Coco & Murray (2007) - patterns in the sand

Question 1

The source and significance of nonlinearities is often related to…

Answer 1

the presence of thresholds activating specific processes, or to the relationships coupling flow motions and sediment response, usually parameterized by relating the sediment transport fluxes to flow velocity taken to some power greater than one (Van Rijn, 1993).

Question 2

The dissipative nature of the nearshore can be easily recognized by considering…

Answer 2

the continuous input of energy in the system (through waves, wind, and tides) and the subsequent loss of this energy because of bed friction, wave breaking, and work done on sediment transport.

Question 3

Feedbacks play a key role in spontaneous formation of patterns.
What do positive feedbacks do?

Answer 3

They amplify the interactions between flow and sediment processes and the evolving morphology, leading to regular morphodynamic patterns.

Question 4

Feedbacks play a key role in spontaneous formation of patterns.
What can negative feedbacks do?

Answer 4

to (statistically) steady-state response and can dampen the growth of new perturbations.

Question 5

What can overwhelm feedbacks?

Answer 5

Very energetic forcing conditions (e.g. a storm) can, however, overwhelm the potential feedbacks, obliterating regular patterns, analogous to the way high thermal energy favours disorder.

Question 6

What are beach cusps?

Answer 6

Alongshore sequences (up to kilometers long) of crescentic shapes developing in the swash zone (the area intermittently covered by water running up and down the beach).

Question 7

What is an individual beach cusp characterised by?

Answer 7

Two raised horns, protruding in the offshore direction, separated by a more gently sloping bay.

Question 8

The physical mechanism leading to the formation of beach cusps was of extreme simplicity:

Answer 8

Random seabed perturbations over an otherwise planar slope affect flow motions by decelerating (accelerating) it
over topographic highs (lows) inducing deposition (erosion) so that highs become higher and eventually develop into beach cusp horns and lows become lower and eventually develop into beach cusp bays.

Where highs or lows develop too close to each other, merging (or splitting) occurs as the growing features affect the flow around themselves and, thus, impinge on each other. The adjustment between neighbors eventually produce a regular alongshore pattern.

Question 9

Give a example of the nature of self-organization.

Answer 9

Beach cusps provide a clear example, local interactions leading to adjustments in search of an optimal, globally ordered configuration.