Difference between revisions of "Dune development"
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| − | Dunes are found worldwide along sandy coasts. They protect settlements in the coastal zone against wave damage and flooding. However, they are not equally well developed everywhere. [[Definitions of coastal terms#Dissipative and reflective beaches|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]]. | + | |
| + | [[File: WorldDuneMap.jpg|thumb|right|350px|Fig. 1. World map of coastal zones where well-developed coastal dune belts occur. Adapted from Martinez and Psuty (2008)<ref>Martínez, M.L. and Psuty, N.P. 2008. Coastal Dunes: Ecology and conservation. Springer-Verlag, Berlin, Heidelberg, Germany</ref>.]] | ||
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| + | 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. [[Definitions of coastal terms#Dissipative and reflective beaches|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; [[Dynamics, threats and management of dunes]] provides an introduction to dune management; vegetation that promotes dune formation is described in [[Shore protection vegetation]]; erosion of the dune under storm conditions is dealt with in [[Dune erosion]]. | ||
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Dune formation is influenced by many factors, in particular: | Dune formation is influenced by many factors, in particular: | ||
* sediment supply | * sediment supply | ||
| − | * | + | * wave climate |
* wind regime | * wind regime | ||
| − | |||
| − | |||
* grainsize | * grainsize | ||
* aeolian sand transport | * aeolian sand transport | ||
| + | * storms | ||
| + | * beach morphology | ||
* vegetation | * vegetation | ||
| − | * overwash | + | * [[overwash]] |
These various factors will be discussed shortly in the following. | These various factors will be discussed shortly in the following. | ||
===Sediment supply=== | ===Sediment supply=== | ||
| − | Coastal dune development generally occurs on prograding or stable coasts and seldom on retreating | + | Coastal dune development generally occurs on prograding or stable coasts and seldom on coasts that are structurally retreating<ref>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</ref>. |
| − | Observations show that accretion rates in foredunes correspond reasonably well to volumes supplied to the beach over the long term<ref>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</ref>. | + | Observations show that accretion rates in foredunes correspond reasonably well to volumes supplied to the beach by onshore bar migration over the long term<ref>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</ref>. Sructural 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<ref>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</ref>. An updrift-located sandy river delta can constitute an important sediment source<ref>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</ref>. 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<ref>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</ref>. Progradation of [[Definitions of coastal terms#Dissipative and reflective beaches|dissipative coasts]] is strongly promoted by welding of [[nearshore sandbars]] to the beach<ref>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</ref>. |
| − | |||
| − | |||
| − | |||
===Wave climate=== | ===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 | + | Waves are the major agent for sediment exchange between beach and shoreface. Onshore or offshore transport can both occur, depending on the current wave climate, water level and beach state. 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]]). In cases where nearshore sandbars move onshore, these bars may weld to the beach, producing strong beach progradation<ref>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</ref> (see [[Nearshore sandbars]]), which is a major factor in post-storm beach and dune recovery. Maximum [[wave run-up]] is dominated by infragravity [[swash]], which transports sand ashore under average and medium energetic conditions<ref>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</ref> (see [[Infragravity waves]]). However, for optimum dune accretion wave run-up should not greatly reduce the width of the dry beach. |
===Wind regime=== | ===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<ref>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</ref>. The longshore wind component increases the fetch, i.e. the | + | 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<ref>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</ref>. The longshore wind component generally increases the fetch, i.e. the length of the wind path over the beach 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<ref> 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</ref>. |
===Grainsize=== | ===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. | + | 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 (see [[Coastal and marine sediments]]). An inverse correlation is generally observed between dune volume and the grain size of non-cohesive beach sediment. |
===Aeolian sand transport=== | ===Aeolian sand transport=== | ||
