Chapter3 Flashcards
Apart from special waves such as
tsunamis
Apart from special waves such as tsunamis, the only thing that produces the waves we see on our coasts is the
action of the wind blowing over the sea surface
Waves arriving on a coast can be generated by
- local wind, in ‘real time’, in which case the waves are called windsea, or
- they can be the result of a wind that blew over the surface of the ocean thousands of kilometres away, up to several days before, in which case they are termed swell or groundswell.
A swell
in the context of an ocean, sea or lake, is a series of mechanical waves that propagate along the interface between water and air and so they are often referred to as surface gravity waves.
Swell waves often have a
long wavelength
Swell waves often have a long wavelength but
this varies due to the:
- size,
- strength and
- duration of the weather system responsible for the swell and the size of the water body
Swell waves often have a long wavelength but this varies due to the size, strength and duration of the weather system responsible for the swell and the size of the water body, e.g.
wavelengths are rarely more than 150m in the Mediterranean
Swell waves often have a long wavelength but this varies due to the size, strength and duration of the weather system responsible for the swell and the size of the water body, e.g. wavelengths are rarely more than 150m in the Mediterranean. Swell wavelength, also,
varies from event to event
swells which are …………………………………… occure as a result of severe storms like ……………………..
longer than 700 m
tropical cyclones
Swells have a
narrower range of frequencies and directions than locally generated wind waves, because swell waves have dispersed from their generation area, taking on a more defined shape and direction.
Swell direction is the
direction from which the swell is coming
Swell direction is the direction from which the swell is coming. It is measured in
degrees (as on a compass)
Swell direction is the direction from which the swell is coming. It is measured in degrees (as on a compass), and often referred to in
general directions, such as a NNW or SW swell.
Large breakers one observes on a beach may result from
distant weather systems over a fetch of ocean
Five factors influence the formation of wind waves, which will go on to become ocean swell:
- Wind speed or strength relative to wave speed — the wind must be moving faster than the wave crest for energy transfer, stronger prolonged winds create larger waves
- The uninterrupted distance of open water over which the wind blows without significant change in direction (called the fetch)
- Width of area affected by fetch
- Wind duration — the time over which the wind has blown over a given area
- Water depth
All of these factors work together to determine the size of wind waves:
- Wave height (from trough to crest)
- Wave length (from crest to crest)
- Wave period (time interval between arrival of consecutive crests at a stationary point)
- Wave propagation direction
A fully developed sea has the ………………………………………… theoretically possible for a wind of a
maximum wave size
specific strength, duration, and fetch
A fully developed sea has the maximum wave size theoretically possible for a wind of a specific strength, duration, and fetch. Further exposure to that specific wind could only cause
a loss of energy due to the breaking of wave tops and formation of “whitecaps”
Waves in a given area typically have
a range of heights
Sea water wave is generated by
many kinds of disturbances such as Seismic events, gravity, and crossing wind
The generation of wind wave is initiated by the
disturbances of cross wind field on the surface of the sea water
Two distinct mechanisms have been proposed to explain the means by which the wind is capable of
generating waves and perturbations on the surface of oceans
Two distinct mechanisms have been proposed to explain the means by which the wind is capable of generating waves and perturbations on the surface of oceans:
The Kelvin–Helmholtz instability (after Lord Kelvin and Hermann von Helmholtz) can occur when
there is velocity shear in a single continuous fluid, or where there is a velocity difference across the interface between two fluids.
The Kelvin–Helmholtz instability (after Lord Kelvin and Hermann von Helmholtz) can occur when there is velocity shear in a single continuous fluid, or where there is a velocity difference across the interface between two fluids. An example is
wind blowing over water: The instability manifests in waves on the water surface. More generally, clouds, the ocean, Saturn’s bands, Jupiter’s Red Spot, and the sun’s corona show this instability.
KHI requires a minimum wind speed of
6 m s-1 to make waves grow against the competing effects of gravity and surface tension.
KHI requires a minimum wind speed of 6 m s-1 to make waves grow against the competing effects of gravity and surface tension. Thus, whilst KHI is relevant to the generation of
large wavelength perturbations
it is the Miles-Phillips Mechanism which is relevant to
low wind speeds, and short-wavelength perturbations.
Thus, whilst KHI is relevant to the generation of large wavelength perturbations, it is the Miles-Phillips Mechanism which is relevant to
low wind speeds, and short-wavelength perturbations.
