Deadly Down Current

[Doc Intrepid]

My only personal experience is trying to swim against reasonably strong outflow at Ginnie (although I'm informed that this year with the drought the outflow is diminished somewhat), trying to swim upstream against the flow of the Columbia river, drift diving against reasonably strong tidal flow in Palau, Guam, and Okinawa, and swimming against the tidal swing at various sites in Puget Sound.

Tidal swings vary, but they start off slow, pick up speed, are intense for a period of time that varies depending on the phase of the moon, then they taper off and finally end.

In terms of evidence regarding downwellings all I have to work with is anecdotal accounts gathered over the previous 4 or 5 years from online boards. Generally they describe a descent which is relatively uncontrolled but which seldom exceeds 130 to 150 fsw, after which the diver can conduct an ascent. (Yes, if they're out of gas then they have other issues as well...)

No CD, I've never been caught in a down-welling. But I've been caught in tidal surge and strong currents quite a few times in different environments, and its been my general experience that hanging on to a handy rock goes a long way to prevent yourself from being rolled around or blown off a dive site by them. I suspect the effectiveness on a vertical plane would be similar to the effectiveness on a horizontal plane.

but I could certainly be mistaken...

 

[Cave Diver]

Tidal swings vary, but they start off slow, pick up speed, are intense for a period of time that varies depending on the phase of the moon, then they taper off and finally end.

Ok, I was trying to gather a frame of reference for this. I dont think the flow at springs can be compared to downwellings. It's an apples/oranges thing.

I've done numerous ocean dives in strong current where the current was present the entire dive (100+ minute runtimes) or where the current was present over the course of several dives on the same site spaced an hour or so apart. (Oriskany and Spiegel Grove dives come to mind.) While these aren't wall dives, they are still ocean dives and I would expect behavior of current in these situations to be a more comparable example than rivers or caves. OTOH, they are in Florida and there may be conditions that affect their duration that aren't present in Mexico, so we might be back to the whole apples/oranges thing.

If there is a reasonable expectation that the current will abate in short time, then tucking yourself away out of the flow would seem a good alternative. I'm just curious how reasonable of an expectation that really is.

It would be interesting to hear from someone with more experience in this event.

 

[Solitude Diver]

I had a similar experience here in Puget Sound. Thought I would not live through it.

I was diving with a buddy off of Blake Island (Vashon Island side) one day around 70-80 feet. The current had stirred up debris, reduced vis, and in moments I was solo diving. Not a big deal except for the fact while I was looking for him, I was starting to get pulled down the slope. While trying to kick back up slope, I was soon winded, and over breathing my reg. (Heavily). I thought I would suffocate before drowning. I was pretty sure at the time that this was my last dive, and wondering if it had all been worth it. Many thoughts went through my head at that moment.

I had to fully inflate my (90 lb-tec) wing to break free, and was at about 110 feet and dropping by the time I did so. As soon as I started going up again, I dumped air and got control by 60 ft, and was able to hang off of my safety sausage on a spool and catch my breath. Although I was barely able to inflate it given my breathing problem, it self- inflated on the way up. I ended up in deco by the time I finally got up to 20, and was glad I had a pony for back up because I really sucked down some air. After doing my deco + 3, I was scolded by the dive leader on the boat for not immediately coming up to find my buddy. My thoughts…. If he was up, then he was safe. If not, no point in putting me in danger too. He was an experienced instructor, and I didn’t know where to find him anyway. As far as them finding me, my sausage was flying high and they were following it. Heavy breathing at 110 feet, quick partial ascent, and deco to boot.... not a good time to blow past even a few minutes of deco. (Especially having already gone for a chamber ride in the past due to DCS).

The point is while trying to suck in enough air to breath, and being pulled further and further down, with nothing to grab onto, there is no way anyone could tell me not to do anything but flare out. In a few more minutes, I WOULD have dropped my weights and solved the immediate problem of no air and going down further, THEN worried about the consequences when I got to the surface. I’d consider it again given the same circumstances too.

Note: with the Ebb the currents around that Island can get so strong they create down-wellings that actually create steps in the surface of the water that you can see (1-2 feet high from what I've heard). It is scary even in a small boat at the wrong times. There is one report of a diver actually getting sucked down there and never being seen again (I believe several hundred feet deep on the West side).

Scary stuff. Commands respect.

 

[String]

There have been several incidents and some fatalities here due to down currents that even with weight ditching and BC inflation prevent an ascent.
Bottom topography combined with tides and winds can cause it.