| − | Supply limitation is a major impediment | + | [[File:BackshoreGroundwaterUpwelling.jpg|thumb|right|250px|Fig. 2. Groundwater upwelling on the upper beach, near the foot of an eroding dune, Aquitaine coast, France.]] |
| − | 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<ref>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</ref>. 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]]. | + | |
| + | The magnitude of aeolian sand transport is approximately proportional to the third power of the wind speed<ref name=VR>Van Rijn, L.C. 2022. A fully predictive model for aeolian sand transport, part 3: Verification and application of model for natural beaches. Coastal Engineering 171, 104051</ref>. Supply limitation is a major impediment to aeolian sand transport on beaches<ref>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</ref>. 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<ref>Van Rijn, L.C. and Strypsteen, G. 2020. A fully predictive model for aeolian sand transport. Coastal Engineering 156, 103600</ref>. However, the influence of moisture becomes minor when wind velocities exceed 10 m/s. The presence of salt crusts, algae, clay, organic matter and calcareous materials also inhibits the uptake of sand from the beach; the presence of coarse materials (i.e. gravel, shells) can lead to an armour layer on top of the finer beach sand. An empirical formula for aeolian sand transport that takes these various factors into account has been developed by Van Rijn (2022)<ref name=VR/>. | ||
| + | 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<ref>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</ref>. 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 (Fig. 2), see [[Submarine groundwater discharge]]. | ||
===Storms=== | ===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. | + | 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. Moderate onshore winds (no strong raise of water levels and wave run-up) can accomplish fast after-storm beach recovery (weeks to months), by returning the eroded sediment from the lower beach and upper shoreface<ref>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</ref><ref>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</ref><ref>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</ref>. However, the much longer timescale of dune recovery (years to decades) entails high sensitivity of dunes to sequences of storms<ref>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</ref><ref name=P17>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</ref>. 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<ref name=P17/>. |
| − | + | ||
| + | Storms are also a determining factor for dune development. Embryo dunes cannot develop into a mature dune ridge if they are too close to the shoreline. This is the case when the erosion frequency is too high to allow dune restoration by aeolian sand transport. Moreover, if the width of the dry beach in front of the dune is small, aeolian transport will be small too. This means that dune development will occur only from a minimum distance from the shoreline, which depends on storm frequency and other site characteristics. | ||
| + | |||
| + | ===Beach morphology=== | ||
| + | A wide subaerial beach is a primary condition for dune formation. Gently sloping flat beaches are a characteristic of dissipative coasts. [[Definitions of coastal terms#Dissipative and reflective beaches|Dissipative beaches]] have approximately 60% higher long-term net aeolian sediment transport compared to reflective beaches<ref name=SH>Short, A.D. and Hesp, P.A. 1982. Wave, beach and dune interactions in southeastern Australia. Mar. Geol. 48 (3–4): 259–284</ref>. On reflective beaches, the presence of steep berms and beach faces disturb the wind flow causing a reduction of the wind velocity. Moreover, the beach width (distance from the high water line to the dune foot) is smaller for a steep beach than for a gently sloping beach; the wind fetch for sand uptake is thus also smaller. So there is less aeolian sediment transport from the beach to dunes, eventually resulting in small foredunes. 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 and greater wind fetch that lead to a greater aeolian sand transport potential and higher/wider foredunes <ref>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</ref>. 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<ref name=SH/>. | ||
===Vegetation=== | ===Vegetation=== | ||
| − | + | Shore vegetation contributes to shore protection in two complementary ways<ref name=F>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</ref><ref>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 </ref>: | |
