location map of sydney’s northern beaches. coasts – issues & processes who & what?
TRANSCRIPT
Location map of Sydney’s Northern beaches
COASTS – Issues & ProcessesWho & what?
Biophysical interactions: how do the four spheres impact
upon coasts?
Atmospheric interactions with the coast:
• Winds generate waves, storm surges and currents.
• Temperature affects the rate of weathering and the amount of river sediment.
• Glaciation (ice age) brings changes in sea levels.
• Precipitation affects channel flow, groundwater and supply of terrestrial (land) sediment.
• Human interactions have caused Global warming and this is predicted to cause sea level rising.
Lithospheric interactions with the coast:
• Tectonic forces cause crustal uplift and subsidence, volcanic activity, earthquakes and tsunamis.
• Different rock types and structures have varied rates of weathering and erosion, ie soft and hard rock. This also effects rates of mass movement.
• Soil degradation, erosion, salinisation, • Humans have built structures such as sea walls
and breakwaters to stabilise shoreline conditions. Humans dredge sand bars (to get rid of excess sand that block lagoons)
Hydrospheric interactions with the coast:
• Together with the atmosphere the hydrosphere creates currents, tides, storm surges, sedimentation, groundwater, channel flow, surface runoff, rivers, lakes, lagoons.
• Human interactions effect flooding, river flow, urban runoff and the storage of water..
Biospheric interactions with the coast:
• The variance in dune vegetation affects the stability of dunes.
• Healthy dune systems should sustain a diversity of Flora and Fauna.
• Human interaction can effect this health, through fires, development, deforestation and introduced species.
Coasts - Energy at the Coast
The coast represents the meeting point between the land and sea. Coasts are very dynamic areas and they are constantly changing. This change is due to 3 main processes which operate at the coast,
1. Erosion;
2. Transport and
3. Deposition.
Examples of coastal landforms formed by erosion and deposition
Agents of Erosion
Waves
The Zones of the Coast
• These 3 processes are all driven by the amount of energy that is available at the coast. The main agents of change at the coasts are waves. Waves are movements of energy throughout the water, but where do waves get their energy from? The answer to this is wind.
Waves
• There are 3 main factors which will affect the strength of a wave and therefore whether it is more likely to erode or build up the coastline
Waves
• (i) the strength and speed of the wind - the faster the wind, the more energy is transferred and therefore the bigger the wave that is produced.
Waves
• (ii) the duration of the wind - this is the length of time for which the wind has blown - the longer the wind blows, the more energy is transferred to the wave
Waves
• (iii) the fetch - this is the distance over which the wind has blown and therefore how far the wave has travelled. The longer the fetch, the larger the wave is likely to be.
• How is energy transferred to create waves?
1. Wind creates friction on the waters surface;
2. Frictional drag between the wind and the waters surface causes water particles to rotate and energy is transferred forward;
3. When the wave reaches shallow water, it slows down due to friction between the base of the wave and the sea bed. The shape of the wave becomes increasingly elliptical;
4. The top of the wave continues to move forward as it is unaffected by the friction with the sea bed. It becomes steeper and steeper and eventually breaks;
5. Water moves up the beach as the swash;
6. Water then returns back down the beach as the backwash.
• As wind blows over the surface of the sea, it creates friction. This frictional drag causes water particles to begin to rotate and energy is transferred forward in the form of a wave. Whilst the water moves forward, the water particles return to their original position.
• As a wave reaches shallow water, friction between the sea bed and the base of the wave causes the wave to begin to slow down and its shape becomes more eliptical. The top of the wave however, unaffected by the friction, becomes steeper until it eventually breaks.
When waves reach the shore
• When the wave breaks, water washes up the beach, this is called the swash. The movement of water backdown the beach is called the backwash.
Rates of erosion
• It is the rate at which waves reach the coast which determine whether the main process acting on the coastline is erosion or deposition. There are two main types of waves:
Constructive waves
CONSTRUCTIVE WAVES
• Tend to arrive at the coast at a rate of less than 8 waves per minute, they are low energy waves and are small in height. They have a strong swash and a weak backwash. This means that constructive waves tend to deposit material and build up a beach.
• small in height • gentle angle • less energy • strong swash and weak backwash (material is
moved up the beach by the strong swash)• ANIMATION• http://www.curriculumbits.com/prodimages/
details/geography/geo0003.html• VIDEO• http://www.youtube.com/watch?
v=EpFykGLsDnM
DESTRUCTIVE WAVES
DESTRUCTIVE WAVES
• Have much higher energy and tend to arrive at the coast at a rate of more than 8 per minute. They are much larger in height often having been caused by strong winds and a large fetch. These high energy waves have a weak swash but a strong backwash, which erode the beach but pulling sand and shingle down the beach as water returns to the sea.
