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Tide Planning

Tide Planning. Mark Powell Mole Valley Sub Aqua Club SAA - 101. 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 de pth of water of launch and dive sites The problems caused for diving by tidal currents.

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Tide Planning

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

  2. 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

  3. Outline • Cause of the tides • Tidal predictions • Height of the tide • Tidal flows

  4. Moon Sun Earth Cause of the 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

  5. Low 6 hours 6 hours High High 6 hours 6 hours Low Daily Tidal Cycles Moon Sun Earth rotates once every 24 hours There are2 lows and 2 highs a day So, the time between low and high is 6+ hours

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

  7. Tidal Terms Flood Ebb High water Range Low water 0 6 12 18 12.5 hours in one Tidal cycle

  8. Spring and Neap Tides

  9. SomeportJuly Time M 1201:16 0.57 Sat07:44 1.60 13:17 0.54 20:30 1.82 1302:16 0.50O Sun08:15 1.66 14:43 0.54 21:25 1.88 Add 1 hour for BST Tide Tables • For a specific Port • For specificdates • 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

  10. SomeportJuly Time M 1501:16 0.84 Sat07:44 1.32 14:12 0.91 20:39 1.35 2203:10 0.50O Sat09:24 1.66 15:52 0.54 22:31 1.88 Add 1 hour for BST Tide Table Examples • Examples: • We need a long slack to dive a wreck. Which of these two weekends would you recommend and why? • If we had to dive at LW on 15 July – what time of day would you plan for LW? • What moon will we see on 15 July? • What will be the states of the tide at 14:00 and at 21:00 on 22 July? • What will be the tidal range on the morning of 22 July?

  11. 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

  12. 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 0600 1300 1300 1900 SOMEPORTand and and and 1.95 1.31 0.67 0.49 1800 0100 0100 0700 Old Haven+0050 –0010 +0100 +0010-0.1 –0.2 -0.4 –0.2

  13. 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 0600 1300 1300 1900 SOMEPORTand and and and 1.95 1.31 0.67 0.49 1800 0100 0100 0700 Old Haven+0050 –0010 +0100 +0010-0.1 –0.2 -0.4 –0.2

  14. 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

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

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

  17. 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

  18. 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 = 1.4 + 2.7 = 4.1m • Depth of wreck = Charted depth + Height of tide = 30 + 4.1 • Depth of wreck = 34.1m

  19. HW Height (m) 2.0 3.0 4.0 Someport (Harbour) 0.0 1.0 2.0 LW Height (m) HW-6 HW-5 HW-4 HW-3 HW-2 HW-1 HW Time Height of tide:Tidal Curves

  20. HW Height (m) 2.0 3.0 4.0 Someport (Harbour) 0.0 1.0 2.0 LW Height (m) HW-6 HW-5 HW-4 HW-3 HW-2 HW-1 HW Time 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? Depth = 2.8 + 25 = 27.8m

  21. 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

  22. Tidal Diamonds

  23. Tidal Diamonds

  24. Strength of Currents • Strongest at … • Headlands • Deep water • Surface • Weakestat ... • Bays • Shallow water • Seabed

  25. 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

  26. Diagrams showing currents … Time based on a Standard Port Arrows indicate strength & direction 3 hours after HW Dover 08,17 HW-1 HW-6 HW-5 HW-4 HW-3 HW-2 HW HW+1 HW+2 HW+3 HW+4 HW+5 Tidal Atlas • 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

  27. HW Dover - 3 HW-1 HW-6 HW-5 HW-4 HW-3 HW-2 HW HW+1 HW+2 HW+3 HW+4 HW+5 02,04 Tidal Atlas Examples • Examples: • You dive at 10am, HW Dover is at 1300. 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? 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 0.2 + 0.4 = 0.3 knots

  28. 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

  29. Tidal races • Overfalls Headland Channel

  30. 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

  31. 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

  32. 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

  33. 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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