Tides by gjjur4356


Section 13
           What are Tides?

• Tides are the vertical movement of water

• Tides rise and fall
      Why Know About Tides

• Clearance under Bridges

• Water Depths in Shallow Passages

• Prevent Grounding

• Mooring and Anchoring
            Causes of Tides

• Moon’s gravitation attracts the ocean
  – Mid-ocean: low tide
  – Shore line: high tide
             Time of Tides

• Moon circles Earth in 24 h 50 min
  – Lunar day is 50 min. LONGER than solar day

• Tides occur 50 minutes LATER each day
                    Types of Tides

In Conjunction or Opposition
   (New moon and Full moon)
• Additive
   – Extra high tides

In Quadrature
• Counteractive
   – Lower tides
          Classification of Tides
• Semi-diurnal
  – 2 high and 2 low tides per day
  – Diurnal inequality:
     • HHW/HW and LLW/LW

• Diurnal
  – Only 1 high and low per day

• Mixed
  – Both semi-diurnal and diurnal at different times
  Varying Depths and Heights

• Depths measured from the LOWEST level
  – Lowest low water

• Heights are measured from the HIGHEST
  – Highest high water
        Sources of Tide Info
• Newspapers and radio
  – Somewhat useful

• Tide Tables
  – Reference ports read directly
  – Secondary ports require calculations
  – Canadian Hydrographic Service produces
    “Canadian Tide and Current Tables”
         Using Tide Tables

 – In STANDARD time
    • In SUMMER – add an hour for DST
         Chart Measurements
• Soundings are from chart DATUM
  – DATUM is a reference level
     • Chart A: Datum is Lowest Normal Tide
             Height of Tides

• Tides are from chart DATUM
  – The height of the tide is the distance of the
    water surface from Datum

                                     High Tide
                                     Lowest Normal Tide

                                       Chart A: Datum
                                       is Lowest Normal
Real Water Depth

   Height of Tide
   Charted Depth
             Range of Tide

• Range of tide:
  – Difference between a high and the next low
    or vice-versa

• Useful to know when:
  – Anchoring
  – Tying to a fixed dock

• Stand: time interval of no vertical water
 movement at turn of the tide
  – HW Stand
  – LW Stand

• Useful to know when:
  – Passing through an area with strong current
     Tide Tables Only a Start

• Tide Tables give times and heights of
 successive highs and lows each day

• Pilot may want to know:
  – Height of tide at a certain time
  – Time where water will be a certain depth
Determining the Height of the Tide

• Make a Graph
  – Good for pre-planning
  – Use data from Tide Tables

• Procedure:
  –   Mark heights on left-hand margin
  –   Mark times along bottom margin
  –   Plot the highs and lows and connect with lines
  –   Divide each range into quarters
Graph – Step 1
            Graph – Step 2

• Draw a short vertical line through each 1st
  and 3rd quarter point

• Along each vertical line, lay off 1/10 of the
  range, upwards near a HW and
  downwards near a LW
          Graph – Step 3

• Draw a smooth curve through these new
 points, the mid-point, and the High and
 Low points

• Charted clearance is the distance from the
 overhead object

• Uses HHW Large Tides
      To Determine Clearance
• Subtract the calculated height of tide for the
  time from the HHWLT level of the day

• Determine the clearance of the bridge above

• Add them together to get clearance

• Add a safety margin of 1 metre to the height of
  the mast

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