Thus, whilst KHI is relevant to the generation of large wavelength perturbations, it is the Miles-Phillips Mechanism which is relevant to low wind speeds, and short-wavelength perturbations. In particular, the Miles-Phillips Mechanism involves a
resonant interaction between the surface of the water and turbulent fluctuations in the air.
To produce waves
the air moving over the surface of the water has to somehow transmit its energy to the water.
To produce waves, the air moving over the surface of the water has to somehow transmit its energy to the water. Just how this happens is a
very complicated process, still not well understood.
To produce waves, the air moving over the surface of the water has to somehow transmit its energy to the water. Just how this happens is a very complicated process, still not well understood. The most accepted theory is the one proposed in the 1950s by J.W. Miles and O.M. Phillips” the
Miles-Phillips theory.
Miles-Phillips theory. The theory describes how
waves are generated from a flat sea using two mechanisms; the first of which produces tiny ripples called capillary waves, and the second of which produces bigger waves called gravity waves.
According to the Miles-Phillips theory,
- capillary waves first begin to grow from an entirely flat sea,
- and then gravity waves are subsequently formed from a sea already containing capillary waves.
Gravity waves and capillary waves are named as such because
the restoring force (the force that returns the sea to an equilibrium position after the wind has lifted it up) is gravity in the case of a gravity wave and capillary action, or surface tension, in the case of a capillary wave.
The initial generation of capillary waves is due to
perturbations in the surface wind, causing irregularities in the water surface.
The initial generation of capillary waves is due to perturbations in the surface wind, causing irregularities in the water surface. The wind does not
blow completely horizontally all the time; it will naturally contain random disturbances that give it small vertical motions as well.
The initial generation of capillary waves is due to perturbations in the surface wind, causing irregularities in the water surface. The wind does not blow completely horizontally all the time; it will naturally contain random disturbances that give it small vertical motions as well. Sometimes, these vertical motions are
enough to create tiny up and down motions on the surface of the water itself.
The wind does not blow completely horizontally all the time; it will naturally contain random disturbances that give it small vertical motions as well. Sometimes, these vertical motions are enough to create tiny up and down motions on the surface of the water itself. This is
the vital beginning which triggers off further reactions and facilitates the flow of energy between wind and water.
Once the sea contains capillary waves, there is
an increase in surface roughness, which allows the moving air to ‘grip’ the surface of the water.
Once the sea contains capillary waves, there is an increase in surface roughness, which allows the moving air to ‘grip’ the surface of the water. There is no longer any need for
small vertical perturbations in the air flow; the horizontally-moving air will now push up the existing bumps in the water surface.
Once the sea contains capillary waves, there is an increase in surface roughness, which allows the moving air to ‘grip’ the surface of the water. There is no longer any need for small vertical perturbations in the air flow; the horizontally-moving air will now push up the existing bumps in the water surface. This second mechanism is
self-perpetuating
This second mechanism is self-perpetuating; the rougher the surface the more
‘grip’
This second mechanism is self-perpetuating; the rougher the surface the more ‘grip’, the more
grip the bigger the waves
This second mechanism is self-perpetuating; the rougher the surface the more ‘grip’, the more grip the bigger the waves, the bigger the
waves the rougher the surface
While the first mechanism causes the waves to
grow at a rate which is linear with time
While the first mechanism causes the waves to grow at a rate which is linear with time, the second mechanism is
exponential with time; the bigger they are the quicker they grow.
The restoring force of these bigger waves is now
gravity, not surface tension.
The restoring force of these bigger waves is now gravity, not surface tension. Eventually a point will be reached where the
wind can’t lift up the surface of the sea anymore”
The restoring force of these bigger waves is now gravity, not surface tension. Eventually a point will be reached where the wind can’t lift up the surface of the sea anymore” the
force of gravity pulls the water back down again at the same rate as the wind lifts it up.
The restoring force of these bigger waves is now gravity, not surface tension. Eventually a point will be reached where the wind can’t lift up the surface of the sea anymore” the force of gravity pulls the water back down again at the same rate as the wind lifts it up. This
natural limit is reached for a given wind speed, so, if the wind gets stronger, the waves will get higher.
The dissipation of swell energy is
much stronger for short waves
The dissipation of swell energy is much stronger for short waves, which is why
swells from distant storms are only long waves
The dissipation of swell energy is much stronger for short waves, which is why swells from distant storms are only long waves. The dissipation of waves with periods larger than ………………… is very weak
13s
The dissipation of swell energy is much stronger for short waves, which is why swells from distant storms are only long waves. The dissipation of waves with periods larger than 13 s is very weak but still
significant at the scale of the Pacific Ocean.
