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Tide Planning Mark Powell Mole Valley Sub Aqua Club SAA - 101

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Tides Objectives At the end of this lesson you should know : How to choose days when the tides are good for diving The importance of the depth of water of launch and dive sites The problems caused for diving by tidal currents

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Tides Outline Cause of the tides Tidal predictions Height of the tide Tidal flows

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Tides Thin layer of water over earth’s surface Gravity of Sun and Moon attract water towards them Moon is closer and therefore stronger than the Sun Cause of the Tides Moon Earth Sun

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Tides Daily Tidal Cycles There are 2 lows and 2 highs a day So, the time between low and high is 6+ hours Earth rotates once every 24 hours 6 hours High Low Moon Sun

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Tides Monthly Tidal Cycle Week 1 : New Moon Week 2 : First quarter Week 3 : Full Moon Week 4 : Third quarter

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Tides Tidal Terms High water Low water hours in one Tidal cycle Range FloodEbb

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Tides Spring and Neap Tides

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Tides Tide Tables For a specific Port For specific dates High and Low : Times in GMT April to October add 1 for BST Depths in metres Phases of the moon Can find Neaps and Springs Someport July Time M 12 01: Sat 07: : : : O Sun 08: : : Add 1 hour for BST

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Tides Tide Table Examples Examples: 1.We need a long slack to dive a wreck. Which of these two weekends would you recommend and why? 2.If we had to dive at LW on 15 July – what time of day would you plan for LW? 3.What moon will we see on 15 July? 4.What will be the states of the tide at 14:00 and at 21:00 on 22 July? 5.What will be the tidal range on the morning of 22 July? Someport July Time M 15 01: Sat 07: : : : O Sat 09: : : Add 1 hour for BST

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Tides Sources of Tide Tables Almanacs: whole UK coast Cheap booklets from marinas Newspapers: good if you know Differences Internet: often free but unknown accuracy Computer programs

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Tides Tide Table Differences Based on a Standard Port – for which there is a tide table For several Secondary Ports – smaller harbour or village Time differences of the 2 typical HWs and LWs Height differences of HW and LW for Springs and Neaps Time Differences Height Differences Place High Water Low Water MHWS MHWN MLWN MLWS SOMEPORT and and and and Old Haven – – –0.2

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Tides Tide Table Differences Examples: HW at Someport is 1.35m at 1321 today. When is HW Old Haven and what height is it? Answers: HW Old Haven is at 13:21 – 00:10= 13:11 The height is 1.35 – 0.2 = 1.15m Time Differences Height Differences Place High Water Low Water MHWS MHWN MLWN MLWS SOMEPORT and and and and Old Haven – – –0.2

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Tides Tides and Depth of Water Chart Datum … Lowest spring tide in average weather conditions Charted depth, High and Low Water measured from Chart Datum Depth of water = height of the tide + charted depth Importance of Depth of Water … Depth of water at dive site Usability and safety of harbours and slips

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Tides Water depth at Dive Site Chart Datum Low water High water Height of the tide Seabed Charted depth Depth of water Chart Datum

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Tides No water to launch into or retrieve from High Water Low Water Sand bar blocking harbour mouth Slip Harbour Trapped inside Water depth at Launch Site Trapped outside Safe depth to cross the bar

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Tides Height of tide: Rule of Twelfths Rise/fall of tidal range is 6 hours Divide range into 12 units Tidal height for each hour Rise/fall for each hour: 1 unit in first hour 2 in the second hour 3 in the third hour 3 in the fourth hour 2 in the fifth hour 1 in the sixth hour Add up rises/falls for hour of tide Subtract fall from HW or Add rise to LW

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Tides Height of tide: Rule of Twelfths Example : HW is 5m and LW at 5am is 1.4m, we want to dive at 9am on a wreck of charted depth 30m. How deep will the wreck be? Tidal range = 5.0 – 1.4 = 3.6m Height of 1 twelfth = 3.6 / 12 = 0.3m Hours after LW = 9 – 5 = 4 hours Rise between 5&6= 1 twelfth, between 6&7= 2 twelfths between 7&8= 3 twelfths, between 8&9= 3 twelfths Rise from 5 to 9am= 0.3m x 9 twelfths = 2.7m Height of tide = LW + Rise from 5 to 9 = = 4.1m Depth of wreck = Charted depth + Height of tide = Depth of wreck = 34.1m

