Dune development

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Fig. 1. World map of coastal zones where well-developed coastal dune belts occur. Adapted from Martinez and Psuty (2008 [1]).

Dunes are found worldwide along sandy coasts, see Fig. 1. They protect settlements in the coastal zone against wave damage and flooding. However, they are not equally well developed everywhere. Dissipative coasts that are subject to energetic waves provide the best conditions for the development of a strong dune belt. This article provides an introduction to the processes involved in dune formation and the conditions that promote dune formation. Several related topics are covered in other articles: Sand Dunes in Europe gives a description of different dune zones; vegetation that promotes dune formation is described in Shore protection vegetation; erosion of the dune under storm conditions is dealt with in Dune erosion.


Processes that influence dune formation

Dune formation is influenced by many factors, in particular:

  • sediment supply
  • beach morphology
  • wind regime
  • wave climate
  • storms
  • grainsize
  • aeolian sand transport
  • vegetation
  • overwash

These various factors will be discussed shortly in the following.

Sediment supply

Coastal dune development generally occurs on prograding or stable coasts and seldom on retreating coasts[2]. Observations show that accretion rates in foredunes correspond reasonably well to volumes supplied to the beach over the long term[3]. Ongoing shoreline retreat is generally linked to divergence of longshore sand transport (see Natural causes of coastal erosion), whereas convergence of longshore sand transport is considered a major mechanism for beach stabilization or accretion[4]. An updrift-located sandy river delta can constitute an important sediment source[5]. Another source of sediment supply is provided by disequilibrium shoreface morphology caused by the delay of shoreface adaptation to evolving hydrometeorological conditions including sea level change[6]. Progradation of dissipative coasts is strongly promoted by welding of nearshore sandbars to the beach[7].

Beach morphology

A wide subaerial beach is a primary condition for dune formation. Gently sloping flat beaches are a characteristic of dissipative coasts. Dissipative beaches have approximately 60% higher long-term net aeolian sediment transport compared to reflective beaches[8]. On reflective beaches, the presence of steep berms and beach faces disturb the wind flow such that they act to reduce the wind velocity and consequently there is less aeolian sediment transport from the beach to dunes, resulting in small foredunes in the long term. In contrast, dissipative beaches display a low gradient, flat to slightly concave sub-aerial beach morphology, no berms, and a wider backshore, providing less reduction in wind velocity across the beach and backshore that leads to a greater aeolian sand transport potential and higher/wider foredunes [9]. Dissipative beaches are frequently characterized by large-scale transgressive dune sheets; intermediate beaches, by a trend from large-scale parabolic dune systems (high-wave energy) to small-scale blowouts (low-wave energy); and reflective beaches by minimal dune development[8].

Wave climate

Waves are the major agent for sediment exchange between beach and shoreface. Onshore or offshore transport can both occur depending on wave climate, water level and beach state at any given time. Whether onshore or offshore transport dominates on a long time scale varies widely along most coasts. However, stable or accreting wide beaches are often found at fine-sandy high-energy coasts, which are typically dissipative coasts. Although such coasts may suffer strong erosion during storm events, fast recovery is often observed (see Dune erosion). Wave run-up is dominated by infragravity swash, which transports sand ashore under average or even energetic conditions[10] (see Infragravity waves). In cases where nearshore sandbars move onshore, these bars may weld to the beach, producing strong beach progradation[11] (see Nearshore sandbars). However, for optimum dune accretion wave run-up should not greatly reduce the width of the dry beach.

Wind regime

Coastal dunes grow by inland-directed aeolian sand transport. The growth rate depends on the strength of the onshore component of the wind vector. The highest sediment flux to the dune occurs for high-speed winds with a moderate shore-oblique incidence angle[12]. The longshore wind component increases the fetch, i.e. the distance it takes for loading the aeolian transport with saltating sand grains. Saltation is the bouncing of sand grains along the surface where the winds are capable to mobilize and transport particles over short distances. The optimum fetch does not only depend on wind strength, but also on sand grainsize, sand moisture and other features such as crusts and shells[13].

