Coasts

Coasts as natural systems

inputs

• precipitation

• river discharge

• currents

• wind

• sediment (originate from outside the system)

Coasts as natural systems

open or closed system?

open

Coasts as natural systems

processes

• LSD

• erosion

• weathering

• mass - movement

• deposition

Coasts as natural systems

outputs

• currents

• evaporation

• land-forms (sediment transported outside of local sediment cells)

Coasts as natural systems

stores

• beaches

• landforms

• oceans

• sediment

Coasts as systems

negative feedback

sub aerial erosion and mass movement in storms can cause cliff collapse and build up of material at the cliff base - protects cliff base from further coastal erosion

Coasts as systems

what is negative feedback?

A process where initial changes trigger responses that counteract those changes, stabilising the system.

Coasts as natural systems

positive feedback

erosion of cliff base by waves releases material, the material further amplifies cliff base erosion through abrasion

Coasts as natural systems

what is positive feedback?

A type of environmental system where changes in one aspect lead to further changes that enhance or amplify the effects, often leading to more erosion or disruption.

Coasts as natural systems

dynamic equilibrium

rate of sediment erosion = rate of sediment deposition

Sources of energy to coastal environments

- waves

Water waves are transverse, transfer kinetic energy not matter, follow an orbital movement

- waves start at sea in a circular orbit blown by prevailing wind

- slow down as they reach shore due to friction with rising floor

- slowing of wave causes a change to an elliptical orbit, top of wave breaks over

- wave swashes up beach, water returns to sea as backwash

Sources of energy to coastal environments

- factors influencing wave energy

Depends on:

- wave speed

- how long the wind has been blowing (wind duration)

- fetch (distance over sea wind has travelled)

Sources of energy to coastal environments

- tides + currents

Tides =

- change in water level of seas/oceans caused by gravitational pull of the moon

spring tide = highest monthly tide, moon sun and earth aligned

neap tide = lowest monthly tide, moon and sun perpendicular

Currents =

- large scale movement of water in oceans, key mechanism in transport of sediment

- distribute global heat

rip currents - powerful underwater currents close to beach - energy source - ouput of sediment

surface currents - wind driven

deep currents - density driven

Sources of energy to coastal environments

- constructive vs destructive waves

Constructive =

- lower wave freq + lower wave height, longer wavelength

- strong swash carries material up onto the shore

- weak backwash therefore little erosion

- form wide sloping beaches

- longer wave period - more time taken for one wave cycle

Destructive =

- higher wave freq, higher wave height, shorter wave length

- weak swash/ little deposition

- strong backwash erodes material on beach

- form steep beach profiles

- shorter wave period

Geomorphical processes

Wave refraction

As waves approach the headland they slow down and increase in height due to friction with sea bed, refraction causes concentration of energy on the headland, results in increased erosion at headland and low energy waves create beaches either side

Sediment sources

• estuaries

• erosion

• offshore sand banks, rivers

Sediment cell

• closed system usually bounded by headlands

• contains processes of erosion, transportation, deposition

• only sediment inputs are from land/sea bed erosion

• sediment movement very limited between cells

• dynamic equilibrium flows of sediment but can be affected by natural variation/ human intervention (rock groynes)

Sediment budgets

= balance between inputs/outputs of sediment into a cell

• rate of erosion and rate of accretion (sediment storage in sinks)

Thermohaline circulation

= deep-global ocean currents driven by differences in water density

• in polar regions cold surface water becomes saltier due to formation of sea ice, water becomes denser and sinks

• surface water is pumped to ocean depths, causes horizontal movement of deep ocean currents

• water rises back to surface closing the cycle at the equator

Geomorphical process

Transportation

= longshore drift - movement of sediment along the coast in direction of prevailing wind , swash 45, backwash 90, zigzag

• traction - boulders pushed along seabed by water force

• saltation - bouncing of pebbles on sea floor

• suspension - fine particles suspended in water

• solution - dissolved soluble materials

Geomorphical processes

Deposition

= reduced flow of energy moving material causes deposition - due to change in shape of coast, loss of sea energy, less wind, shallower floor creating friction, meeting a river

