Water Movements – White Water/Surge/Inlets and Outlets/Tidal Currents/Surf

Because of the force of water movement, a diver can become a hostage to the sea.

White water

This water is white because of the foaming effect of air bubbles. This dramatically interferes with both visibility and buoyancy, as well as implying strong currents or turbulent surface conditions. A diver in white water is a diver in trouble. Under these conditions, the recommendation is usually to dive deeper.

Surge

The to-and-fro movement of water produces disorientation and panic in inexperienced divers, who often try to swim against it. Other divers use the surge by swimming with it, then hold onto rocks or corals when the surge moves in the opposite direction. This approach may be detrimental to the ecology, but good for survival.

Inlets and outlets

Occasionally, there is a continuous water flow, because of a pressure gradient through a restricted opening, which can siphon and hold (or even extrude) the diver. It is encountered in some caves, blue holes or rock areas near surf (an underwater ‘blow hole’), in human-made structures such as the water inlets in ships’ hulls and in outlets in dams and water cocks (taps). The pressure gradient may slowly draw the diver into its source and then seal him or her in, like a bath plug. Protection is by not occluding these inlets and by avoiding the area or covering it with a large grating.

Tidal currents

These currents are very important to the diver. If used correctly, they take the diver where he or she wants to go. Otherwise, they are likely to take the diver where he or she does not want to go. The latter event can be both embarrassing and terrifying, and it can also be very physically demanding.

Frequently, divers are lost at sea because of currents. Sometimes these currents can be vertical and cannot be combated by swimming or buoyancy. Certain popular diving areas, such as at Palau (especially Pelalu), Ras Muhammad, the Great Barrier Reef and Cozumel, are famous for their currents, and multiple fatalities are not uncommon.

Divers sometimes relate their successful swims against 4- to 5-knot currents. In fact, the average fast swim approximates 1.2 knots. For brief periods, it may be possible to reach up to 1.5 knots. The average swimmer can make very slow progress or none at all against a 1-knot current. A half-knot current is tolerable, but most divers experience this as a significant problem, and so it is. They tend to exaggerate the speed of the current as the hours go by, and especially during the après-dive euphoria (1 knot = approximately 2 km/hour).

Tidal currents are usually much faster on the surface than they are on the sea bed because of friction effects. A helpful observation is that the boat will usually face the current with its anchor upstream and the stern of the boat downstream. Any diver worth his or her salt knows that it is safer to swim against the current for the first half of the usable air and allow the current to bring the diver back to the boat for the second half of the dive. The ‘half-tank rule’ is worked out by taking the initial pressure, say 200 ATA, subtract the ‘reserve’ pressure (the pressure needed to charge the regulator), say 40 ATA, i.e. 160 ATA, and divide this by 2, i.e. 80 ATA. Thus, for this example, 80 ATA is used on the outward trip, and then the return is made with ample air to allow for misadventure (e.g. navigational error).

Untrained divers tend to make unplanned dives. They submerge and ‘just have a look around’. While they are having their look around they are being transported by the current, away from the boat, at a rate of 30 metres every minute in a 1-knot current. When they consider terminating the dive, after they have used most of their air, they have a very hard return swim against the current. They surface, because of their diminished air supply, well downstream from the boat and have to cope with a faster, surface current. This is a very difficult situation and far more hazardous, than that of the experienced diver who used the half-tank rule, who surfaced upstream from the boat and floated back to it, but who also had enough air to descend underwater and return with ease if desired or to rescue a companion.

The lines attached to the boat are of extreme importance when there are currents. First, there is the anchor line, and this is the recommended way to reach the sea bed upstream from the boat. The anchor chain should not be followed right down to the anchor because this may occasionally move if the boat moves, and it can cause damage to the adjacent divers. More than one diver has lost an eye from this ‘freak accident’. How may the diver reach the anchor line? A line may be attached to the top of the anchor line, with the other end to the stern of the boat. It should have enough play in it to allow divers to sit on the side of the boat and to hold it with one hand – the hand nearest the bow of the boat – while using the other hand to keep the face mask and demand valve in place. On entry, the diver ensures that he or she does not let go the line. The diver then pulls himself or herself forward to the anchor line and descends.