Grainsize

Fine sand is more easily transported by the wind than coarse sand and therefore contributes more to dune accretion. There is a strong relationship between grain size and beach morphology. Fine sand (average grain size less than about 0.3 mm) is typical of dissipative beaches, while coarser sand and gravel is characteristic of reflective beaches. Dune volume is inversely correlated with the grain size of non-cohesive beach sediment.

Fig. 2. Backshore groundwater upwelling, near the foot of the eroded dune face, Aquitaine coast, France.

Aeolian sand transport

Supply limitation is a major impediment for aeolian sand transport on beaches [14]. Supply limitation is caused by cohesion and adhesion effects that inhibit the mobilization of sand grains by wind stress. Moisture is a major factor; even low levels of moisture can effectively reduce the transport rate of dry sand[15]. The presence of salt crusts, algae, clay, organic matter and calcareous materials also plays a role. Only a dry beach allows the aeolian sediment transport to reach its maximum value. Strong dune accretion therefore requires a dry beach of sufficient width. Moisture can be due to precipitation, to inundation by waves, or to water-table effects[16]. Tidal inundation and wave run-up limit the width of the dry beach. In cases of steep coastal topography, beach sand can be wetted by a high water table and groundwater exfiltration, see Submarine groundwater discharge.

Storms

Storm surge levels that reach to the dune foot can cause severe dune erosion, see Dune erosion. During storm surges with high waves and elevated water levels, wave bores and swash collide with the dune, and the dune responds by slumping or scarping. The removed sand is deposited on the lower beach or moved to the upper shoreface. The magnitude of dune erosion during a storm event increases with the length of exposure of the dune face to incoming waves. The height of the dune foot, i.e. the width and slope of the backshore, are important beach parameters for dune vulnerability to erosion. After-storm onshore winds that induce only a moderate raise of water levels and wave run-up promote fast beach recovery (weeks to months), by returning the eroded sediment from the lower beach and upper shoreface[17][18][19]. However, the much longer timescale of dune recovery (years to decades) entails high sensitivity of dunes to sequences of storms[20][21]. Storms are also a determining factor for dune development. Embryo dunes are washed away by frequent heavy winter storms and are therefore unable to grow into a new row of dunes. A high-intensity storm can set back dune development for many years. Overall, net dune growth depends on the balance between summer growth and winter erosion[21].

Fig. 3. Development of an embryo dune induced by sand trapping marram grass (Ammophila arenaria).

Vegetation

Vegetation is a major trigger for incipient dune formation (next section). Besides, shore vegetation contributes to shore protection in two complementary ways[22][23]):

  1. by decreasing wave runup, due to the frictional effect of stems and foliage;
  2. by decreasing sand loss by wave backwash, due to the effect of plant roots on sand aggregation and fixation.

These functions increase the retention of sand volume of the upper beach and backshore during storms and high water levels. This sand also nourishes further aeolian inshore transport and increases the sand volume of the dunes behind. The suitability of different beach vegetation species for different coastal environments and their dune building capacity is discussed in Shore protection vegetation.

Overwash

Storm impacts that result in dune overwash and inundation are highly detrimental to the dune system. Overwash sediments are not returned to the active coastal zone and thus not available for dune recovery.


