Lab 5A-1
Name:
Stamps
Section:
Write your name in this box.
Due Date:
Waves Lab
Get a big, long wave tank from the cart.
(Do not take one of the smaller tanks yet.)
There should be a triangular sponge and
a small yellow plastic container in the tank.
Collect a cork, a ruler, and a paddle (plastic
rectangle) from the side of the room.
Using the plastic containers at the side of the room,
fill your wave tank until the water level in the tank
is about 3-inches deep.
Names of Group Members:
1.
2.
3.
4.
5.
Activity #1: What is a wave?
Stick the paddle into the groove at one end of the tank (you may need to scoop some sand out),
and paddle gently and steadily (do not change the speed of your paddling).
1. Observe the motion of the cork as waves go by. Describe the motion of the cork.
What is a wave? A wave is a “disturbance” in a
“medium” (e.g., air, water, the Earth). The energy
of the wave is passed from one molecule to the next
via bumping, pulling on one another, etc.
Energy
is here.
2. Consider the motion of the cork. What does it suggest about how molecules of water move as
a wave goes by? Describe the probable motion of the water molecules. In particular, do the
water molecules travel from place to place or do they more-or-less remain in place?
Take the cork out of the water, and put it on the table to dry.
Wave Reflection
Stick the paddle into one end of the tank but not all the way to the bottom; do not put it into the
groove. Gently “flick” the lower end of the paddle to create a big wave crest and then take the
paddle out of the water. Try not to create a tsunami.
3. What happens when a wave hits one end of the tank? What does the wave do?
Lab 5A-2
Wave Interference
Stick two paddles into opposite ends of the tank but not all the way to the bottom; do not put
them into the grooves. Gently “flick” the lower ends of the paddles at the same time to create
wave crests going towards one another and then take the paddles out of the water. Try not to
create tsunami.
4. Describe what happens to the level of the water when 2 wave crests meet.
2 crests
meet
5. Based on the picture on the right,
what happens to the level of the
water when two troughs meet?
2 troughs
meet
What happens to the level of the
water when a crest meets a trough?
a crest meets a trough
6. What happens after 2 wave crests meet?
Do the waves reflect (“bounce back”) or do they just go through one another?
To answer this question, get one of the small rectangular tanks and fill it with just enough
water to cover the bottom of the tank. Then, lift up one corner of the tank and gently set
the corner down. Sketch what you seen into the bird’s-eye-view picture below.
Tank
The sea surface often looks very complex, but it has an underlying simplicity: waves from
different places come together and momentarily interfere, adding (or canceling) and thus
causing the sea surface to jump up or down for a moment. But then the waves move on and
interfere with the next wave in their path, and then the next, and so on.
Lab 5A-3
Waves & Water Motion
Watch the video on Wave Motion.
7. How is a wave different from an ocean current? In other words, describe the motion of the
water below a wave, and how it differs from the motion of water in a current (like a river).
8. How does the water beneath a wave move as the wave goes by?
(Discuss the shape and how the shape changes with depth.)
9. Examine the picture below.
What happens to wave orbitals if the orbitals run into the bottom of the ocean?
Describe the motion of the water below a wave when the waves’ orbitals touch bottom.
Ocean Bottom
Stick the paddle into the groove at one end of the tank,
and paddle slowly and steadily (do not change the speed of your paddling).
10. Look at the sand on the bottom of the tank in the middle of the tank.
Describe the motion of the sand as waves pass over it.
Does the sand actually move anywhere?
Lab 5A-4
Activity #2: Describing Waves (Wave Characteristics)
Spatial Dimensions of Waves: Crest & Trough, Height & Length, Steepness
The top part or high point of a wave
is called the crest of the wave.
The bottom part or low point of
a wave is called the trough of
the wave.
S te
Height
ep n
Crest
ess
Trough
Wavelength
The height of a wave is the vertical distance between the trough and the crest.
The wavelength of a wave is the horizontal distance between two crests.
The steepness of a wave is the height of the wave
divided by the wavelength of the wave.
Wave
Steepness
Height
Wavelength
11. Label the crests and troughs of the wave in the picture below.
12. What is the height of the wave in the picture below?
13. What is the wavelength of the wave in the picture below?
14. What is the steepness of the wave in the picture below?
Water Level (feet)
4
3
2
1
0
10
20
30
40
Horizontal Distance (feet)
50
60
70
Lab 5A-5
15. Examine the picture below.
How is the height of a wave related to orbitals of the wave?
The height of a wave is the __________________________________
of the orbital at the surface of the ocean.
