Topic 2B - Coasts

Positive feedback at coasts

Waves erode cliffs, released material helps erode more

Negative feedback at coasts

Shore is eroded, material makes wave cut platform wider, therefore it can absorb more energy and reduce impact

Littoral zone

The area of coasts that can be affected by wave action (dynamic zone)

4 sections of the littoral zone

• Backshore

• Foreshore

• Nearshore

• Offshore

Geology

The characteristics of the land (rock type and structure)

Concordant costaline

Rock types run parallel to the coast

Discordant coastline

Rock types run at right angles to the coast

Causes of sea level change

• Global warming

• Tectonic activity

• Eustatic/Isostatic change

Eustatic change

Sea level itself rises

Isostatic change

Land rises or falls relative to the sea

Characteristics of high energy coasts

• Destructive waves

• Strong wind

• Long fetches

• High erosion than deposition

• Rocky

• Steep cliffs

• Headlands

• Wave cut platforms

Characteristics of low energy coasts

• Constructive waves

• Sheltered location

• Short fetch

• Higher deposition than erosion

• Sandy beaches

• Salt marshes

• Mud flats

• Gentle relief

• Beaches

• Spits

• Coastal plains

Characteristics of rocky coasts

• Cliffs

• Erosion more than deposition

• More resistant geology (north and west UK)

• Primarily marine and sub-aerial erosion

Marine erosion

Caused by waves

Sub-aerial erosion

Weathering and mass movement

Characteristics of coastal plains

• Land slopes towards sea (low relief)

• Includes sandy and estuarine coastlines

• Low energy (greater deposition than erosion)

• Dunes, wetlands and marshes

Coastal morphology

The shape and form of the coastline, influenced by rock type and structure

Lithology

The geological structure of rocks, the way they are folded or tilted

Characteristics in lithology

• Strata - layers of rock

• Bedding planes - horizontal cracks created by pauses in rock formation

• Joints - vertical cracks caused by tectonic movement or contraction

• Folds - the result of pressure during tectonic movement causing the rock strata to fold

• Faults - the result of stress or pressure causing it to fracture

• Dip - the angle of the rock strata

Cliff profile

Angle and height of the cliff face, includes other features (eg wave cut notches), is heavily influenced by lithology

Coastal recession

Retreat of the coastline inland due to multiple factors

• Erosion (dependent on ithology)

• Sea level rise

• Submergence

Sedimentary rock

Form from pressure (compaction and cementation) AKA lithification

Examples of sedimentary rock

• Shale

• Sandstone

• Limestone

Characteristics of sedimentary rock

• Erode and weather easily (softest rock type)

• Form in layers

• Weak bedding planes

• Heavily jointed and fractured

Metamorphic rock

Forms when sedimentary or igneous rock is altered by heat and pressure without fully melting

Examples of metamorphic rock

• Slate

• Marble

Characteristics of metamorphic rock

• Crystalline structure

• Folded and faulted

• More resistant the sedimentary, less than igneous

• Crystals are parallel aligned

Igneous rock

Forms when molten rock from the mantel hardens and cools

Examples of igneous rock

• Granite

• Basalt

Characteristics of igneous rock

• Erode/weather very slowly

• Two types: Intrusive (forms underground, cools slowly, big crystals) and Extrusive (forms on surface, cools quickly, smaller crystals)

• Interlocking crystals

• Few joints and weaknesses

Differential erosion

Changing rates of erosion of different rock types, leads to complex cliff profiles, extra prominent on discordant coastlines (headlands and bays), also affected by permeability

Vegetation at coasts

Stabilizes coastlines by helping bind soil together, reducing wind speed (therefore erosion) and adds organic matter to help form soil

Pioneer species

First plants to grow in a harsh environment, helps allow others to grow later

Sand dune formation

• Windblown sand is deposited against an obstruction (eg plant)

• Grows in size as more is deposited, form rows at right angles to prevailing winds

• Eventually become fixed by vegetation

Sand dune succession

• Embryo dune

• Fore dune

• Yellow dune

• Grey dune

• Mature dune

Embryo dune

• First step, smallest dune

• High pH

• Little soil content

• Max 1 meter

• Pioneer species only

Fore dunes

• Second step

• Embryo dunes protect from prevailing wind

• Non-pioneer species begin to grow and stabilize the dune, also add organic matter

• Max height 5 meters

Yellow

• Third step

• Yellow-darker greyish with more organic matter

• Flowering vegetation found

• Max 8 meters

Grey dunes

• Fourth step

• Soil acidity higher

• Shrubs begin to appear

• Max 10 meters

Mature dune

• Fifth and final stage

• Most stable

• Several hundred meters from shoreline

• Can support trees

Salt marshes

Form in estuarine areas because of sediment and shelter from large waves, mix of fresh and salt water, algae helps trap sediment and bind it together, grasses start to grow

Wave fetch

Distance travelled in open water

Wave height

Distance from peak to trough

Wave length

Distance from crest to crest

Wave frequency

Number of waves that pass a point over a given time

Constructive waves

• Lower height

• Swash stronger than backwash

• Greater deposition

Destructive waves

• Higher wave height

• Stronger backwash than swash

• Greater erosion

Abrasion (coastal)

