question regarding divers in Tsunami

[NU2DIVING]

After reading the articles regarding the couple that was scuba diving when the tsunami hit, I had a couple of questions. Maybe some of you can answer them. The husband mentioned that the bottom started to churn up to the point of white out and he had the sensation that he was ascending. However when he looked at his gage it was telling him that he was getting deeper. He said that he was getting sucked twords the bottom. I have not seen anything about how deep they were at the time the tsunami hit. I was wondering if you all think he was litterally being sucked or pushed down or do you think it was merely the wave passing over head. My thoughts are as follows. The bottom was starting to churn up. This could have been cuased by water being pulled out to see to feed the wave as it approached the shore. This would also account for the diver feeling like he was ascending at first. Then as the wave got on top of the him he would now be in deeper water as the apex of the wave crossed over him. This could affect his buoyancy and cause him to sink. His gauge would show that he was getting deeper as well as the wave passed over head. Then he said he saw sun light and was able to surface. When the wave completely passed over him he would potentially be close to the depth he was at before the tsunami hit.

I'm just curious if anyone can confirm my thoughts as being accurate or completely wrong.

Mike

 

[sharpenu]

Possible, since according to one article I read, they were diving a wreck. Depending on where they were, the wave height was reported to be up to how high? That would increase your depth by that amount. The wavelength of a tsunami is reportedly long.

 

[lamont]

based on some of the reading i've done so far, i'd guess that tsunami waves tend to 'break' in fairly deep water well offshore. and by 'break' i don't necessarily mean cresting action, but just that the outgoing surge of water and incoming surge of water collide and form a turbulent zone. that's probably one reason why the water is so muddy in all the videos we've seen.

i'm not sure how high a 30 ft tsunami is when its in 100 fsw deep water, but it should be less than 30 ft, so the reports of divers getting pushed down to 100 fsw from 30 fsw wouldn't merely be due to the height of the water overhead changing.

during a tsunami a lot of water has to flow onshore and that water has to come from somewhere and some pretty violent currents will exist where that water is moving rapidly. i'd think there would be more than enough bulk flow of water to kick a diver all over the place, even significantly offshore...

 

[Robert Phillips]

Eathquake induced tsunamis are ground swells and ground swells do not have the same characteristics as surface swells or capillary waves. From what I understand of wave dynamics, these types of swells carry a tremendous force in deep water and are not as noticeable on the surface because of the length of the wave. If you notice on most of the videos you can see on the tele, it seems that the water level raises and then you see the inflow of water. That inflow is the raising of the water level accompanied by the force of the swell. It's similar to a flood. The waves don't act like normal waves that you see at the beach where you see water rushing out to replace the trough that divides the beach from the crest due to the wavelength of these type of waves.
I am not an expert by any means, just remembering things learned ages ago in JC.
Some one feel free to correct me if I'm way off base here.
Robb

 

[lamont]

right, but you've got millions of gallons of water (and current) in those outflows and inflows, and at some point offshore those flows are going to meet and the current will become turbulent.

i read somewhere online that you can actually get a breaking-wave effect from a tsunami as well, but only with certain kinds of shoreline features.

 

[--tom--]

you may wish to look at http://www.noaa.gov/tsunamis.html for more details on tsunamis

 

[Addict]

I have a letter from a couple who were diving off the Similan Islands off of Thailand. They actually were able to maintain a depth of 30ft through the Tsunami which swelled 30 ft higher then the normal surface.

Luckily their entire boat of people made it through, though they surfaced about a mile apart.

Amazing experience. The details are just devastating. I couldn't imagine.

 

[donacheson]

After reading the articles regarding the couple that was scuba diving when the tsunami hit, I had a couple of questions. Maybe some of you can answer them. The husband mentioned that the bottom started to churn up to the point of white out and he had the sensation that he was ascending. However when he looked at his gage it was telling him that he was getting deeper. He said that he was getting sucked twords the bottom. I have not seen anything about how deep they were at the time the tsunami hit. I was wondering if you all think he was litterally being sucked or pushed down or do you think it was merely the wave passing over head. My thoughts are as follows. The bottom was starting to churn up. This could have been cuased by water being pulled out to see to feed the wave as it approached the shore. This would also account for the diver feeling like he was ascending at first. Then as the wave got on top of the him he would now be in deeper water as the apex of the wave crossed over him. This could affect his buoyancy and cause him to sink. His gauge would show that he was getting deeper as well as the wave passed over head. Then he said he saw sun light and was able to surface. When the wave completely passed over him he would potentially be close to the depth he was at before the tsunami hit.

I'm just curious if anyone can confirm my thoughts as being accurate or completely wrong.

Mike

Without digging out my old textbooks, it's probably reasonable to assume that the divers experienced a very complex situation depending upon the depth of the water at their dive site and the bottom contours.

In deep water, wave dynamics are easilyl understood. The motion beneath the wave is roughly circular, the diameter of the circles diminishing exponentially with depth. The longer the wavelength (distance between crests), the deeper the motion penetrates. The pressure change at depth as the wave passes overheads also decays exponentially with depth; in other words, the maximum pressure (in atmospheres) isn't simply the undisturbed depth plus the wave height dived by 33 - it's less and the excess diminishes with depth.

Tsunami's are very long waves, so they reach deep. I'd expect as a tsunami wave approached shore, the circular water motion near the bottom would stir up silt and push a diver about, but the deeper the diver, the less the motion. As the wave continues shoreward, the bottom slows the base of the wave, forcing the wave to build vertically, crest, and eventually break. One sees the same sort of behavior, but on a much smaller scale, when ocean waves approach a beach.

 

[jonnythan]

I have various videos of the tsunamis making landfall. A few large (not gigantic, more like 10-12 feet tall) waves accompany what looks like a simple rise in the water level. This makes sense to me. The "wave" would be hardly noticeable in deep water since the wavelength is incredibly long (hundreds of kilometers) and the height is only 30 feet or so.

Picture a wave 30 feet tall and 100 miles long making landfall. It won't be a large breaking wave, it would appear to be the water simply rising since it's not at all steep. The water did recede slightly beforehand, which also makes sense.. that's simply the bottom of the wave.

It's quite surreal.. bright blue skies, but all of a sudden the ocean rises 30 feet over the course of a few minutes and floods everything in sight.

 

[Seadeuce]

Surface swells affect depth at a ratio of about 2:1, meaning that if the distance between the top of the swells is, say, 100ft, then it will cause a disturbance down to a depth of 50ft. If the swell is shorter, 40ft, the depth is 20ft etc.

It must follow that a tsunami wave, with a much elongated circular motion underwater, will cause disturbance at depths well beyond air diving range. Hence, a diver will experience turbulence,
but unless he is near-shore, will not undergo too great a pressure differential. When an ordinary swell passes over a diver's head he will remain at the same pressure unless he is gripping a shotline.

I believe that being underwater is probably less dangerous, unless of course one is too near the shoreline, when receding water would equal a rapid ascent for the diver.

Seadeuce