MadSci Network: Earth Sciences

Re: Why are sandy beach shore lines often 'scalloped' in their shape?

Date: Wed Jul 20 13:59:20 2005
Posted By: Gene Marlin, Undergraduate, Geosciences/Geology, Mississippi State University
Area of science: Earth Sciences
ID: 1118467747.Es

Hi Brad,

The regular series of points and indentations you have noticed are a phenomenon known as "beach cusps".

To quote an early description of them:

Among the minor forms of the shore zone none has proved more puzzling than the cuspate deposits of beach material built by wave action along the foreshore. Sand, gravel, or coarse cobblestones are heaped together in rather uniformly shaped ridges which trend at right angles to the sea margin, tapering out to a point near the water's edge. (Johnson, 1919, p. 457)

They are usually ephemeral features with a lifetime on the order of hours and seem to occur on beaches all over the world (in fact, the source above mentions an early description from Lake Huron). They tend to occur on steep berms or beach faces when waves are striking the shore head-on. (Ibid.)

Johnson and the other early theorists found them difficult to explain, with ideas focusing on interference of separate wave trains and selective erosion of prior weak points on the beach, which grow by focusing return swash from waves down their axes.

The current view is more complex and involves a phenomenon known as "edge waves". These edge waves are basically outgoing gravity waves that have been refracted by shallow water into occuring perpendicular to the beach, at regular intervals.

As incoming waves approach shore and break, they interfere with these edge waves, to create periodic changes in sedimentation along the beach. Many groups have succeeded in creating beach cusps in wave tanks; one example with a mathematical treatment of edge waves is Guza and Inman, 1975.

The larger features you mentioned along the shore may have any of a number of origins. Though there are "megacusps" out there they are most likely spits which are much larger and have an entirely different origin than cusps, namely deposition of sediment by longshore transport.

The key question would be, are these larger features periodic, or do they occur more or less independantly?


D.W. Johnson, Shore Processes and Shoreline Development, 1919.

Guza, RT; and Inman, DL. "Edge Waves and Beach Cusps", Journal of Geophysical Research, V. 80, pp.2997-3012.

Hyptertext links:

http://www.c ki/Gravity_wave rocesses/Longs horeTransport.aspx

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