Coastal Systems and Landscapes

Systems in Physical Geography

Coasts are open systems as have inputs, components/stores + outputs into the surrounding environment

Inputs - when matter or energy added to system e.g. sediment + energy

Outputs - when matter or energy leaves system e.g. sediment washed out to sea

Flows/tranfers - when matter or energy moves from one store to another within system e.g. sediment erosion + deposition

Stores/components - when matter or energy builds up e.g. beaches/dunes

Coasts in dynamic equilibrium with balance between inputs + outputs

System adjusts through feedback

Positive feedback - progressively greater change away from dynamic equilibrium as change amplified e.g. beach forming slows down waves so more sediment deposited + beach grows

Negative feedback - system returning closer to dynamic equilibrium as change counteracted e.g. eroding beach exposes cliffs which are eroded and deposit sediment to cause beach to grow

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Wind

Wind energy - the primary source of energy for other processes

Wind created by air moving from areas of high pressure to areas of low pressure (therefore higher in storms)

Spatial variation in energy as a result of varying wind strength + duration

The energy of a wave is dependant on wind strength, wind duration + length of fetch

Wind creates waves due to frictional drag

Length of fetch determines size + energy of waves

Many coastlines have prevailing wind direction which determines direction waves approach coast

Fetch - distance of open water which blows uninterupted

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Waves

Wave height - difference between the crest and trough of a wave + determined by wind speed + fetch

Wavelength - difference between crests

Frequency - time lapse between crests

Water moves in circular orbit

Formation - when wind blows over surface friction between wind and surface of sea gives water a circular motion

As waves approach shore they break, friction with sea bed increases, wave slows, wave increases in height + crest of waves rises up and collapses

Swash - water washing up beach

Backwash - water washing back towards sea

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Wave Refraction

Involves waves breaking onto an irregularly shaped coastline

Waves drag in the shallow water approaching a headland so the wave becomes high, steep and short

The part of the wave in the deeper water moves forward faster causing the wave to bend

The low-energy wave spills into the bays as most of the wave energy is concentrated on the headland

e.g. a headland separated by two bays

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Types of Waves

Contructive waves:

  • low
  • long length (up to 100m)
  • low frequency (6/8 per minute) 
  • high swash, low backwash
  • material deposited up beach
  • forms berms

Destructive waves:

  • high
  • steep
  • high frequency (10/14 per minute)
  • low swash, high backwash
  • forms storm beaches
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Currents

Permanent or seasonal movement of water in the seas + oceans

Longshore currents:

  • most waves approach shoreline at an angle
  • creates current running parallel to shore line
  • effect - transports sediment parallel on the shoreline

Rip currents:

  • strong currents moving away from the shoreline
  • due to build-up of seawater and energy along coastline
  • effect - hazardous for swimmers

Upwelling currents:

  • global patterns of currents circulating in the oceans + thermohaline circulation causes deep, cold water to move towards surface, displacing warmer surface water
  • effect - cold current rich in nutrients
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Tides

Periodic rise + fall of ocean surface, caused by gravitational pull of moon + sun

Affect the position waves break on beach

Area of land between maximum high tide + minimum low tide is where most landforms created + destroyed

Tidal range - vertical difference in height of sea level between high + low tide

Spring tide - a tide just after a new or full moon, when there is the greatest difference between high and low water

Neap tide - a tide just after the first or third quarters of the moon when there is least difference between high and low water

Tidal/storm surges - pushing of water against a coastline to abnormally high levels, usually a combination of extreme low pressure + high tides

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Energy of Coasts

High-energy coasts:

  • high inputs of energy from large, powerful waves
  • caused by strong winds, long fetches + steeply shelving offshore zones
  • have sandy coves + rocky landforms
  • rate of erosion higher than rate of deposition
  • e.g. cliffs, caves, stacks + arches

Low-energy coasts:

  • low inputs of energy in small waves
  • caused by gentle winds, short fetches + gently sloping offshore zones
  • some have reefs or offshore islands
  • rate of deposition higher than rate of erosion
  • e.g. saltmarshes + tidal mudflats
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Sediment Sources

Seabed - rising sea levels over last 18,000 years have meant sediment from continental shelf areas has been swept towards the shoreline

Rivers - account for 90% of coastal sediment, a combination of bedload shingle + suspended silt + clay

Cliff erosion - sediment from erosion contributes only 5% or less to coastal systems

Biological origin - e.g. shells + corals

Offshore deposits - waves, tides + currents transport sediment into coastal zone from offshore e.g. sandbanks

