@ Dynamic Coast

The Dynamic Coast

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A presentation about the interaction between the dynamic coast and people

Houses threatened by coastal erosion in California

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People and the Coast

People have always lived by the sea. The sea provides food, opportunities to trade and is a great place to relax…

…but the coast can change and affect people and their

property. Protection from the sea might be necessary.


Cliff erosion along Auckland’s coastline and erosion at Omaha Beach (below).

What do you think causes erosion of the coastline?

What makes it worse?

Can it be stopped or lessened?

Images courtesy of Auckland Regional Council


Natural processes that cause coastal erosion

The coastline

Coastal Geomorphology

•The geology of the coast

•The shape of the seabed

Aeolian (Wind)

The wind determines the waves and also carries fine sediments

Biological

• Weathering can weaken rocks

• Vegetation can strengthen and stabilise slopes.

Hydrological

• Wave energy and force

• Movement of sediment

• Currents

• Changes in sea level


Erosion and deposition over time

Do you know of a beach that is eroding?

A collapsed cliff face in East Anglia, England.

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.

Time

Storm events

erosion


Examples of coastal landforms

Which landforms are caused by erosion?

Which coastlines are growing?


Can you name these landforms and describe how they might have been caused?


Look carefully at this coastline

Why has this part eroded?

This part of the coastline has remained stable.

Why has it not eroded?

This dotted line is the original coastline


Stacks – a well known coastal landform

Old Man of Hoy, Scotland

You’ve probably seen pictures of stacks before, but did you know that they are a

landform caused by coastal erosion?

The Twelve Apostles, Australia

Ha Long Bay, Vietnam


The Dynamic System of the Beach

Why do some coastlines erode away? Why do some grow?

All beaches exist in a dynamic equilibrium involving four factors:

1.The supply of sand2.The energy of the waves3.Changes in sea-level4.The location of the shorelineIt is the balance of these four factors and how they interact with each other that determines whether a beach erodes or grows.

This sand dune pictured above is evidence that this beach has lots

of sand.

How might this sand have been deposited here?


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


Storms – a major cause of coastal erosion

Higher than usual wave heights, and thus, greater wave energy

Erosion of a beach is often in spurts, with high rates of erosion occurring during a storm, and lower rates occurring in calm periods.

Storms bring:Waves containing sand that has been churned up from the sea floor, causing them to be more erosive.

Image: Richard Vroom


These photos show a beach on the west coast of the USA.

The top photo shows a cliff face with a beach below.

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


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.


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.

Examples of houses affected by coastal erosion in the United States

Do you know of someone who has property or land affected by coastal erosion?


People versus the Sea: Who wins?

A seawall left battered by the July 1978 storm at Omaha on the east coast of Auckland, New

Zealand.(Image courtesy of NIWA)

Who do you think would win the battle of People versus the Sea?

Which is more powerful?

Hold a quick class survey to see who thinks

the sea will ultimately always win and who

thinks that people can tame the sea and control

coastal erosion.


Humans: Negative 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.

The energy of the waves.

The natural shape of the coastline.

Human activity can cause these factors to be altered, and this will affect the dynamic equilibrium of a beach.

Look at these groynes. They are designed to trap

sand, but in doing so, they are probably starving a beach further down the

coast of sand.


Accelerating Erosion

Can a man-made structure designed to prevent coastal erosion actually achieve the opposite and accelerate the rate of erosion?

Example: Wave Energy

When a wave strikes at a beach all its energy has to go somewhere. If a large wave in a storm hits a straight wall, its energy is concentrated and it can do more damage, often undermining the wall.

Study these two diagrams of coastlines and discuss which one dissipates wave energy and which one concentrates wave energy


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

What effect do you think this structure has? Have you seen something similar?


Why might some structures work and others not?

Although hardening of the shoreline may temporarily protect property, it often results in beach loss on chronically eroding shores.

Images: Coastal Lands Program, Hawai’i

In this photo, there was not enough distance between the building and the shore, and people have put sacks down to try and stabilise the beach.


Coastal Management

Over the course of history, humans have interacted with the coastline in a variety of ways.

There are three main approaches to managing shoreline erosion:

1. Hard Stabilisation2. Soft Stabilisation3. RelocationThe approach that is taken can depend on the cost, how many people or properties are affected, and the rate of erosion.

