CoastsProcesses and Forms

Understanding the Physical Geography of Coasts

1. Tides & Waves

2. Transport of Sand

3. Coastal Erosion

4. Coastal deposition and beaches

5. Sea level change & coasts

6. 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

Wave Refraction (bending)

Concentrates energy at headlands

Spreads out (dilutes) in bays)

Wave Refraction (bending)

More wave refractionWaves 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

Santa Barbara

Dredge

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.

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.

October 1997

April 1998

Images from USGS

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)

Old Man of Hoy, Scotland

The Twelve Apostles, Australia

Ha Long Bay, Vietnam

Coastal Erosion and PeopleOver 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

Image: Coastal Lands Program, Hawai’i

If this was your house, what would you do?

Sometimes the sea just jumps over the wall!

Images: Coastal Lands Program, Hawai’i

Hard Stabilization Methods

Groin

Shore protection (a revetment) by massive concrete armour units fronting a resort in Japan. (Photo: Jess Walker, courtesy of NIWA)

Soft StabilizationA 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 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 Dynamic

Equilibrium is affected

by…

The Waves

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 Sea Level

In New Zealand the sea level is rising at approximately 15 cm/100yrs. Climate change may cause this to rise further.

The location of the shoreline

Shorelines move back and forth between storms and their location can either increase or decrease erosion rates

Animation in Classroom Resource Folder

Even seasons alter sand equilibrium

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 sand-transporting 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. PadreIsland,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 BaySydney, Australia

Coast rising up form Marine Terraces (ancient, uplifted wave-cut platforms)

1st -Waves erode platform in high sea level time2nd -Tectonic uplifting during low sea level time

1st -Waves erode platform in high sea level time2nd -Tectonic uplifting during low sea level time

California Coast (usually occupied by cities) Old Sea Stack

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)

Fringing Reef

Holand Island

Grand Cayman Is.

Tureia, Coral Atoll Oeno, Fringing Reef

Bora Bora Atoll

Online Video

Online K-8 Lessons

Online AnimationsLongshore Drift and Depositional Landforms (Coasts)http://serc.carleton.edu/NAGTWorkshops/visualization/collections/

longshore.htmlVisualizations for Teaching Tideshttp://serc.carleton.edu/NAGTWorkshops/visualization/collections/tides.htmlVisualizations for Teaching Rocky Coastlines and Erosional Landformshttp://serc.carleton.edu/NAGTWorkshops/visualization/collections/rocky_coastlines.htmlCoastal Wave Mechanicshttp://serc.carleton.edu/NAGTWorkshops/visualization/collections/coastal_wave_mechanics.htmlCoral Reefs and Atollshttp://serc.carleton.edu/NAGTWorkshops/visualization/collections/coralreefsatolls.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