Sediment and rock are picked up by waves and wear away at cliffs

Hydraulic action (coastal)

Force of waves forces high pressure air into crack at cliffs and weakens to rock, causing joints to expand

Corrosion (coastal)

Weak acids in water dissolve rock particles

Attrition (coastal)

Rocks bash into each other when moved by water and gradually get smaller and rounder

Factors affecting coastal erosion

• Wave type

• Wave size

• Lithology

Traction

Large heavy material is dragged along the sea floor

Saltation

Smaller material is lifted by water and “bounces” along the floor

Suspension

Small particles are carried within the water

Solution

Smallest material is carried dissolved in water flow

Types of sediment transport

• Traction

• Saltation

• Suspension

• Solution

Longshore drift

Main process of deposition and transport along the coast

Surface currents

Formed by wind and close to surface

Deepwater current/thermohaline circulation

Caused by density differences in water, deeper under surface

Tidal range

Difference between high and low tide at a place, larger rang makes stronger tidal currents that can transport more sediment

Drift-aligned beach

Longshore drift moves sediment along the beach , often end in a spit when coastline changes direction

Swash-aligned beach

Form when energy is low, waves run parallel to shore so little sediment movement

Spits

• Created because of longshore drift

• When coastline changes direction the waves loose energy to carry sediment and deposit it, then builds up out to sea

• Often has a curved end due to second prevailing wind

Lagoon

• Small body of water is cut off from the sea

• Usually behind a bar or tombolo

• Way fill with sediment and from new land

Tombolo

When a spit joins the mainland to an island

Sediment cells

• Closed systems as sediment movement is contained

• 11 cells in the UK

• Can be divided into sub-cells where inputs (sources), stores (sinks), transfers and outputs can all be identified

Sediment budget

The balance between inputs and outputs in a sediment system, should be ina. state of dynamic equilibrium but can be disturbed by human activity and climate change

Mechanical/physical weathering

When rocks breakdown without physical changes

Biological weathering

Rocks break down due to organic activity

Chemical weathering

Rocks breakdown due to a chemical reaction

Physical weathering examples

• Wetting/drying

• Exfoliation

• Salt crystallization

• Frost weathering

Chemical weathering examples

• Oxidation

• Hydration

• Carbonation

• Solution

Biological weathering examples

• By vegetation

• By animals

• By humans

Freeze thaw

A form of physical weathering

• Water collects in cracks in the rock

• Water freezes and therefore expands when temps drop, widening the crack

• Repeats

Mas movement

The downhill movement of material due to gravity

Factors affecting type of mass movement

• Angle of slope

• Nature of materials

• Amount/type of vegetation

• Water

• Rock type/structure

• Human activity

• Climate

Soil creep

• Soil expands due to heating or freezing

• This causes it to lift at a right angle to the slope

• When it shrinks it falls down

Flow

• Slopes between 5-15 degrees

• Between 1-15 km/year

• Happens after soil has become heavily saturated

Slide

Movement of material in one group until it reaches the bottom

Fall

• Steep slopes and rapid movement

• Can be caused by extreme weathering, rainfall, earthquakes, and hot weather

Slumping

Weather rock types become saturated and heavy so large areas move down in one piece, leaving a curved surface behind

• Can be rotational (at coasts)

Mass movement land forms

• Rotational scars

• Talus scree slopes

• Terraced cliff profiles

Rotational scar

• Curved

• Un-weathered

• Un-vegetated

• Result of rotational slumping

Talus scree slope

• Fan shaped mound of material

• Made from block fall debris

• Concave profile

Terraced cliff profiles

• Profile of the cliff is stepped

• Result of lithology and rock fractures

Causes of eustatic change

• Changing amounts of ice

• Thermal expansion of the sea

• Tectonic changes (reduces ocean capacity)

Causes of isostatic change

• Post glacial adjustment

• Accretion

• Tectonic activity (moves up land)

Post glacial adjustment

Heavy ice sheets in glacial period push land down, when glacial period ends the land will rebound (isostatic recovery)

Accretion

Within the sediment cell areas of high deposition cause land to build up

Emergent landforms

Can occur in isostatic recovery

• Raised beaches

• Fossil cliffs

Raised beaches

• Emergent landform

• Beaches that are high above high tide level

• Very flat

Fossil cliffs

• Emergent landform

• Steep slope at the back of a raised beach, wave cut notches, caves and arches may be found

Submergent landforms

Happen from a relative rise in sea level, can happen when coastlines flood

• Ria/drowned valley

• Fjords

• Dalmatian coast

Ria/drowned valley

Lower course of river valley floods

• V shaped cross section

• Estuarine

• Most common submergent landform

Fjord

Glacial valleys flood due to eustatic change

• Deeper than rias

• Flat bottomed U shape

• Straight profile

• Can be deeper than the adjacent sea

Dredging

Removal of sediment from river or sea, changes amount of sediment being deposited at the coast

Dams and coastal recession

Construction traps sediment behind the wall (100 billion tonnes, another billion each year, which starves the coast of sediment

Reasons for changing rates of recession

• Wind direction

• Fetch

• Tides

• Seasons

• Weather systems

• Storms

Factors increasing flood risk

• Land height

• Degree of subsidence

• Vegetation removal

• Global sea level rise