Section 2: Tides, equilibrium and dynamic theory

Question 1

What are tides

Answer 1

The regular and predictable rise and fall of the sea caused by the gravitational attraction and rotation of the Earth, moon and sun system
Vertical change in sea level
Tides are normally the most predicable and dominant component variability in sea level change

Question 2

What are tidal currents

Answer 2

The horizontal movement of water due to tides

Question 3

What is sea level

Answer 3

The vertical change in the height of the sea surface which occurs over many different spatial and temporal scales.

Question 4

Why are tides important

Answer 4

Navigation: eg Southampton, double high tide, asymmetric, long flood (high waters) allow large vessels to enter, short ebb
Renewable energy: tidal barrage eg Rance estuary France
Tidal mills
Coastal flooding: eg Thames tidal barrier
Coastal morphology
Coastal erosion
Estuarine flushing
Biology
Habitat creation

Question 5

Zonation of intertidal biology

Answer 5

Subtidal
Mudflats
Low marsh
High marsh

Question 6

What properties do objects need to be to have strong gravitational attraction

Answer 6

very big and/or close together

Question 7

Which side of the Earth has a stronger gravitational attraction to the moon

Answer 7

Which ever side is closest

Question 8

How many times larger is the Earth than the moon

Answer 8

Approx 81 times larger

Question 9

What is the Barycentre

Answer 9

The Earth and moon form a single system, mutually revolving around a common centre of mass (the barycentre). A wobble in the rotation

Question 10

What is the balance between centrifugal force and gravitational force

Answer 10

Centrifugal force the same everywhere on the planet, the rotation creates an apparent forces as CF pushes in the opposite direction of the rotation
Moon gravity pull in one direction CF the other

Question 11

How does a tidal bulge form around the equator

Answer 11

On the side of the moon, the pull is stronger so gravitational forces overcome centrifugal and pull around the equator, on the other side centrifugal over comes gravity so pull outwards on the other side
Poles have lower change in vertical elevation

Question 12

3 assumption of Equilibrium theory

Answer 12

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

Question 13

If the assumptions of Equilibrium theory were true, what would happen

Answer 13

Assume largest tides at the equator, smallest at poles
Should have two peaks per day, semi-diurnal tides

Question 14

How long does it take for the same point on Earth to realign with the moon after a full rotation

Answer 14

Would assume 24 hours, but moon is also moving so takes 24 hours and 50 mins

Question 15

When are the spring tides and what is it?

Answer 15

Constructive alignment of the tidal pull of the sun and the moon, larger bulge, tidal elevation, stronger overall gravitational pull
Two spring tides a month: new and full moon

Question 16

When are the neap tides and what is it?

Answer 16

Moon is out of phase, the interactions cancel each other out, lower tidal elevation
Two neap tides a month: first and third quarter

Question 17

What would be needed for perfect equilibrium of tides

Answer 17

Moon’s orbit exactly circular (its elliptical)
Moon’s orbital plane were aligned to the Earth’s rotational plane
Earth’s rotational plane aligned to the Earth’s orbital plane about the sun
The earth’s orbit about the sun was exactly circular

The earth wobbles so the budges are exactly at the equator

Question 18

What is apogee

Answer 18

The moon is at its furthest point of it orbit around the Earth

Question 19

What is perigee

Answer 19

The moon is at its closest point to the Earth in its orbit

Question 20

What are King Tides

Answer 20

When high tide, spring tide and equinoxes occur at the same time
Extreme high tide

Question 21

If equilibrium theory was correct what would happen

Answer 21

1. high tide would occur at the same time along each latitude
2. Tidal range would be largest at the equator and smallest at the poles
   Semi-diurnal tides at the equator and diurnal tides near the poles