Sediment budget - the difference between the amount of sediment that enters the system and the amount that leaves so coastline builds if positive + vice-versa

Sediment cells:

  • coasts divided into sediment cells
  • lengths of coastline between headlands
  • self contained (closed system which don't affect other cells)
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Coastal Erosion Processes

Corrasion (abrasion) - bits of rock + sediment transported by waves smash + grind against rocks + cliffs, breaking bits off and smoothing surfaces

Hydraulic action - air in cracks in cliffs compressed when waves crash in + pressure exerted by compressed air breaks of rock pieces

Cavitation - as waves recede, the compressed air violently expands, exerting pressure on the rock and causing pieces to break off

Wave quarrying - the energy of a wave as it breaks against a cliff is enough to detach bits of rock

Solution (corrosion) - soluble rocks e.g. limestone + chalk get gradually dissolved by seawater

Attrition - bits of rock in water smash against each other + break into smaller bits

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Transportation

The process of eroded material being moved

Solution - substances that can dissolve are carried along in the water e.g. limestone is dissolved into water that's slightly acidic

Suspension - very fine material such as silt + clay particles are whipped up by turbulence + carried along in water (most common)

Saltation - large particles e.g. pebbles or gravel are too heavy to be carried in suspension so force of water causes them to bounce along sea bed

Traction - very large particles e.g. boulders pushed along sea bed by water

Longshore drift:

  • swash carries sediment e.g. shingles + pebbles up the beach, parallel to the prevailing wind
  • backwash carries sediment back down the beach at right angles to shoreline
  • when angle between prevailing wind + shoreline, a few rounds of swash + backwash moves sediment along shoreline
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Deposition

When material being transported is dropped on the coast

Marine deposition - sediment carried by seawater deposited

Aeolian deposition - when sediment carried by wind deposited

Occurs when sediment load exceeds ability of water or wind to carry it either because sediment load increases e.g. from landslide or wind slows down as less energy

Loss of energy from:

  • increased friction - if waves enter shallow water or wind reaches land, friction between water/wind + ground surface increases which slows down water or wind
  • turbulent flow - if water or wind encounters object e.g. area of vegetation, flow becomes rougher + overall speed decreases 

If wind drops, wave height, speed + energy will decrease

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Sub-Aerial Weathering

Physical weathering - non-chemical/biological break up of rock

Biological weathering - living organisms activity

Chemical weathering:

  • breakdown of rocks by changing its chemical composition
  • carbonation - when carbon dioxide in atmosphere dissolves in rainwater forming weak carbonic acid, reacting with rock containing calcium carbonate e.g. carboniferous limestone
  • oxidation - when rocks containing iron are exposed to oxygen in air + water they react to form iron oxide which is weak + crumbles
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Sub-Aerial Processes

Weakens cliff + makes them more vulnerable to erosion 

Salt weathering:

  • caused by saline water entering pores or cracks in rocks at high tide
  • as tide goes out rocks dry + water evaporates, forming salt crystals
  • as crystals form they expand + exert pressure on rocks

Freeze-thaw weathering:

  • occurs where temperatures fluctuate above and below freezing
  • enters joints + crevices in rocks
  • if temperature drops below 0° the water in cracks freezes + expands, exerting pressure

Wetting + drying - when clay in rocks gets wet, it expands + breaks off fragments

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Mass Movement

When material moves down a slope + more likely to occur when cliffs undercut by wave action

Unconsolidated rocks e.g. clay more prone to collapse as little friction between particles to hold them together

Heavy rain can saturate unconsolidated rock to further reduce friction

Runoff - (flow of water over land) - erode fine particles e.g. sand + silt + transports them downhill

Landslides - cliffs made of softer rock slip when lubricated by rainfall + remaining in tact

Rockfalls - rocks undercut by sea or slopes affected by mechanical weathering

Mudflows - heavy rain causes fine material to move downhill over weak material such as clay

Rotational slip/slumping - where soft material overlies resistant material + excessive lubrication takes place so slip plane forms

Soil creep - very slow movement of soil particles down slope

Run-off - the movement of water across the hard surface, carrying debris

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Coastal Landscape Development

Coastal landscapes reflect the interaction of a range of factors + processes

Geology:

  • coastal configuration - headlands attract energy due to wave refraction
  • rock characteristics (lithology) - resistant rocks such chalk erode slower than weak rocks such as clay 
  • structure - cracks + fissures exploited
  • coastline type - discordant (lines at right angles to coast) + concordant (lines running parallel to coast)