• large in height • steep • lots of energy • weak swash and strong backwash (beach is
scoured and degraded as the strong backwash pulls sand and shingle back down the beach)
• ANIMATION• http://www.curriculumbits.com/prodimages/
details/geography/geo0003.html• VIDEO• http://www.youtube.com/watch?
v=FHvAxYrHdDU&eurl=http://burychurchgeog.blogspot.com/2007/09/waves.html
Wave animations
• http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21401 Summer/winter pattern
Fill in the table
Stronger swash or backwash
Beach gradient
High or low energy
Frequency
Wave period
Wave length
Wave steepness
Wave height
DestructiveConstructive
Waves
• Wave Machine Simulator - create your own ocean wave
• Waves - includes animation of swash and backwash (BBC Bitesize)
Key Term Check:
• Swash - the movement of water and material up the beach (in direction of prevailing wind)Backwash - the movement of water and material back down the beach (straight back down due to gravityConstructive wave - low energy wave with greater swash than backwash - tends to build up the beachDestructive wave - high energy wave with greater backwash than swash - tends to erode beach
Wave of translation animation
• http://www.saddleback.edu/faculty/csolem/Lecture%2012/anim04.swf
(www.class.unl.edu)
Wave terminology
Fetch
Plunging breakers
Plunging breaker
Spilling breakers
Spilling breaker
Wave refraction animation
• http://www.saddleback.edu/faculty/csolem/animations/38_WaveMotion.swf
Wave animations
• Home 1.Beach anatomy, 2.wave anatomy, 3.wave motion, 4.wave refraction, 5.longshore current, 6.rip current, 7.coastal deposition, 8.coastal erosion landforms, 9.coastal erosion: controls Models
• http://www.uky.edu/AS/Geology/howell/goodies/elearning/module14swf.swf
Tides definedTides defined
•• "Tides"Tides" is a generic term used to define " is a generic term used to define the alternating rise and fall in sea level the alternating rise and fall in sea level with respect to the land, produced by the with respect to the land, produced by the gravitationalgravitational attractionattraction of the moon of the moon and the Sun. and the Sun.
((http://http://www.bigelow.orgwww.bigelow.org))
(www.alunatime.org)(www.alunatime.org)
Spring & neap tidesSpring & neap tides
•• Happen on a 28 day Happen on a 28 day cycle, not annually!cycle, not annually!
•• Spring tides lead to a Spring tides lead to a large tidal range, the large tidal range, the highest tides and the highest tides and the lowest tides.lowest tides.
•• Neap tides have a Neap tides have a small tidal range.small tidal range.
(www.the(www.the--riverriver--thames.co.uk)thames.co.uk)
Tides and erosionTides and erosion
•• Neap tides lead to concentrated erosion on one part of Neap tides lead to concentrated erosion on one part of the cliff.the cliff.
•• Tides are not directly linked to wave size (wind) so you Tides are not directly linked to wave size (wind) so you can get some of the most powerful waves combining can get some of the most powerful waves combining with neap tides.with neap tides.
Tides animation
• http://www.ioncmaste.ca/homepage/resources/web_resources/CSA_Astro/files/content/multimedia/unit3/tides/tides.swf
• http://www.onr.navy.mil/focus/ocean/motion/tides1.htm
Coastal Processes: Erosion, Transport and Deposition
• There are 3 main processes that cause a coastline to change:1. Erosion2. Transport3. Deposition.
• There are number of factors which affect each of these processes - we are going to start by exploring erosion processes and the factors that can affect the amount of erosion that may take place along a coastline.
COASTAL EROSION
• Erosion Processes:Erosion is the wearing away of rocks, at the coast there are 6 main types of erosion processes in action (see animations on the following website)
• http://www.bbc.co.uk/schools/gcsebitesize/geography/coastal/coastalprocessesrev4.shtml
ABRASION
1. ABRASION (this is also known as corrasion) - this is where rock fragments are hurled at cliffs by breaking waves, gradually scraping away at the cliff face;
HYDRAULIC ACTION
2. HYDRAULIC ACTION - as waves break against the cliff face, the pressure of the breaking wave can compress air in cracks. This compressed air gradually forces open the crack in the rock - as this process continues, the rock becomes increasingly weakened.
