Houser and Ellis (2013) - beach and dune interaction Flashcards
Ultimately, what does dune development depend on?
The exchange of sediment between the beach and backshore by wind, which varies with the speed and direction of the wind, tidal range, the availability and characteristics of sediment, and the morpho-dynamic state of the beach and nearshore.
Differences in the rate of dune formation and morphology reflect…
differences in the wind regime, wave climate, temperature and precipitation, littoral sediment supply, sediment size and mineralogy, and vegetation type and density.
What does coastal dunes require to develop?
An available supply of beach sediment, a sufficient fetch for the transport of sediment from the beachface to the backshore, and vegetation to capture the sediment.
Intervening factors that may modulate sand transport by wind include…
the intrinsic properties of beach–dune interfaces, such as microscale topography, vegetation cover, moisture content, and extrinsic factors, such as surrounding morphology and storm wave impact on the upper beach and dune front.
Long-term patterns of dune development are dependent on…
sediment supply, which is in turn dependent on the availability of sediment from alongshore, and offshore sources, and sufficient time for dune recovery following storm activity.
Much of the interaction between beach and dune occurs on…
the backshore, a transitional area between the dynamic wave- and tide-dominated surf and swash zones, and the dunes built through aeolian transport.
Transport of sediment on the backshore is dependent on…
the ability of onshore winds to entrain and transport more sediment from the beach to the dune than is lost from the dune during storms that erode the dune.
Wind blowing over a surface leads to the formation of…
a boundary layer, the region closest to the surface, which is slowed by the frictional resistance offered by the sediment and other roughness elements (pebbles, shells, wrack, etc.).
The lower portion of the wind profile over a flat homogenous surface is defined as a log-linear increase in velocity with in- creasing height.
In general, grains will be moved by the wind when…
the fluid forces of lift and drag overcome the effects of the weight of the particle and cohesion between adjacent particles.
Drag force results from…
Lift force results from…
The drag force results from the fluid flow over and around the exposed particle (i.e., the shear stress), whereas the lift force results from lower fluid static pressure above the grain and the steep velocity gradients at the grain surface.
Once entrained, the dominant mode of sand transport is…
Saltation.
What happens when wind encounters a dune?
As the wind encounters the toe of the dune there can be a stagnation (speed-down) at lower wind speeds followed by a compression of the flow and speed-up as the air encounters the stoss slope of the dune.
Erosion of the backshore and dune foot leads to a scarped slope that promotes aeolian sediment transport to the lee slope and the migration of the dune landward.
The effect of acceleration across the backshore, ramp, and dune is partly offset by what?
Vegetation, which extracts momentum from the wind, protects the surface, and traps sediment
The distance required for the transport to reach steady-state from the leading edge of the sand is termed what?
The fetch length.
For wind speeds at or close to the threshold, the length of the fetch is dependent on the ability of saltating grains to further eject sediment downwind, whereas for winds well above the threshold, the length of the fetch is dependent on the rate at which momentum is extracted from the wind