Climate - temperature ranges lead to more freeze-thaw weathering + wet climates lead to slope failure

Nature of tides + waves - wave steepness, length of fetch and sea depth

High + low energy coasts

Human activity + coastal management

Headlands + bays - form where there are bands of alternating hard + soft rok at right angles to shoreline(discordant coastline) so soft rock eroded quickly, forming a bay whilst the harder rock is eroded less and sticks out as a headland

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Cliffs + Wave-Cut Platforms

1) Cliffs form as sea erodes land + retreat over time due to weathering

2) Weathering + wave erosion causes notch to form at high water mark + this eventually develops into a cave

3) Rock above cave becomes unstable with nothing to support it and collapses

4) Wave cut surfaces are left below

5) Wave cut platform is cut into by abrasion + eroded by hydraulic + chemical action

6) Over time waves break further out to sea, wave energy dissipated + erosion is reduced

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Caves, Arches + Stacks

1) Weaknesses such as a joint or fault in a resistant rock e.g. chalk are attacked by waves

2) Erosion (hydraulic action, abrasion, wave quarrying) widens the weakness and undercuts the base to form a cave

3) Erosion processes concetrate on the headland, often a cave meets another cave and a hole through the headland is opened up to form an arch

4) As the cliff face recedes, a wave cut platform develops

5) The arch will eventaully collapse, leaving an isolated stack

6) The sea attacks the base of the stack and eventually a wave-cut notch will undercut the stack and a raised portion of the wave cut platform will be left as a stump

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Beaches

From when constructive waves deposit sediment on the shore (store)

Beach types:

  • shingle beaches - steep + narrow, made up of larger particles, which pile up at steep angles
  • sand beaches - formed from smaller particles + wide + flat

Beach features:

  • berms - ridges of sand + pebbles (1-2 metres high) found at high tide marks
  • runnels - grooves in sand running parallel to shore formed by backwash returning to the sea
  • cusps - crescent-shaped indentations the form on beaches of mixed sand + shingle

Swash-alligned beach - waves break parallel to coast

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Spits

Form when coastline suddenly changes direction e.g. across river mouths

Longshore drift continues to deposit material across the river mouth, leaving a bank of sand and shingle sticking out into the sea

Simple spit - straight spit growing out roughly parallel to coast 

Occasional changes to dominant wind + wave direction may lead to spit having recurved end

Over time, several recurved ends may be abandoned as waves return to original direction 

Compound spit - a spit with multiple recurved ends

Area behind spit is sheltered form waves + often develops into mudflats + saltmarshes

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Offshore Bars + Tombolos

Bars are formed when a spit joins two headland bays together across either bay or river mouth

Lagoon forms behind a bar

Offshore bars - bars can form off the coast when material moves towards the coast (normally due to sea level rises)

A bar that connects the shore to an island (often a stack) is called a tombolo e.g. St Ninians Isle in Sheltands is joined to larger island by a tombolo

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Barrier Islands/Beaches

Long, narrow islands of sand or gravel that runs parallel to the shore and are detatched from it

From in areas where there's a good supply of sediment, gentle slope offshore, fairly powerful waves + small tidal range

Unclear how form but scientists believe after last ice age when ice melt caused rapid sea level rise, the rising water flooded the land behind beached + transported sand offshore where it was deposited in shallow water

Another theory is that islands were originally bars attatched to the coast which were eroded in sections

A lagoon or marsh often forms behind barrier island where coast is sheltered from wave action

e.g. Horn Island off Mississippi

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Sand Dunes

Formed when sand deposited by longshore drift is moved up beaches by wind

Sand trapped by driftwood or berms is colonised by plants + grasses e.g. marram grass

Vegitation stabilises the sand + encourages more sand to accumulate, forming embryo dunes

Over time, oldest dunes migrate inland as newer embryo dunes are formed

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Estuarine Mudflats + Saltmarshes

Form in sheltered, low-energy environments e.g. river estuaries or behind spits

Mudflats develop as silt + mud are deposited by river or tide

Colonised by vegetation that survive the high salinity (halophytes) + long periods of submergence by the tide (pioneer species)

Plants trap more mud + silt + gradually build upwards to create an area of saltmarsh that remains exposed for longer and longer between tides (climax community)

Flocculation - clay particles join together and sinks to bottom of bed

Erosion by tidal currents or streams forms channels in surface of mudflats + saltmarshes which may be permanently flooded or dry at low tide

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Eustatic Sea Level Change

Sea level change caused by a change in the volume of water in the sea, or the change in the shape of the ocean basins

