Coasts
Processes and Forms
Understanding the Physical
Geography of Coasts
1.
2.
3.
4.
5.
6.
Tides & Waves
Transport of Sand
Coastal Erosion
Coastal deposition and beaches
Sea level change & coasts
Your tropical vacation
1. Tides and Waves
Classroom Resource
Navy Animation about Tides
Waves generated by winds
Classroom Resources
Introductory
Movie Clip
Open Ocean
Classroom Resource
Types of Breakers
Concentrates energy at headlands
Spreads out (dilutes) in bays)
Wave Refraction
(bending)
Wave Refraction
(bending)
More wave refraction
Waves bend as they approach the shore
(because they slow)
2. Transport of Sand
Classroom Resource
2 parts: beach drift & longshore current
Consequence
of sand
movement:
Sand Spit
Classrom Resource
Consequence
of sand
movement:
Baymouth
Bar
Classroom Resource
Animation about baymouth bar formation
Consequence of sand movement:
Tombolo (island connected to land by sand spit)
Point Sur, California
Classroom Resource
Movie showing sand piling up behind Getty
and Groin
Consequence of sand movement:
Sand backs up behind groins
Dredge
Santa Barbara
Which way is
the longshore
current?
Why no Columbia
River Delta?
Ocean waves and strong
currents quickly
redistribute the
material that is being
deposited so it allows
for no sediment to
build up and form a
delta rather it
redistributes it.
3. Coastal Erosion
Most coastal erosion occurs during storms. Storms
cause great damage, but often the rock or beach
has been exposed to years of weathering and
erosion by the wind so is already in a weakened
state. The storm really just finishes it off.
These photos show a part of
the coastline of the state of
Louisiana, USA, that was
affected by Hurricane Katrina
The top image, taken in July
2001, shows narrow sandy
beaches and adjacent overwash
sandflats, low vegetated dunes,
and backbarrier marshes broken
by ponds and channels. The
second image shows the same
location on August 31, 2005, two
days after Hurricane Katrina
made landfall on the Louisiana
and Mississippi coastline. Storm
surge and large waves from
Hurricane Katrina submerged the
islands, stripped sand from the
beaches, and eroded large
sections of the marsh.
These photos show a beach
on the west coast of the
USA.
The top photo shows a cliff
face with a beach below and
sea stack.
October 1997
A year later, after a series of
winter storms, attributed to El
Nino, the beach beneath the
cliffs has all but disappeared.
The natural ‘bank’ of sand has
gone, leaving the cliff more
exposed to erosion.
Images from USGS
April 1998
Higher than usual wave
heights, and thus, greater
wave energy
Storms bring:
Waves containing sand
that has been churned up
from the sea floor, causing
them to be more erosive.
Wave cut platform
Wave cut notch
Sea Caves
Sea Stack
Sea Arch
More Sea Stacks (used to be land)
The Twelve Apostles, Australia
Ha Long Bay, Vietnam
Old Man of Hoy, Scotland
Coastal Erosion and People
Over 80% of the world’s shorelines are retreating at a rate from centimeters
to meters per year. Many governments, local authorities and individuals
spend large amounts of money protecting their developments from being
washed away by coastal erosion.
Image: R.K. Smith
www.climatechange.govt.nz
Note the fence out on the right
If this was your house, what would you do?
Image: Coastal Lands Program, Hawai’i
Hard Stabilization Methods
Groin
Images: Coastal Lands Program, Hawai’i
Sometimes the
sea just jumps
over the wall!
Shore protection (a revetment) by massive concrete armour units fronting a resort in Japan.
(Photo: Jess Walker, courtesy of NIWA)
Soft Stabilization
A beach usually erodes because of a lack of sand to protect it. Thus,
soft stabilization methods involve building up the supply of sand and
protecting it.
Examples include:
• Beach Nourishment: Trucking (or dredging in – piping) sand in from one
beach to another
• Sand dune stabilization: Planting grasses and fencing the dunes
Simple fence collecting sand
4. Deposition &
Beaches
Why do some coastlines
erode away? Why do
some grow?
In this case, beach erodes after hurricane
• High energy of the waves
• Storm surge raise in sea-level
• Location of the shoreline erode back
The Waves
The Sand Supply
Sand is food for beaches and
gives shorelines protection from
the waves. Being starved of
sand can cause higher rates of
erosion.
