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Author Topic: Why do different sites along the same coastline have different tidal heights?  (Read 2032 times)

Offline chris

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Why can two areas on the same coastline, but some distance apart, have tidal heights that differ wildly?


 

lyner

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The tidal height at any one place is due to a number of different 'waves' corresponding to harmonics and sub-harmonics of the main tidal cycle and the relationship between the positions of Sun and Moon. These travel (as surface waves do) at different speeds. Hence they arrive at a given point on the coast in different relative phases. The time of high tide is fairly similar for nearby ports but the sum of the waves at the max may be very different because of this.
There is another factor - the rate at which the height can alter will also depend upon the depth and profile of the coast. In coastal inlets, the delay in 'filling and emptying' will affect the max and min heights attained.
It is only since I have been regularly using one particular place to sail that I realised that, at any particular place on the coast, the Spring Highs always occur at the same time of day and the Neap Highs occur about six hours after. The actual time is very dependent of where you happen to be, though.
 

Offline techmind

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Why can two areas on the same coastline, but some distance apart, have tidal heights that differ wildly?
I'm not sure whether you're referring to instantaneous difference in water height (i.e. time-shifting) or difference in tidal amplitudes.

Either way, it's caused by local geography of the coast, channels and/or sea-bed which causes the water to 'slosh' about when driven by the sun and moon, rather than follow the simple motion which you would expect in the open ocean.


It's not as clear as I'd like, but you might take a look at http://www.es.flinders.edu.au/~mattom/IntroOc/lecture11.html
Otherwise find some HMSO tidal charts and you'll soon see how complicated things get in the waters around the UK.
« Last Edit: 16/11/2008 00:30:16 by techmind »
 

lyner

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Quote
the simple motion which you would expect in the open ocean.
Even that's not too simple; the natural period of oscillation in the Pacific is different from that in the Atlantic - they both act like enormous Helmholtz Resonators, apparently.
« Last Edit: 17/11/2008 15:11:07 by sophiecentaur »
 

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