16. Examine the picture below.
How is the wavelength of a wave related to the orbitals of the wave?
The wavelength of a wave determines how far down the water beneath the wave moves.
The longer the wavelength of the wave, the _______________________
the orbitals go down beneath the surface.
Place the sponge into one end of the tank. Stick the paddle into the groove into the other end of
the tank, and paddle quickly and steadily (do not change the speed of your paddling).
While one student is paddling, dip a piece of construction paper quickly into the water
and capture a “snapshot” of the water’s surface with at least 2 wave crests of similar height.
Immediately trace the outline of the waves using a PENCIL before the image blots or dries.
17. Calculate the height, wavelength, and steepness of the wave in the tank.
Use the height of the smallest of the wave crests.
(Larger ones are more likely to result from interference.)
If there are 3 or more wave crests in the snapshot, use the longest wavelength.
(The smaller ones are more likely to represent reflections off one end of the tank.)
Wavelength
Height
Steepness
18. Waves break when their steepness reaches 1/7 (or more). 1/7 is equal to 0.14.
According to your calculations, are your waves breaking?
Lab 5A-6
Temporal Dimensions of Waves: Wave Period and Frequency
Scientists describe how quickly waves cause the sea surface to bounce up and down
in two different ways.
The wave period is the period of time that it takes a wave crest to travel a distance of one
wavelength. In other words, the wave period is “how long” it takes a wave crest to move one
wavelength.
The wave frequency of waves is the number of wave crests that pass a location in a period of
time. In other words, the wave frequency is “how many” waves go by over some time period.
Scientists measure wave period (or frequency) for several reasons. One reason is that wave
period and frequency never change 1 as waves travel, unlike wave height or wavelength.
Secondly, they are much easier to estimate from a distance than wave height or wavelength.
Finally, if you measure the wave period or frequency and know the depth of the water, then
you can calculate the waves’ wavelength and speed 2. Pretty nice, huh?
Stick the paddle into the groove at one end of the tank, and paddle quickly and steadily (do not
change the speed of your paddling). Then, paddle slowly and steadily.
19. At which paddling speed did the resulting waves have a higher frequency?
At which paddling speed did the resulting waves have a longer period?
At which paddling speed did the resulting waves have a longer wavelength?
Use your observations to complete the following table.
Frequency
(high or low?)
Period
(long or short?)
Wavelength
(long or short?)
Paddling
Quickly
Paddling
Slowly
20. What is the relationship between waves’ frequency and period?
The higher the waves’ frequency, the _______________________ the waves’ period.
21. What is the relationship between waves’ period and wavelength?
The longer the waves’ period, the _______________________ the wavelength of the waves.
Remove the sponge from the tank. Squeeze the water out of the sponge and into the tank before
putting the sponge on your table.
1
Wavelength changes as waves approach a beach, because the waves are slowing down. The period does not change
at all; the wavelength does all the adjusting that is necessary. The period does change when the waves break, but by
this point it no longer matters: once they break, they are no longer waves.
2 2
ω = gk tanh(kh), where ω is the angular frequency, g is gravitational acceleration, k is the angular wavenumber
(wavelength), and h is the depth of the water. In deep water, it is simply ω2 = gk. In shallow water, it is ω2 = ghk2.
Lab 5A-7
Wave Speed
Stick the paddle into one end of the tank but not all the way to the bottom; do not put it into the
groove. Gently “flick” the lower end of the paddle to create a big wave crest and then take the
paddle out of the water. Try not to create a tsunami.
Get a stopwatch from the side of the room. Using a stopwatch, time how long it takes a wave
crest to travel the length of tank four times (two roundtrips). In other words, begin timing when
the wave hits one wall of the tank. Let the wave go to the other end of the tank and come back
again once. Then, let the wave go to the other end of the tank and come back again. Stop timing
when the wave crest hits the original tank wall for the third time.
Show your method to your instructor to make sure that you are timing the wave speed properly.
22. How long does it take for the wave to travel the length of the tank four times?
(Do the experiment at least 3 times to make sure that you are getting consistent results.)
Using small plastic containers, remove water from the tank until the water is about 1.5-inches
deep. (Please do NOT scoop out sand and put it down the drain.) Repeat the experiment
described above.
23. How long does it take for the wave to travel the length of the tank four times when there is
less water in the tank? (Do the experiment at least 3 times to make sure that you are getting
consistent results.)
