Big tides at Blakeney Point

The moon’s gravity causes the tides by pulling up a lump of ocean under the moon, and creating another, because of the centripetal reaction, on the opposite side of the earth. As the planet spins, these lumps of water stay aligned to the moon, creating a local sine wave from which the handy “rule of twelfths” approximation is derived (see Albrecht Dürer’s 1525 drawing).

Because the moon’s monthly orbit is in the same direction as the earth’s spin, the harmonic period is a little more than 12 hours, and the two daily high tides get later every day.  The sun’s gravity also contributes, so when the sun and moon are approximately aligned, twice a month at new moon and full moon, you get bigger tides called spring tides.

But tides can get higher than the regular bi-monthly springs. That’s partly because the distances and alignments of the earth, moon, and sun vary on time scales longer than the lunar month, giving extra-high tides, for example when there’s a super-moon. We know when to expect these, but we’re not always so good at predicting the wind, which also has strong effect on the tide. This year’s Storm Antoni brought breaking waves almost over the narrow ridge of the spit, driving seafoam like drifting snow, and propelling a talented kite-surfer across a temporary lake that had formed over the reeds near the Cley windmill. And on the 30^th of September, the full moon spring tide coincided with winds that pushed more water than expected into Blakeney Harbour, and up to and into the Old Life Boat House. It also brought an unusual visitor in the form of a squid, which was caught by students doing the M.Res. in Biodiversity, Evolution & Conservation, and released safely back into the harbour.

By Duncan Greig