Is the water pressure below ocean waves constant?

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Discussion Overview

The discussion centers on whether water pressure at a specific depth below the ocean surface varies with the presence of surface waves. Participants explore the implications of wave crests and troughs on pressure readings at depths, considering factors such as wavelength and water movement.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants suggest that water pressure at a depth of 10m would vary with the height of surface waves, as the depth of water above changes with wave crests and troughs.
  • Others argue that the wavelength of the waves may not significantly affect pressure at that depth, especially if the water depth is much greater than the wavelength.
  • One participant notes that in shallow water waves, water at depth moves due to wave effects, while in deep water waves, this movement is not observed.
  • Another participant questions the necessity of water movement at depth for pressure changes, asserting that incompressibility of water means pressure can change without flow.
  • Some participants reference the dynamics of wave motion and pressure gradients, discussing how these factors might influence pressure readings at depth.
  • A hypothetical scenario involving a hollow cylinder submerged in water is presented to explore how pressure might behave in such a setup.
  • Concerns are raised about the lack of observed flow at depth for deep water waves, suggesting a potential lack of pressure gradient.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between surface waves and pressure at depth, with no consensus reached on whether pressure varies with wave activity. The discussion remains unresolved regarding the influence of wave characteristics on pressure readings.

Contextual Notes

Participants acknowledge the complexity of the topic, including factors like wave dynamics, incompressibility of water, and the effects of wavelength and depth on pressure measurements. Some assumptions about wave behavior and pressure gradients are not fully explored.

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  • #32
olivermsun said:
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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  • #33
Treva31 said:
If you are stationary say 10m below the ocean surface does the water pressure at your location vary with the waves.

For example:

When a wave crest is above you than means perhaps 12m of water is above you.
Then a few seconds later a trough is above you so only 8m of water is above you.

So you might expect your pressure reading to be going up and down with the waves?
No. It is rather complicated, because you have to include dynamic pressure, and not just the hydrostatic pressure (use Bernoulli's Equation). It's not static, if there are waves, and dynamic pressure comes from the fact that the water is moving and accelerating, which requires forces/pressure. How it behaves depends on the depth of the water. See here, e.g. http://web.mit.edu/2.016/www/handouts/Free-Surface-Waves.pdf
 

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