Dune development process

The development of a bare beach to a foredune depends on three stages: dune formation, dune growth and dune survival[24]. Sand grains are lifted from the beach by strong winds and carried until the wind velocity decreases below the transport threshold due to topographic obstacles landward of the driftline such as tidal litter, driftwood, or clumps of vegetation[25]. Dune development begins with the establishment of perennial vegetation on the beach[26]. The vegetation traps and stabilizes the sand, preventing it from being blown away, which results in a small embryo dune (also known as an incipient foredune or nebkha dune, Fig. 3), see Shore protection vegetation. Embryo dunes are often ephemeral, depending on the frequency of swash inundation, storm wave erosion, and overwash[24]. They become established coastal foredunes if sand capture by the vegetation can go on, which requires a sufficiently wide beach[21]. Volume increase of the embryo dunes affects the wind flow pattern and creates preferential zones of sand erosion and deposition. Over time, a dune ridge parallel to the shore may form by coalescence of embryo dunes, turning the previous foredune into a secondary foredune landward of the new primary foredune[25]. These secondary foredunes, being isolated from deposition and accretion from nearshore processes, are characterized by the presence of successive and mature plant communities. Secondary dunes are generally stabilized and composed of multiple parallel and vegetated dune ridges sequenced by dune slacks[25].

Fig. 4. Following the construction of the Rhine discharge sluices ('Haringvliet sluices') in 1970, a strong progradation occurred at the adjacent coast of Goeree. The image from 2005 shows new dune ridges that formed on the accreted strand plain. Photo credit Rens Jacobs, Beeldbank Rijkswaterstaat.


Related articles

Sand Dunes in Europe
Dune erosion
Shore protection vegetation
Shoreface profile
Nearshore sandbars
Infragravity waves


