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Hans
2008-Apr-09, 06:33 AM
Question on the speed of water rise

If you have a large body of water (ocean size) and introduce at one point along its perimeter a volume of water that will raise the entire ocean 10 cm higher how fast does that increase in water height spread?

neilzero
2008-Apr-09, 07:39 AM
The added water would be a wave and waves travel about 20 kilometers per hour = 480 kilometers per day = 3360 kilometers per week. Neil

mugaliens
2008-Apr-09, 05:03 PM
The added water would be a wave and waves travel about 20 kilometers per hour = 480 kilometers per day = 3360 kilometers per week. Neil

Er....

I don't think I've ever disagreed with you before. Perhaps I have, but I don't recall.

The velocity of a water wave is dependant on several factors, including it's height (the amplitude), it's wavelength (measured as crest to crest), and the medium through which it's travelling (let's assume sea water).

I've observed sea water waves travelling at less than two mph (calm beach, small waves). I've also observed them travelling at around 35 mph (Oahu's North Shore).

Tsunamis can travel up to 500 mph.

An aside note - I observed a very small tsunami in real life in California around 2003. That's another story...

I don't know what the formula is for calculating the velocity of a water wave, but from what I've observed, it depends more on it's wavelength than on it's amplitude.

Jeff Root
2008-Apr-09, 07:50 PM
My understanding is that waves are mostly on the surface of the water, but
go down to a certain depth. If the water is deeper than that, the depth of
the water has no effect on the speed of the wave. If the water is shallower
than that depth, the wave will speed up or slow down, causing both the
wavelegth and wave height to change as well.

For the original question, presumeably the new water could be added in such
a way that it makes only a single swell, rather than a series of waves. That
would probably be far easier to do by immersing a carefully-shaped solid body
into the water at just the right speed, instead of adding more water.

-- Jeff, in Minneapolis

mugaliens
2008-Apr-10, 07:45 PM
My understanding is that waves are mostly on the surface of the water, but
go down to a certain depth. If the water is deeper than that, the depth of
the water has no effect on the speed of the wave.

True.


If the water is shallower
than that depth, the wave will speed up or slow down, causing both the
wavelegth and wave height to change as well.

When waves hit shallow water, the water actually increases it's amplitude while simultaneously retarding it's ability to propogate.

Waves are very misunderstood! They don't actually move. Rather, the upwelling results in a horizontal, but very limited displacement laterally, which causes an adjacent upwelling, etc. Thus, they propogate.

When they hit the shore, that lateral displace remains unimpeded on the surface, but is impeded along the bottom. The bottom impediment results in an upward defelection of water, increasing the wave's height. Since the top is unimpeded, the wave breaks (curls).


For the original question, presumeably the new water could be added in such
a way that it makes only a single swell, rather than a series of waves. That
would probably be far easier to do by immersing a carefully-shaped solid body
into the water at just the right speed, instead of adding more water.

-- Jeff, in Minneapolis

That's easy - just mount a pool next to a reflecting wall. Drain the pool then rapidly lower the seaside wall. The water rushes in, reflects off the reflecting wall.

Result: one wave.

They do it in water parks all the time.

This is why tsunamis are so dangerous. Most are caused by undersea landslides (not the actual earthquake - it's just than the earthquake triggers the landslide; a few are caused by upheavals from earthquaks or volcanic explosions, but these are rare).

The earthen matter dislodges in a landslide slides downward. This results in a rather large (diameter) but very shallow (like 300 mm aka about 1 ft) depression in the ocean. In fact, if you were in a boat where occured, you probably wouldn't even notice it unless you were paying very close attention to the horizen - it would rise relative to your position every so slightly.

This downward depretion propogates first, followed by an upwards upheaval, which is again very wide in diameter, but very shallow.

This is why tsunamis are so dangerous. When the downward depression reaches the shore, it simply causes the tide to go out. If you're ever at the beach and this happens rapidly, don't go out and pick up the starfish! Run for high cover, as it's twin reflection, the high portion, isn't far behind!

That's what rolls in. Almost never as a traditional-looking wave so it doesn't look dangerous. But it is. It just looks like the tide rolling in, and that's exactly what's happening. But the surge is a good 20 ft higher than any other tide, so it completely refills what was just let out, the proceeds to flood inward to depths upwards of 10 feet and to distances which can reach miles inland.

Most people survive this part. It's when they, floating in the water amidst all that debris it created get pulled back out to sea that they die.

The exit currents can often be double the speed of the initial incoming current, depending on the toplogy of the land.

Why? Because the incoming current isn't a current. It's a tidal front, which is a kind of wave. The exit current isn't a wave. It's raging, very wide river, and that's what causes the most destruction.

By the way, the universal wave equation is velocity=frequency x wavelength. So, for the same frequency but with a very long wavelength, you get a very fast wave!

And incredibly powerful. In fact, two weeks after the Sumatra tsunami, various instruments showed that the Earth was still ringing like a bell. Not loud, but the evidence of the wave was still travelling around the planet.

Two weeks.

Here's more (http://www.abelard.org/briefings/tsunami.php), if anyone is interested.

Hans
2008-Apr-11, 03:32 PM
Thanks Gentlemen - probably more information than I needed!

hhEb09'1
2008-Apr-11, 04:12 PM
probably more information than I needed!mugaliens loves this stuff

neilzero
2008-Apr-12, 06:02 AM
Only mugaliens and I mentioned numbers, and I was near the middle of his range. It occurs to me that the tide circles the Earth in about 24.5 hours, so we could assume a bit over 1000 miles per hour. In a real experiment, the tide would modify the distribution of the rise, and some oscillation would occur. It appears that your question should have an answer, but we did not find it. If your water addition behaves like a tsamii, and 500 miles per hour is average speed, some locals would experiece the first wave in 24 hours at a distance of 12,000 miles, even sooner, if the tide assisted. Other locations would be skipped (or mostly cancelled by the tide) and the rise might be delayed by weeks. I think we need more analysis. Neil

Hans
2008-Apr-14, 06:11 AM
Keep me informed please, this information is useful in debunking claims of unknown civilizations being "suddenly swamped" by raising sea levels caused by melt water from the ice age

mugaliens
2008-Apr-16, 04:08 PM
Only mugaliens and I mentioned numbers, and I was near the middle of his range. It occurs to me that the tide circles the Earth in about 24.5 hours, so we could assume a bit over 1000 miles per hour. In a real experiment, the tide would modify the distribution of the rise, and some oscillation would occur. It appears that your question should have an answer, but we did not find it. If your water addition behaves like a tsamii, and 500 miles per hour is average speed, some locals would experiece the first wave in 24 hours at a distance of 12,000 miles, even sooner, if the tide assisted. Other locations would be skipped (or mostly cancelled by the tide) and the rise might be delayed by weeks. I think we need more analysis. Neil

Well, the Earth's tide isn't actually a wave, but a bulge caused by a varying gravimetic potentional from the sun and moon. If the Moon were to suddenly vanish, that would create a wave. Don't know what the speed would be, but I doubt it would be 24 hours, as the wavelength would be half way around the Earth. Probably closer to minutes, and the oceans would literally "ring," causing many very fast changes in tides for a while until things settled down.