Waves: Generation and Transformation

Question 1

Why are the waves so big and perfect in Tahiti

Answer 1

Large because: Pacific ocean is so wide
-Fetch: the distance over which the wind blows
-causes big waves

Perfect because: Volcanic island, surrounded by coral (coral ring around Tahiti)
hits the coral and kicks up

Question 2

Why go surfing before a hurricane

Answer 2

faster the windspeed, the bigger the wave

right before a hurricane, the biggest waves come in

Question 3

gravity waves

Answer 3

in deep water, there is no significant transport of mass

in deep water, you just do a perfect circle. if youre in moving, youre in shallow water

for lake and ocean waves, the restoring force is gravity

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)

Question 4

capillary wave

Answer 4

calm water
less than 1 second period
little ripples on the water
restored by surface tension

Question 5

infragravity waves

Answer 5

have periods of more than 20s
restored through gravity

Question 6

wave anatomy

Answer 6

trough - low point
crest - high point
wave length - crest to trough
period - time it takes from crest to crest
inside the wave is orbital motion

on crest of wave, water is moving forward
on trough, water is moving backward

wind moves crest forward, trough is restored in opposite direction

Question 7

wave height and period

Answer 7

crest goes first, trough goes second
(this is wrong, but thats how we look at waves)
should actually be looked at the other way around

Question 8

3 factors that affect wind wave growth

Answer 8

wave growth:
happens in the area of fetch
by the end of the fetch, its a fully developed fetch
outside of the fetch, you get big waves

strength of the wind, wind duration, fetch length

Question 9

where are waves the biggest

Answer 9

antarctic sea
-no land mass
-wind just circling around

next largest are in the north pacific and north atlantic

Question 10

what determines the size of the wave coming right towards the shoreline

Answer 10

-no wind here

-wind speed during a storm
-wind duration
-distance from the edge of the storm to the beach

Question 11

wind wave prediction

Answer 11

wind wave - fetch, duration, speed

swell wave - speed, time, distance

Question 12

why are the waves in lake erie smaller than in lake michigan

Question 13

wave transformation

Answer 13

waves transform beyond the storm

directional spreading
- no wind at the side, spread to the side, turn away from the wind

dispersion
-waves of different lengths and periods travel at different speed

friction
- loses energy

viscous dissipation
- water is sticky, resistant to movement
- waves push against viscousity and lose energy

-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

Question 13

shoaling

Answer 13

as speed slows down, wavelength decreases

period never changes
orbits become much more oval (forward backward motion)
-compressed the motion into very minimal vertical space

-wave height grows
-> to conserve energy

waves over shallow water compress, waves over deep water lengthen (like an accordian)
height is inverse to length and celerity

Question 14

during shoaling, what happens to conserve energy

Answer 14

as waves get slower and closer together, wave height must increase to conserve energy

E is proportional to H^2

Question 15

cross shore transport

Answer 15

once the wave breaks, height decreases
once you get to break point, height decreases
-height energy is transferred to forward kinetic energy

in shoaling zone, sand moves forward
after the wave breaks its surf zone, current starts to develop and sand moves backwards

Question 16

diffraction

Answer 16

waves pass an obstacle such as break wall

Variation in crest and trough
elevations perpendicular to the
direction of propagation results
in energy transfers along the
crest/trough and thus change in
wave height
* Energy is transferred into
sheltered areas behind:

Question 17

refraction

Answer 17

waves turn towards the shallow zone because the deeper water is always going faster

wave energy is converging towards the side 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

Answer 18

as a wave approaches shallow water,
loses energy due to diffraction or turns due to refraction

Question 19

shape of a wave

Answer 19

waves in deep water are sinusoidal (linear)

in shallow water, they are very non linear
-gets more asymmetric vertically
-crest grows

when a wave gets taller in the nearshore, only the crest grows. the trough never gets deeper. size of the trough offshore is identical to the size of the trough in shallow water

Question 20

describe the different shapes of waves

Answer 20

deep water - airy wave, linear - perfectly sinusoidal

gets shallower -> stokes wave

even shallower_> cnoidal wave ->extremely asymmetric

if a wave never breaks -> solitary wave ->(tsunami)

Question 21

stokes waves and stokes drift

Answer 21

change from deep water to shallow

under the crest, water moves forward
under the trough, water moves backwards
in deep water -> perfect rotation

in shallow water -> more forwards because crest is higher and less back
(2 steps forward, one step back)
-now you have a current (a stokes drift) that moves water towards the beach
- drift also is the first ingredient to creating an undertoe)

Answer 22

in a shoaling zone, transport is towards the beach

2 steps towards the beach (in the crest), 1 step back (in the trough)

Answer 23

starts as an airy wave
touches the bottom at half the wavelength
starts shoaling
under shoaling, first thing to change ->speed
second thing > wavelength decreases
-to conserve energy height goes up
energy goes into the crest, becomes a stokes wave
shallower and shallower becomes a cnoidal wave
wave breaks
throughout the shoaling zone, move foreward

Question 24

where is on shore sediment transport the greatest

Answer 24

acceleration - wave comes in fast and hits the sand hard

• gets taller and taller, gets more skewed

becomes more asymmetric, crest is taller & has more acceleration

where is the forward sand motion maximum: at the point it breaks
tallest a wave ever gets is when it breaks

sand moves faster and faster towards the beach, wave breaks, whole thing shuts down and gets slower

on shore sediment transport is at its greatest at the break point

Question 25

breaking

Answer 25

breaking generates current

infragravity waves

height of the waves divided by the depth is where the wave should break

Question 26

breaker types

Answer 26

spilling breaker
- just the top of the wave is breaking
-over a wide distance

plunging (wave gets a little bigger)
-whole wave flips upside down
-best surfing here
-doesnt realize theres a bottom under closer to the shoreline, gets big, then falls

collapsing
-doesnt realize there is a beach until right at the shoreline and crashes

surging
-doesnt realize theres a beach at all
-low energy
-tiny, super long wavelength
-tsunami is a surging wave

the steeper the slope, the more you get plunging and surging, the less steep, the more spilling

for a given 2 out of 3: beach slope, wave period, wave height
as the 3rd increases you get
surging->plunging->collapsing->surging