24. Do your results indicate that waves travel faster in shallow water or deep water?
25. Why do waves move faster in water of this depth?
Waves in deeper water move ________________
Hint: What happens to waves’ orbitals
if the water is too shallow? Why? How
and why might his affect waves’ speed?
than waves in shallow water, because when
waves enter shallow water their orbitals
_______________________________________
and this reduces the waves’ speed.
Characteristics of the medium (e.g., water, air, etc.) like “density” and “depth” determine the
speed, not just the wavelength (or period). When you paddled quickly and slowly, you set the
period by paddling, and the wavelength changed to fit the correct speed for the depth!
Lab 5A-8
Activity #3: Waves on Beaches
Wave Growth on Beaches
Place your wave tank in the center of the two tables. Place the small yellow container beneath
one end of the tank. Stick the paddle into the groove at the other end of the tank. Try paddling at
different speeds, and paddle steadily at each speed that you try.
26. Does the wavelength of waves get longer
or shorter as they approach a beach?
_______________________
27. Why does the wavelength of waves change as they approach a beach?
The wave crest near the shoreline of the tank is in (A)______________________ water
and is therefore moving (B)______________________ than the wave crest farther offshore,
so the wave crest farther offshore (C)___________________________________________
the wave crest near the shore.
A: deeper, shallower
B: faster, slower
C: moves closer to, gets farther away from
Waves grow as they approach the shore, because their energy is “compressed” (squeezed)
horizontally (not just vertically!)
28. Which wave crest is moving slowest, the wave crest closest to the shore (A),
the wave crest farthest from the shore (C), or the wave crest between them (B)?
________
29. Which wave crest is moving fastest, the wave crest closest to the shore (A),
the wave crest farthest from the shore (C), or the wave crest between them (B)?
________
Wave crest B is being pushed forward by the faster-moving wave crest C, but it cannot move
forward fast enough, because it is being blocked by the slower-moving wave crest A. The water
in wave crest B is stopped from going forward and yet is being pushed forward from behind.
30. What direction does the water at B go
in response to these pressures?
A
Beach
___________________________________
B
C
Lab 5A-9
Here is another way to think about why waves grow at a beach:
31. Which moves faster, the front part of a wave
or the back part of a wave? (See the picture above.)
________________ part of a wave
Thus, the back part of a wave crest catches up to the front part of the SAME wave crest,
so the water in the crest gets concentrated in a smaller area.
I always hesitate to use this explanation because many students think that a wave crest that is
“behind” another wave crest can actually “catch up” to the wave crest that is “in front” of it.
32. The crest coming from behind CANNOT catch up with the wave crest in front of it.
Why not?
Note: If two different sets of waves are coming into the shoreline at the same time and they have
different wavelengths, then one may “feel the bottom” more than the other, allowing a fastermoving crest from one set of waves to catch up and temporarily merge with a crest from a
slower-moving set of waves.
Lab 5A-10
Waves Breaking at a Beach
33. In general, waves break when their (A)______________________________ becomes
(B)__________________________ than ______________.
(Hint: See the bottom of page 5.)
As waves approach a beach and the water becomes shallower and shallower water,
the waves’ wavelength and height change. How do they change and do these changes
make the waves steeper or less steep?
As waves approach a beach, their height gets (B)_________________________.
which causes the waves to become (C)_________________________.
As waves approach a beach, their wavelength gets (D)_________________________.
which causes the waves to become (C)_________________________.
A: height, period, steepness, wavelength
B: larger, smaller
C: steeper, less steep
D: longer, shorter
Stick the paddle into the groove at the deep end of the tank (away from the beach), and
paddle steadily (do not change the speed of your paddling). Observe the shape of the
waves from the side as the waves “break.” Try different paddle speeds.
34. Does the shape of the waves become more symmetrical or more asymmetric as they approach
the beach? Sketch the side-view shape of the waves close to your beach in the space below.
Symmetric
Wave Crests
35. Which side of a wave becomes steeper as the wave approaches a beach,
the side of the wave facing the beach or the side of the wave facing away from the beach?
The side of the wave ______________________________________ the beach
becomes steeper as waves approach a beach.
As waves get close to a beach the bottom part of the wave “feels the bottom” more and more
quickly than the top part of the wave which allows the top part of the wave to “get ahead” of the
bottom part of the wave.
Lab 5A-11
There are many ways to describe and classify
breaking waves. For simplicity, we will consider
two broad categories.
A plunging breaker occurs is when the top of the
wave gets far ahead of the bottom of the wave,
causing a “barrel” or arc. In this case, the wave
gets steep very quickly; it rapidly crosses the 1/7th
threshold. Since it takes time for a wave to break,
this kind of breaking waves gets much steeper
than 1/7th before finally breaking.
