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.
Processes that influence dune formation
Dune formation is influenced by many factors, in particular:
- sediment supply
- beach morphology
- wind regime
- wave climate
- aeolian sand transport
These various factors will be discussed shortly in the following.
Coastal dune development generally occurs on prograding or stable coasts and seldom on coasts that are retreating . Observations show that accretion rates in foredunes correspond reasonably well to volumes supplied to the beach by onshore bar migration over the long term. 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. An updrift-located sandy river delta can constitute an important sediment source. 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. Progradation of dissipative coasts is strongly promoted by welding of nearshore sandbars to the beach.
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. On reflective beaches, the presence of steep berms and beach faces disturb the wind flow causing a reduction of the wind velocity. 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 . 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.
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 (see Infragravity waves). In cases where nearshore sandbars move onshore, these bars may weld to the beach, producing strong beach progradation (see Nearshore sandbars). However, for optimum dune accretion wave run-up should not greatly reduce the width of the dry beach.
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. The longshore wind component generally 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.
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. 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. Supply limitation is a major impediment for aeolian sand transport on beaches. 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. 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; substantial amounts of coarse materials (i.e. gravel, shells) may lead to beach armouring. An empirical formula for aeolian sand transport that takes these various factors into account has been developed by Van Rijn (2022). 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. 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.
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. However, the much longer timescale of dune recovery (years to decades) entails high sensitivity of dunes to sequences of storms.
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.
- 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 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.
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.
Dune development process
The development of a bare beach to a foredune depends on three stages: dune formation, dune growth and dune survival. 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. Dune development begins with the establishment of perennial vegetation on the beach. 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. They become established coastal foredunes if sand capture by the vegetation can go on, which requires a sufficiently wide beach. 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. 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.
- Sand Dunes in Europe
- Dynamics, threats and management of dunes
- Dune erosion
- Shore protection vegetation
- Shoreface profile
- Nearshore sandbars
- Infragravity waves
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