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Tides “Eternity begins and ends with the ocean's tides.”

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What are Tides? Shallow water waves generated by: gravitational pull of the moon gravitational pull of the moon –The Sun and Moon actually tug at the Earth’s oceans, causing a tidal bulge (the tidal influence of the Moon is about twice that of the Sun). Centrifugal force produced by the rotation of the earth-moon system around their centers of mass Centrifugal force produced by the rotation of the earth-moon system around their centers of mass

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Variations in Tides Semidiurnal – two high and two low tides per 24 hour period (due to the rotation of the earth around it’s axis) Semidiurnal – two high and two low tides per 24 hour period (due to the rotation of the earth around it’s axis) Diurnal – a single high and low tide per 24 hour period – Occurs in the GOM - usually Diurnal – a single high and low tide per 24 hour period – Occurs in the GOM - usually Syzgy – Sun, Earth and Moon are all aligned – causes Spring Tides Syzgy – Sun, Earth and Moon are all aligned – causes Spring Tides Quadrature – Moon is at 90º to the earth and sun – causes Neap Tides Quadrature – Moon is at 90º to the earth and sun – causes Neap Tides

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Variations: Orbit of the moon Perigee – moon is the closest to the earth. Increases force on tides by 20% Apogee – moon is farthest from earth. Reduces force by 20%

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Declination Movement of the moon up and down with respect to the equator Movement of the moon up and down with respect to the equator –causes daily variation in tidal levels –When the moon is over the equator, declination = 0. No diurnal variation in the two tides - Equatorial tides. –At maximum declination, greatest diurnal variation, tropic tides are produced Equatorial Tides Tropic Tides

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Other factors affecting tides Prevailing winds Prevailing winds Atmospheric pressure Atmospheric pressure Coastal geomorphology Coastal geomorphology Minor astronomical motions Minor astronomical motions

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Why measure tides? Charting Coastal Waters – establishment of a uniform level (datum plane) to which observed water depths can be referred Charting Coastal Waters – establishment of a uniform level (datum plane) to which observed water depths can be referred Provide data for tide and current predictions Provide data for tide and current predictions Investigate fluctuations of sea level Investigate fluctuations of sea level Information for Engineers Information for Engineers Information for legal cases regarding tidal boundaries Information for legal cases regarding tidal boundaries

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Tide levels and Datum planes National Tidal Datum Epoch National Tidal Datum Epoch –19 years of observation –Length necessary due to lunar declination that varies in a regular cycle that has a period of 18.61 years MLLW is used as the datum plane for chart depths (most conservative averages) MLLW is used as the datum plane for chart depths (most conservative averages)

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Tidal Datum references for Dauphin Island for the National Tidal Datum Epoch of 1983-2001

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Predictions of Tide Times and Heights National Ocean Service provides predictions for 50 Stations (Reference Stations) National Ocean Service provides predictions for 50 Stations (Reference Stations) Predictions can be obtained for 2500 subordinate stations Predictions can be obtained for 2500 subordinate stations –Use tidal differences and time differences between the arrival of the tidal wave at two stations –All chart times are local STANDARD time (not daylight savings time!)

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Instruments –Tide Staff – A tide staff graduated in feet or centimeters is installed in a permanent housing mounted next to the tide gauge. zero mark on the staff becomes the vertical reference (MLLW or another datum system) that all subsequently recorded water levels refer to. zero mark on the staff becomes the vertical reference (MLLW or another datum system) that all subsequently recorded water levels refer to. –Tide Gauge – Mechanical or Acoustic Mechanical gauge - features a spring-loaded pulley and wire leading down to a cylindrical float inside a vertical stilling well Mechanical gauge - features a spring-loaded pulley and wire leading down to a cylindrical float inside a vertical stilling well Acoustic - utilize an acoustic “shock-wave” sent down a vertical wave-guide. After striking the water surface, the wave is reflected back to a transducer and microcomputer that converts travel time to distance based on the speed of sound in air. Acoustic - utilize an acoustic “shock-wave” sent down a vertical wave-guide. After striking the water surface, the wave is reflected back to a transducer and microcomputer that converts travel time to distance based on the speed of sound in air. –Advantage - don’t have moving parts to jam or become fouled –Do require compensation for the effects of temperature change on sound speed to maintain their high standard of accuracy. –Stilling Well– A cylinder installed near a body of water is used to hold and protect hydrological sensors. The stilling well allows water to move in and out freely but dampens wave and current action so as to provide a reasonable representation of the level of the water body.

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Figuring Tides at Subordinate Stations Example: What is the height of the tide in Fowl River at 1900 hrs on 11/22/07 ? Mobile Station (known values) Fowl river Substation Tide Height Time 2130 0804 1900 A B C

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Solving the Tide Solution Form 1. Pick a reference station (Mobile) close to your desired subordinate station (Fowl River) 2. Fill in part 1 (reference station) using “Mobile, AL – Times & Heights” 1.Use day before and after desired 2.Start with HIGH WATER! 3.Fill in time and height from chart 3. Subordinate Station – use “Tidal differences & Other Constraints” 1.Find desired subordinate station 2.Fill in time difference for HW and LW 3.Fill in height difference (this is a MULTIPLIER) 4.Copy your original dates

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Tide Solution Form (con’t) 4. Time/Height/Date 1.Subtract your time difference from your original time Make sure your date does not change with your time change! Make sure your date does not change with your time change! 2.Multiply original height and height difference for new height Don’t forget your UNITS! Don’t forget your UNITS! 3.DST – Add one hour for DST time All charts are in standard time All charts are in standard time Check dates AGAIN to make sure they didn’t change with the DST time change Check dates AGAIN to make sure they didn’t change with the DST time change

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Tide Solution Form (con’t) 5. Part II – Height at anytime 1.Refill desired station, date and time Good QA/QC Good QA/QC 2.Duration of rise and fall – subtract HW from LW on day of interest (use DST #’s) “A” on graph “A” on graph 3.Time from nearest tide – subtract nearest tide time from your time of interest “B” on graph “B” on graph 4.Range of tide – subtract HW height from LW height “C” on graph “C” on graph 5.Fill in height of nearest tide Tide Height Time 2130 0804 1900 A B C

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Tide Solution Form (con’t) 6. Correction – use “Table 3” 1.If you are in an area with a semidiurnal tide (WE ARE), you must divide both your duration of rise and fall and your time from nearest tide by 2 when looking up your correction in the upper box!! 2.Find your duration, and time and then move down to the lower box for range of tide (DO NOT DIVIDE THE RANGE by 2) 3.Read off your correction factor and fill in on form 7. Final Height When nearest tide is HW subtract correction. When nearest tide is LW, add correction.When nearest tide is HW subtract correction. When nearest tide is LW, add correction.

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Subtracting Times Follow these steps: 1. Subtract the hours 2. Subtract the minutes –If the minutes are negative, add 60 to the minutes and subtract 1 from hours. Easy example: What is 4:10 - 1:05 ? Subtract the Hours: 4-1 = 3 Subtract the Minutes: 10-5 = 5 The minutes are fine, so the answer is 3:05 Hard example: What is 4:10 - 1:35 ? Subtract the Hours: 4-1 = 3 Subtract the Minutes: 10-35 = -25 The minutes are less than 0, so add 60 to Minutes (-25+60 = 60-25 = 35 Minutes) and subtract 1 from Hours (3-1 = 2 Hours)... answer is 2:35

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