blue steal
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It has been a good thread.
Dealing with Downcurrents
- Thread starter Dave Dillehay
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We are 'vacation' divers - only diving a few times a year, and only as our bosses let us....
With that said, we were in COZ last week (Mar 21 - 28) - enjoyed it much, except got caught in what our divemaster called a 'water tornado'.
Thank you all for posting and making folks aware. I wish I'd read this thread before we went.
We were diving nitrox - for both morning dives - nice, long and mostly shallow dives...when we'd gotten out past the edge of the reef on a dive. Sorry, but I don't remember the site. There were more than a handful of these downwellings surrounding us and the boat. Some appeared to be only a foot to a couple of feet across and others seemed quite big - maybe 2 yards? We were surfacing from our dive when we were pulled down to over 60 feet in a blink of an eye. My instinct was to swim sideways - out of it - but was caught completely by surprise and completely unaware of what "IT" was and how to 'run away'. We did get up OK and had no issues.
Please do share this thread with divers you know - planning and watching for these things should be part of the COZ springtime dive plans / announcements DM's give. Especially for those of us who are 'recreational' divers...
With that said, we were in COZ last week (Mar 21 - 28) - enjoyed it much, except got caught in what our divemaster called a 'water tornado'.
Thank you all for posting and making folks aware. I wish I'd read this thread before we went.
We were diving nitrox - for both morning dives - nice, long and mostly shallow dives...when we'd gotten out past the edge of the reef on a dive. Sorry, but I don't remember the site. There were more than a handful of these downwellings surrounding us and the boat. Some appeared to be only a foot to a couple of feet across and others seemed quite big - maybe 2 yards? We were surfacing from our dive when we were pulled down to over 60 feet in a blink of an eye. My instinct was to swim sideways - out of it - but was caught completely by surprise and completely unaware of what "IT" was and how to 'run away'. We did get up OK and had no issues.
Please do share this thread with divers you know - planning and watching for these things should be part of the COZ springtime dive plans / announcements DM's give. Especially for those of us who are 'recreational' divers...
Skinsfan1311
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On 3/day, I usually use Nitrox on the second dive to reduce the adjust air depth, expecting that the boat will do a shallow reef for the third dive anyway.
More common in Coz is 4/day: 2 - 2 dive trips, one morning and the other afternoon. Expecting the third dive to be deeper than the second, I use Nitrox on the third, but then I carry air in my 19 cf pony - just in case.
DjDiverDan
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I dive Cozumel regularly (one, twice, sometimes three times a year - 4-7 days, depending on when I can get away), and I will not use Nitrox on first dives. First, when you order Notrox from the Dive Operator, you don't know exactly what you are going to get; if you end up with 36% O2, that makes even an 80 foot dive a little iffy, with little room for error. at 90 feet, your 36% Nitrox gives you almost 1.5 AtmPPO2, which is more that I am comfortable with. My normal practice is to only order Nitrox for second dives; usually only on last day of diving, and then, if I get anything more than 32% O2, I make sure to stay above 60 feet.
As to Cozumel downcurrents, the only two locations I've seen/experienced downcurrents are Santa Rosa Wall and Barracuda, and in both locations, I was always forewarned in pre-dive briefings by DMs (with Aldora and Dive with Martin), though I understand that such warnings are not universal among Coz Dive Ops (IMO, they should be). In both locations, in my limited experience, the downcurrents are intermittent and, at depth, do not run right up against the wall. If you are going across the wall at 100', at 3-4 feet away from the wall, you likely won't even experience a downcurrent. At 10-18 ft. away from the wall, it might sweep you down to 130-140' before you know it. Advice I was almost uniformly given on pre-dive briefings was: (1) stay close to the wall; (2) check guages frequently when you approach depth limit; and (3) if caught in a downcurrent, do not try to fight it directly, and don't fill BC with air or drop weights (both are potential recipes for disaster); rather move as close to wall as possible (where almost always you will escape downcurrent), keep moving along wall (downcurrents usually are no more than 25-50 feet wide), and, only if absolutely necessary, use handholds on wall to ascend. I've never experienced a downcurrent when I stayed fairly close to the wall, and the only time I saw a diver get caught in one, she was out 15-18 feet away drom the wall, and dropped down to 140 ft fairly quickly, but escaped it when I went after her & dragged her back to the wall. Once back close to the wall, ascent back up to safe limits was easy.
DjDiverDan
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"The current interacting with the wall is what is causing the downward flow."
