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

2. What are Tides? Shallow water waves generated by:
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

3. 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)
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
Quadrature – 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. Declination
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

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

7. Why measure tides?
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
Investigate fluctuations of sea level
Information for Engineers
Information for legal cases regarding tidal boundaries

8. Tide levels and Datum planes
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 years
MLLW 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 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!)

11. 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.
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
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.

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)
Tide
Height
Time
2130
0804
1900 A B C
Fowl river Substation
Tide

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 desired
Start with HIGH WATER!
Fill in time and height from chart
Subordinate Station – use “Tidal differences & Other Constraints”
Find desired subordinate station
Fill in time difference for HW and LW
Fill in height difference (this is a MULTIPLIER)
Copy your original dates

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

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

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 form
Final Height
When 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 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: = -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