Tide Planning by 5SMUw1

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


                   Outline

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


          Cause of the Tides


        Moon
                      Earth                     Sun


• 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
                        Tides


          Daily Tidal Cycles
                         Low
              6 hours           6 hours

       High                               High

Moon          6 hours           6 hours           Sun
                         Low

• Earth rotates once every 24 hours
• There are 2 lows and 2 highs a day
• So, the time between low and high is 6+ hours
                         Tides


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


              Tidal Terms

                     Flood            Ebb
High water

                                                 Range

             Low water

  0              6               12         18


         12.5 hours in one Tidal cycle
                       Tides


      Spring and Neap Tides

              Neaps             Springs
Phase of      First quarter +   New moon +
moon          Last quarter      Full moon
Tidal range   Small             Large
Currents      Weak              Strong
Slack time    Long              Short
                               Tides


                     Tide Tables
                                       Someport    July
• For a specific Port                          Time     M
• For specific dates                   12      01:16   0.57
• High and Low :                       Sat     07:44   1.60
    Times in GMT
                                               13:17   0.54
    April to October add 1 for BST
                                               20:30   1.82
    Depths in metres
• Phases of the moon                   13      02:16   0.50   O

• Can find Neaps and Springs           Sun     08:15   1.66
                                               14:43   0.54
                                               21:25   1.88
                                       Add 1 hour for BST
                                     Tides


              Tide Table Examples
•    Examples:                                          Someport    July
1.   We need a long slack to dive a wreck. Which                Time     M
     of these two weekends would you recommend
     and why?                                           15      01:16   0.84
2.   If we had to dive at LW on 15 July – what time     Sat     07:44   1.32
     of day would you plan for LW?
3.   What moon will we see on 15 July?                          14:12   0.91
4.   What will be the states of the tide at 14:00 and           20:39   1.35
     at 21:00 on 22 July?
5.   What will be the tidal range on the morning of     22      03:10   0.50   O
     22 July?
                                                        Sat     09:24   1.66
                                                                15:52   0.54
                                                                22:31   1.88
                                                        Add 1 hour for BST
                     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
                                  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
                    0600 1300    1300   1900
        SOMEPORT    and    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
                            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
              0600 1300    1300   1900
  SOMEPORT    and    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
                       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
                          Tides


           Water depth at Dive Site
                                  High water


              Height of
              the tide            Low water
Chart Datum                                    Chart Datum



Depth of
water         Charted
              depth




           Seabed
                             Tides


        Water depth at Launch Site
            Trapped
            outside           Trapped          No water to launch
                              inside           into or retrieve from



High                                 Harbour
Water

Low
Water




         Sand bar blocking   Safe depth to
         harbour mouth       cross the bar          Slip
                                        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
                                    12
• Rise/fall for each hour:
                                    10
      1 unit in first hour
      2 in the second hour             8

      3 in the third hour              6
      3 in the fourth hour             4
      2 in the fifth hour              2
      1 in the sixth hour
                                        0
• Add up rises/falls for hour of tide       1   2 3 4   5 6   7 8 9 10 11 12
                                                     Hours after LW
• Subtract fall from HW or Add rise to LW
                                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 = 1.4 + 2.7 = 4.1m
      Depth of wreck      = Charted depth + Height of tide = 30 + 4.1
      Depth of wreck      = 34.1m
                             Tides


Height of tide:Tidal Curves

                 HW Height (m)
                                          Someport (Harbour)
                 2.0   3.0   4.0




    0.0   1.0   2.0                HW-6 HW-5 HW-4 HW-3 HW-2 HW-1 HW


     LW Height (m)                               Time
                                 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 Height
                    (m)                Someport
                                       (Harbour)
                    2.0   3.0
                    4.0

                                                                Depth = 2.8 + 25
                                                                      = 27.8m

        0.0   1.0   2.0         HW-6 HW-5 HW-4 HW-3 HW-2 HW-1
                                HW
         LW Height (m)
                                              Time
                                   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
     Tides


Tidal Diamonds
     Tides


Tidal Diamonds
                    Tides


      Strength of Currents
• Strongest at …
   Headlands
   Deep water
   Surface

• Weakest at ...
   Bays
   Shallow water
   Seabed
                     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
                                       Tides


                         Tidal Atlas
 • Diagrams showing currents …
                                                                     3 hours after HW Dover



 • Time based on a Standard Port
 • Arrows indicate strength & direction
•Numbers show :
     Speed in tenths of a knot … 12 is 1.2 knots
     At Neaps, Springs … slowest, fastest
                                                                            08,17

•12 or 13 diagrams cover tidal cycle               HW-
                                                   6
                                                         HW-
                                                         5
                                                               HW-
                                                               4
                                                                      HW-
                                                                      3
                                                                            HW- HW-
                                                                            2   1



                                                   HW    HW+   HW+    HW+   HW+ HW+
                                                         1     2      3     4   5
                                 Tides


          Tidal Atlas Examples
• Examples:                                              HW-6   HW-5   HW-4   HW-3   HW-2 HW-1



• You dive at 10am, HW Dover is at 1300. Its
  mid way between Springs and Neaps.                     HW     HW+1   HW+2   HW+3   HW+4 HW+5

• There are 12 diagrams – “HW Dover–6”
  thru to “HW Dover+5” on the atlas.
• Which diagram do you choose?                                    HW Dover - 3




An arrow close to where you want to dive looks like
this.                                          02,04
What will be the speed and direction of the the
current?
Answer : to the west, average of 0.2 + 0.4 = 0.3 knots
                               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
                 Tides


             Tidal races


• Overfall     Headland    Channel
  s
                     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
                         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
                                  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
                          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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