# by decreasing wave runup, due to the frictional effect of stems and foliage; | # by decreasing wave runup, due to the frictional effect of stems and foliage; | ||
# by decreasing sand loss by wave backwash, due to the effect of plant roots on sand aggregation and fixation. | # 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 | + | These functions increase the retention of sand 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]]. The type of dune that develops depends on the type of vegetation. Marram grass (''Ammophila arenaria'') has very strong sand trapping and sand binding capacities and therefore stimulates the development of high voluminous foredunes with a steep slope. The species ''Panicum amarum'' (Bitter Panicum) and ''Uniola paniculata'' (Sea Oats) are good sand binders but have slow lateral spread; foredunes dunes vegetated with these species therefore tend to be steep and narrow<ref>Hacker, S.D., Jay, K.R., Cohn, N., Goldstein, E.B., Hovenga, P.A., Itzkin, M., Moore, L.J., Mostow, R.S., Mullins, E.V. and Ruggiero, P. 2019. Species-Specific Functional Morphology of Four US Atlantic Coast Dune Grasses: Biogeographic Implications for Dune Shape and Coastal Protection. Diversity 11, 82</ref>. Dune vegetation with the species ''Leymus arenarius'' and ''Leymus mollis'' (Lyme grass) generally has a lower density; these species therefore favour the development of wide low foredunes<ref name=P21>Pickart, A.J. 2021. Ammophila invasion ecology and dune restoration on the West Coast of North America. Diversity 13, 629</ref>. |
| + | |||
| + | [[File:NebkhaMarramGrass.jpg|thumb|250px|right|Fig. 3. Development of an embryo dune induced by sand trapping marram grass (''Ammophila arenaria'').]] | ||
| + | |||
| + | Vegetation is a major trigger for incipient dune formation. The development of a bare beach to a foredune depends on three stages: dune formation, dune growth and dune survival<ref name=M13>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</ref>. | ||
| + | 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<ref name=H2>Hesp, P. 2002. Foredunes and blowouts: initiation, geomorphology and dynamics. Geomorphology, 48, 245–268</ref>. Dune development begins with the establishment of perennial vegetation on the beach<ref>Maun, M.A. 1994. Adaptations Enhancing Survival and Establishment of Seedlings on Coastal Dune Systems. Vegetatio, 111, 59–70</ref>. 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<ref name=M13/>. They become established coastal foredunes if sand capture by the vegetation can go on, which requires a wide beach and sufficient distance from the shoreline<ref name=P17/>. 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<ref name=H2/>. 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<ref name=H2/>, illustrated in Fig. 4. | ||
| + | |||
| + | |||
| + | [[File:DuneBeltsGoeree2005.jpg|thumb|center|600px|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 have since formed on the accreted strand plain. Photo credit Rens Jacobs, Beeldbank Rijkswaterstaat.]] | ||
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| + | |||
| + | ===Dune development with fences=== | ||
| + | |||
| + | [[File:NileDeltaDuneFences.jpg|thumb|right|400px|Fig. 5. Dune development with sand fences for coastal protection (Egypt, 2022). Green Climate Fund project “Enhancing Climate Change Adaptation in the North Coast and Nile Delta Regions”.]] | ||
| + | |||
| + | Dune development can be stimulated artificially by installing sand-trapping fences that reduce the wind velocity and thus enhance sand deposition. Fences are generally considered a nature-based technique that mimics the action of vegetation. Fences are used for the development of dunes where they don't yet exist, for reinforcing or stabilizing existing dunes or for reducing inshore sand drift. The effectiveness of fences depends primarily on the porosity and height. A porosity of about 50% is commonly considered optimal for sand trapping. Smaller porosity produces deposition close to the fence, whereas higher porosity allows for sediment accretion further downwind<ref name=ES>Eichmanns, C. and Schüttrumpf, H. 2022. A Nature-Based Solution for Coastal Protection: Wind Tunnel Investigations on the Influence of Sand-Trapping Fences on Sediment Accretion. Front. Built Environ. 8: 878197</ref>. | ||
| + | |||
| + | Fences are usually constructed perpendicular to the prevailing onshore wind direction. With time, two sand deposits emerge on the windward and leeward sides of the fences, windward deposits being smaller than the leeward ones. Sediment accretion increases shortly after installation and decreases slightly or stagnates towards an equilibrium as time passes. The sediment deposit can grow about as high as the fence height. Initially, sediment accretion occurs mainly horizontally and vertically (with fastest growth rates for small-porosity fences), while horizontal dune growth dominates later on if a sufficient sediment supply is provided<ref>Ning, Q., Li, B. and Ellis, J.T. 2020. Fence Height Control on Sand Trapping. Aeolian Res. 46, 100617</ref>. | ||
===[[Overwash]]=== | ===[[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. | + | 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. Overwash deposits can be overgrown with cord grass (''Spartina patens'') that consolidates the overwash deposit but is not effective for dune rebuilding<ref>Stallins, J.A. 2005. Stability domains in barrier island dune systems. Ecological Complexity 2: 410–430</ref>. |
| − | |||
| − | ==Dune | + | ==Dune-beach dynamics== |
