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Published byRosemary Whall Modified over 3 years ago

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**Tides Tide producing forces Semi-diurnal; diurnal tides**

Lunar and solar tides Resonance Shallow water waves Tidal ellipses Tidal residuals Tidal-stream selective transport

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England San Francisco Philippines Korea

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**Not this kind of “tidal wave”…**

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**Moon’s gravitational attraction**

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**The point “Z” stays fixed, and the earth-moon system rotates about it**

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**Force imbalance in earth-moon system creates tidal bulges**

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By the time the Earth has completed one revolution (one day), the moon has moved slightly, so the Earth needs to rotate a bit more to catch up with the moon…

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**M2 period (12.42 hours) Semi-diurnal period**

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**Declination offsets bulge from the equator**

(Diurnal period)

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**Main Tidal constituents**

Origin Symbol period coefficient generation (hours) Semidiurnal tides Moon M semidiurnal principal lunar tide Sun S semidiurnal principal solar tide Moon N elliptical M2 tide Diurnal tides M&S K principal declination tide Moon O diurnal principal lunar tide Sun P diurnal principal solar tide

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**Sun and moon effects can be additive or not…**

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**Amphidromic point (point around which tides propagate and where tidal elevation at a minimum)**

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Kelvin wave: propagates with the coast on its right in the northern hemisphere and its amplitude decays offshore

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**M2 cotidal charts for the Atlantic Ocean **

Amplitude (cm) Phase (degrees)

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Co-tidal Chart for M2 tide

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M2 tidal

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**Tidal Resonance: amplification of tidal signal (e. g**

Tidal Resonance: amplification of tidal signal (e.g., amplitude) that occurs in an embayment or on a shelf, when the “length” of the embayment or shelf is near one-quarter of the wavelength of the wave.

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**Variations in tidal form world wide**

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semidiurnal Mixed, SD dominant Mixed, D dominant diurnal

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**Bay of Fundy – tide High tide Low tide (6 hours later)**

gets huge (15 m) because of resonance High tide Low tide (6 hours later)

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**Some engineering applications using tidal resonance and their effects**

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**Tidal currents (tidal ellipses)**

60m 100m 200m

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**Deep water waves: Wavelength short relative to water depth**

Shallow water waves: Wavelength long relative to water depth

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**Velocity felt all the way to the bottom**

Velocity decays before reaching the bottom

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**Tidal currents – can be very complicated (from near Australia)**

Current hodograph shows direction and speed of slow every hour (each point) over 15 days. Pattern doesn’t even repeat!

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**Biological applications…**

Tidal residuals Transport Retention Selective tidal-stream transport

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**Tidal ellipses Residual (time-averaged) currents**

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**Another way of looking at residual currents**

Another way of looking at residual currents... this time in the English Channel and North Sea

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**Plaice migration: as adults they migrate from the feeding areas to the spawning grounds and back.**

How do they do it?

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**Simulated tracks with day-night (diel) vertical migration: not much net displacement**

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Westward swimming Northward swimming

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