WA L1: How do Waves Work Flashcards
How many oceans in the world?
Only one world ocean
Name the ocean basins. (Past and modern)
Arctic, Atlantic, Indian, Pacific, Southern (modern day)
Describe surface currents.
Currents caused by very large scale, permanent winds that blow over the ocean. They transport both matter (water) and energy.
Describe waves. Technical def’n?
Technical def’n: the mechanical expression of energy.
Caused by winds that blow occasionally or over smaller areas. Waves transport energy in the direction of the wave.
What is a crest?
highest point of a wave
What is a trough?
lowest point of a wave
Define wavelength L (or lambda)
distance between two successive crests or two successive troughs of a wave. Unit: meters
Define wave height H
vertical distance between crest and trough (meters)
Define amplitude (a)
vertical distance between crest calm sea level or between trough and calm sea level or half the vertical distance between a crest and a trough; H ÷ 2; (metres)
Define period T
time for one wavelength to pass a point; (minutes or seconds)
Define frequency f
number of waves passing a point per unit of time; (per minute or second)
Define celerity or speed
distance traveled per unit time; (metre per second)
Define steepness
a measure of the “peakedness” of a wave; the ratio of Height to Wavelength, H / L; (no units)
What is the energy transported by waves related to?
related to wave height H; but this only applies to deep-water waves.
Waves with great heights transport more _____ than waves with shorter heights.
energy
Usually, waves with great heights also have long _____.
wavelengths
At any one instance, some wind waves may carry as much or even more energy than waves generated by earthquakes, storms or even the gravitational attraction between the Sun and Earth and the Moon and Earth (tides). So why is the total amount of energy from the disasters much larger compared to wind-driven waves?
wind waves are at one location for very short periods of time (seconds), while larger waves last for longer (minutes, hours, days).
What is generating force?
the force that disturbs the water or causes waves to form
What are the 4 wave types? (GF)
Wind wave, seiche, tsunami, tide
What is the generating force for a wind wave?
wind over ocean
What is the generating force for a seiche?
changes in atmospheric pressure; storm surge; tsunami
What is the generating force for a tsunami?
Faulting of sea floor;
Undersea volcanic eruption;
Landslide into ocean
What is the generating force for a tide?
Gravitational attraction between Earth-Sun and Earth-Moon;
Rotation of Earth
What is the typical wavelength for a wind wave?
60-150m
What is the typical wavelength for a seiche?
large; variable; a fraction of basin size
What is the typical wavelength for a tsunami?
200 km
What is the typical wavelength for a tide?
1/2 circumference of Earth, 20 000 km
Most common wave type in the ocean? (GF)
wind-generated waves
Which waves are generated by forces that occur episodically?
Seiches and tsunamis
Which type of waves rarely cause natural disasters?
tides
What is restoring force for waves? How many types?
The force that causes water to return to its undisturbed state. Two types for RF.
What are the two RF wave types?
capillary wave, surface gravity waves (comprised of wind wave, seiche, tsunami, tide)
What causes capillary wave?
surface tension
What causes surface gravity waves?
gravity
Typical wavelength for capillary wave?
< 1.7 cm
Typical wavelength for surface gravity wave?
> 1.7 cm
Describe capillary waves
tiny waves with wavelengths about fingernail size. They are important in the generation of wind-driven waves.
How can water remain relatively calm with only fine ripples when only a weak, gentle wind is blowing?
because water has a very high surface tension
Describe how wind-driven waves are generated.
When wind blows over a calm sea, capillary waves are generated first. As the wind continues and strengthens, more energy is transferred to the ocean surface and the waves grow into those that can only be restored by gravity.
What is an assessment of the ocean’s conditions or the roughness of the ocean surface called?
Sea state
When wind blows across the ocean surface, energy is transferred from the _____ to the ocean. The wind pushes the surface “skin” of the ocean and a series of small wrinkles, the _____ waves, develops. As the wind speed __increases/decreases___, or as wind blows over _longer/shorter____ periods of time and over the ____ area, more energy is transferred to the surface ocean and the waves grow ____.
atmosphere; capillary; increases; longer; same; larger
How can the amount of energy transferred to the ocean be estimated?
it can be estimated by the height of the waves produced
What does it mean when the ocean has very tall waves?
It means that lots of energy have been transferred to it
How are very large waves formed?
When the wind speed is very high. In addition, the wind must blow for a long period of time over an uninterrupted distance in the same distance in order to generate these large waves.
What is a “fetch”?
The uninterrupted distance or area where the wind blows.