Wave approaches the cliff. Note cracks exaggerated In size
Wave reaches the cliff & the air trapped by the wave is compressed into the crack.
Wave rebounds from the cliff & the compressed air escapes explosively, enlarging the cracks & rippingbits of rock off.
HYDRAULIC ACTION
CORROSION
3. CORROSION (this is also known as solution) - this occurs where the salt water is able to dissolve some of the chemicals in rocks
SCOURING
• 4. SCOURING - this occurs at the base of the cliff as the waves break and swirl around, gradually removing loose rock.
ATTRITION
5. This is where rock fragments carried by the waves hit against each other and gradually wear down to form sand and silt
WAVE POUNDING
6. The sheer force of waves hitting against the cliff face
Video of headland erosion over a year
http://www.youtube.com/watch?v=ChEHQUMEkXw
Coastal Kung Fu
• http://blip.tv/scripts/flash/blipplayer.swf?autoStart=false&file=http://blip.tv/file/get/Adtastic2001-CoastalKungFu837.flv?source=3
These processes of erosion form a series of distinctive
landforms at the coast
Rates of Coastal Erosion
• So what are the factors that determine how much erosion can take place at the coast?
The Resistance of the Rocks
• - e.g. sandstone, limestone, chalk and granite are resistant rocks (often forming cliffs and headlands) and erode relatively slowly, whilst less resistant rocks such as clay and shale are easily eroded.
The Strength of the waves
• affected by the wind strength and duration and its fetch
The shape of the coastline
• (which is dependent on its geology) - on concordant coastlines, rocks are parallel to the wave front and therefore rates of erosion are similar along the coastline. On discordant coastlines, differential erosion may occur, where bands of hard and soft rock outcrop at right angles to the sea. Consequently headlands and bays form along discordant coastlines and whilst headlands remain exposed to the force of the waves, bays are sheltered.
COASTAL TRANSPORT
• The second process operating at the coast is transport. Material eroded by the sea is carried within the water in a number of ways, minerals dissolved from rocks are carried in solution, whilst small rock fragments, light enough to be held within the water, float in suspension.
COASTAL TRANSPORT
• The largest rock fragments which are too heavy to be picked up by the waves, are transported by the process of traction, this is where they roll along the bed when the waves pick up enough energy.
COASTAL TRANSPORT
• Finally, medium sized rock particles, which cannot be carried by the waves all the time, are moved by saltation. This is where during times of higher wave energy the particles are picked up and then dropped again as the wave looses its energy.
LONGSHORE DRIFT
• The main form of transport operating at the coast is that of LONGSHORE DRIFT.
Longshore drift is the process by which sand and pebbles are moved along a beach by the movement of the waves.
• Amounts of Longshore Drift depend on:
• The strength of the waves, size and amount of material available for the movement.
• Incline (slope) of the beach
COASTAL DEPOSITION
• Material is moved up the beach by the swash at an angle which is controlled by the prevailing wind. The backwash then carries material back down the beach at right angles to the coastline under the influence of gravity. Gradually the material is moved along the coastline, its direction being controlled by the prevailing wind direction.
COASTAL DEPOSITION
• Although some material eroded at the coast is washed out to sea, most of the material is transported along the coast by longshore drift. Deposition will occur when the waves are no longer able to transport material due to a loss of energy.
COASTAL DEPOSITION
• This is the case with constructive waves, where material is moved up the beach in the strong swash, but the weak backwash means material is deposited to build up the beach
COASTAL DEPOSITION
• Deposition commonly occurs:
• where the water is sheltered (e.g. a bay) and the waves lack energy where the coast is shallow and the increased friction between the water and the sea bed reduces the energy available for transport.
COASTAL DEPOSITION
• Deposition of material results in the formation of a number of distinctive features: beaches; spits; bars and tombolos.
COASTAL DEPOSITION• The final process operating at the coast is that of
deposition - this is where material that is too heavy to be transported any more is left behind, building up the beach. Due to the importance of energy in transporting sand and shingle, it is the largest material that is deposited first. A number of distinctive features may form due to coastal deposition.