| − | + | Pellon et al. (2020)<ref>Pellon,E., de Almeida, L.R., Gonzalez, M. and Medina, R. 2020. Relationship between foredune profile morphology and aeolian and marine dynamics: A conceptual model. Geomorphology 351: 106984</ref> proposed a conceptual model of dune-beach dynamics based on hydrogeomorphological observations of several beaches along the northern Spanish coast. They present evidence that dune development for stable beaches (no strong structural erosion or accretion) results from a dynamic feedback involving storm wave runup and beach width. When the beach width is smaller than the long-term equilibrium beach width, the dune will recede as a result of erosion by storm wave runup with a recurrence time smaller than the dune recovery period. However, when the dune recedes, the width of the dry beach increases. Hence, the aeolian transport increases, thus decreasing the dune recovery time, while the recurrence time of storm wave erosion of the dune increases. The dune foot position will thus advance and eventually oscillate around an equilibrium position due to this negative feedback mechanism. This conceptual model also explains why energetic beaches often have a greater dune volume than beaches in a low-wave environment. On energetic beaches, the distance between shoreline and dune foot is large, implying a wide dry beach and thus strong aeolian sand transport. Dunes on energetic beaches therefore grow faster during periods between storm surges than dunes on low-energy beaches. | |
| − | |||
==Related articles== | ==Related articles== | ||
:[[Sand Dunes in Europe]] | :[[Sand Dunes in Europe]] | ||
| + | :[[Dynamics, threats and management of dunes]] | ||
:[[Dune erosion]] | :[[Dune erosion]] | ||
:[[Shore protection vegetation]] | :[[Shore protection vegetation]] | ||
Latest revision as of 19:43, 3 August 2024
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; Dynamics, threats and management of dunes provides an introduction to dune management; vegetation that promotes dune formation is described in Shore protection vegetation; erosion of the dune under storm conditions is dealt with in Dune erosion.
Contents
Processes that influence dune formation
Dune formation is influenced by many factors, in particular:
- sediment supply
- wave climate
- wind regime
- grainsize
- aeolian sand transport
- storms
- beach morphology
- 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 coasts that are structurally retreating[2]. Observations show that accretion rates in foredunes correspond reasonably well to volumes supplied to the beach by onshore bar migration over the long term[3]. Sructural 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].
Wave climate
Waves are the major agent for sediment exchange between beach and shoreface. Onshore or offshore transport can both occur, depending on the current wave climate, water level and beach state. 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). In cases where nearshore sandbars move onshore, these bars may weld to the beach, producing strong beach progradation[8] (see Nearshore sandbars), which is a major factor in post-storm beach and dune recovery. Maximum wave run-up is dominated by infragravity swash, which transports sand ashore under average and medium energetic conditions[9] (see Infragravity waves). 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[10]. The longshore wind component generally increases the fetch, i.e. the length of the wind path over the beach 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[11].
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 (see Coastal and marine sediments). An inverse correlation is generally observed between dune volume and the grain size of non-cohesive beach sediment.
Aeolian sand transport
The magnitude of aeolian sand transport is approximately proportional to the third power of the wind speed[12]. Supply limitation is a major impediment to aeolian sand transport on beaches[13]. 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[14]. However, the influence of moisture becomes minor when wind velocities exceed 10 m/s. The presence of salt crusts, algae, clay, organic matter and calcareous materials also inhibits the uptake of sand from the beach; the presence of coarse materials (i.e. gravel, shells) can lead to an armour layer on top of the finer beach sand. An empirical formula for aeolian sand transport that takes these various factors into account has been developed by Van Rijn (2022)[12]. 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[15]. 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 (Fig. 2), 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. Moderate onshore winds (no strong raise of water levels and wave run-up) can accomplish fast after-storm beach recovery (weeks to months), by returning the eroded sediment from the lower beach and upper shoreface[16][17][18]. However, the much longer timescale of dune recovery (years to decades) entails high sensitivity of dunes to sequences of storms[19][20]. 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[20].
Storms are also a determining factor for dune development. Embryo dunes cannot develop into a mature dune ridge if they are too close to the shoreline. This is the case when the erosion frequency is too high to allow dune restoration by aeolian sand transport. Moreover, if the width of the dry beach in front of the dune is small, aeolian transport will be small too. This means that dune development will occur only from a minimum distance from the shoreline, which depends on storm frequency and other site characteristics.
