Lecture 13 - Tides PDF

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Lecture 13 Tides
Lecturer Tim Ralph...

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Lecture outline:  What is the tide?  What forces the tide?  Lunar and solar orbits  Tidal variation  Tidal constituent  Tidal datums and pattern  Global tides and amphidromic points  Australian tidal ranges  Measuring tides  Importance of tides What is a tide  Tides = periodic movements which are directly related in amplitude and phase to some periodic geophysical force  Periodic = appearing or occurring at intervals  Amplitude = range or magnitude (height of their displacement)  Phase = stage of the tide  Geophysical force = variation in gravitational field on Earth’s surface, cause by regular movements of the moon and sun Bay of Fundy, Canada- biggest Tidal variations in the world- 8m every 6 hours Horizontal waterfalls, WA- also caused by tides- back and forth movement based on differing tide heights between these two different areas. What forces the tide  Moon-majority of the forcing- gravitational force acting on both earth and moon.  Rotation of the earth- centrifugal force  Sun also plays an important role - not as strong as moon, when the sun, moon and earth all in alignment we get strong tides. Causes spring tides. If they are really big- king tides Neap tides- smallest tides  The sun and the moon are not in alignment (they are at right angles to each other) Moon cycle- about 1 month. Different variation in tides in different places. Lunar perigee and apogee When moon is closest to earth- we get strongest tides- (lunar perigee) Apogee- longest distance between the earth and moon- smaller tides (less gravitational force between those) Smallest distance from earth and sun- Solar perihelion- king tides (when the earth, moon and sun are all in alignment) Larger distance from earth and sun- solar aphelion- smaller tides. Tidal variation summary  Tide varies daily due to earths rotation  Every 2 weeks due to full moon (spring tides) and quarter moon (neap tides) Varies monthly due to the rotation of moon around earth- Lunar perigee and apogee

Varies annually due to rotation of earth around the sun- solar perihelion and aphelion. 

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Tidal constituents can be described as sine waves Tidal constituents- have different amplitude. They don't just depend on gravitational forces.

Tidal datums  Vertical references based on local water level measures Tidal ranges  Difference between the height of high water and the next succeeding or last preceding low water Most common tidal pattern  Semi- diurnal= 2 highs and 2 lows, similar amplitude per day  Mixed- 2 highs and 2 lows- less common  Diurnal- 1 high and 1 low tide- only few places- Mexico Other influencers of tide  Tides- amplified on wide continental shelf- larger tides in Northern Australia  Weather, strong winds moving way from coast- lower tides  High/low pressure system- sea level increases  Funnelling effects of base and estuaries (complex)- upto 16m we can get. Global tidal ranges  Nodal points- almost no tidal variation

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Tidal ranges- much higher in northern Australia- wide shelves in the north

Tides based on height Microtidals- difference of 80m Macrotides- large tides- 12m Maeso tides- upto about 3m First attempt at measuring water level  Egyptians started, nilometer Now tidal Gauges are commonly used- placed on piers Problems with float tide gauges  Sinking of the well = higher readings  Sedimentation in the stilling well = higher readings  Friction in float mechanism = sticking  Timing errors e.g. incorrectly set clock or drifting in time  Water level lower than well = cant measure  Flow through orifice during waves  These type of measurements are also not completely accurate. Modern methods  Recording pressure then convert to sea level  Acoustic tide gauges: time for sound to travel to water surface and back  Satellite altimetry- accurate  Can measure accurate tidal variation across the globe Why tides are important Classification of tides

Importance of Tides:

Tidal range- impacts morphological features

Importance of tides  They determine habitats and species  Some species prefer consistent conditions while some are happy with larger variations  Eg: salt marsh which only grows in intertidal zone, species which needs the wetting and drying through different cycles

When storm is about to come, we get storm surge (low pressure) Low tide for storm surge- no flooding High tide for storm surge- flooding (Tsunamis) Tides are also important for shipping and port operations. Importance of tides for generating energy Key points about tides  Driven by gravity of moon and centrifugal force  Alignment of moon with sun drives spring and neap tides  Major cycles • Lunar perigee and apogee (~1 month)  Solar perihelion and aphelion (365.25 days)  Tidal constituents and datums  Tidal patterns: diurnal, semi-diurnal, mixed • Measuring tides: Nilometers, float gauges, then modern methods  Important for morphology, habitats, coastal hazards, shipping, energy, etc....