The sea states depends on what three factors?
Wind strength/speed, fetch, wind duration (length of time that the wind has been blowing over the fetch)
Describe how waves are generated by the wind using the words: ripple, chop, swell…
As wind blows over the fetch, waves are transformed from ripples to chop, then swells. A fully developed sea includes waves of varying heights, lengths, speed, and direction. As waves move away from the fetch (swell), they are well-sorted with long waves leading the wave train.
What is chop?
short waves. capillary->ripples->chop.
When is a fetch in the ocean considered fully developed? Describe this state.
once it has absorbed the max. amount of energy it can contain. In this state, any additional energy input by the wind is balanced by energy dissipated by breaking waves. Waves in this condition are chaotic, where there are waves of varying heights, lengths, speed, direction. The waves of a fully developed sea can achieve the maximum height possible for a given wind speed, fetch and duration of wind.
In which condition are waves chaotic? (where there are waves of varying heights, lengths, speed, and direction)
the fully developed state of a specific fetch.
As the waves move away from the fetch, what are they sorted by? What are these well-sorted waves called?
Their speed and wavelength. Fastest speed or longest wavelength waves leave fetch first. Slowest and shortest leave last. These sorted waves are called swells
What are swells? What is different with them compared to the waves within the fetch?
Well sorted waves (by speed and consequently wavelength) leaving the fetch. They are well sorted, as opposed to the chaotic waves found within the fetch.
What are called groups of swells that propagate away from the fetch?
Wave trains.
The ____ the wind and the ____ the duration the wind is blowing over the largest of fetch, the greater the maximum wave heights achievable in that patch of ocean.
stronger; longer
Why is information on the sea state or the parameters that characterize it important in the real world?
They are important for ocean transportation sector, forecasting storms and their effects on near shores, for sailors, and for surfers around the world.
What is the most commonly tracked wave parameter? Why?
Wave height; it is a good indicator of wave energy and can easily be measured by satellite sensors.
What is significant wave height?
A way to measure wave height. It is the avg height of the highest one-third of the waves (waves measured from trough to crest). Because the significant wave height is an average number, we expect to encounter a group of waves whose maximum wave height can be up to twice the significant wave height
How to estimate significant wave height in simple terms?
a) List all the wave heights observed in descending order (tallest wave to shortest wave);
b) Select n of the tallest waves, where n is equal to one-third of the number of waves observed;
c) Calculate the average wave height of your selected waves;
d) This is the significant wave height.
How do governmental space agencies produce maps of significant wave height?
they produce them from satellite altimetry information
Monthly analyses for the globe show that the average wave height in the tropical Pacific is typically less than _ meters, regardless of season, whereas wave height at high latitudes (nearest the poles) in the winter hemisphere typically reaches _ to _ meters due to large storms. (Winter hemisphere refers to the hemisphere, north or south, that is experiencing winter. So, this would be the Northern Hemisphere from December to February and the Southern Hemisphere from June to August.)
3; 3 to 6
What is a qualitative measure of sea state?
the Beaufort (Wind Force) Scale
Describe the BeaufortScale
Empirical measure of wind force/intensity based on human observation of the sea state. Ranges from Beaufort number 0 (calm) to 12 (hurricane force).
Who invented the Beaufort Scale and why?
Admiral Francis Beaufort. He invented it during a time when devices to measure wind speed had not yet been developed. The thirteen conditions of how a British Naval Frigate would be affected in the open ocean.
Speed of hurricane-force winds (BF scale no.12?)
> 64 knots
What would number 6 or 7 on Beaufort Scale result in?
Issuance of a small craft advisory
What would a force 8 or 9 on the BF scale relate to?
a gale warning
What would a force 10 or 11 on the BF scale relate to?
storm warning
What would above 11 BF scale numbers relate to?
hurricane/cyclone warning
How do water particles move in a wave?
Forward and downward in the direction of the wave propagation as the wave crest passes, then backward and upward as the trough passes. Net movement of water particles is zero.
What kind of motion do water particles in a wave undergo?
circular/orbital motion
As one goes deeper into the water column, what happens to the orbits of water particle path?
The orbits attenuate (decrease)
What is the wave base? How to calculate it?
The max. depth at which a wave’s passing causes water motion. wave base = L / 2 (L is wavelength)
The behaviour of a wave is mainly dependent on how the _____ compares with the ____ of the ocean it is traveling through
wavelength; depth
Describe deep-water waves.
Waves that travel in ocean depths d equal or deeper than L/2. These waves do not feel the bottom, because their wave bases are shallower than the depth of the ocean they are passing through.