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[21]. On reflective beaches, the presence of steep berms and beach faces disturb the wind flow causing a reduction of the wind velocity. Moreover, the beach width (distance from the high water line to the dune foot) is smaller for a steep beach than for a gently sloping beach; the wind fetch for sand uptake is thus also smaller. So there is less aeolian sediment transport from the beach to dunes, eventually resulting in small foredunes. 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 and greater wind fetch that lead to a greater aeolian sand transport potential and higher/wider foredunes [22]. 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[21].
Vegetation
Shore vegetation contributes to shore protection in two complementary ways[23][24]:
- by decreasing wave runup, due to the frictional effect of stems and foliage;
- 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 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. The type of dune that develops depends on the type of vegetation. Marram grass (Ammophila arenaria) has very strong sand trapping and sand binding capacities and therefore stimulates the development of high voluminous foredunes with a steep slope. The species Panicum amarum (Bitter Panicum) and Uniola paniculata (Sea Oats) are good sand binders but have slow lateral spread; foredunes dunes vegetated with these species therefore tend to be steep and narrow[25]. Dune vegetation with the species Leymus arenarius and Leymus mollis (Lyme grass) generally has a lower density; these species therefore favour the development of wide low foredunes[26].
Vegetation is a major trigger for incipient dune formation. The development of a bare beach to a foredune depends on three stages: dune formation, dune growth and dune survival[27]. 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[28]. Dune development begins with the establishment of perennial vegetation on the beach[29]. 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[27]. They become established coastal foredunes if sand capture by the vegetation can go on, which requires a wide beach and sufficient distance from the shoreline[20]. 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[28]. 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[28], illustrated in Fig. 4.
Dune development with fences
Dune development can be stimulated artificially by installing sand-trapping fences that reduce the wind velocity and thus enhance sand deposition. Fences are generally considered a nature-based technique that mimics the action of vegetation. Fences are used for the development of dunes where they don't yet exist, for reinforcing or stabilizing existing dunes or for reducing inshore sand drift. The effectiveness of fences depends primarily on the porosity and height. A porosity of about 50% is commonly considered optimal for sand trapping. Smaller porosity produces deposition close to the fence, whereas higher porosity allows for sediment accretion further downwind[30].
Fences are usually constructed perpendicular to the prevailing onshore wind direction. With time, two sand deposits emerge on the windward and leeward sides of the fences, windward deposits being smaller than the leeward ones. Sediment accretion increases shortly after installation and decreases slightly or stagnates towards an equilibrium as time passes. The sediment deposit can grow about as high as the fence height. Initially, sediment accretion occurs mainly horizontally and vertically (with fastest growth rates for small-porosity fences), while horizontal dune growth dominates later on if a sufficient sediment supply is provided[31].
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. Overwash deposits can be overgrown with cord grass (Spartina patens) that consolidates the overwash deposit but is not effective for dune rebuilding[32].
Dune-beach dynamics
Pellon et al. (2020)[33] proposed a conceptual model of dune-beach dynamics based on hydrogeomorphological observations of several beaches along the northern Spanish coast. They present evidence that dune development for stable beaches (no strong structural erosion or accretion) results from a dynamic feedback involving storm wave runup and beach width. When the beach width is smaller than the long-term equilibrium beach width, the dune will recede as a result of erosion by storm wave runup with a recurrence time smaller than the dune recovery period. However, when the dune recedes, the width of the dry beach increases. Hence, the aeolian transport increases, thus decreasing the dune recovery time, while the recurrence time of storm wave erosion of the dune increases. The dune foot position will thus advance and eventually oscillate around an equilibrium position due to this negative feedback mechanism. This conceptual model also explains why energetic beaches often have a greater dune volume than beaches in a low-wave environment. On energetic beaches, the distance between shoreline and dune foot is large, implying a wide dry beach and thus strong aeolian sand transport. Dunes on energetic beaches therefore grow faster during periods between storm surges than dunes on low-energy beaches.
Related articles
- Sand Dunes in Europe
- Dynamics, threats and management of dunes
- Dune erosion
- Shore protection vegetation
- Shoreface profile
- Nearshore sandbars
- Infragravity waves
References
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- ↑ Eichmanns, C. and Schüttrumpf, H. 2022. A Nature-Based Solution for Coastal Protection: Wind Tunnel Investigations on the Influence of Sand-Trapping Fences on Sediment Accretion. Front. Built Environ. 8: 878197
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