www.challengeddivers.com A simple resource for UK Shore Divers Anyone standing on Blackpool beach wondering where the sea has gone will have experienced the dramatic effect that tides have on the UK coastline. This is due mainly to various alignments of heavenly bodies such as the sun and the moon tugging away at the water on our planet. In the same way that some clever person has worked out that Halley’s Comet will return on the 28th July 2061, it is possible to predict the effect on our tides, and of course like most thing to do with numbers, they make tables.... hence tide tables. Unlike Halley’s Comet, tides are a day-to-day thing, with a monthly influence, but why do we have tables for different days? Well, everything isn’t equal. Firstly a tidal cycle takes about 24hrs and 50 minutes and a monthly cycle around 29.5 days. We work on a straightforward 24hr clock with months of between 28 and 31 days, so of course it’s going to be a bit out of step. The tables may only show one high or low tide in a 24 hour period, but never more than (but usually) two of each. Approximately six and a bit hours between high and low tide. Get the time right Because the world follows the UK, all tide tables are given as GMT (Greenwich Mean Time) with no adjustment for BST (British Summertime). So they work “as is” for the winter, but between the clocks going forward at the end of March, and back at the end of October we have to add an hour on to the published time. Why different places, different times? Because it doesn’t all happen at once. The tides tend to roll around the UK in a clockwise motion, so high tide at St Abbs will be slightly ahead of high tide at Tynemouth. High highs and low lows We know that on average we get two high and two low tides per day. If you look at tide tables over a week you will notice that as the high tides get higher the low tides generally get lower, until the high tide peaks, then it reverses. The highest high tides with the lowest low tides are known as springs, and the lowest high tides with the highest low tides are known as neaps.... clear as mud. Springs follow neaps follow springs follow neaps and so on throughout the year about a week apart. Each month you should expect two springs and two neaps. Springs are usually better for shore diving because of the depth, but can be subject to stronger tidal flows. Diving at Slack Tide You would expect slack (minimal tidal flow) to happen at high or low tide, but due to variations in topography this is not always the case. It can last for an hour and be up to 3 hours either side of high water. Generally this is not an issue for shore diving. So how deep will it be? The chart will give the predicted time and depth above chart datum for the tides on any given day. Basically this is the minimum depth to expect in metres. As tides aren’t instant, the curvy graph gives a representation of the expected depth over the 24hr period. In the example above this would be roughly 3.5 metres at midday. What is the rule of 12th’s? Without the squiggly line on the chart, given the times and depths of high and low tide we can approximate the minimum depth above chart datum for a given time. There is roughly 6 hours between high and low tide (12 x half hours) but the tide does not come in or out smoothly. For the first 3 hours it gathers speed, slowing down for the next three hours. In the example above, to make this work, we have to pretend that low tide was at 4am and high tide 10am, the difference being 6 hours and 4 metres. Divide 4metres by 12 and get a third of a metre. In the first half hour, tide rises by 1/3rd metre (1/12th) In the second half our, tide rises by another 2/3rds metre ( total 3/12ths) in the third half hour, tide rises by another 1 metre ( total 6/12ths) In the fourth half hour, tide rises by another 1 metre (total 9/12ths) in the fifth half hour, tide rises by another 2/3rds metre (total 11/12ths) In the 6th half hour, tide rises by another 1/3rd metre (total 12/12ths) So halfway through at 7am we would expect to have a minimum of 2.8 metres above chart datum to go splashing about in until we reach 4.8 metres at 10am. This works backwards going down to low tide. Shallow bits, chart datum, sandbanks, rocks, wrecks and currents You’ve done the easy bit, and we have some idea of the minimum depth to expect at our planned dive time. Now this is where local knowledge or a good reference book comes in handy. Where’s the best place to park, can I get air fills, what time does the ice cream van turn up? Tidal range over 7 days Chart Datum | Tide Table Links | Reference Books | Weather Prediction Links |