Waves: Generation and Transformation Flashcards
how does wind make a wave
-wind at the top going faster than the wind at the water surface due to friction
-wind flips and punches the water, forces the water down, water rebounds and forms a wave
trough is always the start of the wave (its where the wave is pushing the water down)
what is the definition of a wave and how are waves characterized
Wave: physical process, energy is transported through a medium without any significant transport of the material in that medium
Each kind of wave is characterized by a Restoring Force that leads to the oscillatory behavior
Gravity waves
-the restoring force is gravity
-lake and ocean waves
Infragravity Waves(>20s):
-long period waves
-restored through gravity
Capillary Waves(<1s):
-short period waves
-restored through surface tension
-little ripples on the water
what determines the size of the wave coming right towards the shoreline
there is no wind here, the waves were formed by wind offshore
-length of fetch (longer fetch means bigger waves, fetch is where waves develop so more development in longer fetch)
-wind speed (strength) gives energy to the waves ->more energy = bigger
-wind duration -> time to grow, if winds persist, waves continue to gain size and energy
-distance from the edge of the storm to the beach (swell decays over a longer distance)
what is swell? (Ground swell, Wind swell, tropical storm swell)
A swell is a series of long, rolling waves that travel across the ocean without immediate influence from the wind (beyond the fetch)
Ground swell:
- strong winds form far out (from mid latitude depressions), waves travel long distances
-strong winds, long durations, long fetch
-long wavelengths and periods
-long travel distance, has time to organize into clean, neat waves (shorter messier ones die out)
Wind swell:
-local or nearby winds blowing over a short distance
- strong winds, limited fetch and duration
-enough fetch and duration to form big waves, but not enough to organize
- messy and disorganized
- Short Wave Periods
- Steeper, Less Powerful Waves – Break quickly and close together
- Shorter Travel Distance
Tropical Storm Swell:
-large tropical depressions (tropical storms, hurricanes) produce strong, sustained winds
-large, long-period waves travel outward from the storm centre
organized swells that radiate outward
can travel long distances
-most storms travel east to west, producing more East swells than West swells
wind wave prediction vs swell wave prediction
wind wave - fetch, wind duration, wind speed
swell wave - wind speed, time traveled, distance traveled
how do waves transform beyond the storm
directional spreading
-Loss of energy to sides
- no wind at the side, waves turn away from the wind, spread to the side
dispersion
-waves of different lengths (L) and periods (T) travel at different speed (C)
L = gT^2 / 2π
C = gT / 2π
friction
- largest waves touch bottom (at about 1/2 their wavelength), energy dissipated through friction with bed
viscous dissipation
- water is sticky, resistant to movement
- lose energy to internal friction as layers of the water move past each other
-wave interference
waves of different wave lengths are moving at different speeds
->constructive when waves are in line with each other
->destructive when opposite to each other
Group Velocity
speed at which a wave group (collection of individual waves traveling together) moves across the ocean.
determines how quickly wave energy propagates, different from the speed of individual waves within the group
Group Velocity:
Deep water: ½ of the individual wave velocity
Shallow water: approaches the velocity of the individual wave
what happens during shoaling
waves grow taller and slow down as they move from deep water into shallower water.
Wave Slows Down – wave hits bottom, drags against the seabed, friction, wave slows orbits
->become more oval (forward backward motion rather than circular)
Wave Height Increases – motion compressed into minimal vertical space, waves forced upward to conserve energy
Wavelength Decreases – distance between waves shortens as height increases
Breaking Point – If water gets shallow enough, wave becomes too steep and collapses, creating a breaking wave (surf waves).
waves over shallow water compress, waves over deep water lengthen
Wavelength and celerity decrease as
the wave shoals (inverse to height)
in shoaling zone, sand moves forward
what happens in the surf zone?
breakpoint = end of shoaling, start of surf zone
once the wave breaks, height decreases
-height energy is transferred to forward kinetic energy
wave breaking causes currents to develop
-> sand moves backwards
Undertow & Infragravity waves develop in the surf zone during storms with breaking waves
diffraction
waves spreading around obstacles
obstacle causes variation in crest and trough elevations across the wave’s width
redistributes energy, causes localized wave height changes
- Energy is transferred into sheltered areas behind:
Breakwaters
Islands
Submerged Obstacles
refraction
Wave crest travels different distances
depending on the water depth
(deeper water moves faster)
waves turn towards the shallow zone because the deeper water is always going faster
wave energy is converging towards the side (shallow) and diverging in the middle
if you have conversion and diversion, you have erosion and deposition
if the shoreline is straight, its in equilibrium
waves coming in at an angle throw their water in one direction and create a longshore current
45º along the shoreline has the greatest alongshore current
what changes as waves approach shallow water?