Describe shallow-water waves.
Waves that are traveling n shallow water relative to their wavelengths. Where the water depth d is equal or shallower than L / 20. These waves are said to feel the bottom as the water particles near the wave base are dragging on the bottom as they move.
Particle orbital motion of deep-water wave?
circular
Particle orbital motion of transitional/ intermediate water wave?
circular to elliptical
Particle orbital motion of shallow-water wave?
elliptical to flat
Depth of water of transition/ intermediate water wave?
Depth is greater than depth L/20 and shallower than L/2
Describe transitional/ intermediate waves.
waves that are in transition between deep-water wave and a shallow-water wave. It is only beginning to feel the bottom, but doesn’t fully behave like a shallow-water wave.
What affects the speed of a wave?
The environment
What is another word for deep-water waves?
dispersive waves. the speed of travel is related to their wavelength (distance from crest to crest). Waves with longer wavelengths travel faster than short waves.
Equation to calculate speed of deep-water waves using wavelength?
S or V = sqrt(g*L/(2pi))
S or V is the speed of wave, L is the wavelength, g is the gravitational constant 9.8 m/s^2
Equation to calculate deep-water waves from wave period?
S or V = 1.56 * T
T is in seconds
What is another word for shallow-water wave?
Non-dispersive wave. Its wave speed is controlled only by the water depth
In deep water, do longer waves travel faster or slower than shorter waves?
longer waves travel faster than shorter waves
In shallow water, do waves travel faster or slower over shallower depth?
all shallow-water waves travel at the same speed when they are at the same depth, but deeper is faster.
What is the only thing that affects shallow-water wave speed?
water depth
Equation to calculate shallow-water wave speed?
S or V = 3.1sqrt(d)
d is depth of water
Describe the process of shoaling
characteristic of waves changing as they move from the deep ocean toward the coastline and into shallower water.
What does the depth at which waves become shallow-water waves depend on?
Depends on its wavelength. Long waves become shallow waves farther off-shore (deeper water) than shorter waves.
What is the shoaling process?
Describes how waves change characteristics as they move from deep ocean toward coastline and into shallower water.
How do waves change as they shoal?
- Waves slow down. (All except for capillary)
- Wavelengths shorten. The leading wave, being in shallowest water, now moves slower than the wave behind it. So the waves in the wave train tend to bunch up and wavelength shortens. Crests will soon seem to overtake the troughs.
- Orbitals flatten. Recall that the shallow-water waves’ motion of water particles extend to the bottom of sea floor. The water particles trace flattened orbits (elliptical). At extreme, water particles at the bottom only move forward and back.
- Energy is conserved. Total E is the same. As waves slow, their wave energy is converted to some other energy form when the wave particles drag against bottom of seafloor. Creates friction, converting wave energy to kinetic, heat, and potential energy.
- Wave period remains the same. (measure of amount of time it takes for two successive crests or troughs to pass a point. Although wavelength of shoaling wave becomes shorter, it slows down at the same time. So wave period doesn’t change.
- Wave height increases.. As wave shoals, the water that is moving through is packed into shallower depths. Because energy is conserved, the height of the wave must increase to compensate for the loss in water depth.
- Waves steepen. The bunching up of waves, decrease in wavelength, and increase in wave height H increases steepness (= H/L) of the leading waves. As waves steepen, crests become more peaked (less rounded and not smooth)
What are the only waves that do not slow down during shoaling?
capillary waves
When do waves become too high for their own wavelengths and unstable and on brink of collapse?)
When the steepness (H/L) >= 1/7
What happens when wave’s height becomes too tall for depth of water to support and the wave over-steepens and breaks?
The wave ceases to exist. It has turned into surf.
What is the math def’n for the point where wave becomes surf?
H/d
What is the math def’n for the point where wave becomes surf?
H/d ≈ 3/4
Define surf
turbulent mass of agitated water rushing onshore during and after waves break
Water particle motion of waves just before they break and become surf?
the water particles change from their circular orbital paths to a more horizontally-oriented translational path that hurls the water toward the beach
Describe what happens after the surf stage?
Wave energy transferred to water (allowing it to move), and to the ocean bottom and shore. With production of surf, wave energy converted to kinetic. Energy from waves also transformed to potential energy contained within the shore, ocean bottom and surf.
What point can the water become destructive?
When energy from waves is transformed to potential energy contained within the shore, ocean bottom and surf. Force of breaking ocean waves can destroy just about any structure.