The dissipation of swell energy is much stronger for short waves, which is why swells from distant storms are only long waves. The dissipation of waves with periods larger than 13 s is very weak but still significant at the scale of the Pacific Ocean. These long swells lose
half of their energy over a distance that varies from over 20000 km (half the distance round the globe) to just over 2000 km.
These long swells lose half of their energy over a distance that varies from over 20000 km (half the distance round the globe) to just over 2000 km. This variation was found to be
a systematic function of the swell steepness
These long swells lose half of their energy over a distance that varies from over 20000 km (half the distance round the globe) to just over 2000 km. This variation was found to be a systematic function of the swell steepness:
the ratio of the swell height to the wavelength
This variation was found to be a systematic function of the swell steepness: the ratio of the swell height to the wavelength. The reason for this behavior is
still unclear but it is possible that this dissipation is due to the friction at the air-sea interface.
Swells are often created by
storms thousands of nautical miles away from the beach where they break, and the propagation of the longest swells is only limited by shorelines.
Swells are often created by storms thousands of nautical miles away from the beach where they break, and the propagation of the longest swells is only limited by shorelines. For example
swells generated in the Indian Ocean have been recorded in California after more than half a round-the-world trip.
For example, swells generated in the Indian Ocean have been recorded in California after more than half a round-the-world trip. This distance
allows the waves comprising the swells to be better sorted and free of chop as they travel toward the coast.
This distance allows the waves comprising the swells to be better sorted and free of chop as they travel toward the coast. Waves generated by storm winds have ……………………… and will ………………………
the same speed and will group together and travel with each other, while others moving at even a fraction of a meter per second slower will lag behind, ultimately arriving many hours later due to the distance covered.
The time of propagation from the source t is proportional to
the distance X divided by the wave period T.
In deep water it is
t=4Pix/(gT)
t=4Pix/(gT)
where
where g is the acceleration of gravity
For a storm located 10000 km away, swells with a period T=15 s will arrive
10 days after the storm, followed by 14 s swells another 17 hours later, and so forth.
This dispersive arrivals of swells, long periods first with a reduction in
the peak wave period over time, can be used to tell the distance at which swells were generated.
What we call sea state is the
effect that the local winds have on sea conditions – this is independent of travelling swell waves generated by winds outside of the local area.
Sea state is related to ……………………… which disribe ………………………..
the Beaufort scale which describes the state of the sea.
The Beaufort wind force scale is
an empirical measure that relates wind speed to observed conditions at sea.
Note that wind wave heights are significantly affected by …………………. such as ……………………………………………..
local conditions, such as whether the wind is onshore or offshore and the fetch and its duration.
Note that wind wave heights are significantly affected by local conditions, such as whether the wind is onshore or offshore and the fetch and its duration.
The wind flow, in turn, affects the
the local sea conditions
The wind flow, in turn, affects the local sea conditions. An offshore wind blowing from the land onto the sea is usually
smooth close to shore but noticeably rougher further out because it takes a little distance for the wind to work up waves on the sea surface.
The wind flow, in turn, affects the local sea conditions. An offshore wind blowing from the land onto the sea is usually smooth close to shore but noticeably rougher further out because it takes a little distance for the wind to work up waves on the sea surface. It starts off generating
small wavelets close to the land, then short steeper waves and then bigger less steep waves further out
The wind flow, in turn, affects the local sea conditions. An offshore wind blowing from the land onto the sea is usually smooth close to shore but noticeably rougher further out because it takes a little distance for the wind to work up waves on the sea surface. It starts off generating small wavelets close to the land, then short steeper waves and then bigger less steep waves further out. On the other hand, onshore winds do not have
this fetch limitation. Depending on the wind direction, the more sheltered areas should not be as rough as exposed areas. When interpreting the sea state, the local conditions need to be taken into consideration.
As swell waves typically have long wavelengths (and thus
a deeper wave base),
As swell waves typically have long wavelengths (and thus a deeper wave base), they begin the
refraction process (see water waves) at greater distances offshore (in deeper water) than locally generated waves.
Since swell-generated waves are mixed with
normal sea waves, they can be difficult to detect with the naked eye (particularly away from the shore) if they are not significantly larger than the normal waves.
Since swell-generated waves are mixed with normal sea waves, they can be difficult to detect with the naked eye (particularly away from the shore) if they are not significantly larger than the normal waves. Swells can be thought of as a fairly
regular (though not continual) wave signal existing in the midst of strong noise (i.e., normal waves and chop).
Swells were used by
Polynesian navigators to maintain course when no other clues were available, such as on foggy nights.
