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Ocean Waves at Beaches

09/18/2017 9:23 AM

When at the beach (ocean or gulf), have you noticed the waves seem to always be coming toward shore?

I know the wind blows other directions, so, why are the white caps always toward the beach?

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#1

Re: Ocean Waves at Beaches

09/18/2017 9:39 AM

Waves only (usually) break when they enter shallower water. Also, given randomly-oriented waves, crossing each other in arbitrary directions, at the coast waves perpendicular to the shore generally never break (they're not going into shallower water), and there are no substantial waves moving away from shore as they've had no "run-up", i.e. no time to pick up enough energy from the wind - they'd only be babies (also they're usually going into deeper water).

Take away the off-shore and parallel ones, and you're only left with on-shore waves to break.

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#2

Re: Ocean Waves at Beaches

09/18/2017 9:48 AM

This is because the depth tapers from deeper offshore to shallower onshore. Water moving in "piles up" and spills over as whitecaps; water moving out doesn't "pile up."

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#3

Re: Ocean Waves at Beaches

09/18/2017 10:52 AM

This is a somewhat-related question, based on a statement in a fiction book I'm reading. Are there waves in the Aegean Sea, or at least on the south shore of Crete? That's what this novel says. Does not make sense to me, unless maybe if there's a sheer drop-off where the water meets land?

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#4
In reply to #3

Re: Ocean Waves at Beaches

09/18/2017 11:00 AM
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#5
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Re: Ocean Waves at Beaches

09/18/2017 11:37 AM

These look like waves to me. Thanks!

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#8
In reply to #3

Re: Ocean Waves at Beaches

09/18/2017 12:24 PM

Why would there not be waves, seems like a better question? There is wave energy on the surface of all waters. Some areas more than others.

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#9
In reply to #3

Re: Ocean Waves at Beaches

09/18/2017 1:51 PM

"The winds in Crete usually come from the northwest so most beaches with a west or northwest aspect will be OK for bodyboarding."

Looks like not much wave action on the south coast.

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#10
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Re: Ocean Waves at Beaches

09/18/2017 1:56 PM

Yes, windward and leeward make a difference in wave height, but the question remains, and I think answered, are the waves breaking inward at all points, or do some start at the beach and move outward. I have never witnessed wave breakers moving outward.

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#6

Re: Ocean Waves at Beaches

09/18/2017 12:22 PM

Because the sand does not move enough (usually) to start a wave. The waves start somewhere out there (usually wind-induced), but they have a velocity/depth ratio that prevents them breaking over as white-cap.

The waves do not slow down when they arrive in the shallows near the beach, but here the group velocity/depth ratio is exceeded, and the waves break over.

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#7

Re: Ocean Waves at Beaches

09/18/2017 12:22 PM

It's the difference between having water in a glass and on a plate, the tapered edge vs the straight edge boundary...The water is simply moving up and down in swells and depressions....Sometimes there can be a harmonic wave created resulting in a rogue giant wave that crosses the ocean for miles never reaching shore....The proof is that at the edge of the ocean that the water level doesn't rise, as the waves or swells reach the beach they immediately collapse and flow back into the ocean...

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#24
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Re: Ocean Waves at Beaches

09/20/2017 4:16 AM

"....water is simply moving up and down..."

???

It can be well modeled fairly simply....but not that simply. The water doesn't move simply up and down.

At depths less than 1/2 a wavelength the water moves in a circle. At the surface this circle has a radius of roughly the amplitude of the wave. As depth increases, the circle radius is reduced. If the water depth is less than a wavelength, the vertical component ot the circle is squeezed down and the motion near the bottom becomes linear, about parallel with the floor (above boundary layer, surface effects).

The water level both rises with onshore winds and rises as the crest of a wave hits. The mass of water brought can worsen flooding in hurricanes and is also responsible for riptides as the water returns through limited paths between breakers.

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#11

Re: Ocean Waves at Beaches

09/18/2017 8:00 PM

When at the beach (ocean or gulf), have you noticed the waves seem to always be coming toward shore?

I think the answer is refraction. Regardless of the direction a wave is propagating, the change in depth causes it to bend toward shallower waters, thus toward the shore.

"Wave refraction is the process by which wave crests realign themselves as a result of decreasing water depths. Varying depths along a wave crest cause the crest to travel at different phase speeds, with those parts of the wave in deeper water moving faster than those in shallow water. This process continues until the crests become (nearly) parallel to the depth contours. Rays—lines normal to wave crests between which a fixed amount of energy flux is contained—converge on local shallows and shoals. Therefore, the wave energy between rays is concentrated as they converge, with a resulting increase in wave height."

https://en.wikipedia.org/wiki/Wind_wave

https://www.youtube.com/watch?v=NOnK62gMIH8

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#23
In reply to #11

Re: Ocean Waves at Beaches

09/20/2017 4:03 AM

Winner, winner, chicken dinner!

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#12

Re: Ocean Waves at Beaches

09/18/2017 9:12 PM

This beach slope is the cause of tsumanies as well.

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#14
In reply to #12

Re: Ocean Waves at Beaches

09/18/2017 11:01 PM

"tsumanies" ???

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#15
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Re: Ocean Waves at Beaches

09/18/2017 11:15 PM

He was channeling corporal Klinger.

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#19
In reply to #12

Re: Ocean Waves at Beaches

09/19/2017 9:41 AM

Did you mean jumanji?

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#20
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Re: Ocean Waves at Beaches

09/19/2017 12:50 PM

Wood wave...

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#21
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Re: Ocean Waves at Beaches

09/19/2017 1:21 PM

The wizard of search strikes again, GA!

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#22
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Re: Ocean Waves at Beaches

09/19/2017 1:25 PM

...as in tsumani tutanka owa chi! Dances with wolves is my friend!

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#13

Re: Ocean Waves at Beaches

09/18/2017 10:52 PM

Differential heating causes "sea breeze" that always blows onshore when the sun is up.

https://mgtvwnct.files.wordpress.com/2015/07/sea-breeze.png

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#16

Re: Ocean Waves at Beaches

09/19/2017 7:51 AM

After 4 years of aerology someone has finally asked a question I can answer beside "whats the weather going to be today".

Waves require a fetch area. There is no fetch area on the beach.

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#18
In reply to #16

Re: Ocean Waves at Beaches

09/19/2017 9:41 AM

As in Rover go fetch, or office boy step and fetch, or fetch up enough energy to make a wave, fetch?

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#17

Re: Ocean Waves at Beaches

09/19/2017 8:06 AM

My supposition is that the actual source of the waves is out to sea. Much as dropping a pebble into a pond. All waves travel away from the source. Watching incoming waves, the disturbances are always further out than you can see. If you drove a boat along the shore waves would travel away from the boat in both directions. Again I propose that the source of the wave is out to sea. Wind will just change the wave energy content.

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