What would you spend millions of dollars on protecting from coastal

erosion?

A housing estate.A tourism resort.

A historical Maori Pa site. A golf course.


1. Hard Stabilisation Methods

This involves holding the shoreline in place using fixed structures, usually made out of concrete.

Examples include:

• Sea walls that run parallel to the shoreline and aim to protect buildings. They often require maintenance and can look ugly.

• Revetments are sloping walls usually made of large concrete blocks not cemented together. In New Zealand they are often big boulders.

• Groynes that run perpendicular to the shoreline and aim to prevent sand loss and also deflect wave energy.

• Breakwaters are large concrete structures built offshore that aim to alter the wave direction and dissipate wave energy before it reaches the shore.

Image: Coastal Lands Program, Hawai’i

What would you do here?


Examples of Hard Stabilisation Methods

groyne

Sometimes the sea just jumps over the wall!

Images: Coastal Lands Program, Hawai’i


2. Soft Stabilisation

A beach usually erodes because of a lack of sand to protect it. Thus, soft stabilisation methods involve building up the supply of sand and protecting it.

Examples include:

• Beach Nourishment: Trucking sand in from one beach to another

• Sand dune stabilisation: Planting grasses and fencing the dunes

• Beach Drainage: This involves lowering the water table that is under the beach to enable sand to remain on the beach and not be washed away.

Planting marram grass is a common method of dune

protection.

Can you think of other ways in which dunes are

protected?


3. Relocation

This can be thought of as the “the sea has won” approach, because it is really saying that people should move out of the way of the sea.

There are advantages and disadvantages:

1. The natural events of coastal erosion are allowed to continue.

2. Money is saved by not doing any construction work.

3. Money is lost by not doing anything, as land is lost and people have to relocate.

The management and planning of development along a coastline has a big impact into how a coastline affects human development.

If councils don’t allow anyone to build within say, 500m of an actively eroding coastline, there will be less chance of any building being affected.

What do you think the locals would think of this planning restriction? View this video clip:

“Should they have been allowed to build there?”

Included in retail version


The New Zealand Situation

With over 11 000 km of coastline, New Zealand actually has very little coastal protection, mostly because there is very little concentrated human development on the coastline. Most of the ports have some form of protection and some of the cities, like Auckland, have extensive sea walls. The vast majority of coastal protection works are done by local and regional councils or by individuals.

This has resulted in a variety of structures all over New Zealand’s coast, often protecting baches and small coastal settlements. In some cases the structures are causing erosion to occur.

Think of a beach you visit regularly. Is there any coastal protection at this beach?

Image: University of Waikato, Department of Earth and Ocean Sciences

Old Tyres used as

protection


Image: Auckland City Council

An example of a seawall that doubles as a walkway. Visually pleasing and also practical.

Examples from around New Zealand


Image: R.K. Smith www.climatechange.govt.nz

Note the fence out on the right


Image: Environment Waikato


Matapouri Bay, Image: Northland Regional Council


Image: Coastal Lands Program, Hawai’i

If this was your house, what would you do?


Revision

Go back to considering who wins in “Humans vs the Sea”. What do you think?

1. Can you remember the various processes that cause coastal erosion? How many can you identify?

2. Explain the four factors that make up the dynamic equilibrium of a beach.

3. Describe how a man-made structure designed to protect a beach from erosion, could actually cause erosion to increase.

4. Name the three approaches to coastal management and give examples for each.

5. Choose one fact or piece of information that you have learnt from this presentation and explain what it is.

Right PlaceResourcesRight PlaceResources

We are very grateful for the use of information and images from the following websites and organisations:

• United States Geological Survey: Center for Coastal and Watershed Studies; coastal.er.usgs.gov/

• National Institute of Water and Atmospheric Research (NZ): www.niwascience.co.nz

• The Why Files: www.whyfiles.org

• University of Hawai’i Sea Grant College Program

• Auckland Regional Council: www.arc.govt.nz

• Auckland City Council

• Environment Waikato

• www.climatechange.govt.nz

• Northland Regional Council

• Department of Earth and Ocean Sciences, University of Waikato.

All images are from Wikipedia unless otherwise noted

The Dynamic Coast

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@ Dynamic Coast

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