extra: waves Flashcards
Wave height
distance between the trough and peak of a wave
wave length
distance between two identical parts of different waves
wave period
time taken in between the part of the first wave passing a point and then that same point for the next wave
Zero up-crossing wave
a wave that crosses the average water level in an upward direction
Zero down-crossing wave
A wave that crosses the average water level in a downward direction
Where is wave energy derived from?
Wind by virtue of pressure differences either side of the wave crest
When is the process generating wave energy most powerful and when does it start to diminish and how?
Maximum translation in wave energy occurs when wind speed is 3x wave speed. Any additional wind speed is inefficient translation of energy and is wasted. Instead the wave crest now starts to dissipate and whitecap. However, the waves will nevertheless continue to grow in height just at a less efficient rate
What are the two types of wave in the process that generates waves?
Capillary waves and gravity waves
Capillary waves
like ripples on the surface of the ocean that are heavily influenced by local conditions. This is the infant stage of a wave and so at this point they are small in all their characteristics
Gravity waves
once capillary waves become big enough they can develop their own vertices (eddies). This means their restoring force now becomes gravity and they can grow a lot more from now
What are the main factors that affect the generation of waves?
Wind seed, wind direction, fetch, and duration
What is wave interference?
This is when two waves come in to contact with each other and their unique characteristics come together to influence one another in the form of a new wave
What would happen to two interfering waves of the same height, length and regime?
Because they are in sync this means that only the wave height doubles up and everything else remains constant
What would happen to two waves of the same height and length but inverse regime?
The regime is a key factor. The peaks of one set of waves would fill in at the point where the other waves have a trough and vice versa. This means that the waves effectively cancel each other out and the ocean becomes flat.
What would happen to two waves with completely different everything?
It depends on the characteristics exactly but there would be some form of equilibrium reached
What are sea wave conditions?
In the sea the waves are created and because there is very little else they are solely created based upon local conditions
What are waves like in sea conditions?
The local conditions are fairly disorganised and so the waves produced are quite chaotic.
What are the wave periods of waves generally in sea conditions?
Less than 8-10 seconds
What are waves like in swell conditions?
After a given time period of experiencing sea conditions, the waves will start to form a series of long-crested waves with now a narrower range of different characteristics between them
What happens to multiple waves in swell conditions?
They start to become a lot more similar
What are the general wave periods for waves in swell conditions?
More than 8-10 seconds
What causes waves to switch from their characteristics in sea conditions to those characteristics under swell conditions? Explain the process
Wave dispersion- waves are sorted out according to wavelength
What is a common trigger for wave dispersion?
Storm occurences
How does a storm trigger wave dispersion?
The storm forces waves out from the centre and this process, as with many things in nature, is intended to be as effective/efficient as possible and so the waves that have similar characteristics are grouped together
Compare the wave speeds of single waves versus a group of waves
A group of waves speed is essentially an average of all those singular waves. Each singular wave has its own speed and so the groups speed will , because its an average be different to all the individual single waves
What is the only characteristic of a wave that does not change as the wave approaches the shore (shallower depths)?
Wave period
What happens to wave speed as waves approach the shore?
Their speeds start to become very similar
What does understanding the wave dispersion effect enable?
It aids in the understanding of how storm/weather characteristics will translate in to the resultant waves that are experienced at the coast solely based upon understanding wind speed, direction, pressure differences
What are the 3 methods of wave measurement?
Pressure sensor
Waverider bouys
Satellite
What depth do pressure sensors and waverider buoys operate at optimallly?
Pressure sensor <20m
Waverider buoys <100m
What electromagnetic bands are most commonly employed when analysing waves using satellites?
Microwave, radar, SAR.
Explain wave occurrence in the Southern Ocean?
in the southern ocean waves are more likely to be bigger and stronger because firstly this ocean lies in the latitudes of 40 and 50 degrees where there are the roaring forties and firing fifties wind bands. Furthermore, there is effectively infinite wave fetch as there is no landmass interruption
What are the 3 statistical descriptors of wave height?
Mean wave height
RMS wave height
Significant wave height
Significant wave height
calculate mean from the highest 1/3 of waves
RMS wave height
square values of all waves, add up square values, square root the sum
What are the 2 statistical measures of wave period?
Mean wave period and significant wave period
What are the 2 statistical measures of wave lengths?
wave frequency and peak wave period
wave frequency
the number of waves passing a point in a given time period
peak wave period
the lengths between the peaks of two waves
What is the use of a directional wave spectrum?
We can see the frequency of waves coming from all directions and determine the wave period for all these waves
What are the 4 processes that waves experience as they come in to contact with shallow water?
Shoaling, refraction, diffraction, friction
What is shoaling?
Contact of the water with an increasingly shallower depth of the sea floor up a ramp causes the waves to buckle whereby the wavelength reduces but the wave height increases
What is refraction?
process of waves reforming to the shape of the sea-bed contours
What are the two types of wave refraction?
Divergence and convergence
Wave divergence (refraction)
Waves come in to contact with a part of the sea bed that forces the new waves to bend outwards and consequently the waves are now likely to hit a range of locations
Wave convergence (refraction)
Waves come in to contact with a part of the sea bed that forces the new waves to bend inwards and thus become targeted upon one point (i.e. they converge on that point)
What are the general consequences of refraction around headlands?
Convergence - higher wave heights
What are the general consequences of refraction in embayments?
Divergence - lower wave heights
What is wave diffraction?
As a wave approaches an obstacle it will wrap around that obstacle and thus be directed behind the shadowed area
What process does diffraction normally work in conjunction with?
Refraction
What does friction do to waves in shallower environments?
Rough sea beds will be varied in intensity and location. This causes turbulence and therefore dissipation of wave energy
Describe the wave breaking process at the crest of a wave
The breaking (shoaling) wave will have a higher wave speed at the crest of the wave than the rst of the wave below
Why is the wave speed at the crest of a wave higher than that at the base of a wave?
Because as the wave moves forward and encounters the shallow sea bed it the loss of momentum at the base of the wave is greater than at the crest and so this propagates the crest forwards and trips up thus breaking
How to calculate wave depth?
1/2 wavelength
What water depth does breaking of waves occur at?
depths of 1.3x the wave height
What are the 3 wave breaking types?
- spilling = crest gradually spills over itself
- plunging = barrelling effect
- collapsing = subtle breaking that is effectively more of a surge forward (horizontally) with less vertical collapse
