Wednesday, 30 September 2015

The Tide is High

This year we have seen more of what are called "super high tides". February 21 saw tides of 12.15 metres (39 feet, 10 inches), and on 31st August there was a tide that was 12.1 metres (39 feet,8 inches), while on 28th September we saw a tide reach 12.2 metres (40 ft, 0.3 inches).

These high tides result from an unusual alignment of the sun, moon and Earth so that the gravitational pull on our oceans is higher than usual five more times this year. But is there is risk of flooding? That depends more on pressure and wind.

The tidal gauge in St Helier at Albert Pier is the source of the information that gives us the exact readings of high and low tides.

This records the actual tide as opposed to the forecast tide, which can vary.

Barometric pressure affects the height of the tide, if the air pressure is low the tide will be higher than the tide table prediction. Wind speed and direction is also a prime factor on how such tides will affect the island's coasts. Strong winds can pile up water on coastlines and low pressure systems can also cause a localised rise in sea level.

Fortunately high air pressure and low winds meant no flooding. Instead, we could just enjoy the tide, very high, lapping at the sea wall, which is rather magical.

Jersey marine biologist Andrew Syvret explained there is a common misunderstanding about how the tides work.

He said: “People think the tide rises and falls in the English Channel; that it floods and empties.

“The reality is you have a huge see-saw of water, with high tide at one end and low tide at the other. This massive bulge of water moves backwards and forwards twice each day.

“When it arrives in our corner of the English Channel this bulge of water really hasn’t got anywhere to go. We then find ourselves in a giant anti clockwise tidal gyre – a great whirlpool.”

And gravity over the surface of the land and sea varies due to differences in the subsurface and surroundings -- the greater the mass, the greater the gravity

In addition to that, there is a very small but significant rise in sea levels in recent years. This is measured in millimetres, not metres.

No comments: