coastal landscapes and change

LANDFORMS AND LANDSCAPES

COASTAL RISKS

MANAGING RISKS AND CONFLICT

9.1 MARINE PROCESSES AND WAVES

9.2 TRANSPORT AND DEPOSITION

9.3 SUB-AERIAL PROCESSES

10.1 SEA LEVEL CHANGE

10.2 THE EROSION THREAT

10.3 COASTAL FLOOD RISK

11.1 CONSEQUENCES FOR COMMUNITIES

11.2 HARD, SOFT AND SUSTAINABLE MANAGEMENT

11.3 COASTAL DECISION-MAKING

COASTAL PROCESSES

8.1 THE COASTAL ZONE

8.2 GEOLOGICAL STRUCTURE AND LANDSCAPES

8.3 RATES OF EROSION

the littoral zone is the wider coastal zone including adjacent land areas and shallow parts of the sea just offshore

this can be divided into a number of 'sub-zones'...

backshore zone above high tide level is only affected by waves during exceptionally high tides and major storms

wave processes are normally confined to the foreshore between high and low tide marks

shallow water areas close to land are termed the nearshore; this area is often one of intense human activity but also forms part of the physical systems of the coastline as it transfers sediment by currents close to the shore

sub-aerial processes; surface runoff erosion and mass movement ( landslides ) slowly move sediment downslope

weathering; the chemical, biological and mechanical breakdown of rocks into smaller fragments and new minerals

mass movement; landslides, slumps and rock falls, which move material downslope under the influence of gravity

surface runoff; water, usually during heavy rain, flowing down the cliff ad causing erosion of it

primary coasts are dominated by land-based processes such as deposition at the coast from rivers or new coastal land formed from lava flows

secondary coasts are dominated by marine erosion or deposition

emergent coasts where the coasts are rising relative to sea level - tectonic uplift

submergent coasts are being flooded by the sea due to sea level rising

concordant vs discordant coasts

concordant or pacific coasts are generated when rock strata run parallel to the coastline ( Lulworth Cove, Dorset )

hard portland limestone and resistant purbeck beds protect the much softer rocks landward

marine erosion has broken through the resistant rocks, then rapidly eroded the wider coves behind

white chalk behind the coves prevent erosion further inland

discordant or atlantic coasts form when different rock strata intersect the coast at an angle, so geology varies along the coastline

dominated by headlands and bays

weak rocks have been eroded creating elongated, narrow bays

especially resistant areas remain as detached islands

rias; drowned river valleys

geological structure

the arrangement of rocks in 3 dimensions

strata: the different layers of rock within an area and how they relate to each other

deformation: the degree to which rock units have been deformed ( titled or folded ) by tectonic activity

faulting: the presence of major fractures that have moved rocks from their original positions

cliff profiles are influenced by several aspects of geology; the resistance of the rocks, and the angle of rock strata to the coastline

faults are major fractures in rocks produced by tectonic forces and involving the displacement of rock on either side of the fault line

joints occur in most rocks, often in regular patterns, dividing up rock strata into blocks with a regular shape

fissures are much smaller cracks in rocks, often only a few centimeters long, which represent weaknesses erosion can exploit

rates of erosion

igneous

few joints so limited to weaknesses erosion can exlpoit

interlocking crystals make for strong, hard erosion-resistant rocks

e.g. granite, basalt, dolerite

very slow ( < 1 mm per year )

metamorphic

often folded and heavily fractured, weaknesses erosion can exploit

foliation; crystals orientated in one direction producing weaknesses

e.g. slate, schist, marble

slow ( 1 - 10 cm per year )

sedimentary

many bedding places and fractures are often more vulnerable to erosion

geologically younger rocks meaning they tend to be weaker

e.g. sandstone, limestone, shale

moderate to fast ( 10cm - several meters per year )

permeable rocks allow water to flow through them, and include many sandstones and limestones

impermeable rocks do not allow groundwater flow and include clays, mudstones and most igneous and metamorphic rocks

coastal vegetation

e.g. coastal sand dunes, salt marshes, mangrove swamps

roots of plants bind sediment particles together making them harder to erode

when submerged, plants growing in sediment provide a protective layer so the surface isn't directly exposed to moving water

protect sediment from wind erosion by reducing wind speed at the surface due to friction

waves

directly influence erosion, transport and deposition deposition and help shape the coastline

caused by friction between wind and water; the force of the wind creates ripples in the water, which grows into waves if the wind is sustained

wave size depends on: wind strength, duration of wind blows, water depth and wave fetch

wave fetch: the uninterrupted distance across water over which wind blows

tides

formed by the gravitational pull of the moon acting on water on the Earth's surface

produced by a bulge of water which, as it passes a location on the coast, high tide occurs

large tidal ranges can produce waves capable of erosion, called tidal bores

when storms occur at the same time as spring tides 9 the highest tides ) the risk of coastal flooding is increased

constructive waves have a low wave height and a long wavelength

also called spilling or surging waves

gentle, flat waves with a strong swash but weak backwash

strong swash pushes sediment up the beach creating a ridge

destructive waves have a wave height over 1m and a wave length around 20m

common during storms

also called plunging waves

strong backwash that erodes beach material creating an offshore ridge

beach morphology: the shape of the beach including its width and slope and features such as berms, ridges and runnels. it also includes the type of sediment such as shingle, sand and mud

sediment supply from rivers is reduced, due to the construction of dams

interference in sediment supply along the coast, often as a result of coastal management

changes in climate; stormier winter's may create a more destructive coastline

erosion processes

hydraulic action

abrasion

attrition

corrosion

sediment is moved around by waves, collisions between rocks chipping fragments off, causing sediment to become smaller and more rounded

air trapped in cracks and fissures is compressed by the force of waves crashing against the cliff face

pressure forces cracks open, often dislodging rocks from the cliff

sediment picked up by waves is thrown against the cliff face, chiseling away at the surface, gradually removing smaller pieces

the dissolving of carbonate rocks from the spray of seawater

erosional landforms

blow hole: forms when a coastal cave turns upwards and breaks through the flat cliff top

wave-cut notch: eroded material at the base of a cliff by hydraulic action and abrasion

stacks

arches

stumps

caves

cracks

sediment transport

traction

saltation

suspension

solution

sediment rolls along, pushed by waves

pebbles, cobbles, boulders

sediment bounces along, due to force of water or wind

sand-sized particles

sediment is carried in the water column

silt and clay particles

dissolved material carried in the water as a solution

chemical compounds in solution

longshore drift is a key source of sediment for depositional landforms. sediment is deposited when the force transporting sediment drops

gravity settling occurs when the energy of transporting water becomes too low to move sediment

flocculation is a depositional process that is important for very small particles as they remain suspended in water

depositional landforms

spit

tombolo

barrier beach/bar

hooked/recurved spit

a spit whose end is curved landward, into a bay or inlet.

naturally curves into shallower water

made more pronounced by secondary direction of winds

e.g. Hurst Castle Spit, Hampshire

a sand or shingle bar that attaches the coastline to an offshore island, formed due to wave refraction around an offshore island

an area of calm water and deposition between the island and the coast

e.g. St Ninian's tombolo, Shetland

sand or shingle beach ridge extending beyond a turn in the coastline

longshore drift spreads out and loses energy leading to deposition

e.g. Spurn Head on the Holderness Coast

sand or shingle beach connecting two areas of land with a shallow water lagoon behind

occurs when a spit grows longer so it extends across a bay

e.g. Chesil Beach, Dorset

vegetation plays an important role in stabilizing depositional landforms; plant succession, salt marshes and sand dunes bind the loose sediment together, encouraging further deposition

depositional landforms are therefore at risk when vegetation is damaged due to overgrazing or trampling from tourism and leisure activities

mechanical: breaks down rocks due to the exertion of a physical force - does not involve chemical change

chemical: involves a chemical reaction and the generation of new chemical compounds

biological: often speeds up mechanical or chemical weathering through plants, bacteria or animals

freeze-thaw - water expands in volume when freezing, exerting a force within cracks and fissures

salt crystalisation - the growth of salt crystals in cracks and pore spaces can exert a breaking force

carbonation - the slow dissolution of limestone due to rainfall producing calcium carbonate solution

hydrolysis - the breakdown of minerals to form new clay minerals due to the effect of water and dissolved CO2

oxidation - the addition of oxygen to minerals producing iron oxides and increasing volume contributing to mechanical breakdowns

plant roots - tree and plant roots growing in cracks and fissures forcing rocks apart

rock boring - many species of clams and molluscs bore into rock and may also secrete chemicals that dissolve rocks

isostatic change = a local rise or fall in land level

eustatic change = a rise or fall in water level caused by a change in the volume of water - a global change