Perhaps the most important line, if there is a current, is a float line or ‘Jesus’ line. This line drags 100 metres or more behind the boat, in the direction of the current, and it has some floats to ensure that it is always visible to divers on the surface. It is often of value to have one diver on this line while the others are entering the water. The diver on the line virtually acts as a backstop to catch the odd stray diver who has not followed instructions and is now floating away with the current. The Jesus line is also of immense value at the end of the dive when divers have, incorrectly, exhausted their air supply or when they come to the surface for some other reason and find themselves behind the boat. This would not have happened had a dive plan been constructed and followed correctly. Occasionally, however, it does happen to the best divers, and it is of great solace to realize that the Jesus line is there and ready to save the sinner – irrespective of religious persuasion.

Even divers who surface only a short way behind the boat in a strong surface current may find that it is impossible to make headway without a Jesus line. If this is not available, they can descend and use their compass to navigate back to the anchor line or inflate the buoyancy compensator, attract the attention of the boat lookout and hope to be rescued.

Buddy breathing while swimming against a strong current is often impossible. Even the octopus (spare) regulator is problematic at depth or when two people are simultaneously demanding large volumes of air, typical of divers swimming against a current. An alternative air supply (a reserve or pony bottle) is of value, if it has an adequate capacity.

In dive planning, there should be at least one accessible fixed diving exit, easily identifiable, that serves as a safe haven. This may be an anchored boat, in areas with tidal currents. The safety boat is a second craft – not anchored – and this, like any boat that is driven among divers, needs a guard on its propeller. To attract the safety boat, various rescue options include the following:

  • A towed buoy.
  • An inflatable 2-metre-long bag, called the ‘safety sausage’, to attract attention.
  • Pressure tested distress flare (smoke/light).
  • Personal floatation devices.
  • Personal electronic, sonic or luminous location devices.

Divers can now carry a personal location beacon or emergency position-indicating radio beacon (EPIRB), especially of value if diving in fast currents. These devices need to be pressure protected and are of value only once on the surface.

There are other problems with currents, and these are especially related to general boat safety and ensuring that there is a stable anchorage.

When the current is too strong or the depth or sea bed is not suited to an anchored boat, a float or drift dive may be planned. This requires extreme care in boat handling. Divers remain together and carry a float to inform the safety boat of their position. It allows the surface craft to maintain its position behind the divers as they drift.

The concept of ‘hanging’ an anchor, with divers drifting in the water near it and the boat being at the mercy of the elements, has little to commend it. The raising of the diver’s flag under such conditions, although it may appease some local authorities, is often not recognized by the elements, reefs or other navigational hazards, including moored boats.

Some currents are continuous, e.g. the standing currents of the Gulf of Mexico, the Gulf Stream off Florida and the Torres Strait, but tidal currents are likely to give an hour or more of slack water with the change of tide. At these times diving is usually safer and more pleasant because the sediment settles and enhances visibility. To ascertain the correct time for slack water, reference has to be made to the tidal charts for that area. The speed of the current can be predicted by the tidal height.

Surf

Entry of a diver through the surf is loads of fun to an experienced surf diver. Otherwise, it can be a tumultuous moving experience and is a salutary reminder of the adage ‘he who hesitates is lost’. The major problem is that people tend to delay their entry at about the line of the breaking surf. The diver, with all his or her equipment, is a far more vulnerable target for the wave’s momentum than is any swimmer.

The warning given to surfers, referring to water colour, is that ‘White is right but green is mean and blue is too’. This ensures that the surfer enters the surf and avoids rips. For the diver, it is the opposite. The diver may use the apparently calmer water to ride the rip into the ocean.

When the surf is unavoidable, the recommendation is that the diver should be fully equipped before entry and not re-adjust face masks and fins until he or she is well through the surf line. The fins and face mask must be firmly attached beforehand because it is very easy to lose equipment in the surf. The diver walks backward into the surf while looking over his or her shoulder at the breakers and also toward a buddy. The face mask and snorkel have to be held on during the exposure to breaking waves. The regulator must be attached firmly to the jacket, with a clip, so that it is easily recoverable at all times.

When a wave does break, the standing diver presents the smallest possible surface area to it; i.e. he or she braces against the wave, sideways, with feet well separated, and he or she crouches and leans, shoulder forward, into the wave. As soon as possible, the diver submerges and swims (in preference to walking) through the wave area. If the diver has a float, then this is towed behind. It should never be placed between the diver and the wave.

Exit should be based on the same principle as entry, except then the surf is of value. The wave may be used to speed the exit by swimming immediately behind it or after it has broken. The float then goes in front of the diver and is carried by the wave.