TIDES

Periodic short term changes in the height of the ocean surface at a

particular place

TIDAL MARSH

Moon's Gravity Pulls Oceans - Near-side Bulge is Easy to Understand

Moon and Earth actually orbit around the Earth-Moon Center of Mass (about 1500 km beneath the surface of the Earth)

Motion of Earth Around Center of Mass Creates a Bulge on the Far Side of the Earth

Both Moon and Sun Cause Tides

DEFINITIONS• Tidal day

– 24 hr 50 min – Time between successive phases of moon over a

given point on the earth• Tidal Period

– Time between two successive high or low tides • Tidal Range

– Difference between highest and lowest tide levels• Daily inequality

– Difference in height  between successive high orlow tides

The Tidal Cycle• In general, a complete tidal cycle takes 24 hours

and 50 minutes. • This is the time it takes for the Earth to rotate on

its axis back to its original position with respect to the moon, the primary tide-causing force.

• Because it takes the moon about 27.3 days to complete one orbit around the Earth, the moon moves a little bit further around the Earth each day.

• Thus, the time of the tides advances about 50 minutes each day.

TIDES• Periodic changes in sea level relative to land

along a coast• Daily or Diurnal Tides

– One high and one low tide each day• Semi-daily or Semidiurnal Tide

– Two high and two low tides of approximate equal heights occur each day

• Mixed Tide– Two high and two low tides of unequal heights

(HHW, LHW, HLW, LLW)

TIDES

• Many other factors influence the nature and intensity of the tides, including the shape of the ocean basin and the Coriolis effect.

• These factors create high and low tides. Depending on the position of the Earth with respect to the moon and the sun, differences in the height of sea level during the high and low tides may be great or small

AMPHIDROMIC POINT• As the tidal bulge moves across the Atlantic it

encounters theAmerican Continents

• Because the Moon keeps on moving overhead, the tidal bulge gets left behind and the tidal wave is reflected back into the Atlantic

• The lagging bulge and the reflection of the tidal bulge give rise to different types of tides depending on the dimensions and shapes of the basins.

AMPHIDROMIC POINT• As the tidal bulge moves across an ocean and is

reflected back from the opposite side, the Coriolis Effect causes the moving water to be deflected.

• The peak of the tidal bulge moves around the basin rather than just  straight back and forth across it.

• In an open ocean the crests and troughs of the wave actually rotate around a point near the center of the ocean.

• This point is called the amphidromic point.

SPRING AND NEAP TIDES

• Spring Tides– Occur at Full and New Moon Sun, – Moon and earth in a line – Greatest tidal range

• Neap Tides – Occur at the first and third quarter of moon– Least tidal range

TIDAL RANGE

The Bay of FundyNova Scotia, Canada

In the Bay of Fundy the tidal range can be up to 16m

TIDAL CURRENTS

• Horizontal water movement caused by tides

• Tides are like Shallow water waves • Orbital motion of water is highly elliptical:

can be assumed to be to and from motion • Flood tides when water moves in • Ebb tide when water moves back

TIDAL BORE

Tidal Friction• Tides stretch the oceans, and to a small extent,

the solid mass of a planet or satellite. • In one complete rotation, the planet material

keeps deforming and relaxing. • This takes energy away from the rotation,

transforming it into heat. • In effect, this is a frictional loss, like a giant

brake on the planet. • Over the centuries, the moon's rotation on its

own axis has slowed until it presents essentially the same face to the earth.

• Each century, the day increases by about 3 milliseconds.

• Over 100 million years, the day will increase by about an hour.

Tidal Friction

Rotation and Friction Causes Tides to Lead Moon

Bulge Pulls Moon, Throws into Larger Orbit

Friction Slows Earth

Precambrian (900 m.y.): Year = 500 Days, Day = 18 Hr., Month = 23.4 Days

Cambrian (500 m.y.): Year = 400 Days, Day = 22 Hr.

Predicting Tides• Predicted tidal heights are those expected under

average weather conditions. • When weather conditions differ from what is

considered average, corresponding differences between predicted levels and those actually observed will occur.

• Generally, prolonged onshore winds (wind towards the land) or a low barometric pressure can produce higher sea levels than predicted,

• While offshore winds (wind away from the land) and high barometric pressure can result in lower sea levels than predicted.

Tidal Power• The potential energy contained in a volume of

water is– E = hMg

• where h is the height of the tide, M is the mass of water and g is the acceleration due to gravity.

• Therefore, a tidal energy generator must be placed in a location with very high-amplitude tides.

• Suitable locations are found in the former USSR, USA, Canada, Australia, Korea, the UK and other countries

Tidal Power

• In 1966, France built the World’s first tidal power station on the river Rance, in the process constructing 24 earth dams which generate approximately 502 millions KW/H of electrical power/year.

Severn Barrage, UK• John Hutton, Secretary of State for Business,

Enterprise and Regulatory Reform, announced a further feasibility study on 25 September 2007.

• The proposal for a hydro-electric barrier to generate 8.6 GW and meet five percent of Britain's power needs, is being opposed by environmental groups

• Power would be equivalent to about 18 million tons of coal or 3 nuclear reactors.

• This decreases the output of greenhouse gases into the atmosphere.

Operating Stations

• SITE Tidal Range (m) Power (MW)

• Cook Inlet   • Knik-Arm 8,4 1440 • Turnagain-Arm 8,4 9002• Rio Gallegos 7,6 7003• Golfo de San Jorge 4,2• Bahia Sao-Jose 5,6 7000 • Golfo-Nuevo 3,6 110005• Belem 5,9 306• Bay of Fundy   • Cumberland Bay 9,9 1080 • Cobequid Bay 11,8 4030 • Shepody Bay 9,6 15507• Annapolis-Royal (1984) 6,4 208

• SITE Tidal Range (m) Power (MW)• Ungava Bay   9 dam sites 7,7 92609• Severn River 8,3 720010• Strangford-Lough 3,0 32011• Solway-Firth 5,1 683012• River Rance* (1966) 8,5 24013• Chausse de Sein 8,5 1200014• Cotentin Peninsula 8,0 5000015• Kislaya Guba (1968) 2,3 0,416• Lumbovskiy Bay 4,2 67017• Mezenskiy Guba 6,0 1520018• Guba Penzhinskaya:   • south range 6,2 87400 • north range 6,2 2140019• Tugurskiy Bay 4,7 1030020• Inchon 6,050021• Tsien-tien (1980) 5,0 322• Gulf of Cambay 6,8 736023• Gulf of Kachchh 5,3 160024• Walcott Inlet 5,0 125025• Security Bay 5,6 570