Know Your Tides
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SCUBA SCHOOL – KNOW YOUR TIDES Hardly a summer goes by without someone leaving their boat at anchor, only to return later to find that their RIB has been left high and dry on the beach. Not only beached, but a six-metre 140 horse powered bouncy castle for a company of five year-olds re-enacting scenes from Pirates of the Caribbean. Subsequent attempts to drag the half-ton RIB through the sand in a futile attempt to catch the receding tide only add to the fascination. Comments of no use whatsoever usually ensue, "heh mate, I watched the Royal Tattoo last Saturday and the Field Gun race wasn't half as good as this"! Eventually a local makes the sensible observation, “It'll be best to wait for the tide to come in”. “What time will that be?” “'bout 9 o'clock I reckon”. “but I've got to be in Manchester by 6!” At the least a poor understanding of tides can spoil the enjoyment of diving at the coast, at worst can ruin it through incidents such as lost diver. Four things divers need to know about tides: 1. Time of high and low water - you can then plan when to launch or recover a boat, or conduct a shore dive. 2. Height of tide - to enable more accurate calculation of depth. 3. Speed and direction of currents - for planning drift dives. 4. Time of slack water – when you don't want to dive in a current. WHERE'S THE WATER GONE? Around the complex coastline of northern Europe and the UK the gravitational pull of the sun and moon causes dramatic changes of sea level in just six hours. Such a colossal movement of water in a short period of time creates a current often too great for divers to swim against. Most divers will find a current of more than 0.5 - 1 knot (nautical miles per hour) off-putting and a rate of three knots is generally regarded as a safe limit for most drift diving situations. There are many areas of the UK where divers can experience current rates greater than this but such currents are usually produced by the flow of water local to complex reefs and headlands. The resulting turbulence can produce uncontrollable depth changes during a dive. On 'The Fangs' reef, Anglesey you can expect to be taken from 20m to the surface in just a few seconds! The localised rough sea state can also present a danger for boats trying to navigate the area. In the huge areas of the open oceans these effects are less pronounced. The rise and fall of water may be slight, perhaps a couple of metres, and occurs over a longer period (twelve hours). Currents caused by wind as opposed to rise and fall can still be strong and set in a particular direction for weeks on end. The currents in the Red Sea being a good example. Around the UK currents tend to flow in a particular direction for about six hours before stopping and turning (‘slack water’). The period of slack water depends on the area and can be anything from a few minutes to a few hours. The phrase, "We missed slack again", is common terminology usually associated with that elusive wreck, "HMS In The Vicinity"! Information on currents is readily available from admiralty publications such as charts and tidal stream atlases. However the most important sources are local skippers and dive centres because tidal predictions are, after all, just that - predictions. There is no substitute for local knowledge. TIDE TIMES Times and heights of tides (in metres) are published in the form of a calendar for numerous ports and can also be obtained from websites such as 'easytide'. Dec 1st Mon Time M 0021 5.65 0623 0.98 1240 5.70 1857 0.90 In this example low water occurred at 0623 and 1857 hours, with high water at 0021 and 1240 hours. In the summer don’t forget to add an hour for BST. The heights in metres are simply added to the depth as shown on an Admiralty chart. For example the wreck of the Lady Meath near Holyhead is charted as being at a depth of 17m. You could actually expect the diving depth at high water to be 22.65 metres. At low water you could expect a diving depth of 17.98 metres. Tidal currents can cause soft sea beds such as sand to be 'scoured' from around wrecks. It would be prudent to assume a diving depth even deeper than that indicated from this simple calculation. Whenever possible, go back over the site and get a profile of the dive site using the boat's echo sounder. This will allow you to check the diving depth against your planning. Spring Tides and Neap Tides If you take a closer look at tide tables and compare the tides of one day with another there are some significant differences. Firstly, the times of high water and low water advance by about one hour per day. Secondly, the height of tide changes as well. Compare this example with the previous one: Dec 7th Sat Time m 0230 1.86 0845 4.57 1505 2.15 2128 4.75 On 1st December the rise and fall (‘range’) of tide was as much as 4.8m. But on the 7th of December a similar six hour period saw a range as small as 2.6m. A large tidal range one week is followed by a smaller tidal range the next; i.e. ‘Spring Tides’ and ‘Neap Tides’ (respectively). Spring Tides feature a greater volume of water moving in the six hour period compared to Neap Tides. Spring tide current rates are faster and when slack water does occur it will also be significantly shorter. For example, the wreck of the 'Kyarra' near Swanage can have slack water lasting 45 minutes on a Neap tide but only 25 minutes on Spring tides. For a site like the Kyarra (depth 30m) 25 minutes is often enough for many divers. However extended dives with decompression stops may give surfacing times well after the end of slack water. It’s the acceleration of the current that can catch you out. Generally speaking neap tides give you a better chance of avoiding ‘lost diver’ situations. The success or failure of any diving expedition to the coast is often decided months in advance simply by the choice of date and time. If you are planning dives you clearly need to give priority to those trips that will take your divers to areas with strong tidal influences. Budding expedition leaders have sometimes found to their frustration that the best liveaboard expedition boats are booked up as much as three years in advance for trips to tidal areas such as the Channel Isles. Scuba Training from Bubblemaker to Instructor Level. Head Office: 50 Mainstone close, Redditch B98-0PP Facebook: Scuba School - Twitter: @scuba_school Website: www.scubaschool.org.uk Tel:07773-966-835 [email protected] : [email protected] .