Photograph by AP
Read Caption

The historic abbey of Mont Saint-Michel became an island on March 21 after a rare “supertide” flooded a causeway.

Photograph by AP

Sun and Moon Create Europe's 'Tide of the Century'

Rare “supertide” forms temporary island in the English Channel.

Thousands of tourists in northern France were delighted this weekend by a so-called “tide of the century,” which scientists say actually happens every 18.6 years.

The visual highlight of Saturday's rising water was the formation of a temporary island, crowned by the picturesque Mont Saint-Michel, a medieval abbey in Normandy that is also a UNESCO World Heritage Site.

“Several factors came together to cause a special alignment that made higher and lower tides than normal,” says Stephen Gill, a chief scientist with the National Oceanic and Atmospheric Administration.

Tides are caused by the gravitational pull of the sun and moon on the Earth’s ocean. Every 18.6 years, Gill says, that effect is more pronounced because of two factors. First, the sun, moon, and Earth all line up on the same plane, magnifying the gravitational effect. (The alignment was also responsible for the eclipse seen across parts of Europe on Friday.)

Second, the moon is closer to the Earth than at any other time in its monthly rotation, causing a greater gravitational effect on the ocean.

“Each of the oceans and gulfs responds differently to tides,” says Gill. (Learn about king tides.)

Northern France along the English Channel already has some of the highest tides in the world, which are made even more dramatic when forces align.

The last “tide of the century” occurred on March 10, 1997, and the next will be on March 3, 2033. Strong tides were also seen over the weekend on the England side of the channel and in the Netherlands.

View Images

Tourists at Mont Saint-Michel wait for rising water during a high tide on March 21.

At Mont Saint-Michel, water as high as a four-story building covered a causeway, delighting thousands of tourists who had flocked to the site in anticipation. Before the causeway was built, the abbey was routinely cut off by more typical high tides.

A fisherman in France’s Landes region was swept out to his death by the high tide, according to local officials.

Follow Brian Clark Howard on Twitter and Google+.