The Sea Level
The erosive action of a wave is
greatest when the wave is high.
The angle at which they strike a
beach and how much sand they
are carrying can also influence
the rate of erosion.
The
Dynamic
Equilibrium
is affected
by… The location of the shoreline
In New Zealand the sea level
is rising at approximately 15
cm/100yrs. Climate change
may cause this to rise further.
Shorelines move back and forth
between storms and their location
can either increase or decrease
erosion rates
Even seasons alter sand equilibrium
Animation in
Classroom
Resource Folder
Human Impact
A poor understanding of how a shoreline works can actually
cause human activity to accelerate the rate of erosion.
Key factors often over-looked are:
 The supply of sand (cut off by paving over rivers)
 Altering natural shape of the coastline & altering sandtransporting currents
Look at these groins.
They are designed to trap
sand, but in doing so,
they are probably starving
a beach further down the
coast of sand.
Must mention rip currents
Classroom Resource
Animation
5. Coasts and Sea Level Change
Former coastline in glacial time
Barrier Islands – former beach
sand migrates with sea level rise
S. Padre
Island,
TX
http://earth.jsc.nasa.gov
Classroom Resource
Movie clip discussion Florida Barrier Islands
Barrier Islands – slowly move onshore
Cape Hatteras
Coast Rising (and
sea level goes up
and down with
glacial/interglacial
times)
Coast Stable, but
sea level goes up
and down
(e.g. Atlantic
seashore)
RIA – drowned river valley
Coast drowned by sea level rise after last
glaciation – river valleys invaded (ria)
Chesapeake Bay
Sydney, Australia
Coast rising up form Marine Terraces (ancient,
uplifted wave-cut platforms)
1st -Waves erode platform in high sea level time
2nd -Tectonic uplifting during low sea level time
1st -Waves erode
platform in high sea
level time
2nd -Tectonic uplifting
during low sea level
time
California Coast (usually occupied
Old Sea Stack
by cities)
6. Your tropical vacation
This NOAA animation shows the dynamic process of how a coral atoll forms. Corals (represented in tan and purple)
begin to settle and grow around an oceanic island forming a fringing reef. It can take as long as 10,000 years for a
fringing reef to form. Over the next 100,000 years, if conditions are favorable, the reef will continue to expand. As
the reef expands, the interior island usually begins to subside and the fringing reef turns into a barrier reef. When
the island completely subsides beneath the water leaving a ring of growing coral with an open lagoon in its center,
it is called an atoll. The process of atoll formation may take as long as 30,000,000 years to occur.
http://www.oceanservice.noaa.gov/education/kits/corals/media/supp_coral04a.html
Classroom Resource
Alternative Animation Showing Atoll
Formation (in classroom resource folder)
Grand Cayman Is.
Fringing
Reef
Holand Island
Tureia, Coral Atoll
Oeno, Fringing Reef
Bora Bora Atoll
Online Video
Online K-8 Lessons
Online Animations
Longshore Drift and Depositional Landforms (Coasts)
http://serc.carleton.edu/NAGTWorkshops/visualization/collections/longshore.ht
ml
Visualizations for Teaching Tides
http://serc.carleton.edu/NAGTWorkshops/visualization/collections/tides.html
Visualizations for Teaching Rocky Coastlines and Erosional Landforms
http://serc.carleton.edu/NAGTWorkshops/visualization/collections/rocky_coastli
nes.html
Coastal Wave Mechanics
http://serc.carleton.edu/NAGTWorkshops/visualization/collections/coastal_wav
e_mechanics.html
Coral Reefs and Atolls
http://serc.carleton.edu/NAGTWorkshops/visualization/collections/coralreefsatol
ls.html
Imagery seen in this presentation is courtesy of
Ron Dorn and other ASU colleagues, students
and colleagues in other academic departments,
individual illustrations in scholarly journals such
as Science and Nature, scholarly societies such
as the Association of American Geographers,
city,state governments, other countries
government websites and U.S. government
agencies such as NASA, USGS, NRCS, Library
of Congress, U.S. Fish and Wildlife Service
USAID and NOAA.c