Scubadoc’s Ten Foot Stop

June 27, 2009

Problems With Moving Water

Filed under: Article — admin @ 10:47 am
Problems With Moving Water


Avoid this Disaster

Accidents caused by divers being carried away by currents have been reported since the early days of diving in the 1950’s. Most of us have heard about the horror story of the five Japanese divers, who came up after a very beautiful and enjoyable current dive off Peleliu, only to find that they had not only missed their boat but had missed the end of the island. All of these divers died after a prolonged float in what must have been a strong current. The current was not at fault, however, but poorly qualified dive operators and faulty equipment, an uncertified diving guide, a small faulty single engine vessel without a radio and novice divers –all pointed toward this disaster. See Guidelines for the Abandoned Diver

Diving in currents is easily one of the most enjoyable – but can be one of the most dangerous dives that a diver encounters. It is a low energy high speed trip that allows you to see much more of the rapidly passing reef. The flip side of this is that you miss a lot of small life, and the hazards of loss of control and any efforts to work against the currents are high energy and stress producing. Recognizing that a problem exists is the first thing a diver has to face. Divemasters usually are on top of the problem and brief you appropriately.


Signs of A Current

Each diver can detect signs of current, such as:

1). fast moving surface water;

2). which way the boat is facing (depending on whichever is the stronger, the current or the wind);

3). movement of floating material on the water or in the water;

4). movement of divers away from the boat rapidly on entry;

5). bubbles moving away from a diver at an angle underwater;

6). if the plant life and soft coral underwater are laying down.

The contour of the ocean bottom will change currents, often dramatically. A diver can adjust his speed by moving to the bottom, slowing behind coral heads and outcroppings or holding on to permanent objects, all the time presenting the smallest frontal surface area to the current. Because of a “boundary layer” condition, water molecules that are closest to a surface move the slowest due to their nearness to the surface. Other areas of calmness are areas behind obstacles and the sides of walls. Getting close to the bottom and using your finger or dive knife to stabilize you is usually all you need to hold your position.

As the diver moves through the water column he encounters resistance-and this increases by the square of the velocity of the current as it passes over the body. As a diver works against the current along the bottom, a good indication of the amount of work being performed is by monitoring the respiratory rate, this rising exponentially as exertion increases. “Bottom crawling” is a technique that may have to be used when the swimming exertion level rises. This is easily done in rocky areas but can be a real challenge in sandy bottoms-where a good dive knife comes in handy as an anchor.

If conditions exist so that exertion levels continue to rise, it’s better to surface, inflate and wait. Here is where a safety sausage comes in handy. This is an inflatable, long red plastic tube that juts above the water 6-7 ‘ and can be easily seen as much as a half a mile or more. Shining a flashlight in the bottom of the sausage at night provides a long red light that can be seen at a great distance.

Surface floats are other techniques used in diving in currents. A line attached to a float and to a diver give two advantages; the dive group is marked for the boat operator and any tired or nervous divers can hang onto the line and rest. When drifting free without a line, the boat operator relies on visualizing the diver’s bubbles for location; a surface chop can make it difficult for the boat operator to see the divers bubbles,

Strong currents can rip away a mask or snorkel when turning sideways or looking up. Snorkels do better stored in the BC or under a leg strap underwater so as to reduce drag on the mask. Place a little extra tension on mask straps if strong currents are expected.

The dive group should have a clear understanding of exiting and entering the water in conditions of current. Divers should try to let the current work for them by initiating dives into the current on the first part of the dive and planning the return with the current. Float lines are essential for an orderly entry for the divers to hold position for descent and ascent, otherwise divers will be strung out too far apart for a safe dive. Entries should be timed so that there will be no stragglers and the descent be made under control. Exits are also planned so that the float line can be used to pull against the current rather than having to swim against it.

Currents are usually generated by wind and tides or a combination of the two. Predicting what you will encounter generally depends on using information from the local weather service combined with tide information from local dive operators. In certain areas, such as “The Great White Wall” in Fiji and “Blue Corner” in Palau, the currents are almost always due to tidal action and are fairly predictable. One should always go with local experience in making decisions regarding diving into currents.


Surges

Underwater motions occur in areas where swells are forced against a barrier of some sort, such as a beach, wall, rock, or wreck. A surge is a to-and-fro action complicated by indentations such as caves, rocks or large holes in wrecks. Surges can be used to aid in your movement, carrying you forward in one direction where you can stabilize yourself as the surge retreats, and move forward again with the next surge. They are also dangerous since they create huge forces that can carry you into places that you don’t want to go. Divers should remain distant from diver-size holes in wrecks and caves and learn to use surges as a method of aid in movement, either upward to get back into the boat or to move onto a ledge.

WRECKS

Wrecks create special problems with currents. Frequently the boat will anchor on the wreck, playing out enough scope so that divers can easily descend on the line. A tag line is helpful placed between the anchor line and the stern of the boat, facilitating the descent from the dive platform. Divers who lose contact with the line run the risk of being swept away from the wreck and the diveboat, sometimes requiring rescue after coming up predictably exhausted from fighting the current.

Adapted from Glen Egstrom, Ph.D.
Medical Seminars, Inc

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