References

  1. ↑ Martínez, M.L. and Psuty, N.P. 2008. Coastal Dunes: Ecology and conservation. Springer-Verlag, Berlin, Heidelberg, Germany
  2. ↑ Costas, S., Bon de Sousa, L., Kombiadou, K., Ferreira, O. and Plomaritis, T.A. 2020. Exploring foredune growth capacity in a coarse sandy beach. Geomorphology. 371, 107435
  3. ↑ Aagaard, T., Davidson-Arnott, R., Greenwood, B. and Nielsen, B. 2004. Sediment supply from shoreface to dunes: linking sediment transport measurements and long-term morphological evolution. Geomorphology 60: 205–224
  4. ↑ Fruergaard, M., Kirkegaard, L., Oestergaard, A.T., Murray, A.S. and Andersen, T.J. 2019 Dune ridge progradation resulting from updrift coastal reconfiguration and increased littoral drift. Geomorphology 330: 69–80
  5. ↑ Castelle, B., Guillot, B., Marieu, V., Chaumillon, E., Hanquiez, V., Bujan, S. and Poppeschi, C. 2018. Spatial and temporal patterns of shoreline change of a 280-km long high-energy disrupted sandy coast from 1950 to 2014: SW France. Estuar. Coast. Shelf Sci. 200: 212–223
  6. ↑ Kinsela, M.A., Daleaya, M.J.A. and Cowell, P.J. 2016. Origins of Holocene coastal strandplains in Southeast Australia: Shoreface sand supply driven by disequilibrium morphology. Marine Geology 374: 14–30
  7. ↑ Brodie, K.L., Palmsten, M.L. and Spore, N.J. 2017. Coastal foredune evolution, Part 1: environmental factors and forcing processes affecting morphological evolution. In: US Army Corps of Engineers Coastal and Hydraulics Engineering Technical Note, pp. 1–10
  8. ↑ 8.0 8.1 Short, A.D. and Hesp, P.A. 1982. Wave, beach and dune interactions in southeastern Australia. Mar. Geol. 48 (3–4): 259–284
  9. ↑ Moulton, M.A.B., Hesp, P.A., Miot de Silva, G., Keane, R. and Fernandez, G.B. 2021. Surfzone-beach-dune interactions along a variable low wave energy dissipative beach. Marine Geology 435, 106438
  10. ↑ Cohn, N., Ruggiero, P., Garcia-Medina, G., Anderson, D., Sefarin, K.A. and Biel, R. 2019. Environmental and morphologic controls on wave-induced dune response. Geomorphology 329: 108–128
  11. ↑ Aagaard, T., Davidson-Arnott, R., Greenwood, B. and Nielsen, J. 2004. Sediment supply from shoreface to dunes: Linking sediment transport measurements and long-term morphological evolution. Geomorphology 60(1): 205–224
  12. ↑ Bauer, B.O. and Davidson-Arnott, R.G.D. 2002. A general framework for modeling sediment supply to coastal dunes including wind angle, beach geometry, and fetch effects. Geomorphology 49: 89–108
  13. ↑ Sherman, D., Ellis, J.T., Li, B., Farrell, E.J., Maia, P. and Granja, H.M. 2013. Recalibrating aeolian sand transport models. Earth Surf. Landforms 38, 169–178
  14. ↑ de Vries, S., van Thiel de Vries, J., van Rijn, L.C., Arens, S. and Ranasinghe, R. 2014. Aeolian sediment transport in supply limited situations. Aeolian Research 12: 75–85
  15. ↑ Van Rijn, L.C. and Strypsteen, G. 2020. A fully predictive model for aeolian sand transport. Coastal Engineering 156, 103600
  16. ↑ Davidson-Arnott, R.G.D., MacQuarrie, K. and Aagaard, T. 2005. The effect of wind gusts, moisture content and fetch length on sand transport on a beach. Geomorphology 68: 115–129
  17. ↑ Morton, R.A., Paine, J.G. and Gibeaut, J.C. 1994. Stages and durations of poststorm beach recovery, southeastern Texas coast. Journal of Coastal Research 10: 884 908
  18. ↑ Phillips, M.S., Harley, M.D., Turner, I.L., Splinter, K.D. and Cox , R.J. 2017. Shoreline recovery on wave-dominated sandy coastlines: the role of sandbar morphodynamics and nearshore wave parameters. Marine Geology 385: 146–159
  19. ↑ Brooks, S.M., Spencer, T. and Christie, E.K. 2017. Storm impacts and shoreline recovery: Mechanisms and controls in the southern North Sea. Geomorphology 283: 48–60
  20. ↑ Houser, C., Wernette, P., Rentschlar, E., Jones, H., Hammond, B. and Trimble, S. 2015. Post-storm beach and dune recovery: Implications for barrier island resilience. Geomorphology 234: 54–63
  21. ↑ 21.0 21.1 21.2 van Puijenbroek, M.E., Limpens, J., de Groot, A.V., Riksen, M.J., Gleichman, M., Slim, P.A., van Dobben, H.F. and Berendse, F. 2017. Embryo dune development drivers: beach morphology, growing season precipitation, and storms. Earth Surf. Process. Landf. 4144
  22. ↑ Feagin, R. A., J. Figlus, J. C. Zinnert, J. Sigren, M. L. Martínez, R. Silva, W. K. Smith, D. Cox, D. R. Young, and G. Carter. 2015. Going with the flow or against the grain? The promise of vegetation for protection beaches, dunes, and barrier islands from erosion. Frontiers in Ecology and the Environment 13: 203–210
  23. ↑ Bryant, D.B., Anderson-Bryant, M., Sharp, J.A., Bell, G.L. and Moore, C. 2019. The response of vegetated dunes to wave attack. Coastal Engineering 152, 103506
  24. ↑ 24.0 24.1 Montreuil, A.-L., Bullard, J.E., Chandler, J.H. and Millett, J. 2013. Decadal and seasonal development of embryo dunes on an accreting macrotidal beach: North Lincolnshire, UK. Earth Surface Processes and Landforms 38: 1851–1868
  25. ↑ 25.0 25.1 25.2 Hesp, P. 2002. Foredunes and blowouts: initiation, geomorphology and dynamics. Geomorphology, 48, 245–268
  26. ↑ Maun, M.A. 1994. Adaptations Enhancing Survival and Establishment of Seedlings on Coastal Dune Systems. Vegetatio, 111, 59–70


The main author of this article is Job Dronkers
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