Tides and currents are important considerations for any diver, but this is especially true for shore divers. Dr Richard Smith explores the connecton between tides and divers.
SHORE DIVING can be one of the most rewarding and cost effective ways of getting your scuba fix. While some sites are ideal for instructors wanting a sheltered spot to take their students, others can be much more challenging and suited to experienced divers. Tides and currents are important considerations for any diver, but this is especially true for shore divers. An understanding of tides and their associated currents allows shore divers to enter the water at exactly the right time to get the best out of the site and marine life.
HOW TIDES WORK
The gravitational pull of the sun and moon are a major influence on the sea level’s rise and fall. Without the influence of any celestial bodies, the level of Earth’s oceans would remain at more or less static due to our planet’s own gravitational pull and the centrifugal force produced as it spins on its axis. Centrifugal force is often overlooked as another major influence on tides.
The moon’s gravitational pull is strong enough to draw water towards it creating a ‘bulge’, which moves with the moon as it orbits the Earth. The regions of the Earth where these bulges occur experience a high tide, and other areas experience a low tide. Interestingly, however, water on the opposite side of the planet, furthest away from the moon, also experiences a high tide at the same time as a result of the centrifugal force created as the Earth spins.
The sun’s effect is comparable to that of the moon, exerting similar forces on our planet’s oceans. The gravitational force exerted by the sun, however, is less strong than that of the moon due to the sun’s relative distance from us. Considering that both the sun and moon have an influence on the tides, it is logical that their combination can also have profound effects. When the sun aligns with the moon and the Earth, during a full or new moon, the effect on the tides is intensified, resulting in spring tides. The term “spring” does not relate to the season, rather it is named due to the tides springing higher than usual. Conversely, when the moon is in its first or third quarter, the sun counteracts some of the pull from the moon with gravitational forces coming from opposing directions and a lower than expected high tide results. These smaller high tides are known as neap tides.
TIMING IS EVERYTHING
Tides can heavily influence our experience as divers. During spring tides, currents tend to be much stronger, as the difference between high and low water is at its greatest, and a larger than average volume of water moves from one part of the ocean to another. Animals also react to these changes and many fish spawn on a full moon to exploit these strong currents. Strong currents help to carry spawn over greater distances, helping a species to colonise new reefs and genetically mix its populations.
Tides generally rise and fall twice each day, which accordingly affect both visibility and the currents experienced on a dive site. Correctly evaluating the tides is vital for many shore dives. Usually, the best time for a tidally dominated site is high slack. This is the short period of time when the height of the tide has been reached and before it starts to fall again, providing a window of opportunity when there should be very little water movement. Visibility is also usually at its best during the high slack, due to the abundance of fresh, unagitated waters.