Presentation on theme: "“Eternity begins and ends with the ocean's tides.”"— Presentation transcript:
1 “Eternity begins and ends with the ocean's tides.”
2 What are Tides? Shallow water waves generated by: gravitational pull of the moonThe 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
3 Variations in TidesSemidiurnal – 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 - usuallySyzgy – Sun, Earth and Moon are all aligned – causes Spring TidesQuadrature – Moon is at 90º to the earth and sun – causes Neap Tides
4 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%
5 DeclinationMovement of the moon up and down with respect to the equatorcauses daily variation in tidal levelsWhen 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 producedEquatorial TidesTropic Tides
7 Why measure tides?Charting Coastal Waters – establishment of a uniform level (datum plane) to which observed water depths can be referredProvide data for tide and current predictionsInvestigate fluctuations of sea levelInformation for EngineersInformation for legal cases regarding tidal boundaries
8 Tide levels and Datum planes National Tidal Datum Epoch19 years of observationLength necessary due to lunar declination that varies in a regular cycle that has a period of yearsMLLW is used as the datum plane for chart depths (most conservative averages)
9 Tidal Datum references for Dauphin Island for the National Tidal Datum Epoch of 1983-2001
10 Predictions of Tide Times and Heights National Ocean Service provides predictions for 50 Stations (Reference Stations)Predictions can be obtained for 2500 subordinate stationsUse tidal differences and time differences between the arrival of the tidal wave at two stationsAll chart times are local STANDARD time (not daylight savings time!)
11 InstrumentsTide 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.Tide Gauge – Mechanical or AcousticMechanical gauge - features a spring-loaded pulley and wire leading down to a cylindrical float inside a vertical stilling wellAcoustic - 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 fouledDo 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.
13 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)TideHeightTime213008041900ABCFowl river SubstationTide
14 Solving the Tide Solution Form Pick a reference station (Mobile) close to your desired subordinate station (Fowl River)Fill in part 1 (reference station) using “Mobile, AL – Times & Heights”Use day before and after desiredStart with HIGH WATER!Fill in time and height from chartSubordinate Station – use “Tidal differences & Other Constraints”Find desired subordinate stationFill in time difference for HW and LWFill in height difference (this is a MULTIPLIER)Copy your original dates
15 Tide Solution Form (con’t) Time/Height/DateSubtract your time difference from your original timeMake sure your date does not change with your time change!Multiply original height and height difference for new heightDon’t forget your UNITS!DST – Add one hour for DST timeAll charts are in standard timeCheck dates AGAIN to make sure they didn’t change with the DST time change
16 Tide Solution Form (con’t) Part II – Height at anytimeRefill desired station, date and timeGood QA/QCDuration of rise and fall – subtract HW from LW on day of interest (use DST #’s)“A” on graphTime from nearest tide – subtract nearest tide time from your time of interest“B” on graphRange of tide – subtract HW height from LW height“C” on graphFill in height of nearest tideTideHeightTime213008041900ABC
17 Tide Solution Form (con’t) Correction – use “Table 3”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!!Find your duration, and time and then move down to the lower box for range of tide (DO NOT DIVIDE THE RANGE by 2)Read off your correction factor and fill in on formFinal HeightWhen nearest tide is HW subtract correction. When nearest tide is LW, add correction.
18 Subtracting Times Follow these steps: 1. Subtract the hours 2. Subtract the minutesIf 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 3:05Hard example: What is 4:10 - 1:35 ?Subtract the Hours: 4-1 = 3 Subtract the Minutes: = -25 The minutes are less than 0, so add 60 to Minutes ( = = 35 Minutes) and subtract 1 from Hours (3-1 = 2 Hours) ... answer is 2:35
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