Erosion + deposition

Topic 3 - Coasts

constructive waves

low frequency, long and short wave length

• form gently sloping beaches - deposit material (stronger swash than backwash)

• move in elliptical motion - break with little height

destructive waves

high frequency, steep and high

• circular motion - break with height so plunge into beach

• erosional - stronger backwash than swash

erosional processes

• corrosion

• hydraulic action

• abrasion

• attrition

corrosion

a chemical reaction between seawater (containing weak acid) and susceptible rocks

• e.g. limestone

hydraulic action

repeated action of water forced in and out of the cracks in the rock leads to the breakdown of the surrounding cliff

• due to force of water being compressed into rock cracks

abrasion

pieces of rock are picked up by wave and hit against seabed/beach/cliffs

• wears them away over time

attrition

pieces of bedload (material carried in the water) are hit against one another

• causes them to break apart and become smaller and more rounded

cliff erosion - wave cut notch

• destructive waves lead to abrasion

• undercuts cliff face

• curved indent forms in cliff

• creates wave cut notch

cliff erosion - collapse of wave cut notch

• continual erosion of wave cut notch

• rock above becomes less stable and collapses

cliff erosion - wave cut platform

• rock collapses, process begins to create new notch

• multiple collapsing events leads to cliff retreat

• leaves wave cut platform

headland formation

• destructive waves erode weaknesses in headlands

• weaknesses widen, abrasion becomes more important

• leads to erosion into coastal landforms

erosional landform - cave

• erosion of small weaknesses

• crack widens

• small hollowed out area - cave formed

erosional landform - arch

• caves eroded through headland

• arch created

• e.g. durdle door, Dorset coast

erosional landform - stacks

• gravity collapses top of an arch

• leaves a column behind = stack

erosional landform - stumps

• continued erosion and weathering of a stack

• causes breakdown into a stump

• submerged under high tide

• will eventually erode to nothing

longshore drift

transports material along coastlines when waves approach beach at an angle

• prevailing wind pushes wind at and angle to coast

• moves swash up the beach at an angle

• gravity pulls backwash at 90’ to coastline

• moves sediment along beach

depositional landforms - spit

long finger of sand sticking out from one side of a coastline

• when coast changes direction at an estuary LSD moves sediment across

• river has energy to stop spit joining other side

• may have salt marshes behind = sediment settles and is colonised

depositional landforms - recurved spit

wave refraction carries material round tot eh sheltered side of the spit, curving the end

depositional landforms - double spit

2 spits extending from both ends of a bay

depositional landforms - bar

when spit grows across a bay

• lagoons may form behind bars

depositional landforms - beach

consecutive waves deposit sand/shingle inland

• steeper beaches often formed from pebbles

depositional landforms - tombolos

if a spit stretches across a mainland and joins an offshore island

depositional landforms - cuspate foreland

sediment deposited across a bay caused by LSD from both directions

• leads to 2 spits forming that then meet, trapping sediment

• forms new land

sediment cell concept

a closed system operated by sources, transfers and sinks

• driven by erosion, transportation and deposition processes

sediment cell - sources

• subaerial processes

• erosional processes

• sediment brought to the coastline

sediment cell - transfers

• LSD

• onshore/offshore winds and tides

sediment cell - sinks

• depositional landforms

sediment cells

uk has 11 sediment cells

• each has its own SMP (shoreline management plan)

chemical weathering

rocks are broken down because of their chemical composition being changed

• carbonation

carbonation

co2 dissolves in rain

• makes weak carbonic acid

• acidic rainwater dissolves vessels types of rock containing calcium carbonate

biological weathering

when rocks are broken down by living things

• plant roots in rocks enter cracks and push them apart

• algae can release chemicals that breakdown rocks

• some animals secret acid on rocks along coasts e.g piddock shell

mechanical weathering

rocks are broken down without their chemical composition being changed

• freeze-thaw

• salt-weathering

• wetting and drying

freeze thaw weathering

• water enters a crack in rock, freezes and expands in cold weather (by 10%)

