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2024 Coastal ocean processes > Section 2: Tides, equilibrium and dynamic theory > Flashcards

Section 2: Tides, equilibrium and dynamic theory Flashcards

1
Q

What are tides

A

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

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2
Q

What are tidal currents

A

The horizontal movement of water due to tides

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3
Q

What is sea level

A

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

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4
Q

Why are tides important

A

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

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5
Q

Zonation of intertidal biology

A

Subtidal
Mudflats
Low marsh
High marsh

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6
Q

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

A

very big and/or close together

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7
Q

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

A

Which ever side is closest

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8
Q

How many times larger is the Earth than the moon

A

Approx 81 times larger

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9
Q

What is the Barycentre

A

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

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10
Q

What is the balance between centrifugal force and gravitational force

A

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

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11
Q

How does a tidal bulge form around the equator

A

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

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12
Q

3 assumption of Equilibrium theory

A
  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
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13
Q

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

A

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

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14
Q

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

A

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

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15
Q

When are the spring tides and what is it?

A

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

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16
Q

When are the neap tides and what is it?

A

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

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17
Q

What would be needed for perfect equilibrium of tides

A

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

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18
Q

What is apogee

A

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

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19
Q

What is perigee

A

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

20
Q

What are King Tides

A

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

21
Q

If equilibrium theory was correct what would happen

A
  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
22
Q

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

A

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

23
Q

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

A

Right

24
Q

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

A

Left

25
Q

Main reasons for observed tide being different to the equilibrium

A
  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
26
Q

What does dynamic theory suggets

A

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

27
Q

What are amphidromic systems

A

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

28
Q

What are co-tidal lines

A

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

29
Q

What are co-range lines

A

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

30
Q

What is the impact of friction in an amphidromic system

A

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

31
Q

What is a degenerate amphidromic point

A

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

32
Q

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

A

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

33
Q

What is a progressive wave

A

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

34
Q

What produces an observed wave

A

Reflected and incident waves combining together

35
Q

How does a standing wave form

A

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

36
Q

What is a constructive alignment

A

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

37
Q

What is destructive alignment

A

the two waves cancel each other out

38
Q

**The differences between standing and progressive waves

A

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

39
Q

What is the natural period of oscillation

A

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

40
Q

The period of standing waves in enclosed basins vs Open coasts

A

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

Open coasts: period = 4*L/√ gd

41
Q

What impacts does resonance have

A

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

42
Q

Why are tides in continental shelves larger than open ocean

A

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.

43
Q

What should be considered why predicting tidal elevation

A

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

44
Q

When does a hydraulic jump occur

A

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

45
Q

What happens to a flow velocity around an irregular coastline

A

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

2024 Coastal ocean processes (9 decks)

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