Skin divers should understand the mechanics of waves to enhance dive safety. They should look for where the waves barely break on one side of cove to identify ideal spots to begin their descent to reefs.
Watching a wave and accurately translating what you see into a go/no go decision is an invaluable skill. Some days knowing how waves work will keep you out of the water and other days it will aid your decision to try a new dive spot.
There are lots of oceanography books containing complex wave theory. But, for divers, the basics are what you need to make good decisions. Local knowledge about a diving site often translates complex wave physics into local dos and don’ts. For example, do not try to scuba dive in Laguna Beach, California during a south swell or on the front side of Catalina Island during a northeaster. Given the angle of the beaches and the waves, there will be big to giant surf and a very long surge. If you are coming to Laguna, listen to or watch the weather channel and, if you hear “south swell,” grab your surf-board, not your scuba gear. That bit of information has saved savvy Southern California divers lots of freeway time and disappointment.
A common misconception is that a wave is a unit that moves with the molecules of water sticking together the entire distance it travels. In reality, a wave is energy that can move across great distances but the water molecules mostly move up and down in a circular motion.
For example, pretend you are standing on a small cliff on a breezeless day watching a series of regular waves entering a cove. The waves enter the cove, move across it and break on the beach. You throw a stick in the water and observe that it rises and falls, and moves back and forth as waves pass but doesn’t move toward the shore. The stick and water around it move in a slow circular oscillation as each wave passes by.
Next you observe the seaweed stalks growing from the bottom. The water is clear so you can see the seaweed’s movement with each passing wave. As the crest of a wave approaches, the upper part of the seaweed moves toward it. As the crest passes, the seaweed continues to point toward it until a new crest approaches. Broken seaweed fragments suspended in the water move in a slow vertical circle as each wave passes. You have just observed a basic principle of hydrodynamics – objects in the water tend to do what the water they displace would have done.
Wave height Wave breaking depth (feet) (feet)
3 3.9 4 5.2 5 6.5 6 7.8 7 9.1 8 10.4 9 11.7 10 13.0
Now that you know water oscillates in a circular pattern, it is easy to understand what happens to wave energy. As a wave approaches the shore, the depth of the water decreases until it is impossible for the oscillating water particles to complete their orbits. When the orbits break, the wave breaks and the crest tumbles forward, falling into the trough ahead as foaming white water. The forward momentum carries the water to the shore as the wave’s remaining energy is expended. With a swish, the water rushes up on the sandy beach or the shoreline rocks and disappears from sight. This is the surf zone, where the wave gives up its energy and where the rhythmic, orderly water motions give way to sometimes violent turbulence.
As the swell from the deep water moves toward the shoreline, it commonly travels at a speed of 10 to 20 mph, so the changes that occur at the surf zone happen rapidly.
On gently sloping beaches waves roll across the bottom and begin to spill in the trough ahead in a leisurely cascade when the depth of the water is about 1.3 times the height of the wave. The energy slowly drains away. This is sometimes called a spilling break. There can be a series of breaks as the wave dumps, reforms and eventually makes it to shore. Surfers like these for long rides. Most divers have a hard time going through a series of surf zones during a long surface swim.
Another common shoreline wave is a plunging breaker. On a steeper beach such as Monastery Beach in Monterey, California or Hawaii’s Waimea Bay, a wave quickly meets water too shallow to sustain its advance. The crest arches forward unsupported and collapses around a pocket of air. Plungers break in a much shorter distance, providing thrilling rides for expert surfers but dangerous conditions for a scuba dive.
READING A BEACH
When you arrive at a dive site, take a few minutes before unpacking your gear to watch the waves. How many seconds are there between crests? Can you move through the surf zone in that amount of time?
If you have been diving at this location before, how has the surf zone changed? The slope and amount of sand on a beach change from summer to winter. In Southern California, summer beaches have a gentler slope and broader sand. Winter storms remove beach sand, making the slope steeper and waves seem to come in faster and break nearly at the shoreline.
Watch where the waves are breaking across the width of the cove. The point where the waves fall over first has the shallowest bottom. It could be a sand-bar, rocks or a reef that catches the bottom of a wave and causes it to collapse. If the waves barely break on one side of the cove, you know the water depth is greater. This may be the best channel for your underwater swim to the reef.
It is easier and safer to spend as little time as possible in whitewater. With all equipment, including fins, in place, time your entry through the surf during a lull between waves. If you see a wave coming that will break on top of you, duck under, so the majority of the turbulent froth passes over you. Do not make a deep, head first dive toward the bottom, as it is likely shallow; also, the bottom may be littered with boulders waiting to grab any dangling hoses or goodie bags. The bubbling froth of the surf zone makes it difficult to see the bottom and avoid entanglements. When ducking a wave, many divers put one hand over their mask to hold it in place.
Pop up as quick as you can on the backside of a wave and kick vigorously. Hopefully, you can get into deep enough water to avoid being clobbered by a second breaking wave. Transiting through the surf zone is a deliberate, quick action. It is every diver for him/herself, as it is virtually impossible to stop in tumbling surf to help someone else. Once on the backside of the breaking surf, stop, inflate your BC and rest before continuing the dive.
When surveying a beach, you might see a break in the middle, a smaller, later break on one side and an area of confused water on the other side, next to a rocky point. If possible, avoid the confused water next to the rocky point. The dynamics of waves striking, bending and rebounding off a rock wall can make you feel as if you are in a washing machine. You have little control and may get smashed against the rocks when the next set rolls in.
When you first survey a beach, look for any wash rocks offshore. It is a good bet that a rock reef finger extends from near the shore toward the wash rock. Depending upon which direction the waves are coming from, you may be able to get underwater on the leeside of the finger reef. This means Jess surge and an easier swim to the dive area.
How you dive a beach may change every time you visit it. Tides, surge, wave size and speed may necessitate different entry and exit points than usual. You may not be able to dive part of a reef if it is low tide and the waves are breaking, reforming and breaking again.
Getting knocked down in the surf zone or losing equipment is no fun. If you take the time to watch the waves and make conscious decisions about your entry and exit points and use common sense versus macho pride, beach diving will be easier and safer. There is enormous energy in a wave and a diver is seriously outmatched. After packing and driving to a planned dive site it is disappointing to be blown out by waves. If your reading of the waves says, “no scuba,” consider skin diving as an option. Moving through the surf zone is much easier in only skin diving equipment and it is a confidence builder for future scuba dives. If diving is out altogether, perhaps a little boogie boarding or surfing is an option!