Has global effects

Causes:

  • changes in climate:
    • increase in temperature - causes melting of ice sheets which increases sea level + causes water to expand
    • decrease in temperature - causes more precipitation to form as snow which increases volume of water stored in glaciers and so reduces volume of sea, decreasing sea level
  • tectonic movements of earths crust that alter the shape of ocean basins e.g. sea floor spreading increases volume of basin + decreases sea level
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Isostatic Sea Level Change

Caused by vertical movements of the earth relative to the sea

Has local effects

Any downward movement of the land causes sea level to rise locally, while uplift of land causes sea level to fall

Effects:

  • isostatic rebound - uplift of earths crust due to accumulation of or melting of ice sheets that can continue for thousands of years after the weight of a retreating glacier has gone
  • depression of earths crust occurs from accumulation of sediment e.g. mouths of major rivers
  • subsistance of land due to shrinkage after absraction of groundwater e.g. drainage of marshland
  • tectonic processes e.g. one plate forced between another at a plate margin
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Sea Level Change Over Last 10,000 Years

Sea level varies on a daily basis with the tidal cycle

Onshore winds + low atmospheric pressure systems also cause sea surface to rise temporarily

During last glacial period (from roughly 110,00 to 12,000 years ago) water was stored in ice sheets so sea level was lower that present

At last glacial maximum (21,000 years ago) sea level was about 130m lower than present

As temperature started to increase (about 12,000 years ago) ice sheets melted + sea level rose rapidly

Present level reached about 4000 years ago

Over last 4000 years, sea level has fluctuated around present value

Since 1930, sea level has been rising

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Climate Change Impact on Sea Level

Over last century, global temperature has increased rapidly from + this is global warming (1.08° rise between 1990 + 2016)

Consensus amongst scientists that fast changes in climate over last century as a result of human activities such as deforestation + burning fossil fuels

These activities increase concentration of greenhouse gases in the atmosphere which absorb outgoing long-wave radiation so less is lost to space + more is trapped 

Increases in temperature are likely to cause increases in sea level through melting of ice sheets + thermal expansion

Global sea level is currently rising at almost 2mm each year

If greenhouse gas emissions remain very high during 21st century, sea level likely to increase to between 8 + 16 mm a year by 2100

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Climate Change Impacts on Coastal Areas

Storms likely to become more frequent + more intense due to changes in ocean circulation + wind patterns, causing damage to coastal ecosystems + settlements

If sea level rise continues as predicted, it will have major impacts on coastal areas:

  • more frequent + more severe coastal flooding, especially of low lying areas e.g. Kings Point in New York State flooded around 80 times from 1995-2004 but 160 times from 2005-2014
  • submergence of low lying islands e.g. if sea levels rises by 0.5m from its current level then most of Maldives will be submerged
  • changes in coastline - as sea levels rise the coastline changes - islands created + area of land decreased e.g. if sea levels rise by 0.3m from current level then 8000 km² of land in Bangladesh will be lost
  • contamination of water sources + farmland - salt may enter bodies of fresh water e.g. lakes + rivers near the coast, damaging ecosystems + making water unsustainable for use whilst salt water entering soils may damage crops + make land impossible to farm

Sea level rise + increased storminess will increase coastal erosion, putting ecosystems, homes + businesses at risk

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Sea Level Fall Impacts

Results in coastlines of emergence

Raised beaches - formed when fall in sea level leaves beaches above the high tide mark + over time beach sediment becomes vegetated and develops into soil

Wave-cut patforms - exposed after sea level fall

Relict cliffs - when cliffs above raised beaches no longer eroded by sea get covered in vegetation + often see wave-cut notches, caves, arches + stacks

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Sea Level Rise Impacts

Sea level rise results in coastlines of submergence

Rias:

  • formed when river valleys partially submerged
  • have gentle long + cross profile, wide + deep at mouth, becoming + shallower further inland they reach
  • e.g. Milford Haven in Wales

Fjords:

  • like rias but drowned glacial valleys not river valleys
  • straight + narrow with steep sides + very deep firther inland
  • shallow mouth caused by raised bit of ground (threshold) formed by deposition of material by glacier
  • e.g. Sognefjorden in Norway over 1000m deep in places

Dalmation Coastlines:

  • in areas where valleys lie parallel to the sea, an increase in sea level can cause dalmation coastline
  • valleys are flooded, leaving islands parallel to the coastline
  • e.g. Dalmation Coast in Croatia
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Processes Altering Landforms + Landscapes

