask et al., 2009; Soomere and Healy, 2011).

    • Coastal Dunes

      2022, Treatise on Geomorphology
    View all citing articles on Scopus
    View full text