Coasts

Unit 2Thursday 22nd May 2014

Constructive Waves Characteristics

Destructive Waves Characteristics

Constructive and Destructive Waves

LowerLonger

Bigger swash than backwash

Less than 10 per minute

Steeper

Shorter

Bigger backwash than swash

More than 10 per minute

Creates a trough offshore

Hydraulic Action

Abrasion

Caused by waves picking up stones and hurling them at the cliff face,

wearing it away.

Attrition

Any material, but usually rocks and pebbles will become rounder and smaller over time as it

collides with other rocks and pebbles.

Corrosion

The dissolving of rocks and minerals by sea water.

Headland

Bay

Headlands and BaysIs all to do with geology!

Geology = the type of rock

Soft rocks get eroded quickly creating bays

Hard rocks erode more slowly creating headlands

Cliffs are steep rock faces along the coastline, they form along coastlines with resistant harder rocks such as chalk

Wave-cut platforms are rocky ledges on the beach left behind after a cliff collapses. They become smooth over time with erosion.

Cliff Retreat and Wave Cut Platforms

1) During times of high tide the waves attack the base of the cliff to form a wave cut notch.

2) As time passes corrosion and hydraulic action widen the notch causing the cliff to become undercut

3) Eventually the influence of gravity becomes too great and the cliff collapses.

4) This causes the cliff to retreat inland.

5) The rocks left at the base of the cliff are eroded by corrosion and abrasion until a smooth rocky platform is left, called a wave cut platform.

Cave, Arch, Stack, Stump

Weathering and Erosion

Weathering – the breaking down of rock without any involvement of moving forces

Erosion – the breaking down of rock by a moving force such as a wave

Coastal WeatheringFreeze Thaw Solution Biological

Temperatures drop below freezing at night and then

rise during the day

Any water held in cracks in the rock freezes, expands

and then thaws.

This happens over and over until pieces break off.

All rain is slightly acidic.

Polluted air can be very acidic

When rain falls on rocks, the acid reacts with some of the

minerals causing them to dissolve

The roots of vegetation grow into cracks in the rock.

This splits the rock apart.

Mass Movement• Bottom of the cliff eroded by waves and undercut

• Rainfall saturates cliff and makes it heavier

• Water trapped between layers of rock in the cliff acts like a lubricant

• Rocks slide down cliff in a rotational manner

• Often happens in clay

• Rocks are weathered on the cliff face

• They break loose and fall under the influence of gravity

Clay

Mass Movement - slumping

Slumping Explained

For slumping to occur soil must be on top of an impermeable rock such as clay (that is rock that does not allow water to

soak into it.

During times of heavy rainfall the soil above the clay can get saturated. At

the same time waves undercut the base of the

cliff.

A layer of water builds up between the soil and

the clay, where it cant sink it, lubricating it.

Under the influence of gravity the soil slumps in

layers down the cliff.

Coastal Recession is affected by:

Fetch – the bigger it is, the bigger the waves will be and the more the cliff will erode

Geology – cliff recession is faster in soft rocks such as sand or clay, slower in hard rocks like chalk and limestone

Coastal defences – if they protect the cliff, erosion will be less severe and the cliff will retreat more slowly.

Coastal Flooding• Thames barrier protects London from

flooding; closed during times of high tide and severe storms.

• A series of new flood walls will also be installed in 2012

• The Environment Agency monitors the weather and tides 365 days a year and sends out warnings if coastal flooding is likely

• People in Bangladesh build their houses on stilts to protect their properties from flooding

• Also, the Coastal Embankment Project has led to the building of many flood walls, and 500 flood shelters have been built .

• The local people are now being better educated about what to do in a flood and flood warning systems are being put in place

Prevailing wind – the direction the wind blows from MOST OFTEN!

In the UK our prevailing wind direction is from the South West.

It is this that makes swash and Long shore Drift go onto the beach at an

angle.

Longshore Drift (LSD)

1) The waves approach the beach at an angle (in the UK it is 45o) depending on the direction of the prevailing wind.

2) The swash goes up the beach at the same angle carrying sediment and sand with it.

3) The backwash carries the sediment and sand back down the beach at 90o under the influence of gravity.

4) In this way sediment and sand is transported down the coastline.

Coastal Engineering

Hard engineering is anything that is man-made, usually out of concrete or wood

Soft engineering is using anything that is already there in the landscape to try and manage the risk

Hard Engineering – Sea Wall

Pros• Strong• Long lasting• Reflects the wave energy

and protects the base of the cliff

• Cheap

Cons• Looks ugly• Not popular with tourists

Hard Engineering – Rip rap

Pros• Strong• Long lasting• Absorbs wave energy and

reducing erosion

Cons• Not always pretty• Very expensive• Restricts access to the beach

Hard Engineering – Revetments

Pros• Absorbs wave energy and

reducing erosion• Not too ugly• Cheap

Cons• Has to be replaced quite

often

Hard Engineering – Off-shore Reefs

Pros• Absorbs wave energy and

builds up the beach• Not too ugly

Cons• Very expensive• Unpopular with boats and

fishing vessels

Hard Engineering – Groynes

Pros• Stop longshore drift and so

build up the beach• Cheap• Popular with tourists

Cons• Have to be replaced quite

frequently• Do not stop erosion

Soft Engineering – Beach Replenishment

Pros• Builds up the beach again• Very popular in seaside

resorts• Creates a natural barrier to

erosion

Cons• Has to be replaced quite

frequently• Extremely expensive• Can make erosion worse at

other places down the coast

Soft Engineering – Cliff Regrading

Pros• Reduces the risk of slumping• Looks natural• Cheap

Cons• Does not stop erosion• Not guaranteed to work

Soft Engineering – Managed Retreat

Pros• Doesn’t cost anything• Very natural

Cons• Loss of land and homes• Have to compensate

people who loss their land

Swanage

• Managed due to weaknesses in the cliff and soft clays and the risk to houses and hotels on the cliff.

• 90,000m3 of sand pumped onto the beach (beach replenishment)

• 18 new groynes in 2005 to replace old timber ones

• Cliff regrading and drainage to lower the risk of slumping

• Cost £2.2 million in total