Geomorphical processes

Sub-areal weathering

action of rainwater and sun upon land-forms in the coastal landscape causing disintegration of rock in situ

Geomorphical processes

Physical/mechanical

= freeze thaw - water enters cracks, freezes, expands

= crystallisation - salt crystals grow in cracks of rock full of sea water - growth pressures rock joints

= wetting/drying - expansion and contraction of rocks as they wet and dry causes cracks

Geomorphical processes

Chemical weathering

= oxidation - rusting of rocks with iron compounds - disintegrations

= carbonation - dissolving of chalk/limestone coasts in acidic rain/seawater - forms Karst landscapes

- increased CO2 conc in atmosphere, rainwater contains more carbonic acid - accelerates erosion

Geomorphical processes

Biological weathering

= expansion of plant roots on cliffs creating fissures

= surface animals nesting in cliffs - create burrows

Geomorphical processes

Erosion

= breakdown and subsequent removal/transportation of material

Geomorphical processes

Erosion = hydraulic action

• high energy waves exert pressure on rock surface as air forced into cracks, breaks off rock particles

Geomorphical processes

Erosion = abrasion

• high energy waves hurl sediment at cliffs, scratches away at the surface

Geomorphical processes

Erosion = corrasion

• eroded sediment dragged on sea floor - mechanical erosion of sea floor surface smooths it

Geomorphical processes

Erosion = attrition

• pebbles round by crashing into each other

Geomorphical processes

Erosion = solution

• dissolving of chalks/limestone

Geomorphical processes

Erosion = quarrying

• destructive waves in storms directly hit cliff face

• smaller weathered fragments removed

Geomorphical processes

Mass movement

= movement of consolidated and unconsolidated material due to gravity

Geomorphical processes

Mass movement = rock falls

• sudden collapse/breakaway of individual rock fragments

• mech.weathering

• build up of scree at cliff base

Geomorphical processes

Mass movement = landslides

• block of rock moves downhill along a slide plane, heavy rainfall that lubricates slip plane

Geomorphical processes

Mass movement = run off

• overland flow of precipitation over cliff face dislodges small rock particles, can contaminate coast system if contains toxic chemicals

• erodes cliff face

Geomorphical processes

Mass movement = slumping

• permeable rocks on top of impermeable rocks builds up weight due to water infiltration, weak unconsolidated clay slip down over rock, sharp break of slope

Coastal Landscape development

- concordant + discordant coastlines and coastlines and erosion

concordant = bands of rock type run parallel to coast

discordant bands of varying rock type run at a right angle to the coast - differing levels of erosion resistance causes formation of headlands and bays

Coastal Landscape development

- high + low energy coastlines and erosion

Low Energy coastline =
- sheltered areas from winds, low energy constructive waves
- rates of deposition exceed erosion
- formation of depositional landforms: beaches, spits

High Energy coastline =
- exposed to strong prevailing winds - generate high energy destructive waves
- greater rates of erosion that deposition
- formation of erosional landforms : headlands, cliffs

Coastal Landscape developments

- headlands and bays

• softer sections of discordant coastlines are less resistant, erode faster, resistant rock remains protruding as headland

• wave energy concentrated on headlands due to wave refraction, bays either side are low energy - deposition builds beaches

Coastal Landscape developments

- headland features

example: Durdle Door, Dorset, headland of highly resistant limestone
notch = small dent in cliff face from erosional processes
cave = higher energy waves attack notch, widened, cliff face undercut into a cave
blowhole = vertical erosion in cliff, vertical shaft of sky to sea, expels sea spray in storms
arch = 2 caves either side of headland meet in the middle
stack = tall pillar isolated from headland, arch collapses due to sub aerial weathering/gravity
stump = wave cut notch/ undercutting causes collapse of stack, small portion left behind