Individual landforms e.g. spits + arches combine to form landscapes, generally erosional + depositioal processes

Processes operating in coastal systems can create new landforms or change existing landforms e.g. relict landforms still weathered by salt + freeze-thaw

Coastal landscapes are therefore often made up of a mixture of active + relict landforms that reflect different periods of change e.g. beach being formed may be backed up by a relict cliff from earlier time of higher sea level

Changes occur over a wide range of spatial scales + temporal scales e.g. changes can vary from short + sporadic (e.g. storms that last for a few hours) to long + gradual (e.g. tectonic uplift over thousands of years)

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Coastal Management

Aim of coastal mangement is to protect homes, businesses + environment from erosion + flooding

This is because flooding + erosion of the coastline can have severe social, economic + environmental impacts

Limited amount of money avaliable so not all coastal areas protected

Choosing which places are defended is done on cost-benefit analysis so usually used to protect large settlements + important industrial sites rather than small settlements

Four options for coastal management:

  • hold the line - maintain existing coastal defences
  • advance the line - build new coastal defences further out to sea than existing line of defence
  • do nothing - build no coastal defences at all + deal with erosion + flooding as it happens
  • managed realignment - allow shoreline to move, but manage retreat so it causes least damage e.g. flooding farmland rather than towns
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Hard Engineering

Sea wall - wall reflects waves back out to sea, preventing erosion of the coast + prevents flooding. Expensive to build + maintain + creates strong backwash which erodes under wall

Revetment - slanted structures built at foot of cliff made out of concrete, wood or rocks to absorb wave energy. Expensive to build but cheap to maintain + creates strong backwash

Gabions - rock filled cages built at foot of cliffs to absorb wave energy. Cheap but ugly

Riprap - boulders piled up along coast to absorb energy. Fairly cheap but can shift in storms

Groynes - fences built at right angles to coast that trap beach material transported by longshore drift, creating wider beaches which slow waves + gives greater protection from flooding + erosion. Quite cheap but starve down-drift beaches of sand so greater erosion + flooding

Breakwaters - concrete blocks or boulders deposited off the coast forcing waves offshore to reduce energy. Expensive + can be damaged in storms

Earth bank - mounds of earth act as a barrier to prevent flooding. Quite expensive + can be eroded

Tidal barrier - rectractable floodgates across river estuaries built to prevent flooding from storm sturges. Very, very expensive

Tidal barrage - dams built across river estuaries to trap water behind dam at high tide + controlled release of water through turbines generates electricity. Very expensive + increased erosion elsewhere in estuary

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Soft Engineering

Beach nourishment - sand + shingle are added to beaches from elsewhere e.g. dredged from elsewhere to create wide beaches + reduce erosion

Beach stabilisation - done by reducing slope angle + planting vegetation or putting stakes + old tree trunks to stabilise to the sand which creates wide beaches + reduces erosion of cliffs

Dune regeneration - sand dunes created or restored by either nourishment or stabilisation of the sand + dunes provide a barrier between land + sea absorbing wave energy

Land use management - vegitation needed to stabilise the dune can be trampled + destroyed so wooden walkways + fenced off areas reduce vegitation loss

Creating marshland - from mudflats by planting appropriate vegitation e.g. glassworts that stabilises the sediment + reduces wave energy

Coastal realignment - (managed retreat) involves breaching an exiting defence + allowing sea to flood the land behind so vegetation will colonise the land and it'll become marshland

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Sustainable Coastal Management

Hard engineering expensive + can disrupt natural processes

Soft engineering cheaper + require less time + money to maintain than hard engineering schemes

Soft engineering designed to integrate with the natural environment + creates marshland + sand dunes

Soft engineering more sustainable than hard engineering as lower environmental impact + economic cost

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Sustainable Approaches to Coastal Management

Shoreline management plans:

  • coastline split into stretches by sediment cells
  • for each cell a plan is devised for how to manage different area with aim of protecting important sites without causing problems elsewhere in sediment cells
  • for each area within cells, authorities decide to hold, advance or retreat line or do nothing
  • overall plan for sediment cell is called SMP

Integrated coastal zone management:

  • considers all elements of coastal system when coming up with coastal management strategy
  • aims to protect coastal zone in relatively natural state, whilst allowing people to use it and develop it in different ways
  • environment viewed as a whole - land + water interdependent
  • different uses considered e.g. fishing, industry + tourism
  • local, regional + national levels of authority
  • dynamic strategies - decisions re-evaluated if environment or demands of area change
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