Downdraft currents such as laminar flows affect wall diving. Divers should be ready to adjust their equipment, particularly buoyancy compensators, to counteract the downward pull of such currents.
Most of the physical properties that can “get you” when diving are either visible or you can see their effects – such as wind, waves, surf, animals, rocks or sunlight. There are a few, however, such as currents, that come out of nowhere; they function like invisible hands that grab and take you where they want to go.
You may not have ever heard of two currents that affect wall diving: down-welling, the most common, and laminar flow, the technical one. Laminar flow is “a smooth, streamline type of viscous fluid motion characteristic of flow at a low to moderate deformation rate. The name derives from the fluid’s movement in orderly layers of laminae without the formation of small eddies or irregular fluctuations.”
Laminar flow excites fluid dynamic engineers but downwelling affects divers more. In a laminar flow, as the rate increases, the streams continue to flow parallel until a velocity is reached at which the stream wavers and suddenly breaks into a diffused pattern. While you probably have heard of or been moved around by a downwelling current, you would not recognize a laminar flow if it hit you.
When a wall is relatively close to shore and a tidal change is taking place, there can be a backwash of water that seems to flow downhill and wash over the edge of the wall. It is also possible for the wind to whip up waves, which then strike a reef and cause a bottom current. Some of these current paths are well established and divers with local knowledge avoid the areas. Others can be random and it can be hard to predict the beginning time, duration or intensity. When diving areas are known to have currents, it is advisable to consult and/or dive with people with local knowledge. So, what does all this mean for divers once they are in the water? Since you can’t fight ’em, you must learn how to get out of the force of downwelling or laminar currents.
If you are on the flat before the drop-off, you can make a change of direction to the right or left and leave the path of the current. These types of unexpected currents tend to be relatively narrow bands – assuming you are not diving in the Atlantic Coast Gulf Stream or one of the other major currents found around the world.
By swimming at the same depth but 100 to 200 feet to the right or left, you can usually move to the side of the current. You also have the option of getting above the current by decreasing your depth, although this may be mid-water and not your idea of a great dive spot.
If you are at the upper lip of a wall and start to get sucked over the edge and pulled down, there are a number of things to do. First, add a little air to your buoyancy compensator while swimming away from the drop-off. Second, monitor your depth gauge to be sure you are holding at a certain depth. Once you get out of the current, there will be a tendency to rise, owing to positive buoyancy. You have extra air in the BC and, from being stressed, you may be taking bigger than normal breaths. When you bleed off a little air from your buoyancy compensator, it is easy to start sinking. You may release a little too much air, plus your breathing will return to normal volume and frequency.
Sometimes it feels as if you are falling when you wall dive. Our brains are used to seeing the ground on the bottom, not on the side. When wall diving, it is common to not see the bottom at all. Changing your visual references can be disorienting and cause you to overcompensate when adjusting buoyancy or judging your relative change in depth. When making a wall dive, most divers are surprised upon checking their computers or depth gauges – they find they have changed depth and are usually deeper than expected.
If you are on the face of a wall and swim into a downcurrent, follow the same procedure of inflating your buoyancy compensator and swimming away from the face of the wall. Sometimes downcurrents flow over the lip and hug the face of the wall. If you back away and dive 15 to 30 feet from the face, you may be out of the downcurrent. While this eliminates the close-up view of life on the wall, it can salvage a dive. If visibility is good, it is fun, with or without currents, to back up and enjoy the panoramic beauty of the wall.
The farther you move from the wall, the more critical it is to monitor your depth. When swimming in bottomless water, it is incredibly easy to descend 20 feet and not realize it.
General guidelines for current diving apply to wall diving. Currents are streams of water. If you swim perpendicular to the flow of the current, you can usually escape it. Since water is so much denser than air, it takes very little current to affect your dive. Two knots is a lot of current.
You may experience some equipment changes as you approach the edges of a current. If your snorkel is attached to the outside of your mask strap, it will start to vibrate. This is a good early warning sign that you should stop swimming in the direction you are headed. Stop moving and look ahead. Are the plants bent over or vibrating in the flow? Are the fish tucked into holes, rather than swimming above the reef?
If you proceed into more current, you may develop mask leaks, especially if a snorkel is tugging on it. Many experienced current divers do not attach snorkels to their masks for this reason. If the current strengthens or you stay sideways to the flow, your mask can be completely pulled off. This author’s introduction to West Palm Beach’s Gulf Stream current included losing a mask in an estimated two knot current and completing an instructor certification course dive without it. I was a student and not about to fail the course because I couldn’t finish a required dive!
Other current-related concerns are regulator freeflow, body heat loss, exhaustion and being swept away. For short term freeflow problems, put your hand over the purge button and deflect the flow of water slamming into the regulator. You can also swim backward for short distances. Position yourself as if you were on the surface swimming on your back – only do it underwater so the current hits your back. It is not very hydrodynamic – the amount of drag is intense – but your regulator will work properly.
Loss of body heat is a problem anytime you are in moving water. Your body has to keep heating up new volumes of water inside your suit as the old water gets flushed out by the current. People who current dive frequently in cold water use drysuits. In warmer water you will want to wear a wetsuit with long legs and sleeves, rather than a shorty wetsuit or a Lycra skin. In moving water your bottom times will be shorter owing to the chill and a higher consumption of air.
Intense work underwater is exhausting. If you are out of shape or accustomed to calm water diving, working against a current will wear you out. Exhaustion creeps up suddenly and affects your ability to kick and make appropriate decisions. Much of current diving education and experience is about learning not to fight them (you always lose), how to conserve energy and how to get away from the current’s pull or ride with the flow.
When you become exhausted or separated from your group, you risk being swept away. On the surface a signaling device will aid in your recovery. At depth, communication is a problem among the members of a group. If you suddenly are swept downward, your buddy may have a moral, physical or physics dilemma about swimming down to assist you. Buddies who frequently dive deep or in currents often discuss their feelings about obligations and limitations to aid each other where the rescuing diver is subjected to substantial potential physical injury or death. It is a very personal decision but one you and your buddy need to discuss long before there is a need to act.
Today’s buoyancy compensators have tremendous lift capacities. If you get caught in a downcurrent, adding a little air and then more air as needed, will counteract the downward pull. If your vest is fully inflated and you are still sinking, ditch your weightbelt and continue to swim perpendicular to the current. With all the drag from a fully inflated vest, it may take a while to swim a long distance. This is a scary situation but the above technique has worked effectively for hundreds of other divers, so keep kicking. Stay focused on kicking and look for signs the current is lessening. You’ll need to react quickly to bleed off BC air as soon as you escape the current or you’ll start rising.
Wall diving is spectacular and hundreds of safe dives are made annually. Knowing about downcurrents, you won’t be surprised if you start to feel your snorkel vibrating or feel as if you are sinking – because you know what to do: swim away from the drop-off and add air to your buoyancy compensator. If there is a wall no one ever dives ask “Why?” before you go exploring. Local knowledge may include history that the area is ripe for downcurrents.