Deadly Down Current

[Ayisha]

There were a couple of threads about downcurrents a couple of years ago at http://www.scubaboard.com/forums/accidents-incidents/115020-drag-down-current-cozumel-anywhere.html and http://www.scubaboard.com/forums/basic-scuba-discussions/48956-what-proper-procedure-downcurrent.html
Here's an article by docvikingo Upwellings and Downwellings, how to get out of trouble; February, 2001 Undercurrent feature story

Downcurrents are apparently created when the usual horizontal current converges with a current going down a wall, and this causes a massive fastmoving downcurrent.
I knew a couple who got caught in one in Cozumel. You're usually told to stay close to the wall where the current is slower, but I guess we have to be cautious as to how close we stay and to be aware of what the currents are doing in front of/around us.

As some others have said, these downcurrents are usually narrow and you can kick across them to get out of the downcurrent. Kicking up/fighting is less effective and tiring.

To avoid a downcurrent, you should:
-watch what the currents are doing in front of/around you
-watch the particles/sealife in the water for any unusual activity
-stay slightly away from the wall

If you're caught in a downcurrent, apparently you should:
-move across the downcurrent to get out of it
-inflate your BC
-create drag by spreading out your body horizontally
-inflate your lift bag if you have one

As Lynn said, I would be leery about dropping weights at depth, because you will have a whole other problem once you're out of the downcurrent - controlling your ascent and doing any safety stops. If you've been pulled to any significant depth, you may have obligatory deco stops as well. Rather than drop weights, do everything else you can.

 

[DandyDon]

Thanks for the UC link, Ayisha. I did not carry my pony on shallow dives at Cozumel until I got pulled off of the top of a wall - from 30 ft to 100 ft alone. :11: I agree with the author - swim away from the wall. I stated such in the Coz forum and was told by a local expert that I was a fool, swim to the wall - but I don't want to be between the two, trying to go up the wall and hit the down current again. I also do not want to argue with a local expert, nice to have the article from a renown diver.

 

[Doc Intrepid]

"...Would be curious as to how this occurence takes place in the ocean???"

Often, but not always, near the islands of the South Pacific these phenomena result from tidal displacement.

Think of islands and the coral reefs around them as high points in the ocean. When the tide goes out, large amounts of water move from the high points to lower points.

Most tidal swings are relatively low. Many "drift dives" are in fact dives where the diver is carried along by a relatively benign movement of water ebbing in or out with the tide.

But if you look at any set of tide tables, you will notice pronounced variance in them. Occasionally there are days with significantly higher water movement, concurrent with phases of the moon. On such days truly massive amounts of water may be moving with the flow of the tides. The significantly higher amounts of water "draining off the high points" (the islands and their reefs) as the tide goes out can result in downwellings along the walls and shallower points of the reefs.

Hope that explanation makes sense.

Doc

 

[Dadvocate]

Great stuff! Thanks a lot everyone for giving me resources to look throuhg and for putting this happenstance in perspective. I'm going to ask my friend to have a look as well. I think he'll be interested in the info you've provided.

Cheers!

 

[mkutyna]

If you're caught in a downcurrent, apparently you should:
-move across the downcurrent to get out of it
-inflate your BC
-create drag by spreading out your body horizontally

Er? That doesn't make any sense to me unless it means for you to flare out after you get out of the downcurrent. Flaring out in the middle of the downcurrent is just going to provide more surface area for the downcurrent to grab on to and pull you down faster. I hope I'm reading that wrong.

Are those steps to be taken in order or just multiple things that can be done?

 

[Ayisha]

Flaring out in the middle of the downcurrent is just going to provide more surface area for the downcurrent to grab on to and pull you down faster.

No, streamlining yourself vertically would make you drop quickly. Just as when you start a descent, streamlining everything in a straight line vertically and pointing down while exhaling makes for an efficient, effective descent using less weight - you'll drop like a rock if you let yourself. Counter to that, flaring out horizontally creates drag when slowing an ascent OR descent.

The suggestions are just multiple things that can be done. Time is of the essence - do whatever you remember as soon as you remember.
If you have a good horizontal profile in the water, that should help right off the bat. Inflating when negative should be second nature.
Swimming UP when pulled down would be intuitive, and this is where the brain has to kick in more to swim ACROSS the downcurrent.
Do whatever you can.
Try to be aware and avoid it if you can.

 

[jeckyll]

Ayisha: Flaring out when descending is done to change your speed to get closer to the vertical speed of the water around you (i.e. zero the water is not moving up or down).

If the water is moving at a rapid pace, and you flare out, you will again approach the speed of the water around you (i.e. going down faster).

Edit: To provide a visual reference, in the first case, think of a parachute on a drag racer, used to slow down.
In the second case, think of a sail on a sailboat. More sail = more surface area = more effect from the wind.

 

[Doc Intrepid]

If you're diving on a coral wall and you find yourself caught in a down-welling, don't try to swim anywhere, flare, or inflate anything. Its a bit tough to fight against tons of ocean water. Your CO2 levels will skyrocket.

Grab on to the wall.

Hang on.


The force of the current will diminish eventually, and you'll be fine.

 

[Ayisha]

There are several suggestions regarding creating drag. I have never been caught in a downcurrent, and that is why I stated "If you're caught in a downcurrent, apparently you should:" I have experience with current, and have tried to be careful for the last four years, since my friends were caught in a downcurrent. I was simply summarizing information. Whether any or all of them work is unknown to me. At the time you're swimming across, you're probably not creating drag anyway, and swimming across and out of the downcurrent is your ultimate goal.

People also say to hold onto almost anything you can to slow/stop your descent.

 

[Cave Diver]

The force of the current will diminish eventually, and you'll be fine.

Is this personal experience? How long will the current last before diminishing? How do you know? What happens if its pushed you deep and you incur deco obligation and/or start to run low on air while waiting for it to subside?

Just curious.