Journal Review: Alongshore Variation in Beach Cusp Morphology in A Coastal Embayment

Earth Surface Processes and Landforms Journal. Vol 24. Page 335-347.1999.


Gerhard Masselink


What is his research type?
His research is to examine the beach cusp characteristic in 1 km embayment beach (Pearl Beach, New South Wales)


Is he in "Edge-Wave-Theory" or "Self-organization theory"?
He said: Edge wave mechanism of beach cusp formation COULD NOT account for the observed trend in cusp spacing.


Why?
Because, there is NO RELATIONSHIP could established between the spacing of the cusp and the gradient of the beachface.
It said that beach cusp develop and are maintained by 3D swash flow circulation pattern characterized by wave uprush.


History of Research
Standing Edge Waves Theory
According to this theory, swash superimposed on the motion of standing edge waves and produce a systematic alongshore variation in swash height. This variation will result in regular erosional perturbation. The swash cirsulation will enhance topographic perturbation.
Two edge wave type that generally implicated are:
a. Mode Zero, subharmonic edge waves
b. Synchronous edge waves

In edge wave theory, can predict a cusp spacing as a function of incident wave period and beach gradient.


The self-organization theory
This theory said that:
Beach cusp formation consider beach cusp to be the result of feedback processes between morphologic and swash flow.
Positive feedback : Enhances RANDOM MORPHOLOGIC IRREGULARITIES
Negative feedback : Inhibits EROSION AND ACCRETION on well develop cusps.
In this theory, dimension of rythmic morphology is a function of horizontal swash excursion.

How to validate those theories?
There are two avenues of investigation:
1.Monitoring of the water motion in the swash and surf-zones during beach cusp formation.
2.Comparing the equation of cusp spacing in both theories.


Please think this phenomena!
In coastal area,
Wave height, beach gradient, sediment size, cusp spacing, and swash excursion are VARY ALONG ITS SHORELINE.
in contrast the wave period remain UNIFORM
This clue is an ideal opportunity to investigate the two models of beach cusp formation.
So, if egde wave is valid,
The alongshore variation in cusp spacing should be explained in terms of the beach gradient.
However, if the self-organization theory valid,
The alongshore variation in cusp spacing should be accounted for by the alongshore variation in the horizontal swash excursion.


Field Experiment on Pearl Beach
There are 45 beach cusp appear on Pearl Beach with cusp spacing of 22-23 m.
In each profile, were surveyed:
1. Berm Crest
2. Mid-beach face position
3. Down rush limit (the lowest point on the beachface that falls dry during swash action).


Four Parameters were derived from data set:
1. Cusp spacing
2. Beach face gradient
3. Cusp prominence (a function of beach slope at beach cusp horn and bay)
4. Horizontal Swash excursion


How to determined?
1. Cusp Spacing :
From alongshore distance between 2 cusp horn
2. Beachface gradient :
Computed for each mini-profile from the berm crest to the downrush limit.
It obtained by averaging the gradient of the cusp horn and embayment.
3. Cusp Prominence, by computation
4. Horizontal swash excursion, by statistical parameter.
Swash motion monitored at least 30 minute before the swash length parameter can be computed. (Parameter : Mean, max and significant swash excursion)

Result
1. Beach cusp have formed at the preceding day and/or night
2. Edge wave model can only be made to fit the morphological data by invoking several episodes of beach cusp formation.
3. Does the mean of the cusp horn and embayment gradient represent the beachface gradient during cusp formation?
Cusp horn result from a local steepening of the formation of cusp embayment. Mean of the horn and embayment gradient is characteristic of pre-cusp brachface gradient.


S U M M A R Y
There is a strong relationship between cusp spacing and swash excursion.