Shifting Dunes...
Aeolian transport, as opposed to tidal, wave, or current activity, is what causes coastal dunes to form, which sets them apart from most other constructional coastal landforms. Coastal dunes are ridges that develop at the back of a beach. Wind speed, sediment properties, beach shape, moisture content, and the degree of roughness of any present materials (such as driftwood and plants) all affect how quickly aeolian transportation begins. Coastal dunes frequently occur as a result of sediment migration into the back beach environment.
Transport of Sand by Wind (Aeolian Transport)
There are three forces that determine if, and how, an individual grain of sediment will be transported by wind (Figure 1). Gravity acts to keep the grain at the ground surface (FG), there is a drag component that moves the gain along the ground surface (FD) with the wind direction and a lift force (or shear stress: FL) produced by pressure differences created by the movement of wind over the surface of a stationary grain. This lift force is the result of a relatively high velocity flow across the top of the particle, and lower velocities across the middle and lower parts of the sand grain. A stationary grain will begin to first vibrate, and then move when the shear stress at the grain surface exceeds a specific critical value (i.e., the vertical lift exceeds gravitational force, FL>FG) (Figure 1). This value is known as the critical shear stress, or the threshold velocity, and results in the grain being lifted vertically into the air.
(Figure 1)
Transport by wind takes place almost entirely within 0.5 m of the ground surface with nearly 90% of this movement within 2.5 cm of the surface. Once the grain is lifted it is transported downwind in a parabolic trajectory. The speed and distance that the grain will be transported are determined by grain size, shape, and density in relationship to the drag component (FD). For fine-grained sand, the threshold wind velocity for sand is approximately 10 km/hr (about 5 meters per second at 1 m above the surface).
Once the threshold value is exceeded, sediment can be transported in one, or a combination of, four main ways: traction or creep, saltation, reputation, and/or suspension (Figure 2).
Traction (creep) load: The traction load refers to particles that are too large and/or dense to be entrained by the lift component (FL) or are just pushed along by other grains colliding with the surface grains and are "rolled" along the surface due to the traction of the drag force (FD).
Saltation: Saltation refers to the process when the sand grains are lifted into the air and suspended for a short distance before falling back to the surface with a parabolic trajectory. The height and time that the grains remain suspended is determined by grain size, shape, and density in relation to the lift and drag force, and the turbulence and velocity of the wind flow.
Reputation: During saltation, grains in motion may collide with particles on the ground and set them in motion at wind velocities lower than those required to move them by wind alone (i.e., the impact force reduces the lift force required to entrain sediment). When this occurs, it is termed reptation.
Suspension: A very small amount of dust (clay and silt-sized particles) and the finest sand particles may be carried in the air for some distance downwind without following the typical bouncing motion of saltating or reptating grains. Silt and clay particles may be carried to much greater heights, but this sediment, if present, is usually transported beyond the area of sand dunes.
(Figure 2)
Regardless of the mechanism, as sediment is transported downwind it produces bedforms known as ripples and dunes. Both aeolian ripples and dunes display an asymmetrical form comprising a longer, lower gradient (1°–8°) upwind slope known as the stoss slope, and a shorter and steeper downwind side or lee slope (32–36°) (Figure 2). The difference is a matter of size: ripples are small ridges of sand with a height (amplitude) typically less than 4 cm and wavelengths (crest to crest) less than 60 cm, while dunes have the same morphology (or form) as ripples but they are larger scale structures (photo 1). For both ripples and dunes, sediment is generally moved along the stoss to the crest, and when the net accretion of material reaches a critical state of overload (brink point) sediment avalanches down the lee of the dune (slip face). In classic transverse dunes — dunes formed at right angles to the dominant wind, and no vegetation present — this produces tabular cross-strata with a uniform dip direction parallel to the dominant wind direction as the dune migrates downwind (Photos 1 and 2).
(Photo 1)
(Photo 2)
The availability of sediment for dune building is a function of sediment transport in the nearshore environment. The amount of sediment supplied from the beach to the back-beach environment is dependent on wind and wave power, tidal range, and beach type. In general, sediment is transported landward via wave motion and swash to the foreshore, beach face, and backshore environments during periods of beach accretion. During low tides, this sediment dries and is susceptible to aeolian (wind-blown) transportation. In addition to an abundant sediment supply, there are numerous factors that control the availability of sediment to the backshore environment, the main ones being wind velocity and direction, moisture content, vegetation, and beach morphology.
The moisture content of the sand controls the cohesion between individual grains. An increase in moisture content will significantly increase the cohesion of the sediment and reduce the capacity of the wind to initiate transportation. Accordingly, deflation (wind erosion) normally only occurs down to the level of damp sand, some vertical distance above the water table. As the water table in permeable sand forms a horizontal or very gently sloping surface, deflation basins within dune fields, and the sand plains at the rear of transgressive dunes or between dune ridges, are usually relatively flat.
Vegetation is a significant controlling factor influencing the formation and morphology of coastal dunes. The coastal zone can be a very hostile environment for flora with high sedimentation rates, low nutrients, high salt spray, susceptibility to storm erosion and human recreation, and agricultural activity. Only a small group of plants are capable of colonising sand dunes. Nevertheless, in contrast to most desert dunes, vegetation usually plays an important role both in the initial formation and subsequent development of coastal dunes. In the coastal environment, plants are the most common roughness element that can cause a reduction in the wind velocity and reduce the capacity of the wind to maintain aeolian transport, and significantly increase the potential for trapping sand. For example, wind velocities decline rapidly close to the ground surface: if the ground is vegetated, the zone with very low, or zero, wind velocities increase in height from ~0.4 mm to ~10 mm above the ground surface, increasing the potential for sediment trapping and dune accretion. The growth of the pioneer species is stimulated by sand deposition, and plant species type is important in determining the morphology or form of the dune. For this reason, artificial plantings of dune grass are a common method to encourage dune formation and growth.
Beach morphology also plays an important role in the supply and transportation of aeolian sediment into the back beach environment and dune formation. Changes in the slope and morphology of the beach face and back-shore profiles can increase or decrease wind velocity, as well as produce a roughness element, that can result in the deceleration of wind velocity and create turbulence resulting in variations in the rate of deflation, transport, and accumulation of sediment. For example, dissipative beaches, with a wider foreshore and low gradient, provide less resistance to wind flow, and are more conducive to aeolian sand transport than the steeper reflective beaches. In addition, dissipative beaches generally comprise finer sand which requires a lower threshold velocity to entrain sediment. In contrast, reflective beaches are generally composed of coarse grained material requiring higher threshold velocity to overcome the gravitational force and static friction. The steeper profile and the more irregular nature of the reflective beach also results in a zone of reduced wind speeds at the rear of the foreshore which is less conducive to aeolian transport.
Reference: Coastal Dunes: Aeolian Transport By: Craig R. Sloss, Patrick Hesp & Michael Shepherd © 2012 Nature Education Citation: Sloss, C. R., Hesp, P. & Shepherd, M. (2012) Coastal Dunes: Aeolian Transport. Nature Education Knowledge 3(10):21
Whilst out looking for this cache, observe the dunes around you...
