Supertides are real!

“But less intelligible still was the flood that was caused by forty days’ rain, and forty nights’. For here on the moors there were some years when it rained for two hundred days and two hundred nights, almost without fairing; but there was never any Flood.” –Halldór Laxness

Once every 18 years, a French Abbey — Mount St.-Michel — becomes inaccessible, as the English Channel rises to such levels that the causeway that normally reaches it becomes engulfed by the surrounding waters.

Image credit: Associated Press.
Image credit: Associated Press.

You might think this is due to the tides, where the Earth, Moon and Sun align, but then shouldn’t this happen twice a month, during Spring Tides?

Image credit: © 2002 By Keith Cooley, via http://home.hiwaay.net/~krcool/.
Image credit: © 2002 By Keith Cooley, via http://home.hiwaay.net/~krcool/.

As it turns out, the effects are much more subtle, and involve the Moon’s elliptical orbit and the equinoxes as well, but when they all align, once every 18 years, a supertide is the result, and Mount St.-Michel becomes an island!