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Science Modules using the Mini Wave Tank (working document)
Jack A. Puleo, Ph.D. University of Delaware, Center for Applied Coastal Research
View of a breaking wave from below the surface
What are Waves
Wave Description
Period and Frequency
Energy Transport
Wave Speed
Shallow versus Deep Water
Tsunami
Beach Profile
What is a Wave?
Waves are everywhere in nature
Most people think of waves on the ocean when someone mentions waves
But, they exist in many other places:
Here are a few examples: Can you think of more?
http://lh3.ggpht.com
http://image.tutorvista.comhttp://www.dermawave.com
Wave DescriptionA wave is a disturbance that travels through some medium (typically air or water) from one place to another.
There are a variety of wave types, but here we will only discuss surface waves
Surface waves travel on the surface of a fluid where particle undergo a circular motion
For simple surface waves, the water particles have no net displacement. The individual particles return to where they started from.
WAVE PARAMETERS
Wave elevation profile: deep water = sine wave
Wavelengtham
plit
ude
Crest: location of maximum upward displacementTrough: location of maximum downward displacement Amplitude (a): amount of maximum displacement from rest position Height (H): vertical distance between trough and crest Wavelength (L): The horizontal length of a complete wave cycle
Trough
Crest
Rest position
Hei
ght
WAVE PARAMETERS
Period (T): time is takes for one cycle to completeFrequency (f): refers to how often something happens, usually cycles per second. The unit cycles per second is called Hertz (Hz)
Tfand
fT
11
WAVE PARAMETERS
Velocity (C): distance divided by time
Crests can be seen to move along the ocean, how long it takes the crest to get from one point to another is the velocity.
Velocity (C) = Wavelength/Period = L/T
Or
Velocity (C) = Wavelength * frequency = L*f
Moving with velocity, C
WAVE PARAMETERS
Energy (E): From the movie we saw that particles return to their original position.
So, waves transport energy, NOT matter.
Wave energy is proportional to the amplitude of a wave SQUARED
2aE
Low energy
High energy
WAVE PARAMETERS
Water particle motion within a wave train decays exponentially with depth and varies in shape depending on relative depth.
“Shallow” “Intermediate” “Deep”
Dean and Dalrymple Coastal Engineering textbookwhere h is the water depth
220
Lh
L
2
Lh
20
Lh
WAVE PARAMETERS
This suggest that the water particle motion changes with depth.
It also suggests that parameters such as velocity and wavelength change with depth
g is gravity; g = 32.2 ft/s2.“Deep Water” “Intermediate
Water”“Shallow Water”
Wavelength (L)
Wave velocity (C)
2
2gTL
L
hgTL
2tanh
2
2
ghTL
ghC 2
gTC
L
hgTC
2tanh
2
WAVE PARAMETERS
T = 10 seconds, H = 6 ft.
Intermediate region
Intermediate region
TSUNAMI
Tsunami are waves that often cause destruction.
They are often incorrectly referred to as tidal waves, but they are not associated with the moon or sun that forces tides.
Tsunamis typically occur after an earthquake where parts of the ocean floor are raised or lowered very rapidly.
They can also be caused by a landslide where lots of dirt and rock quickly enter the ocean.
TSUNAMI
The wavelength of a tsunami as it travels across the ocean can be 300-500 MILES.
A typical depth for an ocean basin may be 3 miles.
http://www.jobsletter.org.nz
Tsunami height can increase drastically when the water depth becomes very shallow
BEACH PROFILE
The beach profile is the change in elevation or depth as a function of cross-shore distance.
Example: it usually gets deeper as you go farther offshore.
Imagine staring straight offshore and taking a slice down through the beach.
http://myweb.cwpost.liu.edu/vdivener/notes
BEACH PROFILE
The profile shape is governed by variable waves and currents
During large waves, sand tends to move offshore. Sometimes termed “winter” profile
During small, long period waves sand tends to move onshore. Sometimes termed “summer” profile
SUMMER WINTERcoastalchange.ucsd.edu
BEACH PROFILE
Changes in the beach profile can be simulated in the wave tank by running waves of different size for periods of time.
BEACH PROFILE SPREADSHEET
Screen shot of the Excel spreadsheet.
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