# Unit 8 - Tides.

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Unit 8 - Tides

What are Tides? Tides are the periodic raising and lowering of ocean sea level Tides are very long shallow water waves Caused by combination of gravity and motion between Earth, Moon, and Sun Isaac Newton’s gravitational laws explain the tides

Gravitational Forces Gravitational force is greatest at zenith – closest to moon Gravitational force is lowest at nadir – furthest from moon and opposite zenith

Centripetal Forces Centripetal force tethers the Earth and Moon to each other (keeps them in orbit) Centripetal force is equal on all parts of the earth

Resultant Forces produce tides
The resultant force is the difference between gravitational and centripetal forces It is the relatively small force that produces the tides

Resultant Forces The resultant force pushes water into two simultaneous bulges One toward Moon One away from Moon

Tidal Bulges – Moon’s Effect

Tidal Periods Tidal period – time between high tides
Lunar day - Time between two successive overhead moons equal to 24 hours, 50 minutes High tides are 12 hours and 25 minutes apart (2 per lunar day)

Tidal Bulges – Sun’s Effect
Similar to lunar bulges but much smaller (only 46% of the lunar bulge) The moons effect on tides is greater because it is so much closer than the sun

Monthly Tidal Cycle Spring tides Neap tides
New or full moons when sun and moon are in alignment Tidal range is the greatest Syzygy – when celestial bodies are in alignment Neap tides Quarter moons when sun and moon are at right angles relative to the Earth. Tidal range is the lowest Quadrature – when celestial bodies are at right angles

Spring and Neap Tides

Complicating Factors Declination – Angular distance of the Moon or Sun above or below Earth’s equator Sun to Earth: degrees north or south of equator Moon to Earth: 28.5 degrees north or south of equator Lunar and solar bulges shift from equator This produces unequal (mixed) tides at different spots around the world

Declination and Tidal Bulges

Effects of declination on tides

Earth around Sun: Tidal range greatest at perihelion (January) Tidal range least at aphelion (July) Moon around Earth: Tidal range greatest at perigee (Moon closest to Earth) Tidal range least at apogee (Moon furthest from Earth) Perigee–apogee cycle is 27.5 days

Relative sizes of the moon

Effects of Elliptical Orbits

Actual tides are more complex
Continents and friction with seafloor modify tidal bulges Tides are shallow-water waves with speed determined by depth of water Idealized tidal bulges cannot form because they cannot keep up with Earth’s rotation

Where do the tides start?
Tides start in the centers of the world’s oceans. Tides act like water swirling in a bowl. There will be a node in the center where the water level never rises or falls.

Amphidromic Points Cotidal map shows tides rotate around amphidromic points. There are 140 amphidromic points in the world’s oceans. Amphidromic Points Figure 9-14

Cotidal Lines Cotidal lines show where high tides occur at the same time

“if you don’t understand amphidromic points I’m going to bite you!”
Spidey Says… “if you don’t understand amphidromic points I’m going to bite you!”

Tidal circulation Tides progress around basins, clockwise in S hemisphere and counterclockwise in N hemisphere

Animation of Tidal Elevations in the Pacific

Tidal Patterns Diurnal One high tide and one low tide per day
Semidiurnal Two equal high and low tides per day Mixed Two unequal high and low tides per day (most common type)

Tidal Patterns in the U.S.

Tidal Graphs Tidal graphs show all the tidal patterns for a location over time. Spring and neap tides Diuranal, mixed, or semidiuranal tides.

Flood and Ebb Currents Flood current is when the tide is coming in.
Ebb current is when the tide is going out

Tides in Coastal Waters
Tide waves are reflected by coastlines producing complicated effects This can amplify the tidal range Cook Inlet Alaska and the Bay of Fundy in Nova Scotia are examples of extreme tides caused by coastal effects

Cook Inlet Tides

The Bay of Fundy Tidal Range is the difference between high tide and low tide. The Bay of Fundy in Canada has the greatest tidal range on the planet -56 ft. Nova Scotia bends when the tide comes in! As 14 billion tons of sea water flow into Minas Basin twice daily, the Nova Scotia countryside actually tilts lightly under the immense load ! Bay of Fundy tidal bore

Bay of Fundy – World’s Largest Tidal Range

Tidal Bores The wave on the incoming (flood) tide in certain rivers is known as a Tidal Bore. Tidal bores occur in about 100 rivers throughout the world. The Qiantang Bore in China reaches heights of 15 ft. and travels 15 mph. The Pororoca, in the Amazon River, forms waves 12 ft. high and can reach speeds of 20 mph. The Severn Bore in England is a popular one to surf as shown below. Qiantang Bore Surfing the Severn Bore

Whirlpools! Rapidly spinning seawater
Occur in restricted channels connecting two basins with different tidal cycles The Maelstrom in Norway is the world’s largest whirlpool

Tide-Generated Power Renewable energy source but not yet widely used.
Possible harmful environmental effects from blocking tidal currents Oldest was built in France in the 1960s Largest was finished in South Korea this year (2011)

Power Plant at La Rance, France