as a wave approaches shallow water,
loses energy due to diffraction or turns due to refraction
shape:
deep water: sinusoidal (linear)
in shallow water: asymmetric vertically
-crest grows but not trough
- trough stays the same size as it was offshore
describe the different shapes of waves
airy wave
-linear - perfectly sinusoidal
- H/L ->0
-Deep water wave: depth greater than 1/2 wavelength
-no net sediment transport (onshore velocity the same as offshore)
stokes wave
- non linear (larger crest than trough - shoaling)
- H/L -> large
- deep water but getting shallower
-sediment transport (orbital velocities greater under crest, +ve orbital skewness, more onshore transport)
shallow -> cnoidal wave ->extremely asymmetric
H/L very large
if a wave never breaks -> solitary wave (tsunami)
T and L -> ∞
describe the transformation from airy to stokes to cnoidal
> starts as an airy wave
touches the bottom at 1/2 wavelength
starts shoaling
under shoaling, speed changes
next, wavelength decreases
next, height goes up to conserve energy
energy goes into the crest, becomes a stokes wave
shallower and shallower becomes a cnoidal wave
wave breaks (height of the waves divided by the depth is where the wave should break)
throughout the shoaling zone, move forward
why do waves break
as waves enter shallow water and begin to shoal, their speed decreases and height increases
as wave height increases, orbital speed increases
as a wave approaches a shoreline, orbital speed is increasing
wave is slowing down as it approaches the shoreline
orbital speed exceeds wave speed and breaks
breaking is initiating when wave height is 80% of the water depth (H/h = 0.8)
breaker types and the 3 variables
spilling breaker
- just the top of the wave is breaking
- Remains oscillatory, Little mass transport
-transition to breaking is slow and continuous, breaks over a wide distance
-dissipative and intermediate
plunging (wave gets a little bigger)
-slope of nearshore is steep so transition to breaking is quick
-breaks in a single spot
-curling surfing waves
-intermediate to reflective
collapsing
-very steep, wave breaks right at shoreline
-reflective environment
-transition between plunging and surging
surging (eg tsunami)
-extremely steep, doesnt break
-low energy
-small, very long wavelength, low frequency
-Short distance of energy dissipation
Breaker type varies with:
§ Wave height
§ Wave period (wavelength)
§ Beach slope
for a given 2 out of 3: beach slope, wave period, wave height
as height decreases or slope or period increases, you get
surging->plunging->collapsing->surging
how does tide impact waves
The tide changes the positions of shoaling, breaking and swash
high tide will break closer to shoreline
-will be more collapsing and reflective
-Bar migrates onshore during high tide
low tide breaks further offshore
-will be more, spilling, dissipative
-Bar erodes offshore during low tide
set up and set down and its relation to undertoe
set down: drop in mean water surface as wave height increases during shoaling
-maximum at break point (also where stokes drift reaches max)
set up: breaking causes an increase in elevation of mean water level landward of breaking
breakpoint is the transition from set down to set up:
seaward of the breaker you have set down, landward of the breaker you have set up
-> the larger the wave height during breaking, the greater the amount of excess water thrown landward of breaking
slope of beach causes water to travel back as a current (bed return flow or undertoe)
undertoe at a maximum at break point where both stokes drift and breaking are also at a max
undertoe speed not very strong (spread out along shore)
->if in channels, can significantly increase the speed (rip currents!)
how does wave interference form infragravity waves
Two waves with different periods move at different speeds to create a wave group (big waves followed by small waves)
superimposed long wave
-> crest where waves are small and trough where waves are large
->larger waves equivalent to set down, creating a trough of long wave, smaller waves equivalent to set up, creating a crest of the long wave
long wave =Infragravity waves
-period >20s
-frequency lower than the typical surface gravity waves
released when the gravity waves break
-it doesnt break and continues to travel towards shore
-it reflects and travels back offshore
if incident long wave and reflected long wave are in phase, they create a standing wave
->creates a variation in water level at low wave frequencies
3 possibilities for a long wave released at the break point:
leaky wave vs edge wave vs forced wave
LEAKY WAVE
-long wave perpendicular to the beach will reflect and its energy will be lost offshore
-can escape from the nearshore
EDGE WAVE
-long wave approaches coast at an angle, reflected wave refracts and turns back to the beach
-if it gets trapped in surf zone, it is an edge wave
- trapped waves are in phase->wave resonates
-Ability of reflected long wave to escape surf zone depends on relationship between wavelength and beach slope
if surf zone is wider than wave is long, gets trapped through refraction
if long enough, steps out of bounds before refraction, escapes
if it has a high enough frequency, it may move faster than refraction occurs and leak out
some trapped waves interact with themselves in phase and resonate, if out of phase, wave dissipates
FORCED WAVE
-trapped to the shoreline but decay from turbulent dissipation
-trapped waves are out of phase->wave dissipates
energy at shoreline dominated by infragravity waves
standing waves
edge wave creates standing wave pattern in nearshore
standing wave oscillation creates a drift current that deposits sediment towards nodes and antinodes
standing wave composed of antinodes (oscillate up and down) and nodes (no movement)
small sediment is suspended and lifted at antinodes and deposited at nodes
larger sediment (moves as bedload) is deposited at antinodes
beach cusps (common on reflective beaches)
- associated with edge waves
- evidence that edge waves dont create the morphology but that the morphology may determine if the long wave is turned into an edge wave
->reinforce bar morphology but not necessarily the cause
what is the morphological importance of edge waves
edge waves result in drift patterns
form landforms:
Cuspate Beaches
(between dissipative and intermediate)
Crescentic Bars
Longshore Bars
Rip Currents
(intermediate)