Question 22

Tidal ranges of:
Micro-tide
Meso-tide
Macro-tide

Answer 22

Micro-tide: <2m
Meso-tide: 2-4m
Macro-tide: >4m

Question 23

In the Northern hemisphere, which way does Coriolis force deflect tides

Answer 23

Right

Question 24

In the Southern hemisphere, which way does Coriolis force deflect tides

Answer 24

Left

Question 25

Main reasons for observed tide being different to the equilibrium

Answer 25

1. Amphidromic systems 2. Standing waves -resonance 3. Shallow water effects (oceans not actually deep enough for tides to be classed as a deep water wave, so friction occurs etc) 4. Topography

Question 26

What does dynamic theory suggets

Answer 26

Tide from east to west, Coriolis defects to the right of propagation in the N.hem, to the left of direction of propagation in S.Hem

Question 27

What are amphidromic systems

Answer 27

The combines constraint of ocean basin geometry and the influence of Coriolis Rotates around a point that has relatively minimal change in elevation

Question 28

What are co-tidal lines

Answer 28

experience high tide at the same time, but different heights Lines of constant phase

Question 29

What are co-range lines

Answer 29

experiences the same height tide, but at different times Lines of constant range

Question 30

What is the impact of friction in an amphidromic system

Answer 30

Alters the speed of the tide, slows it down Fastest at the place it enter the system, a gradual build-up of friction The amphidromic node (center) is shifted (to the west of the basin in the Northern Hemisphere) The outgoing tidal wave has less energy than the incoming wave

Question 31

What is a degenerate amphidromic point

Answer 31

In a strongly dissipative system, the amphidromic point can be shifted onto the land eg Stonehenge, Dublin

Question 32

What direction to amphidromic systems travel in the N.Hem and S.Hem

Answer 32

N.Hem: clockwise S.He,: anti-clockwise

Question 33

What is a progressive wave

Answer 33

The disturbance produced in the medium travels onwards, handed from one particle to the next. Each particle executes the same type of vibration as the preceding one, though not exactly the same c = √ gd

Question 34

What produces an observed wave

Answer 34

Reflected and incident waves combining together

Question 35

How does a standing wave form

Answer 35

Two waves of identical frequency interfere with one another whilst travelling in opposite directions along the same medium

Question 36

What is a constructive alignment

Answer 36

the two waves come together and create a higher crest and/or trough when they align

Question 37

What is destructive alignment

Answer 37

the two waves cancel each other out

Question 38

**The differences between standing and progressive waves

Answer 38

Progressive: highest tidal currents take place at high tide, mid-tide have slack water Standing tide: maximum velocity at the flood, lowest at high or low tide

Question 39

What is the natural period of oscillation

Answer 39

The time it takes for a wave to travel from one boundary and return after reflection at the second boundary

Question 40

The period of standing waves in enclosed basins vs Open coasts

Answer 40

Enclosed basins, Lakes etc: period = 2*L/ √ gd Open coasts: period = 4*L/√ gd

Question 41

What impacts does resonance have

Answer 41

Systems that are forced by oscillation close to their natural periods (resonance frequency) have large amplitude responses ie large tidal ranges E.g Bay of Funday: dimensions of the phase's exact set up for resonance because of the frequency of the times coming in. The constructive alignment of the incoming wave (the tide) and the reflected tide, increases amplitude of the highest and lowest tide

Question 42

Why are tides in continental shelves larger than open ocean

Answer 42

Shallow water wave speed, friction at wave bed, wave length reduces so height increases in compensation Celerity of the wave (function of the water depth) decreases by shallow water wave theory. Wave compresses.

Question 43

What should be considered why predicting tidal elevation

Answer 43

Shape of the basin, the reflectivity, resonances, relative position of the tide, location of amphidromic point, friction within the basin

Question 44

When does a hydraulic jump occur

Answer 44

Open channel, when the Froude number is a ratio of the flow velocity to the square root of gd, celerity of a shallow water wave, if equal to 1, hydraulic jump occurs A sudden increase in water elevation, can cause a tidal bore

Question 45

What happens to a flow velocity around an irregular coastline

Answer 45

Flow velocities increase around headlands, reduced cross-sectional area, accelerations of the tide, can form eddies This can rapidly move sediment