Spilling
Breakers
Bubbles
Plunging
Breakers
"Gently" Break,
always close to 1/7
"Fast" Break,
quickly go over 1/7
A spilling breaker, on the other hand, gets steeper slowly. As its steepness barely grows above
1/7th, water at the crest begins to slide (“spill”) down the front of the wave, reducing the wave
height and thus lowering the steepness back down to 1/7th. As the wave approaches the shoreline
a little more, it grows a little more, which causes it to break a little more. In other words, the
steepness “hovers” around 1/7th.
In summary, a spilling breaker breaks slowly and gently over a long time,
while a plunging break breaks quickly (suddenly, all at once).
Waves break differently on beaches depending on their initial wavelength and height when they
approach the shore, and the slope of the beach. Let’s examine how the slope of the beach affects
how waves break.
Stick the paddle into the groove at the deep end of the tank (away from the beach), and
paddle steadily (do not change the speed of your paddling). Observe how the waves “break.”
Try different paddle speeds.
Then, have one student hold up one end of the tank while another student paddles.
(Do not hold it up so high that water spills out of the tank!) Observe how the waves “break.”
36. In which case do the waves look more like plunging breakers,
when the slope of the tank is steeper or less steep?
37. Are plunging breakers more common on a beach with a shallow slope or a steep slope?
Why?
Plunging breakers are more common on beaches with a (A)___________________ slope,
because the waves’ steepness increases (B)____________________,
and goes far above ________, the point at which waves begin to break.
The steepness changes rapidly because waves move from deep water to shallow water
very (B)____________________, so the waves (C)______________________ very quickly
and therefore their wavelength (D)____________________ quickly and
their height (D)____________________ quickly.
A: shallow, steep
B: quickly, slowly
C: speed up, slow down
D: increases, decreases
Lab 5A-12
Remove as much water as possible from the wave tank using small plastic containers until there
is only a small amount of water left on the bottom (a quarter inch or so). Try not to remove sand
from the tank and dump it in the sinks because it can clog the drains. Put the sponge and the
small yellow container into the wave tank and then place the wave tank back on the cart. Return
the remaining materials; put them in the appropriate stacks.
Wave Refraction
Waves turn towards the coast as they approach the shore. This is called “wave refraction.”
This is why waves almost always head more-or-less directly towards the shoreline of a beach
instead of approaching at a steep angle.
38. Do waves speed up or slow down
as they get closer to the shore? Why?
Beach
A
wa
ve
cr
es
t
39. Which part of the wave, A or B, is traveling faster?
Why is this end of the wave crest traveling faster?
B
Since this part of the wave crest is traveling faster, it covers a larger distance than the other end.
This turns the line of the wave crest towards the shore.
Wave refraction causes waves to bend to “match” the shape of the shoreline as they approach the
shore. For example:
Cove
Headland
Land
Wa
ve C
Land
Wa
r es
t
ve C
r es
t
Lab 5A-13
Longshore Transport
Watch the video on Longshore Transport (LST).
40. What is longshore transport?
Motion of
the Sand
41. What causes longshore transport?
Beach
Break
ing W
ave C
Wave
rests
Direction
42. Describe the motion of the sand along the shoreline.
Why does the sand move up the beach slope? In other words, what pushes or pulls it?
Why does the sand move down the beach? In other words, what pushes or pulls it?
43. Why does longshore transport only happen in the surf zone (where waves are breaking)?
(Hint: Think about how the water is moving beneath non-breaking waves.)
44. When I ask “What pushes sand down the shoreline?” students often answer “longshore
transport.” Longshore transport does NOT cause sand to move along the shoreline.
Why not? What is wrong with this answer?
Lab 5A-14
Waves Conditions Along the Coast of Southern California
Most ocean waves are created by winds blowing over the surface of the ocean,
especially the winds of storms.
Examine the data in the chart below.
Winter Swell Summer Swell
22 Jan. 2001
14 Sept. 2000
West-facing Beaches of Santa Barbara County
13 feet
3.5 feet
South-facing Beaches of Santa Barbara County
3 feet
2 feet
West-facing Beaches of Ventura County
5 feet
3 feet
South-facing Beaches of Ventura County
3 feet
4.5 feet
West-facing Beaches of Los Angeles County
5 feet
2.5 feet
South-facing Beaches of Los Angeles County
3 feet
3.5 feet
Orange County
3 feet
4.5 feet
Santa Barbara
County
Ventura
County
Santa
Monica
Channel
Islands
Los Ange le s
County
O range
County
Palos Verdes
Long
Beach
45. During which season, summer or winter,
do the west-facing beaches tend to have the largest waves?
