Physics Photo of the Week

March 14, 2013

Scalloped Beach
Ocean beaches, pounded by relentless large waves, often exhibit a scalloped shoreline.  The scalloped beach is highly visible in the center of this picture of a gravel beach on the Pacific Ocean at Lima, Peru.  Click on the image for a zoomed-in view.

I don't know the physics of what creates the scalloped appearance of beaches, but I have my hunches from my very limited knowledge of hydrodynamics and similar phenomena elsewhere.  I believe that these wavy contours where the surf meets beach are caused by lateral currents in the ocean, possibly by oblique incidence of the surf.  The surf this day intersected the beach parallel to the shoreline, but the scallops could have been made from an earlier oblique surf and the beach hasn't had time to re-form.  It could also be due to a lateral current.  Whenever fluids of different densities and resistivities flow past each other, waves tend to form at the interface with the waves perpendicular to the relative direction of flow.  Leaf-litter on a street after hard rains is an example (see PPOW for October 19, 2007).  Wave clouds are a similar example (PPOW February 26, 2010).  Sand dunes in a desert are another.

The scalloped beach also occurs frequently on a smaller scale.  The image at the right is a beach in a cove on a lake.  If the beach is bombarded by repeated waves from passing motorboats, the waves strike the shore from an angle and the scalloped shoreline soon appears.  The lake was otherwise calm.  The foot prints on the beach in the photo at right gives the scale - much smaller than the Lima beach where cars and people are small dots in the distance.  Click on both images for larger views.


Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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