# Why does the speed of the water wave decreases as depth decreases?

1. Jul 24, 2013

### tlfx1996

Why does the speed of the water wave decreases when the water wave approaches the shallower region? Is it because of the frictional force between the water molecules and the seabed? I have searched the net but i didn't get any clear answer to this questions. Please explain in details of the phenomenon without solely based on the formula, thx.

2. Jul 24, 2013

### HallsofIvy

Yes. The friction between the water and the sea bed robs the wave of energy so that it cannot raise up as high.

3. Jul 24, 2013

### tlfx1996

but I still couldn't get a clue how does the change from the orbital movement to the elliptical movement will causes the speed to slow down.

4. Jul 24, 2013

### Naty1

That's a complex phenomena..as is evidenced by continued failures to prevent erosion....

but there are some good insights here:

http://web.utk.edu/~cnattras/Physics221Spring2013/modules/m11/Water_waves.htm [Broken]

More here: http://en.wikipedia.org/wiki/Water_waves#Wave_shoaling_and_refraction

I find this second article strange in that they neglect to mention the effects of opposing currents in the very first section but do properly attribute effects of currents in 'shoaling'. I can tell you from long personal experience that opposing current in shoal AND deep water significantly effects wave heights. In fact, current alone usually causes waves to form.

Last edited by a moderator: May 6, 2017
5. Jul 24, 2013

Viscosity (essentially friction) would certainly play a part. The other issue that immediately comes to mind is that as waves come to shore and grow, they become nonlinear and end up Rollin over and creatin whitecaps. These will dissipate a fair amount of energy.

6. Jul 25, 2013

### Lsos

I don't know much about waves, but looking at this GIF from Wikipedia:

Might it have to do with the fact the wave has to keep the same frequency? I don't know if that's true under certain circumstances. If so, then looking at the GIF it kind of makes sense that the amplitude would rise due to the shallow water. The rise in amplitude would shorten the wavelength, and in order to keep the same frequency, the speed has to slow down.

Don't know if any of what I wrote is true or makes sense...

Edit: Following the Wikipedia article, I reached another article on "wave shoaling". I just skimmed over it, but it seems that the answer would lie somewhere in that term.

I suspect friction is not the prime driver for the reduction in speed....

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