5A_3 – Slide 1
Topic 5A:
Waves, Part III
Online Lecture:
The Life of a Wave –
How Waves Change as
They Travel Across the Ocean
and to the shore
Most Waves Are Made
by the Strong Winds of Storms
Blow over the Paper
ce
Pie
er
p
a
of P
5A_3 – Slide 2
Effects on the Ocean:
● Currents
● Gas Exchange
● Life
Bernoulli Effect Experiment
Winds and Waves
Wind Speed
5A_3 – Slide 3
The Largest Waves are
made close to the
Poles during Winter.
Wave Height
Waves Spread Outwards from the Storm like… 5A_3 – Slide 4
Storm
As waves move away
Asfrom
waves move
from the
the away
storm,
their
storm, their energy is spread
energy
is spread
over
a wider and
wider area. over
a wider & wider area.
Ripples get smaller as
energy spreads out,
but waves do not
“lose” any energy.
5A_3 – Slide 5
Wave Dispersion
Waves “separate”:
Long-Period waves
leave the short-period
waves behind.
faster,
long-period
waves
“Swell:”
Long-period waves that
are not longer “mixed”
with short-period waves
slower,
short-period
waves
Waves on Beaches: Change in Wavelength
○ Does the wavelength of waves get
longer or shorter as they approach a beach?
Bottom of the T
5A_3 – Slide 6
Shorter
an k
○ Why does the wavelength change?
The wave crest closest to shore slows downs, because
its orbitals “feel the bottom.” The wave crests farther
from the shoreline begin to “catch up.”
Never “Catch Up”
Not “hitting”
the bottom &
bouncing up!
5A_3 – Slide 7
Beach
Slowest
Crest
“Squeezed”
Crest
Waves on Beaches: Change in Height
Fastest
Crest
○ Why do waves grow as they approach a beach?
The crest closest to the beach slows down the most since
the water is very shallow, & the crest that is farthest from
the beach is moving fastest, because it is in deeper water.
The crest in the middle is squeezed horizontally:
the only direction that the water in the middle can go is up
Waves on Beaches: Kinds of Breaking Waves 5A_3 – Slide 8
Plunging Breakers
“Fast” Break,
quickly go over 1/7
Plunging
Breakers
Spilling Breakers
“Gently” Break,
always close to 1/7
"Fa
quickl
Spilling
Breakers
Bubbles
Other words are used to describe breaking waves…
"Ge
alway
Waves on Beaches: Breaking Waves
5A_3 – Slide 9
○ Why do waves break?
become “too steep” (> 1/7)
As waves approach a beach, wave height gets larger
(prior to breaking) & wavelength gets shorter.
Both cause the wave to get steeper.
○ Waves break differently
on different beaches
depending on:
S te
Height
ep n
e ss
Wavelength
– their initial wavelength & height, tide level,…
– the slope of the beach
Steep Beach Slope:
waves slow down quickly
when they feel bottom,
squeezed upwards suddenly
Be
a
ch
Orbitals suddenly
"feel the bottom"
Storm
Waves almost always
come almost straight
towards a beach. Why?
Ocean
Land
Why don't waves ever
go down the coast?
5A_3 – Slide 10
Why do waves always come
directly into the beach?
Waves Change Direction: “Refract”
5A_3 – Slide 11
Turn Towards the Coast
Refraction
5A_3 – Slide 12
○ A line of soldiers walks into the mud.
Soldiers on one end of the line move
slower than soldiers at the other end.
They fall behind, causing the line to bend.
Slows
down
first.
Mud
Grass
5A_3 – Slide 13
Wave Refraction
Why?
orbitals “feel
the bottom”
wa
ve
cr
es
t
The part of the wave crest
that is closer to shore is
in shallower water, so it is moving slower.
fa
ste
r
slow down
slo
we
r
○ Do waves speed up
or slow down as they
get closer to the shore?
Beach
The end in deep water travels faster than
the end in shallow water, so it covers a larger distance,
causing the entire line to bend towards the coast
Pictures of Wave Refraction
wave crests
turn to match
the shape of
the shoreline
5A_3 – Slide 14
Refraction usually
5A_3 – Slide 15
“stretches” wave crests:
Spreads Out Energy → Smaller
Effects of Refraction
“Wrap
Around”
Effect
Headland
Island
"Wave Shadow:"
Island has
“Wave
blocked
Shadow”
the waves.
Farther
= Deep
= Faste
= Turn
Waves can bend
towards each other:
Interfere → Higher
Waves May Not Completely Refract
(Match the Shoreline) Before Breaking:
Results in “Longshore Transport”
5A_3 – Slide 16
○ Longshore transport is the movement of sand down
the shoreline. The sand is pushed by the waves.
5A_3 – Slide 17
Longshore Transport
○ When a wave gets
close to the shoreline,
the wave breaks.
○ The water
surges forward,
pushing sand up the
beach at an angle.
Motion of
the Sand
Beach
Direction of
Longshore
Transport
Break
ing W
ave C
Wave
rests
Direction
○ Then, the water
& sand slide back
into the ocean,
pulled downhill by gravity.
○ This happens again and
again, so the sand slowly
moves along the shoreline.
The water no longer moves back
and forth, staying in one place,
like it does further offshore where
the waves are not breaking.
Longshore transport does not cause
sand to move along the shoreline.