• expansion exerts pressure in rock, widens crack

• repeat breaks rock into scree

salt-weathering

• salt water evaporates, leaves crystals behind

• crystals grow and exert stress in rock

• can corrode rock if containing iron

• breaks rock

wetting and drying

frequent cycles of wetting and drying are common on the coast

• clay rich rock expand when wet

• contract as they dry

• causes cracking and breaking

mass movement

when rocks and loose material shift down slopes

• occurs when gravity overcomes the force supporting the material

• common where material is saturated

mass movement - slides

when material moves down a slope in a straight line

• rock slides (landslide)

• rock falls

slides - rock/debris slides (landslide)

rocks with joints or bedding planes parallel to cliff are vulnerable

• inc. water reduces friction - causes sliding

• slabs of rock can slide over underlying rocks on a slip plane

slides - rock falls

on slopes over 40’, when steep jointed rock faces are mechanically weathered

• broken material either bounces away or falls to form scree

• blocks can fall off in a single piece due to rock jointing

mass movement - flows

• soil creep

• solifluction

• earthflows + mudflows

flows - soil creep

• slow and almost continuous

• very slow downhill movement of individual soil particles

flows - solifluction

• avg. 5-100cm annual movement

• mainly in tundra areas when frozen ground thaws

• saturated surface area flows over permafrost

flows - earth flows/mudflows

• inc. in amount of water reduces friction

• causes earth/mud to flow over underlying bedrock

• material becomes jumbled up - doesn’t move in tact (=slide)

mass movement - slumps

• involves rotational movement in saturated conditions

• moderate to steep slopes

• common where softer material overlies resistant/impermiable rock

• causes rotational scars and terraced cliff profile

cause of waves

• wind creates frictional drag, moves upper water surface, circular orbit

• closer to coats, shallow water change the shape to elliptical

• wavelength and velocity decreases

• wave heigh increases, water backs up from behind

• force pushes the wave higher, becomes steeper before breaking onshore

wavelength

distance between the crest of 2 waves

wave period

time interval from crest to crest in seconds

• intervals between 1-20 seconds

fetch

the distance of open water over which a wave moves

• greater = larger wave

summer beach profile

• steeper beach profile

• constructive waves more common

• constructive swash creates a berm

berm

deposit of larger material (shingle) that forms at the top of a beach by a constructive wave

• as berm builds, backwash weakens as it drains by percolation

• can only move small material, so smaller material found closer to shore

winter beach profile

• more destructive waves, more frequent

• erodes berms, transports sediment offshore into offshore bars

• can form a rip

rip

• occurs during winter

• backwash exerts a current

• drags sediment back as the next wave arrives over the top

differential erosion

the erosion of areas of weakness, as opposed to more resistant areas/rock types

blowhole

overlying rock of a cave collapses

• blowhole develops

• hole in ground that connects to an underground partially submerged ocean cave

• air/water forced out hole as waves crash

tides

changes in the water level of seas and oceans

• caused by gravitational pull of the moon

• 2 high and 2 low tides daily

tidal range

the difference in height between high and low tides

• high ranges create powerful tidal currents as they rise and fall

sediment transportation

• traction

• saltation

• suspension

• solution

traction

large and heavy rocks are rolled along the seabed

saltation

smaller and lighter rocks bounce along the seabed

suspension

lighter sediment is carried in water by suspension

solution

dissolved sediment carried in a solution

swash aligned beach

sediment moves up and down the beach with little lateral transfer

drift aligned beach

sediment is transferred along the coast with longshore drift

• finer particle carried further by LSD

• become rounder as they move

salt marshes

areas of flat, silty sediment that accumulate around estuaries or lagoons

• in sheltered areas of deposition

• where salt and fresh water meet

• no strong tides/currents to prevent sediment deposition

• covered at high tide, exposed at low tide

• pioneered by halophytes

sediment budget

the amount of sediment available within a sediment cell

sediment cell feedback

• positive = budget falls, waves transport sediment, sea has surplus energy so erosion inc.

• negative = budget inc, sea has less energy, so more deposition

• cells divided into sub-cells