Swell/Waves and rapid ascent

[Deefstes]

Interesting discussion. Could you perhaps clarify this for me, because I had an argument with a know-it-all DM a while back and I decided to to press the point as he was the DM and I was the lowly diver.

If you're on a 3m safety stop and a swell passes over you, do you move up/down with the swell relative to the surface or do you remain relatively motionless in the water but the amount of water above you increases/decreases?

 

[sschlesi]

Deefstes,

Generally your distance from the bottom will not change, the height of the water above you will change. If you are near a wall or the bottom, it is easy to observe you are not moving up and down much. As you near the surface you will start to move up and down with the waves, but even then you will not maintain a constant amount of water overhead.

Of course the specifics all depend greatly on the height, and speed of the waves. The higher and faster the wave, the deeper it will project it's force.

 

[Thalassamania]

sschlesi, that's not exactly accurate.

If you are a neutrally buoyant particle in the water you describe a circular path that maintains approximately constant distance to the surface. As you descend towards the bottom this circular path becomes an ever more flattened oval, till at the bottom it is virtually horizontal in the form of surge that runs with the direction of the waves. If a wave, however, breaks while it is over you there will be a sudden change in pressure and that's the concern as is the change in pressure as you approach the bottom and the crests and troughs pass over you.

 

[sschlesi]

sschlesi, that's not exactly accurate.

this circular path becomes an ever more flattened oval, till at the bottom it is virtually horizontal in the form of surge that runs with the direction of the waves.

You mean the bottom of the wave, not the bottom of the sea?

Granted in shallow water especially approaching shore the bottom of the wave and sea may be the same.

 

[scuba4]

Thanks to all for the great info.
It was Swell wave, not wind wave so the max swell was only every now and then during the sets. (swell here is total of trough to peak so as max of less than 2.5 not 5M) Also was mainly problem with the swell forming up on the reef, ocean floor around the reef was 16M and not too bad. Boat was well away from reef.
We did abort the dive due to the surge and changing pressure - only spent short time in the worst zone as we decided was not worth continuing the dive. It was a mistake to start the dive, would have been worse mistake to continue the dive. The pressure change was significant as ears required equalizing when the larger swell that came through a couple of times we were near the reef.

As most have advised, (and we have learnt from experience now without injury) we will not dive so shallow in any significant swell. Anything 2M or over is not worth it (unless we are diving much deeper I suppose)

Good news is the local conditions will improve for next 6 months, minimal swell YAY!

Thanks

 

[captain]

You mean the bottom of the wave, not the bottom of the sea?

Granted in shallow water especially approaching shore the bottom of the wave and sea may be the same.

The bottom of the sea. At that point the effect of the wave is to push you back and froth horizontally with little vertical motion. Think of a wave as a rolling tire. Just as the tire thread is mashed parallel to the road and that bit of tire thread is moving parallel the the road so is the bottom of the circular motion of the wave mashed flat by the sea bed.

If the depth of the water is greater than the circular motion of the water in the wave then the bottom has little to no influence on the wave. It's like a spinning tire that is not touching the road.

 

[Deefstes]

Just as the tire thread is mashed parallel to the road and that bit of tire thread is moving parallel the the road

Could you explain this because I really don't know what you mean?

 

[captain]

Could you explain this because I really don't know what you mean?

The part of the tire that is resting on the road is flat to the road surface. Asume that 6 inches of the tire thread is flat on the road. Although the tire is rolling the 6 inches that is touching the road is not moving in a circular motion, it is moving horizontally until it is no longer in contact with the road then it resumes circular motion until it contacts the road again. What don't you understand.

 

[Thalassamania]

Good simulation video

 

[sschlesi]

Great video. That simulation makes it easy to see how the effect of the waves drops considerably with depth. It's a bit hard to read but it appears that video is simulating waves between 3.4m and 6.8m high, I don't think I'd be diving that day.