IF caught in one there is advice around on how to swim to get out of these but sometimes it just isnt possible, especially when narcosis starts to kick in if its a deep bottom.

 

[BigBubbaJ]

That makes me wonder if breathing gas with a MOD (for recreational divers) is more harmful given that this is always unexpected.

Was diving on nitrox 32 all last week, with some pretty steep slopes and dropoffs in a few spots. I was at 72ft on our deepest five (ie not very deep), and a 30 ft vertical downcurrent would have put me dangerously close the the MOD of nitrox 32... so add oxygen toxicity to the mix and it seems that it is very worth it to take note of these techniques.

 

[LarryHinDC]

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

This is good advice for slowing an uncontrolled ascent, but I think it's exactly the wrong idea here. Since the water is pushing you down, going horizontal is just going to give the water more surface area to push on. You need to minimize your profile to give the water less to work with.

 

[Doc Intrepid]

That makes me wonder if breathing gas with a MOD (for recreational divers) is more harmful given that this is always unexpected.

Was diving on nitrox 32 all last week, with some pretty steep slopes and dropoffs in a few spots. I was at 72ft on our deepest five (ie not very deep), and a 30 ft vertical downcurrent would have put me dangerously close the the MOD of nitrox 32... so add oxygen toxicity to the mix and it seems that it is very worth it to take note of these techniques.

At the risk of pontificating, this is sort of the key point:

Tide swings should not come as a surprise to any diver.

Tidal swings are predictable.

Tide charts are available on the internet for nearly any region on the globe.

Divers who dive in areas where tidal swings are powerful should be mindful of the timing of the tides, and should plan to dive the slack.

Tide charts provide a graphic that predicts the rising and falling of the tides, typically over 8 hour columns or so, for each day. Sport boaters, commercial fishermen, yachtsmen, scientists, and other communities of users rely on them. The graphic shows the hour the tidal swing will begin, the time the tide will be running, mean slack tides, and the time of day the water level will be highest and lowest. The relative intensity of the flow can be seen by the relative steepness of the curve - which represents water movement over time. The steeper the curve, the more intense the movement of water during that period of time (although flow can vary based on other factors during each period). Tide tables provide the same information, but using matrices of numbers.

Here are some sources that provide both charts and tables:

WWW Tide/Current Predictor - Site Selection

Tide Location Selection for California

Tide Tables, Charts and Graphs

NOAA Tides and Currents - Home

Here is one that I use that fits onto a Palm device that you can buy inexpensively off eBay and take out onto the charter boat with you:

Tide Tool for the PalmPilot

Here is the charter schedule for one of my favorite PNW charter boats:

deepseacal: November 2007

You might note that the Captain has noted on the calendar that some weekends offer “Good Tides”.

This means that the tidal swing is minimal, with a relatively longer slack period (which is either the top of each curve up, or the bottom of each curve down. "The slack" is the period of time during which the tide is neither coming in nor going out. It offers safer diving, generally speaking).

For divers, lower tidal variance means better visibility. High tidal variance means the rushing water picks up all sorts of particulate with a resulting drop in visibility. The higher the tidal swing, the lower the visibility.

So…the idea is that when you plan a charter trip you consult the tide charts, identify the weekends with the least tidal variance, then see if you can’t find a charter boat that has slots available on those weekends. (You’ll find that lots of other guys do this too – so weekends with desirable tides are the ones filled up first.)

OTOH, if you don’t do this, and are unaware of the times the tide will be running and how swift or intense the run will be on any given day, then you might find yourself caught somewhere in a down-welling and in the middle of what we call “an adventure”!



But generally speaking its better to plan to avoid conditions that lead to down-wellings, than to endure them.

As in all things, your mileage may vary…

 

[jeckyll]

Doc: To add a more international flavour, here is a link to the Environment Canada Tides and Currents page

CHS-Tides, Currents, and Water Levels

 

[Cave Diver]

At the risk of pontificating, this is sort of the key point:

Tide swings should not come as a surprise to any diver.

Tidal swings are predictable.

Tide charts are available on the internet for nearly any region on the globe.

Divers who dive in areas where tidal swings are powerful should be mindful of the timing of the tides, and should plan to dive the slack.


But generally speaking its better to plan to avoid conditions that lead to down-wellings, than to endure them.

As in all things, your mileage may vary…

Good post Doc!

 

[Dadvocate]

That is a great post, Doc. Anyone have ideas where I can get this info for the Philippines?

Cheers!