Types of cliffs

gentle cliffs

- weaker rock less resistant to erosion and more prone to slumping

- low energy environments

- large beaches reduce wave energy, preventing cliff formation via erosion

steep cliffs

- strong rock erosion resistant

- high energy

Wave cut platforms

- top of the cliff weakened through sub- aerial weathering (freeze thaw/biological)

- bottom of cliff experiences concentrated erosion energy (hydraulic action, abrasion), cliff is undercut, wave cut notch formed

- notch increases, stability of cliff decreases, cliff collapses

- wave backwash carries away eroded material, leaves a wave-cut platform at cliff base, cliff face retreats

Marine platform

- gently sloping surface at base of a cliff eroded over time

- exposed at low tide

- indicate eroding/retreating coastline

Coastal Landscape development = beaches

- depositional landform found in low energy coastlines

- beach profile affected by sediment type - backshore sediment has a larger calibre, supports steep slopes and wave type

- 3 sections: backshore, foreshore, nearshore

- material comes from rivers, cliff erosion, offshore

- storm beach: material ridge deposited at the back of beach due to strong swash in storm condition

swash aligned beach - parallel to incoming wave crests, little long shore drift

drift aligned beach - parallel to direction of longshore drift, high rates of sediment transport

Coastal Landscape development = spits/tombolos/bars

Spits =

- elongated narrow ridge of land, joined to mainland at one end but projects out to sea

- material transported in direction of prevailing wind by longshore drift

- when coastline changes shape/estuary is met a loss of energy causes deposition of material carried by waves

- material builds up to form a spit projecting off coastline

- refraction at spit end causes it to curve in towards the coastline (recurved end) - hooked end

• sheltered area behind spit, low energy waves, deposit fine silts/clays, deposits build up + vegetation growths - creates salt marshes

- tombolo = spit that joins an island to the mainland

- bar = spit that stretches across a bay - creates a lagoon behind

compound spit - multiple curved ends

- formed through repeated spit formation and re curvature

Coastal Landscape Development = offshore bar/barrier beach

Offshore bar =

sand deposited as waves dont have enough energy to carry sediment to shore

- absorb wave energy

- waves may pick up sediment from bar - sediment input to coastal zone

- formed by backwash removal of beach sediment

Barrier beach = elongated bank of deposited sand parallel to coastline, not submerged by tides,

- LSD transports sediment along coastline in prevailing wind direction

- coastline shape changes, deposition forms a spit

- spit develops across bay - barrier beach formed

- colonisation of vegetation on barrier beach traps sediment and stabilises barrier beach

- sheltered area between barrier beach and land = lagoon/coastal marsh

barrier island = barrier beach separated from mainland

Coastal Landscape Development = sand dunes

= accumulation of dome shapes of sand, dynamic landform

- sand trapped by obstacles at back of beach

- embryo dunes develop, colonised by vegetation - stabilises surface - roots of marram grass bind dunes

- psammosere - sand dune succession: ecological succession on newly formed sand dunes - eventually reach

- yellow - grey dunes : sand turns to soils due to breakdown of organic matter increasing nutrient content of hummus - wider variety of plant life supported

- dunes become fixed over time, organic layer develops with new vegetation types

- gradual sloping windward side, sediment blown up by prevailing wind, steep sheltered back side over the crest

Sand dune management =

dune stabilisation = planting marram grasses, roots bind together sand to support dune

dune fencing = keeps animals and humans out to protect dune environment

dune thatching = creates a barrier against low winds to protect fore dunes

boardwalks = encourage specific paths, protect ecosystems from major disturbance

system:

inputs - sand supply, strong onshore winds, large tidal range, obstacle to trap sand, vegetation growth

Mudflats

- found on sheltered shorelines with no powerful waves, low lying areas made of clay/silt submerged at high tide

- develop in estuaries with slow flow of fresh river water

- slow river and slow sea meet and fine sediments are deposited, heavier clays aggregate together and sink to bed

- flocculation = particles suspended clump together and settle - deposition

Salt marshes

- develop in sheltered estuaries behind spits

- silt deposited by river is deposited forming inter-tidal mud flats

- vegetation grows up, initially low lying vegetation colonises, followed by pioneers, plants that can tolerate salt - salt grasses/ cord grasses

