Hesp (2011) Dune Coasts

Question 1

What are the four main coastal dune types?

Answer 1

Foredunes
Blowouts
Parabolic dunes
Transgressive dunefields

Question 2

What is the crucial first factor in determining whether dunes can form or not?

Answer 2

Wherever there is a minimal sand supply to build a beach, there will exist the potential for a sand dune to form.

Question 3

What is the dominant force in driving initial sedi­ment supply?

Answer 3

Sea-level change, and for many of the Holocene coastal dunefields of the world, the postglacial marine transgression has been the fundamental driving force.

Question 4

How have surf zones, beaches, and barriers developed?

Answer 4

Where the rising sea level has driven sediment landward from a glacial low stand resulting in gradual coastal barrier and beach translation or rollover.
Once present sea level was reached, all the available sediment was utilized to form a surf zone, beach, and backshore.

Question 5

What determines whether its just a surf zone and beach formed or whether more and larger dunes become possible?

Answer 5

Sand-sized sediment supply then becomes vital.

If only limited sand supply was available (surf zone and beach) but as the sand supply increased…

Question 6

As sea levels reached the present (at different times on diverse coasts), sand supply was initially driven by factors such as…

Answer 6

Shoreface slope.
Availability of sediment to be reworked/redistributed on the shoreface.
The local and regional supply from rivers.
Wave energy.

Question 7

Variations in sand-sized sediment supply to a beach may depend on…

Answer 7

Relative sea level (stable, rising, or falling)
Longshore drift
River supply
Continued adjustments to an equilibrium condition on the
shoreface
In situ production of sediment

Question 8

What is the next factor crucial for dune development (not sand supply)?

Answer 8

The wind energy above a minimum threshold velocity.
The higher the wind energy, the greater the potential for dune development, particularly for those coasts with prevailing onshore winds.

Question 9

What does the degree and type of dune development depend on the ability of?

Answer 9

Pioneer plants to colonize the backshore and maintain overall stability even during periods of storm erosion, or rapid accre­tion/progradation, or not.

Question 10

At a very general level, the dominance of plants colonizing and maintaining a presence on the backshore has led to the development of…

Answer 10

foredunes and foredune plains.

Question 11

Disturbance of the vegetation cover by wind, wave, or climate mechanisms led to the development of…

Answer 11

blowouts and parabolic dunes.

Question 12

In some cases, a high sediment supply, or climatic conditions (e.g., arid or semi-arid), restricted plant cover, and high wave and wind energy led to the development of…

Answer 12

mobile (transgressive) dunefields.

Question 13

Where Do Sand Dunes Form?

Answer 13

If the wind is above thresh­ old velocity and sand is moving, then a slight variation in the horizontal wind speed alongshore or across-shore may result in the deposition of a strip of sand.

Question 14

What is a protodune?

Answer 14

A bedform that has migrated away from the initial depositional site and which does not have a slipface.

Question 15

How does a protodune grow in height?

Answer 15

Once the sand strip or protodune is formed, there is a positive feedback
between form and flow.
The presence of the strip of sand will alter the local wind velocity such that the air flow will be forced to rise slightly over the depositional strip or protodune, decele­rate, and deposit more sand.

Question 16

What does a protodune grow into?

Answer 16

A transverse protodune or dome dune

Question 17

What does a transverse protodune grow into?

Answer 17

The development of a slipface, and the eventual formation of a single, discrete barchan or a transverse dune

Question 18

Where and why are coastal sand dunes not commonly found?

Answer 18

In high latitudes due to the pre­ dominance of glacially derived sediment and coarse sediment sizes

Question 19

What are foredunes?

Answer 19

Foredunes are shore-parallel dune ridges formed on the back- shore by aeolian sand deposition within vegetation.

Question 20

What are the two main foredune categories?

Answer 20

Incipient and established fore- dunes, within which there can be wide morphological and ecological variations.

Question 21

What are incipient foredunes?

Answer 21

Incipient foredunes are new or developing foredunes that form within pioneer plant communities.

Question 22

How may incipient foredunes be initiated?

Answer 22

They may be initiated by sand deposition around obstacles or within discrete clumps of vegetation.

Question 23

The subsequent morphological development of incipient foredunes depends on…

Answer 23

Plant species, density, height and cover, wind velocity, rates of sand transport, and beach progra­dation rates.

Question 24

What does the evolution of incipient foredunes depend on?

Answer 24

econdary factors such as the rate of occurrence of swash inundation, storm-wave erosion, overwash incidence, the presence of debris and wrack, and resultant wind direction.

Question 25

Why do variations in plant species, density, and distribution strongly influence foredune morphology?

Answer 25

By affecting the mean wind flow and turbulence within the plant canopy.
e.g if plants are low prostrate species, their aerodynamic roughness is low and their effect on sand deposition is also low.

Question 26

Plant density influence on foredunes?

Answer 26

The lower the plant density, the less the near-surface and within-canopy drag and the lower and wider the foredune.

Question 27

Incipient foredunes typically adopt one of three forms…

Answer 27

Ramps, terraces, and ridges

Question 28

What are established foredunes?

Answer 28

Established foredunes develop from incipient foredunes and either occupy the seaward-most position at the rear of the beach or are situated behind an incipient foredunes.

Question 29

What are established foredunes distinguished by?

Answer 29

The growth of intermediate, some­ times woody plant species, and by their greater morphological complexity, height, width, age, and geographical position.

Question 30

How are beach ridges formed?

Answer 30

Wave action can form a ridge near the upper reach of wave movement, waves deposit sand and other material at this height.
When the water level drops, a ridge often remains.
More than one ridge may be formed, taking place at different times. Tides and wave height determine the height of beach ridge formation.

Question 31

What is a blowout?

Answer 31

A blowout is a saucer shaped depres­sion formed by wind erosion on a preexisting sand deposit.
The adjoining accumulation of sand, the depositional lobe, derived from the depression and possibly other sources, is normally considered part of the blowout landform.

Question 32

Where are blowouts common?

Answer 32

In coastal dune environments, particu­larly where beaches are eroding and/or receding, but also in high-energy wind and wave environments.

Question 33

Why are blowouts initiated?

Answer 33

Blowouts tend to form when wind erodes into patches of bare sand on stabilized vegetative dunes.

Question 34

Why do blowouts become when they become larger over time?

Answer 34

Parabolic dunes.

Question 35

What are parabolic dunes?

Answer 35

U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes.

Question 36

How can you distinguish between parabolic dunes and blowouts?

Answer 36

On the basis of the presence or absence of the trailing ridges. Parabolic dunes typically have vegetated trailing ridges; blowouts typically do not.

Question 37

Where do blowouts and parabolic dunes commonly form?

Answer 37

On eroding coasts where the foredune stability is reduced.

Question 38

Climbing and cliff top parabolic dunes may be formed…

Answer 38

(1) during low sea levels
(2) during present (high) sea levels where steep terrain lies adjacent to the beach
(3) where rapid erosion of unconsolidated to poorly indurated cliffed coasts is occurring

Question 39

Rates of parabolic dune advance or migration vary considerably depending on…

Answer 39

The morphology, slope, and type (e.g., sandy vs. rocky) of terrain the dunes are moving across, the vegetation cover and type (e.g., woodland vs. grassland), wind velocities, and directional variability of the wind.

Question 40

What are transgressive dunefields?

Answer 40

Transgressive dunefields are aeolian sand deposits formed by the downwind or alongshore movement of sand over vege­tated to semi-vegetated terrain.