Dive safety requires knowledge and awareness of underwater conditions on the part of both dive leaders and divers. Dive leaders are required to inform their divers of environmental conditions at a dive site while divers have to study every available information about a site.
Knowledge and awareness of underwater conditions are important in creating and maintaining dive safety. This does not mean divers must constantly be thinking about temperature, tides, currents, visibility and other water phenomena. The secret of a long lasting and loving relationship with any environment is to develop an awareness of conditions and any changes that might influence the dive and safety.
Awareness also leads to a greater appreciation of the astonishing and intricate relationship among the existing biota. Listen carefully and you will hear – Seawords. We all know the sea is not a silent world. It will speak to you.
Most destination dive sites and live-aboards in the Pacific, both north and south of the equator, are in waters adjacent to or part of coral reef systems. Probably more than half of the diving on these ecosystems will be done in groups led by divemasters or dive instructors. They will have a knowledge of tides, currents and other marine related hydrographic and weather elements. Because of this, it is not as critical for visiting divers to have this data. Awareness and a good working knowledge of environmental conditions will, however, improve opportunities for a more relaxed and enjoyable dive. Dive leaders, whether divemasters, boat skippers or instructors, can add to the enjoyment by making divers aware of existing environmental conditions or any changes that might be expected.
Individual divers or buddy teams also have benefits in their more individualistic approach to underwater exploration. The flip side of this adventuresome coin is being on their own with dive planning and safety. Instantly, there is a need to know as much as possible about everything concerning the site. Of course, they can stumble their way through whatever happens and will probably survive. Most unusual and exciting dive experiences have a way of being fail-safe. But, don’t count on all of them being so. Make plans based on what is known. It is imperative that conditions be recognized and accepted for the way they really are.
Coral reefs are the wondrously exciting creation of millions of tiny marine animals. Coral atolls are even more extraordinary. They are the inspiring, natural wonders of the oceans.
There are two principal kinds of islands or island groups in the Pacific basin: mountains spawned by volcanic action and coral atolls. The volcanic mountains may rise barely above water or tower thousands of feet above the sea. Coral atolls may be only 10 to 30 feet above sea level. There are many kinds, sizes and shapes of coral reef systems associated with the two types of islands. They can be categorized as fringing, barrier and atoll systems. The coral growths around an atoll are sometimes referred to as barrier reefs. Most offer diving opportunities that vary from good to excellent to nearly perfect. There are almost no significant diving hazards as related to the waters and their movements, either tidal or from ocean currents, within coral reef formations. Diving in the waters seaward of the reef system is another story.
Fringing reef systems are associated with mountainous islands and may be attached to the rocky shore or, more usually, form a short distance offshore with a relatively narrow lagoon between reef and shore. Most, but not all, such systems in Hawaii are fringing reefs.
Barrier reefs generally run parallel to but offshore the land mass(es). Sometimes a wide barrier lagoon is present between the reef system and the shore. The Great Barrier Reef off Australia and a similar reef system off the coasts of southern Mexico and Belize are examples of large barrier reef systems.
An atoll is a reef system that surrounds a lagoon. The entire system is completely surrounded by open sea. The lagoon is usually void of any land mass except isolated, coral formed islets. Atolls may be small or, in some cases, more than 100 miles long.
There are some characteristics identical to all coral reef systems, whether fringing, barrier or atoll. These identical characteristics make it relatively easy to plan and execute a safe dive. Three of these characteristics are closely related. First, all coral formations are in a tropical setting; usually between 30 degrees north and south of the equator. Also, this puts most, but again not all, coral reef systems within trade wind weather patterns. Finally, the constant trades establish predominately westward setting currents that wash across or around the coral formations.
There is an old saying that coral grows best where the breakers are the heaviest. This is definitely not true. Generally, coral formations will be at their best where diving is best, i.e., calm, relatively shallow reef areas where there is a steady but moderate, almost gentle flow of clean water. Coral will not survive in stagnant water. Also, most corals cannot survive in heavy breaker areas because of the scouring action of suspended sand particles.
About 90 percent of the weather pattern will be trade winds. Be warned, there is nothing gentle about trade winds. In either hemisphere, they may average 10 to 15 knots in the summer and 15 to 25 knots in the winter. Even higher trades are not unknown. Heavy seas and breakers develop on the windward shore of islands and lagoons. In the northern hemisphere trades will be from the east to northeast; in the southern hemisphere, from east to southeast. Also, along a ten degree wide belt just north and south of the equator in the summer, there will be a belt of light to variable winds known as the doldrums. Dive sites in these areas will experience calmer waters but considerably more rain. These conditions exist with little variation across the entire Pacific Ocean until the volcanic, mountainous areas near the Asiatic continent are encountered.
The trade wind weather pattern generates ocean currents with a westerly set that impact coral islands. Similar ocean currents, known as the equatorial counter currents, set in an opposite direction in the doldrums. Current velocities of one to two knots have been reported in both trade wind and doldrum areas. Both trade wind driven and equatorial counter currents tend to wrap around the island and atolls and converge on the leeward side of the land masses, often with an increase in drift. (Set is the direction in which a current flows and is the opposite to wind direction, that is, an easterly wind blows from the east while a current with a westerly set flows toward the west. Drift is the rate of flow.)
When reef areas have navigable passes or channels, they will usually be on the leeward side of the island or atoll. There are exceptions in volcanic islands and in several of the very large atoll reef systems.
In addition to ocean currents in nearly all the Pacific coral islands, local tidal currents can affect dive planning and safety. The mean range of tides is relatively low. For example, the mean range of tide at Johnston Atoll is two to three feet. Tidal currents are relatively weak. Ebbing tidal currents are usually the strongest.
A combination of ocean currents and wind driven waves causes ocean swells and breakers to pile over the eastern end of the barrier reef into the windward side of the lagoon. This will cause an increase in tidal range and in the speed of currents flowing through channels on the leeside of the atolls. Tide tables and tidal current tables will be helpful in dive planning but will not be completely accurate under those conditions.
Usually the best and safest diving is inside the reef systems; whether fringing, barrier or atoll. Diving in passes and along the open sea sides of reefs must be carefully planned and adequate support and flotation equipment made available. If properly planned and adequately supported, a drift or float dive with the sometimes swift current in a channel can be an exciting experience. Seaward of the passes, particularly on the leeward side of coral formations, diving the walls can also lead to an inspiring dive bonus.
High water within coral formations owing to strong, steady prevailing winds usually does not generate strong currents within a lagoon. However, the strong flow of water out of the lagoon in some of the leeward passes can be potentially hazardous. In almost all cases, just seaward of the water flowing out of a coral formation, this enhanced tidal current tends to swing south (in the northern hemisphere) and combines with the wind driven ocean current to develop a drift that could be hazardous to an improperly planned or poorly supported dive. Diving in the passes or outside any reef system should be attempted only when adequate, well maintained and operated, powered flotation systems are available.