| ||||||||||
|
||||||||||
| ||||||||||
| ||||||||||
|
| |||||||||
| ||||||||||
|
| |||||||||
|
FIFTEEN YEARS OR MORE AGO, members of a certain British dive club invited me to make up numbers and join them on a dive off the south coast. I asked what time "slack" water was expected. I wanted to know what time we would be hitting the water. The dive marshal replied that the boat skipper would know that. It was a fishing boat. When we reached the site, I noted that the buoy on the shotline had a bow-wave like a speedboat's and was being pulled under. The skipper obviously did not know when "slack" water would be, but the dive marshal insisted that it was OK to dive. Against my better judgment, but as the most experienced diver of the group, I decided that if anyone on the boat should try getting into the water, it should be me. I selected the biggest, strongest young man available as my buddy. We spent an exhausting 15 minutes hanging like flags on the shotline, trying not to get our masks ripped off by the flow, before the boat was suitably positioned for us to be picked up safely. No-one else wanted to try. A classic example of getting it wrong. Spring tides The basic effect of the moon's gravitational pull on the oceans of Planet Earth is explained in every dive-training manual. Most divers understand why water levels rise and fall in each tidal period of a little over six hours, and boat cox'ns need to know how much water they will have under their keels and when. However, the real effect for divers is often overlooked. We live on a small island that stands on the continental shelf, a shallow area of seabed that joins Britain to mainland Europe. When the level of the deep Atlantic rises, the water floods over that shelf. When it falls, the level ebbs back. So we get a change in levels and, more importantly, feel the effect of the "tidal" flow, or current. The sun also exerts a gravitational pull on the oceans, but when sun and moon are in opposition, this is partially cancelled out, with less difference between the height of "high" and "low" water. We call this a neap tide, and you will notice a half-moon effect at this time. When sun and moon pull together, when there is either a full or new moon, the effects are more dramatic. We call this a spring tide, but it has nothing to do with the season. There is a much greater difference in the height of low and high water, and this also affects the amount of water that floods in and ebbs out over the continental shelf in the same six-and-a-bit-hour period. On a spring tide, more water is moved and the currents are stronger. This can make diving very hazardous, if not impossible. It's called a "spring" tide only because the Christian church decided that Easter should fall on the first Sunday following the first ecclesiastical full moon on or after the vernal equinox, which is fixed at 21 March. The aim was to make sure that Easter always fell between 22 March and 25 April. So the springtime tide is always a high spring tide. And for divers, the Easter holidays, traditionally when many out-of-practice divers start a new diving season, always offer the most challenging conditions in the water. Regional variations The seabed is not flat. It has natural undulations, reefs and wrecks. As the water is pushed onto the continental shelf it has to squeeze round, up and over or even through these obstacles and, like air passing over a plane's wing, must speed up to do so. Elsewhere vortexes - areas of low sideways pressure where there is little movement - are produced in the lee of a current. Wreck-divers can often avoid a current by sheltering from it in the lee of a hull. Leave the wreck and you might cover a lot of ground before reaching the surface. You get no sensation of this movement, because the water and everything suspended in it, including you, moves along together. Tide tables can predict the rise and fall of sea level at various ports but may not suffice for divers who need to know what is happening at some mid-Channel site. This is where the local knowledge of a skilled dive-boat cox'n is essential. Some places have only small differences between high and low water levels, even during spring tides, while others, such as Jersey in the Channel Islands, are famous for huge tidal differences. Because of local topography, similar regional variations apply to "slack water". This is the pause in water movement that occurs once the tide has reached full height and before the ocean starts to draw it out again, and it also occurs at the point of lowest tide. Currents abroad Places as far away as Palau in Pacific Micronesia and the Seychelles in the Indian Ocean sit on continental shelves and are affected in the same way as Britain. Aldabra is famous among divers for the currents that flow through its channels as the changing tidal height affects the water in its lagoon. This empties or refills every six-and-a-bit hours, with dramatic consequences caused by the changing height of the ocean outside. Many other famous destinations are oceanic by nature. Out in the ocean, tidal predictions are less important, and usually denote only a few metres' difference in water depth. Yet ocean currents, driven by seasonal changes in the ocean wind, the jetstream and convection caused by seawater temperature changes near the Poles, can still be much stronger than anything around Britain. January, for example, is a time of very strong currents in the Maldives. These carry plankton and are well-liked by marine life, so provide high-voltage diving. Similarly, in Polynesia, those who want to see big animals insist that diving on slacks is a waste of time. So if it's pelagic life that interests you, you will seek out the strongest currents, whereas if you're a wreck-diver, you will prefer to dive on slacks. And in Britain slack water is more often than not a requirement. So if you are diving this Easter after a lay-off, treat the tidal flow with due respect!  |
|
| |||||||
 |
DIVER this month
|
Latest News
|
Holiday Offers
|
Competitions
|
Travel
|
Equipment
|
Forums
|
Learn to dive
|
Wrecks
|
UK Boat Spaces
|
Centres Overseas
|
Centres UK
|
Personal ads
|
Weather
|
Careers
|
U/W Photography
|
Marine life
|
Dive Shows
|
Dive Wear
|
Sharks
|
Diving know how
|
Opinion & more
|
Subscribe
|
Books & DVDs
|
Links
|
Contact us
|
About DIVER group
|
Divermart
|