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.