Is the water pressure below ocean waves constant?

  • #26
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let's take inputs to the extreme, say the pressure meter is just an inch under the trough hight, and in opposite extreme , presume it's 3km deep. I think it's obvious that whatever going on the water surface won't affect the readings at that depth. So, in your example , there will be fluctuation in pressure but not entirely equal to the wave hight. The deeper you go the less the fluctuation will be , wave length and speed could slightly contribute to the fluctuation value (remember that there's tons of dynamic involved)
 
  • #27
sophiecentaur
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I stand by my comment. A tsunami is a shallow water wave. As you agree.
If you are using the term 'shallow', referring to the depth with respect to the wavelength and the amplitude then I agree. The effect that you get with ordinary waves, breaking on a gently sloping beach is the same. The height of a Tsunami wave as it travels over the deep ocean may be very great and the effect on a boat out at sea will be relatively mild at all as the acceleration is very low. The path of the water particles is the same for all deep water waves - vertical circles with a radius that decreases with depth and the shape of the path becomes flattened and the motion is mostly forward and backward rather than up and down. This hyper physics link gives a simple model of a continuous wave. If the water at depth is moving in a closed curve then there must be acceleration and that implies there must be varying force (therefore varying pressure) at depth.

The peakiness of a wave is brought about for high amplitude and also when the water becomes shallow. the speed gets slower for shallow water and the peaks overtake the troughs to make a breaking wave. The water flows backwards in front of the crest and accounts for the draining of water away from the coast just ahead of the breaking tsunami wave.
 
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  • #28
anorlunda
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Don't forget that tides are also waves. And even seich waves in oceans with one year periods cause seasonal variations (see #9)

Go back to @jbriggs444 posts in this thread. I'm convinced that his is the right explanation, and his reasoning includes the wavelength between crests.
 
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  • #29
olivermsun
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If you are using the term 'shallow', referring to the depth with respect to the wavelength and the amplitude then I agree. The effect that you get with ordinary waves, breaking on a gently sloping beach is the same. The height of a Tsunami wave as it travels over the deep ocean may be very great and the effect on a boat out at sea will be relatively mild at all as the acceleration is very low.
FWIW, the vertical displacement due to tsunamis traveling in the deep ocean is typically very small. According to Wikipedia, the 2004 Indian Ocean tsunami was observed 2 h after the earthquake in deep water with a height of 60 cm. Conservation of wave action/conservation of energy combined with the slowdown in wave group speed caused the tsunami to reach heights of 24+ m as it shoaled.

The path of the water particles is the same for all deep water waves - vertical circles with a radius that decreases with depth and the shape of the path becomes flattened and the motion is mostly forward and backward rather than up and down. This hyper physics link gives a simple model of a continuous wave. If the water at depth is moving in a closed curve then there must be acceleration and that implies there must be varying force (therefore varying pressure) at depth.
The (exponential) decay scale is in terms of wavelengths, so practically speaking it's very difficult to measure the bottom pressure fluctuation due to short/"deep water" waves. By contrast, long/"shallow water" waves are routinely observed with bottom pressure sensors.
 
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  • #32
sophiecentaur
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FWIW, the vertical displacement due to tsunamis traveling in the deep ocean is typically very small.
It's down to the total volume of water displaced, I suppose. If the deep water height is 1m and the wavelength is 1km then it stands on its end when entering shallow water. You can easily get tens of metres of height.
But, apart from the dimensions, the tsunami has to behave the same as small waves breaking on the shore. Water waves are only different at the very small scale, where surface tension dominates (capillary waves) there is no difference in the physics of straight 'gravity' waves.
Having observed many bow waves from ships as they meet a shallow shore, I have noticed the very same effect that tsunamis cause - once launched, the moderate bow wave carries on for a great distance and then peaks as it reaches the beach etc. etc.. Interestingly, the dissipation is not inverse square because the spreading is not over a sphere and it's not even 1/r because the wavefront of an established bow wave is more or less a straight line. I like to think in terms of Huygens construction (personal observation but justified, I think). A tsunami can be generated by a large aperture source like half an island appearing or disappearing so I imagine the source can be looked at as several wavelengths. Hence less dispersion of energy.
 
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