Tides

Question 1

Why are changes in sea level important?

Answer 1

• Navigation
• Renewable energy
• Coastal flooding
• Coastal morphology (erosion)
• Estaurine flushing
• Biology (mangroves/saltmarsh)
  *

Question 2

What are tides?

Answer 2

The regular and predictable rise and fall of the sea caused my the gravitational attraction and rotation of the Earth, Moon and Sun system. TIdes are used to refer to the vertical change in sea level.

Question 3

What are tide currents?

Answer 3

The horizontal movements of water.

Question 4

What is sea level?

Answer 4

The vertical change in the height of the sea surface which occurs over all time and space scales from many different mechanisms (including waves, seiches, storm surges, tsunamis) with tides being the most predictable and the dominant component of sea level variablility in many parts of the word’s oceans and coasts.

Question 5

What is the universal law of gravitational attraction?

Answer 5

The force of gravitational attraction of two particles of masses M1 and M₂ separated by a distance r is given by:

F=GM₁M₂/r².

Question 6

What is the barycentre?

Answer 6

The common centre of mass of the earth and moon, with a period of 27.3 days.

Question 7

What are tides generated by?

Answer 7

The combined forces of the gravitational attraction between the earth and moon &sun and the rotation of the earth-moon and earth-sun systems, i.e., the balance between centrifugal and gravitational forces.

Question 8

How do you calculate the gravitational force between a particle of mass m_p towards the moon?

Answer 8

F=G(m_pm₁)/(r-a)² where a is the radius of the earth, r is the distance between the earth and moon and g is the gravitational constant 6.67x10-11.

Question 9

What is the tide generating force?

Answer 9

The difference between the gravitational force and the average gravitational force on the earth.

Question 10

What is the tide generating force proportional to?

Answer 10

The mass of the moon/sun and the inverse cube of the distance between the earth and moon/sun.

Question 11

What are the three principle assumptions for the equilibrium theory of tides?

Answer 11

1. The Earth has no continental land masses and is covered by an ocean of uniform depth.
2. There is no inertia in the system and oceans respond immediately to the tide-generation force.
3. The coriolus and friction effects can be neglected.

Question 12

What is the length of the tidal cycle?

Answer 12

12 hours and 25 minutes.

Question 13

What is the equation for the tidal constituent?

Answer 13

Hcos(ωt-P), where H=amplitude, P=phase lag, ω=angular speed and t=time.

Question 14

What causes semi-diurnal and diurnal tides?

Answer 14

The declination of the moon.

Question 15

What causes monthly/seasonal changes in the tides?

Answer 15

The elliptical path of the moon and sun, and the declination of the earth.

Question 16

What are the assumptions required for the dynamic theory?

Answer 16

1. The prescence of land prevents the tidal bulges from directly circumnavigating the earth.
2. Rotation of the earth on its axis is too rapid for the inertia of the water masses to overcome in sufficient time to establish an immediate equilibrium tide. The wavelength of a tide is long relative to depth in the occeans, so they travel as shallow water waves (speed = √gh).
3. Water movements induced by tide generating force are subject to friction and Coriolis.

Question 17

Which direction does Coriolis deflect in the Northern hemisphere?

Answer 17

To the right.

Question 18

Which direction does Coriolis deflect in the Southern hemisphere?

Answer 18

To the left.

Question 19

What are cotidal lines?

Answer 19

Lines connecting points that will have the same high tide at the same time.

Question 20

What are corange lines?

Answer 20

Lines connecting areas with the same tidal range or amplitude.

Question 21

What does friction in a basin lead to?

Answer 21

Outgoing energy is less than ingoing, so there’s a smaller amplitude. The amphidromic point is shifted to the left, so the tidal range on the right is greater than the one on the left.

Question 22

How do you calculate the period of standing waves in the ocean?

Answer 22

T=4L/√gd where L=length

Question 23

What is the equation for the speed of a shallow water wave?

Answer 23

c=√gh

Question 24

What is the cause of double high or low tides?

Answer 24

The additional shallow water tides.