Movements of Ocean Water
Basics about wave-
Wave-
A wave is a disturbance in a medium that carries energy without a net movement of particles and they are the energy, not the water as such, which moves across the ocean surface
Crust and Trough-
- The highest surface part of a wave is called the crest, and the lowest part is the trough
Wave height -
- It is the vertical distance from the bottom of a trough to the top of a crest of a wave.
Wave amplitude -
- It is one-half of the wave height.
Wave period -
- It is the time interval between two successive wave crests or troughs as they pass a fixed point.
Wavelength -
- It is the horizontal distance between two successive crests.
Wave speed -
- It is the rate at which the wave moves through the water, and is measured in knots.
Wave frequency -
- It is the number of waves passing a given point during a one second time interval
The size and force of a sea wave depends on following factors -
- Velocity of the wind.
- The length of time the wind blows and Distance that the wind has travelled across the open sea. This is called a fetch.
Tides-
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by The Moon and The Sun on The Earth
The factors responsible for varying tides-
- The location of the sun, the moon and the earth in relation to each other which is rarely in a straight line.
- The distances of the sun and the moon from the earth are not constant.
- Our globe is not entirely covered with water and the landmass creates obstruction in the movements of the waves and tides are formed.
- Changes in position of the sun and moon in relation to the earth.
Causes of Tides-
- Major cause of Tides on The Earth is Gravitational pull of The Sun ,The Moon on earth.
- Along with this Centrifugal forces of The Earth due to rotation of earth upon own axis is also the major cause
Process-
- On the surface of the earth, nearest the moon, pull or the attractive force of the moon is greater than the centrifugal force, and so there is a net force causing a bulge towards the moon.
- On the opposite side of the earth, the attractive force is less, as it is farther away from the moon, the centrifugal force is dominant.
- Hence, there is a net force away from the moon.
- It creates the second bulge away from the moon.
- On the surface of the earth, the horizontal tide generating forces are more important than the vertical forces in generating the tidal bulges.
- When tidal bulges hit the mid-oceanic islands they become low.
- The shape of bays and estuaries along a coastline can also magnify the intensity of tides
Types of Ocean Tides-
Diunral Tides-
- They occurs at an interval of 24 hours 50 minutes.
- One High and One Low tide takes place in this types of Tide.
- The successive high and low tides are approximately of the same height.
- It generally takes place in High Latitudes.
Semi-Diunral Tides-
- They occurs at an interval of 12 hours 25 minutes.
- Two High and Two Low tide takes place in this types of Tide.
- The successive high and low tides are approximately of the same height.
- It generally takes place in low Latitudes.
Mixed tides-
- These types of tides have alternate and unequal wave height and period
Spring Tides and Neap Tides-
Spring Tide-
- When the sun and the moon are in a line as on a new moon (Amavasya) or a full moon day (Purnima) both of them pull together at the same time in the same direction.
- This combined pull produces an extra large tide and called as Spring Tide.
This position of sun, moon and earth in a straight line is called syzygy and it is also further divided into Two parts-
Conjunction and Opposition-
- When sun and moon are in one side of earth (solar eclipse position), the position is called conjuction.
- However when earth is between sun and moon, the position is called opposition.
- They occur twice a month, one on full moon period and another during new moon period
Neap Tides-
- In its first quarter (Asthmi Shukla Paksha) and the third quarters (Asthmi-Krishna Paksha) the gravitational force of the two heavenly bodies is at right angle and this alignment of sun, earth and moon is called Quadrature.
- In this case, the tidal bulge is lower on both sides facing sun and moon as the gravitational forces of sun and moon work against each other.
- This happen twice the month
Note- The normal Interval between the spring tides and neap tides is of 7 days
Perigee and Apogee tides-
Perigee-
- when the moon’s orbit is closest to the earth it is called as perigee and unusually high and low tides occur.
- During this time the tidal range is greater than normal
Apogee-
- It is opposite of Perigee and generally takes place after Two weeks of Perigee
Note- Both of Perigee and Apogee are monthly movements
Equinoctial tide-
- A tide occurring when the Sun is near equinox i.e.equator , characterized by greater-than-average ranges of the spring tide.
- It takes place Twice the year I.e. during the Spring and Autumn equinox (March 21 and September 23)
Solstices tides-
- On June 21 and December 22 the summer and winter solstices, respectively the sun is at its maximum declination and is its largest angle to the equator I.e. at tropic of capricorn and cancer.
- Due to this there is tidal buldges because of this shift
Perihelion and Aphelion tides-
Perihelion tides-
- When the earth is closest to the sun (perihelion), around 3rd January each year, tidal ranges are also much greater, with unusually high and unusually low tides.
Aphelion tides-
- When the earth is farthest from the sun (aphelion), around 4th July each year, tidal ranges are much less than average and Perihelion and Aphelion tides occurs respectively
Flood and Ebb Tide-
Flood Tide-
- The change of tides from low to high tide is called the flood tide
Ebb Tide-
- The change of tides from high tide to low tide is called the ebb tide
Symmetrical tides-
- If the amplitude of ebb and flood tidal phases is same, the tides are classified as symmetrical. In a mixed tide, the ebb and flood phases have different amplitude
Some of the Terms Related to Tides
Tidal range-
- The difference in height between the high tide and the low tide is called the tidal range.
Tidal bore-
- A tidal bore is a wall of water that carries the tide up the mouths of some rivers
Double ebb-
- An ebb tidal current having two maxima of speed separated by a smaller ebb speed
Tidelands-
- The zone between the mean high water and mean low water lines
Tides in the World and The India-
- The highest tides in the world can be found in Canada at the Bay of Fundy, which separates New Brunswick from Nova Scotia.
- In India Gulf of Kachchh, the Gulf of Khambhat and the Sunderbans are the macro tidal regions. Rest of the coast of the India have very low tidal range
Famous Theories of oceans tides and these are-
Progressive wave theory of William Whewell propounded in the year 1833 and the canal theory of G.B. Airy postulated in the year 1842
Stationary wave theory by R.A. Harris
Effects of Tides-
- Generation of Tidal energy.
- Helpful in Maintenance of Coastal biodiversity.
- Tidal flows helps in navigation.
- Helpful in desilting the sediments and in removing polluted water from river estuaries.
- It increases the depth of water which help ships to move safely to the ports