Section 2: Tides, equilibrium and dynamic theory Flashcards
What are tides
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
What are tidal currents
The horizontal movement of water due to tides
What is sea level
The vertical change in the height of the sea surface which occurs over many different spatial and temporal scales.
Why are tides important
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
Zonation of intertidal biology
Subtidal
Mudflats
Low marsh
High marsh
What properties do objects need to be to have strong gravitational attraction
very big and/or close together
Which side of the Earth has a stronger gravitational attraction to the moon
Which ever side is closest
How many times larger is the Earth than the moon
Approx 81 times larger
What is the Barycentre
The Earth and moon form a single system, mutually revolving around a common centre of mass (the barycentre). A wobble in the rotation
What is the balance between centrifugal force and gravitational force
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
How does a tidal bulge form around the equator
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
3 assumption of Equilibrium theory
- The Earth has no continental land masses but is covered by an ocean of uniform depth
- There is no inertia in the system and the oceans respond immediately to tide-generation force
- The Coriolis and friction effects can be neglected
If the assumptions of Equilibrium theory were true, what would happen
Assume largest tides at the equator, smallest at poles
Should have two peaks per day, semi-diurnal tides
How long does it take for the same point on Earth to realign with the moon after a full rotation
Would assume 24 hours, but moon is also moving so takes 24 hours and 50 mins
When are the spring tides and what is it?
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
When are the neap tides and what is it?
Moon is out of phase, the interactions cancel each other out, lower tidal elevation
Two neap tides a month: first and third quarter
What would be needed for perfect equilibrium of tides
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
What is apogee
The moon is at its furthest point of it orbit around the Earth
What is perigee
The moon is at its closest point to the Earth in its orbit
What are King Tides
When high tide, spring tide and equinoxes occur at the same time
Extreme high tide
If equilibrium theory was correct what would happen
- high tide would occur at the same time along each latitude
- 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
Tidal ranges of:
Micro-tide
Meso-tide
Macro-tide
Micro-tide: <2m
Meso-tide: 2-4m
Macro-tide: >4m
In the Northern hemisphere, which way does Coriolis force deflect tides
Right
In the Southern hemisphere, which way does Coriolis force deflect tides
Left
Main reasons for observed tide being different to the equilibrium
- Amphidromic systems
- Standing waves -resonance
- Shallow water effects (oceans not actually deep enough for tides to be classed as a deep water wave, so friction occurs etc)
- Topography
What does dynamic theory suggets
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
What are amphidromic systems
The combines constraint of ocean basin geometry and the influence of Coriolis
Rotates around a point that has relatively minimal change in elevation
What are co-tidal lines
experience high tide at the same time, but different heights
Lines of constant phase
What are co-range lines
experiences the same height tide, but at different times
Lines of constant range
What is the impact of friction in an amphidromic system
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
What is a degenerate amphidromic point
In a strongly dissipative system, the amphidromic point can be shifted onto the land eg Stonehenge, Dublin
What direction to amphidromic systems travel in the N.Hem and S.Hem
N.Hem: clockwise
S.He,: anti-clockwise
What is a progressive wave
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
What produces an observed wave
Reflected and incident waves combining together
How does a standing wave form
Two waves of identical frequency interfere with one another whilst travelling in opposite directions along the same medium
What is a constructive alignment
the two waves come together and create a higher crest and/or trough when they align
What is destructive alignment
the two waves cancel each other out
**The differences between standing and progressive waves
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
What is the natural period of oscillation
The time it takes for a wave to travel from one boundary and return after reflection at the second boundary
The period of standing waves in enclosed basins vs Open coasts
Enclosed basins, Lakes etc: period = 2*L/ √ gd
Open coasts: period = 4*L/√ gd
What impacts does resonance have
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
Why are tides in continental shelves larger than open ocean
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.
What should be considered why predicting tidal elevation
Shape of the basin, the reflectivity, resonances, relative position of the tide, location of amphidromic point, friction within the basin
When does a hydraulic jump occur
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
What happens to a flow velocity around an irregular coastline
Flow velocities increase around headlands, reduced cross-sectional area, accelerations of the tide, can form eddies
This can rapidly move sediment