- roots of pioneers bind together mud and deposits, vegetation closes over mud, allows marsh grass/ sea lavender growth

- salt marsh colonised by halophytic vegetation (adapted to high salinity)

- complex creek systems develop, channel tides, land slows rises above sea level, more complex vegetation and trees establish

- land rarely covered by sea anymore

Sea level change = eustatic

Sea level change = isostatic

= local changes in sea level as a result of land falling/rising relative to sea

- isostatic readjustment - movement of land areas back to their previous levels following the melt of ice from last glacial period 10,000 years ago -

- causes emergent/submergent coastal land forms

- southeast of British Isles sinking (land lowered by 1mm/ year ) whilst northwest rises: ice sheets were thickest in Northern Scotland (land rising by 1.5mm/ year)

Sea level change = tectonic

= sea level change caused by tectonic activity at plate boundaries

- dramatic/immediate changes to land relative to sea level

- sudden plate movements from earthquakes can cause a rise/fall of the seabed - changes sea level

Emergent and Submergent coastlines - emergent

Emergent and Submergent coastlines

Submergent = exhibits features due to rising sea levels/ falling land

Emergent = exhibits features due to falling sea levels/rising land

- raised beaches with relict cliffs = left over degraded cliffs set back from the sea level, raised beach on a terrace above active cliff, result of isostatic processes and land rising

Emergent and Submergent coastlines

• raised beaches

• rias/fjords

• Dalmatian coasts

- rias = drowned river valley - only highland left visible,

Kingsbury estuary in South Devon, cross profile typical of a river valley

- fjord = drowned glacial valley flooded by the sea, U-shaped valley filled with water - Norwegian fjords - straight profile with truncated spurs, often deeper than adjacent sea due to glacial erosion cutting deep into landscape

- Dalmatian coast = series of "islands" that run parallel to coastline, formed through flooding of mountain range parallel to coast - synclines (valleys) flood and anticlines (folded ridges) remain visible above sea - Croatian coast

Impact of climate change/sea level rise on the coasts

- CC/ SLR impacts

climate change = warming of earths surface and atmosphere as a result of anthropogenic carbon emissions enhancing the greenhouse effect, accelerating sea level rise through melting of land ice and thermal water expansion

sea level rise impacts =

- increased coastal flooding and erosion

- land subsistence in south - loss of low lying land

- salinisation of ground water supplies, impacts habitats and reduces quality of water extracted for irrigation/domestic use

Coastal management and intervention = soft engineering

= use of natural systems for coastal defence, beaches/dunes/marshes to absorb wave energy

- beach nourishment to replace sediment lost in lsd

- dune regeneration: access restriction, boardwalks, stabilisation

- managed retreat: create new flooded marsh areas to protect

Coastal management and intervention = hard engineering

= man made structures to disrupt natural processes

- groynes: 90 degrees to coast to trap sediment transported by lsd - starve coast further up of sediment - cheap

- sea wall: curved barrier to reflect wave energy back to beach

- rock armour: absorb destructive wave energy - dissipated erosional force

- off shore break water: partly submerged rock barrier to break waves before coast

Coastal management and intervention = Integrated Coastal Zone Management - ICZM

= dynamic process to promote sustainable management of coastal zones

bringing together stakeholders involved in a coastlines used

- combines different views/opinions

- coordination of tourism, nature protection, agriculture, industry, infrastructure, climate change mitigation, local activity

- equilibrium between economic development and environmental conservation

Coastal management and intervention = Shoreline Management Plans - SMPs

= detailed documents containing strategies to manage flood risks and erosion for a particular stretch of coastline/sediment cell system

- hold the line: maintains current coast with hard engineering

- advance the line: extension of coastline through beach renourishment, groynes etc

- managed retreat: allowing coastal retreat, eg. breaking flood banks to create salt-marshes

- do nothing: low value areas of land allowed to flood and eroded, existing defences collapse