Swell/Waves and rapid ascent

[scuba4]

Hi
Not sure if correct location for this post?

Diving in swell of 2.5Metres on a reef that was around 12 meters deep. (a few holes/caves etc. My dive computer was going crazy every time a large swell passed over head as it though I was doning a rapid ascent. The waves were probably building to 3 metres on top of the ree (but not breaking). My depth gaguge was changing rapidly from 12 to 15m and then back again to 12M (obviouvsly as the pressure was changing due to the wave height). Because of the rapid chagne back from 15 to 12Metres the compluter was advising it was a rapid ascent. (each time a wave passed - only the larger ones)

It was not a very good dive (due to the surge & reduced viz due to the swell) so I woul dprobably not dive in similar situation again.

Question - Is there a impact on dive profile as these are actually very rapid pressure changes down and then back up? The computer advised mandatory stop at 3-6M for 5 minutes due to the 'rapid' ascent'. We extended our safety stop over this time just to be sure.

Any comments/information relating to pressure changes/wave etc?

 

[captndale]

You are not going to have decompression issues on a 12 metre dive. Your biggest problem will probably be holding your position and buoyancy in the surge and rapidly changing pressure.

 

[cloudswinger]

The depth gauge is being accurate, the pressure is increasing because there's more water above you, hence your depth is changing. If you do go out in swell that big again, go for a deeper dive, since depth will mitigate the effects of the surge.

 

[HowardE]

Diving in 8 foot (2.5 M) seas is questionable at best in the first place. At a depth of 40' (12 M) your depth is changing by a net of 16 feet with each passing wave. Since the wave has 2.5 M up and 2.5M down. So in essence, you were constantly bouncing between 32 and 48 feet (9.5 - 14.5M)? This is a lot and ultimately can effect your profile, and bubble formation, even at that relatively shallow depth. Your computer was right to freak out. Surface conditions should always be considered before getting in the water... AND... the question shouldn't just be "how are the conditions now", but also, "how do I think the conditions will be in an hour"? Are the conditions getting worse? What if when you're done, they are 3-3.5M swells?

 

[rstofer]

There was a discussion a while back concerning beach entries through heavy waves. One thing that came up was using a snorkel rather than a regulator. If you were 3 or 4 feet under water when you took a breath from the reg and then, when the wave passed, you were back at the surface, you had a real good chance of getting a gas embolism.

I don't know why it would be different in the case of being beyond the surf line. I would be very concerned about the rapid changes in depth. Certainly, as you go deeper, the percent change in depth is much less but doing a safety stop at 15' might not be safe at all.

Keep that airway open!

Richard

 

[gcbryan]

I wouldn't want to dive in conditions like that especially if you couldn't get deeper and had to spend the entire dive like that.

I've been in conditions where we did out extended safety stop at 30 feet because of excessive up and down movement at 20 fsw and above.

Being stuck at that depth would be good at all (or fun).

 

[sschlesi]

Diving in 8 foot (2.5 M) seas is questionable at best in the first place. At a depth of 40' (12 M) your depth is changing by a net of 16 feet with each passing wave.

Your depth may change by 5m (assuming nice symmetrical waves), but the pressure on your body is NOT changing by 0.5bar. The weight(pressure) of the wave does not act straight down to the seafloor, it is diffused across all the water underneath it. If you are 1cm. underwater, and a 1m wave passes over you then you will experience an increase in pressure very close to ~0.1bar. However as you get deeper under the water the pressure difference caused by the wave will rapidly diminish.

A common diving situation which demonstrates this to be true is doing a 3m safety stop underneath 1m (trough to peak) waves. If the pressure acted straight down, everyone's pressure gauge would be rapidly swinging up and down by 1m. In reality the pressure changes so little, you would be hard pressed to notice any change on the gauge.

 

[cloudswinger]

Diving in 8 foot (2.5 M) seas is questionable at best in the first place. At a depth of 40' (12 M) your depth is changing by a net of 16 feet with each passing wave. Since the wave has 2.5 M up and 2.5M down. So in essence, you were constantly bouncing between 32 and 48 feet (9.5 - 14.5M)? This is a lot and ultimately can effect your profile, and bubble formation, even at that relatively shallow depth. Your computer was right to freak out. Surface conditions should always be considered before getting in the water... AND... the question shouldn't just be "how are the conditions now", but also, "how do I think the conditions will be in an hour"? Are the conditions getting worse? What if when you're done, they are 3-3.5M swells?

It's not 2.5 m up and 2.5 m down(unless you're in Hawaii, where they measure waves differently!). It's 2.5 m total. The trough is generally below the sea surface and the peak above the sea surface. Like a sine wave, there's a base line, peak and trough.

Your depth may change by 5m (assuming nice symmetrical waves), but the pressure on your body is NOT changing by 0.5bar. The weight(pressure) of the wave does not act straight down to the seafloor, it is diffused across all the water underneath it. If you are 1cm. underwater, and a 1m wave passes over you then you will experience an increase in pressure very close to ~0.1bar. However as you get deeper under the water the pressure difference caused by the wave will rapidly diminish.

A common diving situation which demonstrates this to be true is doing a 3m safety stop underneath 1m (trough to peak) waves. If the pressure acted straight down, everyone's pressure gauge would be rapidly swinging up and down by 1m. In reality the pressure changes so little, you would be hard pressed to notice any change on the gauge.

At 1m wave height, the pressure change is not really measurable, at least with analog gauges, but with 2.5m wave height, the pressure change is more obviously measurable. Anyway, that's fairly big seas to be going diving in, so I wouldn't. Generally there are small craft advisories at about 4-5 feet seas, which is smaller than that size. I don't know any dive boats around here that would go out in seas that big, it's just generally unpleasant to be in a boat, much less try to go diving.

 

[captain]

There is a difference between a wave and a swell. Swells have a long period compared to waves.
A 2 meter wave with a 5 second period is rough as hell but a 5 meter swell with a 30 second period would be very easy to handle.

 

[sschlesi]

There is a difference between a wave and a swell. Swells have a long period compared to waves.
A 2 meter wave with a 5 second period is rough as hell but a 5 meter swell with a 30 second period would be very easy to handle.

Good point, and the longer the period, the more the swing in pressure it will cause at a given depth.