Actually, that's not really right. It's the current coming away from the shoreline (really, return of ocean surge), usually funneled by structure on the sea floor, and coming over the top of the wall, and interacting with the surge coming in toward shore than causes the downwelling. And, as a matter of basic hydrodynamics, the irregularity of the wall surface (i.e., coral growth, nooks, crannies, & outcroppings, etc.) causes turbulence in the boundary area which interferes with any downcurrent, usually forcing it out at least 4-6 feet away from the wall surface, and, at a minumum, making any downcurrent significantly weaker at the wall surface. If you want to check the hydrodynamics, check on the flow of water forced through a smooth 6" pipe; the flow is NOT uniform; turbulence at the boundary area (i.e., closest to the inside surface of the pipe) will significantly slow the flow in that area, while forcing a venturi effect, speeding and spinning the current in the middle of the flow. I have some experience with instrumentation used in flow meters for the chemical and petroleum industry; the mathematics used to model fluid flows through a closed system is really fascinating.
Actually, that's not really right. It's the current coming away from the shoreline (really, return of ocean surge), usually funneled by structure on the sea floor, and coming over the top of the wall, and interacting with the surge coming in toward shore than causes the downwelling. And, as a matter of basic hydrodynamics, the irregularity of the wall surface (i.e., coral growth, nooks, crannies, & outcroppings, etc.) causes turbulence in the boundary area which interferes with any downcurrent, usually forcing it out at least 4-6 feet away from the wall surface, and, at a minumum, making any downcurrent significantly weaker at the wall surface. If you want to check the hydrodynamics, check on the flow of water forced through a smooth 6" pipe; the flow is NOT uniform; turbulence at the boundary area (i.e., closest to the inside surface of the pipe) will significantly slow the flow in that area, while forcing a venturi effect, speeding and spinning the current in the middle of the flow. I have some experience with instrumentation used in flow meters for the chemical and petroleum industry; the mathematics used to model fluid flows through a closed system is really fascinating.
Prop Joe
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When diving up north in the "washing machine" current that is sometimes present we are always told in the pre-dive to stay close to the reef as possible without hitting anything. We always duck into nooks and cranny's to get out of the current when able at Barracuda. There can be some interesting things hiding in the lee of the current. Barracuda reef can be a wild ride ride though. San Juan has humbled me with a ripping current before. One day at San Juan I was stuck at about 80-90' and had to swim laterally to get unstuck from the bottom as the reef was ending and the dive was called. We were on the big boat that day and there was another group behind us. One gent was stuck in the same place, same down current, he did the same as me to get out of it. I could tell by his wide eyed boarding of the boat that he hit the same down currents that I did. My rescue training dive master (Liang) was also on the dive and I thanked her for the training she had given me to react correctly. That being said I've been to San Juan on days when you could do a resort course because there was so little current. Just depends on the day.
Edit:
Check out this video taken at San Juan. Its only about 30 seconds. Notice how fast we are going on the beginning then it slows. Bubbles are going sideways as well. Not a good day to stop and take pictures.
vf31joe's Channel - YouTube
Edit:
Check out this video taken at San Juan. Its only about 30 seconds. Notice how fast we are going on the beginning then it slows. Bubbles are going sideways as well. Not a good day to stop and take pictures.
vf31joe's Channel - YouTube
Last edited:
DjDiverDan
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When diving up north in the "washing machine" current that is sometimes present we are always told in the pre-dive to stay close to the reef as possible without hitting anything. We always duck into nooks and cranny's to get out of the current when able at Barracuda. There can be some interesting things hiding in the lee of the current. Barracuda reef can be a wild ride ride though. San Juan has humbled me with a ripping current before. One day at San Juan I was stuck at about 80-90' and had to swim laterally to get unstuck from the bottom as the reef was ending and the dive was called. We were on the big boat that day and there was another group behind us. One gent was stuck in the same place, same down current, he did the same as me to get out of it. I could tell by his wide eyed boarding of the boat that he hit the same down currents that I did. My rescue training dive master (Liang) was also on the dive and I thanked her for the training she had given me to react correctly. That being said I've been to San Juan on days when you could do a resort course because there was so little current. Just depends on the day.
Edit:
Check out this video taken at San Juan. Its only about 30 seconds. Notice how fast we are going on the beginning then it slows. Bubbles are going sideways as well. Not a good day to stop and take pictures.
vf31joe's Channel - YouTube
Nice video, Joe - that is the kind of current that I would just as soon avoid; it's too fast to let you see anything, and if I want a roller coaster ride, I'll go to Six Flags. But to each his own; I'm certain some divers consider these dives thrilling. Let me add San Juan (along with Barracuda) to my list of Cozumel dive sites that I'll take a pass on when the current is running fast.
Mike
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How you gonna know?
DjDiverDan
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Maybe ask the DM? Seems to me that a pre-dive check by the DM at sites like these would be a good idea.
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