_______________________
46. During which season, summer or winter,
do the south-facing beaches tend to have the largest waves?
_______________________
47. During which season do we get most of our biggest waves here in southern California?
What direction do they come from, the west or the south?
The biggest waves come from the _______________ during the ____________________.
Lab 5A-15
Examine the maps of average wind speed and wave height in the ocean at the side of the room.
48. Where are the largest waves created, at the poles or in the tropics?
_________________
When are the largest waves created at this location, during the
summer or winter? (Hint: When are storms more common?)
_________________
49. Write “biggest waves in June”
and “biggest waves in December”
into the map on the right.
Note: When it is summer in the
northern hemisphere, it is winter
in the southern hemisphere.
When it is winter in the southern
hemisphere, it is summer in the
northern hemisphere.
50. Why do the west-facing beaches tend to have the largest waves during one season and the
south-facing beaches tend to have the largest waves during the other season? In other words,
why don’t all the beaches along our coast have big waves at the same time of year?
The biggest waves come from the _______________ during the ____________________.
and the _______________ during the ____________________.
51. Why were waves larger on the south-facing beaches of Ventura County during the summer
than along the south-facing beaches of Santa Barbara and Los Angeles Counties?
(You might expect that the wave height would be similar since the same waves
are heading towards southern California everywhere along the coast.)
The waves are larger along the south-facing beaches of Ventura County during
the summer, because big ocean waves from the south are not being blocked by:
_______________________________________________________________
52. Why are the waves coming from the south during the summer (winter in the southern
hemisphere) typically smaller than the waves coming from the northwest during the winter?
(Note: Waves lose little or no energy as they cross the ocean.)
● The North Pacific Ocean has stronger storms and winds than the South Pacific Ocean.
● The water is shallower in the South Pacific Ocean,
so the waves lose more energy from “rubbing” against the bottom of the ocean.
● The waves’ do not gain much energy from the weak winds of summer as they travel,
unlike the winter when winds are stronger.
● The waves are coming from farther away during the summer,
so the waves are more likely meet other waves as they travel.
● The waves are coming from farther away during the summer,
so the waves’ energy spreads out over a larger area before they reach land.
Lab 5A-16
Activity #4: Waves Across the Ocean
Deep-Water Wave Speed
The chart below shows how the speed of waves varies with the wavelength of the waves
when waves’ orbitals cannot reach the bottom of the ocean (i.e., when the waves are “deepwater” waves).
53. According to the chart above, which move faster in deep water,
long-wavelength waves or short-wavelength waves?
The winds of storms generate most ocean waves. Waves grow until their speed matches the
speed of the wind, so the stronger the winds, the longer the wavelength of the waves.
The winds of storms generate waves of many wavelengths during this process: some waves are
just beginning to grow, and other waves been growing for a while. Once waves reach the edge of
the storm, they no longer grow and just continue moving across the ocean unchanged (exception:
their height does decrease as their energy spreads out).
54. Which waves will arrive at the coast first, long-period waves or short-period waves?
Explain your reasoning.
Lab 5A-17
Wave Energy and Height
The energy in waves spreads out waves travel across the ocean, which causes waves’ height
to get smaller as they travel outwards from their source – like ripples on a pond. In general,
waves lose little or NO energy as they travel across the ocean.
55. How deep is the ocean, on average?
_______________________
56. A big wave might have a wavelength of 100 feet.
Wave orbitals go down into the ocean a distance of about half of the wavelength of the wave.
How deep do the orbitals of this wave penetrate into the ocean? _______________________
Can this wave’s orbitals reach the bottom of the ocean?
_______________________
Unlike the waves in our wave tanks which rub against the bottom and sides of the tank, there is
nothing for waves rub against in the middle of the ocean. The wave energy keeps traveling like
the energy moving down a row of dominos in a line, falling one by one.
57. What happens when 2 waves meet in the middle of the ocean? In other words:
Do they destroy one another? Do they reflect? Do they go through one another?
58. How or why can wave interference cause waves to lose (some) energy in the middle of the
ocean? In other words, why might wave interference cause the waves to break?
Wave interference can cause the waves’ height to (A)___________________ for a moment
which may make their steepness go above ___________, causing the waves to break
for a moment and thus lose a little energy.
A: increase, decrease
Watch the video on Rogue Waves.
59. What is a “rogue wave”?
60. What can cause “rogue waves”?
Lab 5A-18
This page was intentionally left blank.