tide measurement

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and Tide Measurement

Tides

DOMINGUEZ · BOTOR · JAMBALOS

04/08/2023 2Tides and Tide Measurement

Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth.

What are tides ?

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Factors affecting Tide measurements are:Shape of the landShape of the ocean floor (bathymetry)Depth of waterRestrictions to flow (narrow inlets to bays, etc.)Local winds

Local Influences on Tides

Tides and Tide Measurement


The tidal range changes depending on the relative positions of the moon and the sun.

Spring Tide – higher than normal tide, that is if the moon and the sun are acting together.

Neap Tide – lower than normal tide, that is if the moon and the sun are acting opposite to each other.


Can tides be predicted? How ?How are tides reported?Are tides the same everywhere on Earth?

Some Questions about Tides


Can tides be predicted? How ?Yes, tides are easily observed and predicted. Scientists use a mathematical formula to

predict the earth’s tidal schedules. They use the lunar day, which is 24 hours and 50minutes long.

Scientists use a lunar day because the moon (luna) is the strongest influence on the earth’s tides. These extra 50 minutes cause tides to be at different times each day.


How are tides reported ?


The moon’s gravitational pull is not the same at all locations

Semidiurnal/semidaily tides - two high tides and two low tides each day

Diurnal/daily tides - one high and one low tide each day

Are tides the same everywhere on Earth?


Tide Staff – A tide staff graduated in feet or centimeters is installed in a permanent housing mounted next to the tide gauge. After installation, the zero mark on the staff becomes the vertical reference (the station datum) that all subsequently recorded water levels refer to.

Terms defined


Tide Gauge – Mechanical devices for recording water level are still in use but these are mostly being replaced by acoustic water level meters that measure and record data electronically


Tidal Bench Mark – Should anything happen to the staff, a permanent mark such as a brass disc set in concrete prevents the loss of the station datum (tide staff zero) after a leveling survey between the mark and the staff. This is especially important at stations where the calculation or transfer of a tidal datum, such as mean high water or mean low water, is required.


Setup


Stilling well, a device specifically designed to admit only low-frequency oscillations in water level inside the sensing environment – a tube or pipe of fixed inside diameter.

The bottom of the tube is usually sealed except for a single small orifice that limits the rate at which water can enter or exit the tube.


FORMULA:

H=Zo+∑ fn Fn cos [Ent + Vn + un - gn}

Representations: H = height of the tide at any time (t). Zo = mean height of water level above the adopted datum of predictions. Fn = mean amplitude of nth component. fn= factor for reducing Fn to the year of prediction. En = speed of nth component. t = time reckoned from some initial epoch (such as the commencement of the year of

predictions). Vn =Value of equilibrium argument of nth component when t = 0. un = Nodal angle in degrees of nth constituent and gn = Phase-lag of nth component.

How do the hydrologists compute for the HEIGHT OF TIDE ?


Disadvantage of tide staffs – measurements can only be taken if an observer is present

Tide observations is time-consuming.

The Transition to Recording Tide Gauges


Joseph Saxton, 1851

Uses a float that rose up and down with the tide inside of a stilling well. The float was attached to a wire that connected to the mechanical pulleys and gears that drove a stylus across a strip chart. The movement of the stylus essentially created a graph of the tide over time. Spring-wound clocks were used to advance the strip-chart at an even rate so that the values could be read and correlated with time.

The Saxton Gauge


The Standard Tide Gauge


The sensors of STGs were floats attached by wire to a gearing mechanism in a stilling well. The gearing mechanism drove the location of a pencil relative to a rotating drum covered with a paper tide record sheet. The stilling well configuration calmed the waters around the float, to allow an accurate reading in the presence of waves. A typical stilling well consisted of a 12-inch wide pipe. Inside the stilling well, an eight-inch diameter float was hung by wire from the recording unit above.



The Standard Tide Gauge was used until the late 1960s, when Analog-to-Digital Recorders were introduced



Portable Tide Gauge• Developed for short-term deployments (for a few days to a few months) during hydrographic surveys

•These mechanical gauges had spring-wound clocks and used wax-marigrams (strip charts) as the medium on which a metal stylus traced a curve in response to the rise and fall of the tide. The marigrams had to be replaced every seven days and the tide curves were read manually from the strip charts and processed just as for fixed Standard Tide Gauges.


Analog-to-Digital Recorder (ADR) – a major breakthrough in tide gauge technology, 1966.

The Transition to the Digital World


Includes a float and wire and a stilling wellUses a punch paper tape system to record the

water level every six minutes. The punch paper provided a computer

compatible data recording, compared with earlier analog gauges which drew lines on a paper chart. The ADR paper tapes were read by an optical reader and translated onto nine-track magnetic tape for loading onto a computer system for processing.

ADR Features


Stations were subject to recording errors and marine fouling and were constantly in need of maintenance. In addition, the measurement and data processing equipment could not provide users with information until weeks after the data were collected.

ADR Limitations

tide measurement

Documents

y tide staff

y neap tide tides

y formula

y scientists

y factors

y spring tide higher

present y tide observations

tide measurements