• http://www.bbc.co.uk/schools/gcsebitesize/geography/coastal/coastalprocessesrev5.shtml
Key Terms Check:
• Erosion - the wearing away and removal of materialDeposition - the dropping of materialAbrasion - the wearing of rock due to rock fragments being hurled against cliffsAttrition - the breakdown of rocks as they hit against each otherHydraulic Action - the force of waves causing rocks to split apart as waves compress air in cracks in the rocksWave Pounding - sheer force of water hitting rocksSolution - where minerals in rocks are dissolved by the action of sea waterScouring - occurs where water and broken rock fragments swirl around at the base of cliffs gradually wearing rock away.Longshore Drift - the movement of material along a coastline
Coastal Erosion Landforms - Features and Formation
• Coastal Erosion Features
There are 3 main groups of coastal features which result from coastal erosion:1. Headlands and Bays2. Caves, Arches, Stacks and Stumps3. Cliffs and Wave-cut platforms
1. HEADLANDS AND BAYS
• Headlands are resistant outcrops of rock sticking out into the sea, whilst bays are indents in the coastline between two headlands.
So how do headlands form?
• - Headlands form along discordant coastlines in which bands of soft and hard rock outcrop at right angles to the coastline.- Due to the presence of soft and hard rock, differential erosion occurs, with the soft, less resistant rock (e.g. shale), eroding quicker than the hard, resistant rock (e.g. chalk)
So how do headlands form?
• - Where the erosion of the soft rock is rapid, bays are formed- Where there is more resistant rock, erosion is slower and the hard rock is left sticking out into the sea as a headland.- The exposed headland now becomes vulnerable to the force of destructive waves but shelters the adjacent bays from further erosion.
2. CAVES, ARCHES, STACKS and STUMPS
• Once a headland has formed it is then exposed to the full force of destructive waves and it gradually begins to erode. you need to be able to describe the erosion of a headland and the features that form.
So how does a headland erode and caves,
arches, stacks and stumps form?
• Firstly, the sea attacks the foot of the cliff and begins to erode areas of weakness such as joints and cracks, through processes of erosion such as hydraulic action, wave pounding, abrasion and solution;
• - Gradually these cracks get larger, developing into small caves;- Further erosion widens the cave and where the fault lines runs through the headland, two caves will eventually erode into the back of each other forming an arch, passing right through the headland.
• A combination of wave attack at the base of the arch, and weathering of the roof of the arch (by frost, wind and rain), weakens the structure until eventually the roof of the arch collapses inwards leaving a stack, a stack is a column of rock which stands separate from the rest of the headland.- The stack will continue to erode, eventually collapsing to form a stump which will be covered by water at high tide.
Natural Arch
• Eventually the cave deepens enough for it to pass through the headland, or it meets another cave coming the opposite direction.
• London Bridge, Australia
Stack
• Continual erosion of the arch causes the roof to become unstable & collapse
• The Twelve Apostles, Australia.
• How did the original headland shape become eroded to the present coastal landscape?
• A number of stages are involved.
• All rocks have lines of weakness. The sea and its waves use hydraulic action, abrasion, attrition and solution to erode along any lines of weakness. Undercutting takes place all around the headland.
• These lines of weakness get enlarged and develop into small sea caves.
• The caves are deepened and widened on both sides of the headland until eventually the sea cuts through the headland, forming an arch.
• The rock at the top of the arch becomes unsupported as the arch is enlarged, eventually collapsing to form a stack.
• The stack gets eroded until only a stump remains. • Over time the stump will disappear. • As the headland retreats under this erosion, the gently
sloping land at the foot of the retreating cliff is called a wave-cut platform.
Stages in coastal Development
• a) geological weakness (e.g. fault) forming a geo.
• b) formation of sea cave by marine erosion.c) enlargement of cave to form arch.
• d) collapse of arch to form stack.e) removal of stack to create a stump.
3. CLIFFS AND WAVE-CUT PLATFORMS
• Cliffs are steep rock faces along the coastline, they tend form along concordant coastlines with resistant rocks parallel to the coast.
So how do cliffs and wave-cut platforms form?
• The erosion of a cliff is greatest at its base where large waves break - here hydraulic action, scouring and wave pounding actively undercut the foot of the cliff forming an indent called a wave-cut notch whilst the cliff face is also affected by abrasion as rock fragments are hurled against the cliff by the breaking waves.
• This undercutting continues and eventually the overhanging cliff collapses downwards - this process continues and the cliff gradually retreats and becomes steeper.- As the cliff retreats, a gently-sloping rocky platform is left at the base, this is known as a wave-cut platform which is exposed at low tide.
Smartboard Interactive
• Coastal_erosion_features_summary.notebook
Coastal Deposition Landforms: Features and Formation
• Material that is transported by the waves along a coastline is eventually deposited forming distinctive deposition features. There are four main deposition features that you need to learn the formation of. These are:
• These are:
1. Beaches2. Spits3. Bars4. Tombolos
Depositional Landforms
Beaches
• A beach is defined as the gently sloping area of land between the high and low water marks. Remember, beaches are not permanent features as their shapes are altered by waves.
