Beach-cusp formation

Marine Geology



Field experiments on beach-cusp formation were undertaken to document how the cuspate form develops and to test the edge-wave hypothesis on the uniform spacing of cusps. These involved observations of cusps forming from an initially plane foreshore.

The cuspate form was observed to be a product of swash modification of an intertidal beach ridge as follows. A ridge, cut by a series of channels quasi-equally spaced along its length, was deposited onto the lower foreshore. The ridge migrated shoreward with flood tide, while the longshore positions of the channels remained fixed. On ebb tide, changes in swash circulation over the ridge allowed the upwash to flow shoreward through the channels and the channel mouths were eroded progressively wider until adjacent mouths met, effecting a cuspate shape.

Measured spacings of cusps, ranging in size from less than 1 m to more than 12 m, agree well with computed spacings due to either zero-mode subharmonic or zero-mode synchronous edge waves. Edge-wave-induced longshore variations in run up will cause water ponded behind a ridge to converge at points of low swash and flow seaward as relatively narrow currents eroding channels spaced at one edge-wave wavelength for synchronous edge waves or one half wavelength for subharmonic edge waves. The channels are subsequently modified into cusp troughs as described above.

Publication type Article
Publication Subtype Journal Article
Title Beach-cusp formation
Series title Marine Geology
DOI 10.1016/0025-3227(79)90100-2
Volume 29
Issue 1-4
Year Published 1979
Language English
Publisher Elsevier
Contributing office(s) Coastal and Marine Geology Program
Description 15 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Marine Geology
First page 23
Last page 37
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