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Port and Airport Research Institute, Japan

World - Supermoons Carve Away at Sandy Beaches by Commandeering the Tides

A quarter century of shoreline measurements show that the supermoon’s gravitational force drives more erosion.

As the moon orbits the Earth and the Earth orbits the sun, gravitational forces of these heavenly bodies drag the ocean back and forth in a never-ending dance. A full moon makes both high and low tide even more extreme. A few times a year, an even larger "supermoon" pushes the tides into overdrive by heightening this effect. Now, 25 years of research into moon and tidal cycles on a beach in Japan shows that supermoons can exert enough pull on the ocean to erode the shoreline.  

“The supermoon is the full moon that is closest to Earth,” said study leader Masayuki Banno, a coastal engineer with Japan’s Port and Airport Research Institute. Supermoons typically look about 14% larger than a normal full moon and occur when the moon’s elliptical orbit brings it nearest to Earth.

Banno said that previous observations showed the extreme “king tides” that happen during a supermoon heightened the risk of coastal flooding. “But the beach morphology changes caused by the supermoon had not been investigated.”

Every weekday from March 1986 to December 2010, researchers tracked the shore elevation of a sandy beach facing the Pacific Ocean on the island of Honshu in Japan. They measured the height at 5-meter intervals along a 500-meter stretch from dry sand out to the bottom of the “swash zone,” where waves crash after breaking.

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Read also Supermoon Drives Beach Morphological Changes in the Swash Zone, Masayuki Banno & Yoshiaki Kuriyama, Nov. 5, 2020