Ocean Tides

• Introduction • Definition and terms • Tidal characteristics and tidal terms • Tides and tidal ranges • Tidal phenomenon • Types of tide • Tide producing forces • Tidal bore • Tide prediction tidal theories

Tidal bore

• Is the very rapid rise of tide in which the advancing water forms an abrupt front.

• It is common in rivers which are shallow, sleep and funnel shaped

• The formation of tidal bore depends on the strength of incoming tidal wave slope and depth of channel river flow.

• In india tidal bore is observed in Hoogly river• In west coast it is formed near the entrance of sabarmathi

and mahi river.• The bore in mahi river is found to be of 2.5m length and

18km/h speed.

• Tide: are bulges of water produced by the gravitational attraction of the sun and moon on the ocean and by the centrifugal force resulting from rotation of the Earth-sun and Earth-moon systems.

Why tides occur

• Refers to cyclic rise and fall of sea water.• Variations in gravitational attraction between

the Earth, moon and sun.

The Moon• Primary factor that controls the

rhythm and height of the tides• 2 tidal bulges due to

gravitational attraction• Side of Earth closest to moon -

seawater drawn towards moon.

• Opposite side - bulge produced away from the moon.

• Timing of tides is related to the Earth’s rotation, and the rotation of the moon around the Earth.

The Sun

• The second factor controlling the tides is the sun’s gravity.

• Average solar tide = half average lunar tide.• At certain times, the direction of the moon’s

gravitational attraction is aligned with the sun’s. The two act together to produce highest and lowest tides of the year.

SPRING TIDES

• Highest and lowest tides of the year. (sun and moon aligned)

• Occur every 14-15 days during full and new moon.

PROXIGEAN SPRING TIDES

• Rare, unusually high tide.

• Occurs when the moon is both unusually close to the Earth, and in the new moon phase (between the sun and the Earth).

• Occurs at most once every 1.5 years.

NEAP TIDES

• The gravitational pull of the sun and moon are at right angles to each other.

• Daily tidal variations on Earth are at their least.

• Occur during first and last quarter of the moon.

TYPES OF TIDES

• Geometric relationship of sun and moon to locations on the Earth’s surface creates three different types of tides.

1. DIURNAL TIDES: one high and one low water tidal per day. Northern Gulf of Mexico and SE Asia.

SEMI-DIURNAL TIDE:Two high and two low waters per tidal day.Common on Atlantic coasts of the United States and Europe.

3. MIXED TIDES:• Successive high-water

and low-water stands differ appreciably.

• Higher high water and lower high water, as well as higher low water and lower low water.

• West coast of Canada and the US.

Types of tide

Tidal characteristics and tidal terms

• Tide alternative rise (high) ad fall low of sea level due to gravitational forces exerted on earth by moon and sun.

• Senuduyrbak tides (two highs,twolows• Diurnal tides (one high and one low in a day)• Tige has two cycles per lunar day (244.50m)• Solar tide (two cycles per day (24hr)• Tidal range is the difference in height between high and low tides• Daily change in sea level because of tides is a tidal curve.• Solar tides-produced by sun approximately at same time each

day• Lunar tide-occurs 50mts later each day

Tidal phenomenon

Tides and tidal ranges • As the earth revolves on its axis coastal areas periodically pass through the tidal

bulges and experience high and low tides. • As the crest of a bulge passes, high tide occurs. Troughs between bulges result in

low tides. • Tidal range is the difference in height between high and low tides. A continuous

record of the daily change in sea level because of tides is a tidal curve. • Tidal curves show that tides change daily in range and time of occurrence. The

variation results from the complex interaction of the earth sun and moon. • The interaction of these there bodies is so complex that a given coast may

experience the same tidal conditions only once each 1600 years. • Solar tides: Is the tides produced by the Sun which occur approximately at the

same time each day because the earth rotates on its axis once each 24 hours. • Coastal areas experience a solar high or low tide each 12 hours. • Lunar tides are more complex because the Moon moves forward in its orbit as

the earth rotates on its axis. Thus, a point on the earth directly below the Moon requires 24 hours and 50 minutes before it is again below the Moon.

• Lunar tides occur 50 minutes later each day. Coastal areas experience a lunar tide each 12 hours and 25 minutes internel.

Tide producing forces

• Gravitational attraction of sun and moon• The force of attraction of moon is higher than the sun

because of its proximity to earth.• Centrifugal force• Terms• Hightige: As the crest of bulge passes is called hightide• Troughs between bulges result in low tide• Tidal range: difference in height between high and low

tides• Tidal curve: Continuous record of daily change in sea

level because of tides is tidal curve.

Tide prediction

Tide theories

• Equilibrium theory developed by Newton• Dynamic theory of tides • Modern tidal theory

Tidal current

• Periodical horizontal movements of the sea due to tide generating forces.

• Tide generating forces are thought about by relative position of moon, sun, and the earth

• High and low tides-setup flood and abbcurrency in narrow straits and channels.

• The factors which affect the tidal currents are shape of bay, river flow, and shape of the channel and friction.

• These currents sometime move large sediments which may black the harbor.