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-   -   Waves on beaches (stupid question?) (http://www.sailnet.com/forums/general-discussion-sailing-related/81970-waves-beaches-stupid-question.html)

joelsanda 12-21-2011 03:21 PM

Waves on beaches (stupid question?)
 
Twice in the last few years I've been on islands in the Pacific where the waves felt the same whether the beach I was on was windward or leeward. In both cases I was on a small boat about 100 +/- yards offshore, messing around on the beach and snorkeling. On both sides of the island the waves were nearly identical in size - height and wavelength.

This struck me as a little odd as I'd expect the waves to come onto the islands in the same direction of the wind. But when I experienced nearly identical waves on the leeward side I figured I was wrong.

So ... my question: are these waves coming from the natural uplift of air that brings breezes and winds onto the island, from the low pressure system over most islands? Otherwise one would expect there to be no waves on the leeward side, right?

RichH 12-21-2011 03:45 PM

Not uncommon to have waves hitting the lee-side of a small island and at 180 from the direction of their formation/propagation, ..... sometimes larger than the amplitude of the waves hitting the 'windward' side. Rarely will there be a 'shadow zone' on the leeside of a small island where the waves are 'calm'.

This physics concept of 'wave diffraction & wave refraction' (or 'turning of the waves') is explained and illustrated: Refraction/Diffraction figures 2 & 3.

:-)

Faster 12-21-2011 04:08 PM

In addition, swells can be developed in the open ocean by surprisingly distance storm systems.. these can roll into the otherwise 'leeward' sides of islands.

This phenomenon can often make normally sheltered harbours on certain Caribbean islands less comfortable or even untenable.

joelsanda 12-21-2011 04:15 PM

Quote:

Originally Posted by RichH (Post 809274)
This physics concept of 'wave diffraction & wave refraction' (or 'turning of the waves') is explained and illustrated: Refraction/Diffraction figures 2 & 3.

Refraction could explain what I saw in Mexico. The island was very narrow and I tried to snorkel on the ocean side, but the waves to were too difficult. They were only slightly better on the other side.

Would be interesting to know how much energy is in an ocean wave. Judging by how much the sound waves carry in our home from my son's small electric guitar amp sound waves can pack a punch :-)

joelsanda 12-21-2011 04:19 PM

Quote:

Originally Posted by Faster (Post 809283)
In addition, swells can be developed in the open ocean by surprisingly distance storm systems.. these can roll into the otherwise 'leeward' sides of islands.

Having snorkeled in Mexico and Tahiti I certainly prefer the islands with a barrier reef. The waves in Mexico - exposed on the Pacific side with a red flag warning the whole time I was there - were awesome. There was a storm 'at sea' according to the locals; but 150 yards off shore the waves were barely noticeable on our boogie boards. Move in closer to where the breakers formed and it looked like some kind of life and death struggle!

sawingknots 12-21-2011 04:20 PM

theres way too many varibles to completely figure,the wind, the natual current,how far distant the disturbance and the proximity to other land,in the philippines i've noted that the small[banka boats] often seek shelter behind islands during squals,about every island that i've noticed anywhere has large sandy beaches on one side and mucky mangroves on the other

sawingknots 12-21-2011 04:20 PM

oh yeah i forgot reefs

killarney_sailor 12-21-2011 05:32 PM

It is interesting when you are trying to find an anchorage on one of these islands. From the chart and wind direction it looks like there should be no wind waves since they would have to 'bend' (refract) much more than 90 around the point of the island to get you - but they do.

I agree also about the effect of swells. In deep water you have quite long easy swells but when you get close to shore the wave length shortens and the waves get higher.

paulk 12-21-2011 08:33 PM

power
 
Quote:

Would be interesting to know how much energy is in an ocean wave. Judging by how much the sound waves carry in our home from my son's small electric guitar amp sound waves can pack a punch :-)
I believe there was a calculation made which showed that a wave crashing into a mile of beach represents more energy than man has used since he first learned how to light a campfire. Can't find the right wording to locate it in Google.

casey1999 12-22-2011 01:51 PM

Quote:

Originally Posted by paulk (Post 809408)
I believe there was a calculation made which showed that a wave crashing into a mile of beach represents more energy than man has used since he first learned how to light a campfire. Can't find the right wording to locate it in Google.

Man I wish that were true, we could solve the energy crisis. Here is an equation to calculate wave energy:
http://en.wikipedia.org/wiki/Wave_power
CACR Wave Calculator

Also, this site is great for wind and wave prediction:
Sailing Weather - Marine Weather Forecasts for Sailors and Adventurers - PassageWeather

Use the animate button to make the waves move, you can see how they propagate around islands. Noice that if an island is surrounded by real deep water, the wave will not refract so much to the opposite side, if shallow water wave will refract around.

There are 3 basic types of waves, those generated far off shore from large storms that bring in a ground swell and when that hits the beach, very large waves. This happens in Hawaii when storms around Alaska generate swell. It can be a calm sunny day with no waves at the beach and within 1 hour you could have 20 foot wave faces breaking- a lot of people die this way. Then there is the local sea breeze- air heats up on land, rises, and creates a wind heading to the land which generates a wind chop. Then the tidal wave which is generated by earthquake or underwater land slide. Also you could have a local storm (low pressure area) that would generate waves, but generally it will also be stormy at the beach also (wind and or rain/couds).
Regards


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