New Moon and Monster Tide

Wed 14th February 2024

Winter storms, low pressures, new moon and high tides. These are the ingredients for the super high tides we have recently seen on the Menai Strait.

The perfect alignment of Sun, Earth and Moon happens two days after a full or new moon and that’s been this week when we had the big ‘Spring’ tides. On Anglesey that tide is a wall of 7.5 metres of water, slowly filling the Menai Strait. Every 6hrs 15 mins approximately 30 millions cubic metres of water passes either in or out of the Strait. That’s 12,000 Olympic swimming pools or 20 Millennium Stadiums full of water - four times a day.

The Swellies is the narrowest, shallowest and most volatile section on the whole Strait. That wall of water is squeezed between the famous bridges at speeds up to 10kts (11.5 mph) To compare - Michael Phelps top swimming speed ever was 6mph, or if you were a runner, this would mean a marathon in 2.17hrs. Try keeping that up for six hours.

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The Belgium Promenade

So, while we contemplate this perfect brew of tides and storms our biggest floods can also come without the help of the biggest tides. Atmospheric pressure can heighten or lower tidal heights by 1cm per millibar above or below the average of 1012mb. So, yesterday on Anglesey it was reading 990mb, meaning the low pressure was letting the water rise an extra 22cm.

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Belgium Promenade in Menai Bridge

A final and unique feature of the Menai Strait is that unlike most tidal estuaries, it has two ends, or mouths feeding it. In simple terms, if the wind is strong from the south, it pushes more water in from Aber Menai all the way up to Beaumaris where the axis of the two tides lives. That same wind also pushes more water up the Irish Sea, some of which will wrap itself around Puffin Island and flow South to Beaumaris. (This meeting point of the tides is what created Gallows Point) Once the tide starts heading out, that same wind stops the water flowing out of the Strait, creating an Aber Menai Plug and raising levels particularly around Menai Bridge and Felinheli.

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Bench at High Water

All in all a perfect recipe for a RibRide through the Swellies. We are open all half term and there is no such thing as bad weather, just bad clothing. So dress warm, give us a call and step aboard for another great Adventure with RibRide.