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Tides Height of tide:Tidal Curves HW-6 HW-5 HW-4 HW-3 HW-2 HW-1 HW Time LW Height (m) Someport (Harbour) HW Height (m)

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Tides Height of tide:Tidal Curves Example: Dive at 9am in 25m charted depth, HW at 11:20am, LW Height 0.5m, HW Height 3.5m What’s the depth of water when we dive? HW-6 HW-5 HW-4 HW-3 HW-2 HW-1 HW Time LW Height (m) Someport (Harbour) HW Height (m) Depth = = 27.8m

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Tides Importance of Tidal flows Selecting best days to dive – Neaps Working out the time of Slack water Working out good & safe drift dives Identifying hazardous currents … Fast currents – lost on surface Tidal races – threaten boats, danger to dive Rip currents – threaten boats, danger to dive

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Tides Tidal Diamonds

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Tides Tidal Diamonds

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Tides Strength of Currents Strongest at … Headlands Deep water Surface Weakest at... Bays Shallow water Seabed

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Tides Slack Water “Slack ” - when speed of tidal flow is low Can dive if less than 0.5 miles per hour Generally at local HW or LW Not necessarily at HW / LW of port on tide table Slack happens during the reverse of direction Slack must last long enough to dive

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Tides Tidal Atlas Diagrams showing currents … Time based on a Standard Port Arrows indicate strength & direction 08,17 3 hours after HW Dover Numbers show : Speed in tenths of a knot … 12 is 1.2 knots At Neaps, Springs … slowest, fastest 12 or 13 diagrams cover tidal cycle HW- 6 HW- 5 HW- 4 HW- 3 HW- 2 HW- 1 HWHW+ 1 HW+ 2 HW+ 3 HW+ 4 HW+ 5

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Tides Tidal Atlas Examples Examples: You dive at 10am, HW Dover is at Its mid way between Springs and Neaps. There are 12 diagrams – “HW Dover–6” thru to “HW Dover+5” on the atlas. Which diagram do you choose? 02,04 HW Dover - 3 HW-6HW-5HW-4HW-3HW-2 HW-1 HWHW+1HW+2HW+3HW+4HW+5 An arrow close to where you want to dive looks like this. What will be the speed and direction of the the current? Answer : to the west, average of = 0.3 knots

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Tides Tidal races Places were tidal flow speeds up to pass obstruction Result of races … Faster currents Confused water, eddies Short, steep seas Problems for : Boats: capsize or swamping - rough water, short & steep waves Divers: less buoyancy control, pressure damage, separate from boat

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Tides Tidal races Overfall s HeadlandChannel

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Tides Rip currents Strong local currents Also caused by onshore wind Waves break over sand bar Excess water returns thru gap Dangerous - Keep a look out

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Tides Summary Tidal information for dive boat planning: Use Tide tables to find … Times and heights of HW Springs and Neaps Depth of sea water of launch and dive sites Importance of long slack times and slow currents Tidal hazards

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Tides Self check Define HW, Height of tide, Ebb, Range, Neaps, Slack water How long is the tidal cycle from HW to HW or LW to LW? At home: how to find out HW time and height for a site at a date? Name 2 ways of identifying for Neaps Name 2 reasons for the height of the tide being important to divers Height of tide and depth of water measured relative to what? What 2 components contribute to the depth of water at a site? In which direction does the tide flood in the UK? When does slack occur? Why aren’t tide tables useful for finding slack? Name 3 physical features that reduce tidal flows Name 4 types of hazardous current

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Tides Self check 2 Define Difference, Standard Port, Secondary Port, Tidal Curve What’s the purpose of tide table differences? Name 2 ways of finding the height of the tide LW = 1m, HW=3.4m. What is range? A twelfth? Height at LW+4? At home: how to find out the direction and speed of flow?

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