• Beaches are the main feature of deposition found at the coast, these consist of all the material (sand, shingle etc.) that has built up between the high and low tide mark.
• Beaches are made up of material lying between the high and low tide mark. There are four main sources of beach material.
• 1. Material deposited at the mouth of rivers • 2. Cliff erosion, provides rock fragments that will
build up the beach• 3. Constructive waves have a strong swash
pushing sands / pebbles up the beach• 4. Longshore drift carries material from
elsewhere along the coast
• As constructive waves build up beaches, they often form ridges in the beach known as berms. The berm highest up the beach represents the extent to which the water has reached during high tide.
SPITS
• Spits are long narrow ridges of sand and shingle which project from the coastline into the sea.
• How are Spits formed?
SPITS
• Longshore drift transports material along the coastline. Spits are formed in areas of relatively shallow and sheltered water where there is a change in the direction of the coastline. Deposition occurs resulting in the accumulation of sand and shingle.
SPITS
• The material initially deposited is the largest material, dropped due to the reduction in energy. Finer material is then deposited, helping to build up the rest of the spit. As the spit continues to grow outwards, a short term change in wind direction may result in a change in the direction of the spit forming a curved end (recurved laterals).
SPITS
• If growing across a river estuary, the length of the spit will be restricted by the river outlet washing sediment away. A salt marsh may form in the sheltered, low energy zone behind the spit.
BARS
• These form in the same way as a spit initially but bars are created where a spit grows across a bay, joining two headlands. Behind the bar, a lagoon is created, where water has been trapped and the lagoon may gradually be infilled as a salt marsh develops due to it being a low energy zone, which encourages deposition.
BARS
• Over time the lagoon will become infilled by deposition as either rivers flows in to it depositing material, or as waves break over the top of the Bar depositing coastal material
TOMBOLOS • What are tombolo's and how are they
created?• Tombolo's are ridges of sand and shingle which
join the mainland to an island. • Tombolo's are created through the process of
longshore drift. Where there is a change in the shape of the land, a spit forms in the shallow / sheltered water. A tombolo is formed where the spit continues to grow until it reaches an island, forming a link with the mainland.
Coastal Defence
• When managing the coastline there are two main options:1. HARD ENGINEERINGThis is where man made coastal defence structures are used to reflect large amounts of wave energy and hence protect the coastline.
SOFT ENGINEERING
2.This is where beaches or naturally formed materials are used to control / re-direct erosion processes. Eg a sand dune.
1. Re-Curved Sea Wall
Re-Curved Sea Wall
• - concrete wall which is curved on the underside to deflect the power of the waves
• these can be very expensive and the deflected waves can scour material at the base of the wall causing them to become undermined
these are however a very effective means of preventing erosion and they reflect rather than absorb wave energy.
Rip Rap
• large boulders on the beach absorb wave energy and break the power of the waves- although movement of the boulders is expensive this can be a much cheaper method than some other solutions- the boulders can however be undermined easily by waves washing away sand and shingle beneath them. They also can be quite ugly, changing the appearance of a coastline.
3. Groynes
• these structures (usually either wooden or steel) are designed to top longshore drift and therefore act to build up and anchor beach material, protecting the base of cliffs.
Groynes
• they are effective at reducing erosion in the area they are constructed in by causing significant build up of beach material- groynes may however starve areas further down the coast of material by stopping longshore drift, resulting in an increase in erosion in these areas
4. Gabions
Gabions
• these cages of boulders are built into cliff faces to protect the cliff from the force of the waves;- they are cheaper than sea walls and can be very effective where severe erosion is a problem- they are however visually intrusive
5. Revetments
Revetments
• these wooden structures break the force of waves and beach material builds up behind them- they are cheap and effective at breaking waves- as well as being visually intrusive however they do need replacing more frequently than most other defence methods.
Soft Engineering Techniques
Soft Engineering Techniques
• Soft engineering includes beach replenishment in which beach material is added to provide a "natural solution". Environmentally this is a preferred option as it maintains the beauty of the landscape and avoids visual intrusion, however it can be expensive to maintain as longshore drift continues to move beach material down the coast and therefore regular replenishment is required.
• Sand Dunes and salt marshes can also be encouraged to act as natural barriers to the waves.
COASTAL MANAGEMENT
Groynes
Gabion Cages
Gabion Cages
Rip Rap
Sea Wall
Sea Wall
Sea Wall
Revetment
Beach Feeding
