Science Questions

Why are Carribean tides so small?

Tue, 19th Apr 2016

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David Jackson asked:

Why are Carribean tides so small?


Kat Arney put this to Chris Smith...Beach, Desert Island

Chris - Okay. The likely reason for this is the same reason you donít see very high tides in other parts of the earthís surface, and that is itís all down to how the water moves.  The reason water moves around the earth is because the earth is spinning inside the orbit of the moon, the moon is gravitationally attracting the bulge of water towards itself on the side of the earth closest to the moon, and thereís also another bulge on the opposite side of the earth.  The earth then turns through that bulge which effectively moves across the surface of the earth and itís going to interact with land masses.  So, if you have a certain coastal configuration that means that the water all heaps up in one place because itís got nowhere to go, youíre going to see a bigger tide there than if the water can distribute and flatten out easily. A really good example of this is the Severn Estuary in the Bristol Channel.  Youíve got the patch of the coastline of Wales and the north of Devon and Cornwall which narrows in like a funnel to a very narrow patch of coast.  All of the incoming tide gets funnelled into a very small part of the estuary which heaps up the water there, drives a lot of water inland very fast and then it comes out again, and thatís you get these very big tides.  In some places, when I was in Australia last year, I went to somewhere where some of the highest tropical tides are, in Northwestern Australia, up where the horizontal falls are, and itís between 10 and 13 metres the tide there.  So thatís the reason - itís because the water having little place to go and being funnelled from a massive ocean into a relatively narrow section of the earthís surface and, if you have a lot of water entering a small area, youíre going to get a very radical tide height change.


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You don't need to go as far as the Carribean there is a null point in the middle of the North Sea where there is no vertical tidal movement. This can be seen in one of the tidal charts of the North Sea available from Admirality Chart Agents or on-line. The times of high tides 'rotate' about this point. JohnM, Thu, 19th May 2016

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