coasts eq2

what causes waves

Friction between wind and water, with some energy from the wind being transferred into the water.

● Waves can be generated by storms thousands of miles away and continue under their own momentum creating swell waves.

● Swell is the largest wave.

● Dominant winds- create the largest most damaging waves for a location.

● Prevailing winds- the most frequent wind direction.

● West UK they're the same (South-West) but for East the dominant waves are North-East.

● Wave energy is proportional to the height and each water molecule goes in a circle.

● Different winds create different waves, some combine to make bigger waves, this is interference.

● Constructive interference- waves meet in phase (rouge wave).

Destructive interference- waves meet out of phase (don't always increase wave height

factors impacting wave size

o Strength of wind

o Duration the wind blows for

o Water depth

o Fetch

o Largest waves in Cornwall due to Florida→Cornwall fetch of 4000km

why are waves generally larger in the southwest

· The UK's largest waves are generally experienced in the Cornwall (Southwest).

· This is because the dominant and prevailing winds comes from the south-western direction.

· Larger waves are generated by a sustained south-westerly wind because they have a great distance over which to grow, this transfers kinetic energy from the wind to the water.

· There is also a large fetch from Florida to Cornwall (over 4000 km). There are no land-masses between there and America, so the waves from the Atlantic ocean aren't slowed down before they reach the coast.

describe tsunami waves

Average wavelength- 200km

Average height- 1m

Average height on coast- 30m

● Periods average 15 minutes

● One tsunami event can consist of 3-8 individual waves

● Can be produced by underwater landslides

● Can be produced by underwater earthquakes only if they produce vertical displacement of the seafloor

describe constructive waves

spilling breakers

● Low wave height (less than 1m)

● Long wave length (up to 100m)

● Less frequent (8 or less per minute)

● Elliptical motion, giving strong forward movement

● Forms beach cusps if approaching at angle

● Gentle, flat waves with strong swash but weak backwash which pushes sediment up the beach, depositing it as a ridge of sediment (berm) at the top of the beach. Most of the backwash percolates into the beach

Constructive waves have strong swash and weak backwash. Deposition is greater than erosion.

· Constructive waves have long wavelengths and low frequencies, since only about eight to ten waves occur per minute.

· This low frequency is what helps prevent intensive erosion from taking place, since it hinders the removal of sediments from the coastlines.

· Because constructive waves lack a breaker component, these waves can simply move up beaches and deposit material at the beachfront.

· The weak backwash and suppressed wave gradients prevent any sediments from being carried back into the sea after deposition

describe destructive waves characteristics

(plunging breakers):

● High wave height (over 1m)

● Short wave length (20m)

● More frequent (over 15 per minute)

● Common during storms and are called plunging waves

● Strong backwash that erodes beach material and carries it offshore

● Throws material up a beach to form a berm or storm ridge

Beaches experience both during time.

● As slope of the beach increases ripple marks appear.

Beach cusps appear as coarser material is absorbing wave swash

describe destructive waves

Destructive waves have a weak swash and strong backwash. Erosion is greater than deposition.

· Destructive waves have short wavelengths and high frequencies since around ten to fourteen waves can occur every minute.

· Because of the presence of a breaker, these waves will eventually plunge towards beaches after reaching a certain height.

· Because of this, destructive waves do not usually travel far up the shorelines.

· the high frequency allows for the erosion of materials and sediments back into the ocean since there is usually more water to transport these materials out to sea.

· outcome, is a destructive phenomenon that erodes beaches and can sometimes damage coastal landscapes. Less sediment, sand and geological material becomes available to build-up the beach due to the increased erosion brought about by destructive wave

describe wave motion in deep water

In deep water, water particles move in a circular motion, with this movement decreasing with depth.

The energy, not the water itself, moves forward.

Wave frequency (or wave period) refers to the time it takes for successive waves to pass a given point.

describe wave behaviour in shallow water

Shallow water is defined as when the depth is less than the wave's wavelength.

As waves move into shallower water: Water particle motion shifts from circular to elliptical due to friction with the seabed. Friction slows the base of the wave, causing the crest to move faster, leading to wave breaking.

what is a beach profile

Beach profiles describe the distribution, shape, type and size of deposited material (deposited by wave action). It is dependent on distance from the shoreline (the sediment is graded).

Finer shingle particles are commonly observed along drift-aligned beaches. They get carried further by Longshore drift, and will also become increasingly rounded as they move.

why do beach profiles change

● Sediment supply from rivers is reduced, for instance due to the construction of dams on rivers that traps sediment upstream

● Interference in sediment supply along the coast, often a result of coastal management in one place having an effect on processes further along the coast

● Changes to climate; for instance, if global warming made term-15the UK climate on average stormier, then destructive waves and 'winter' beach profiles would become more common.

when does most erosion occur

● Waves are at their largest, which is influenced by wind speed and fetch, meaning they have a lot of energy and can pick up sediment and hurl it at a cliff face

● Waves approach the coast at 90° to the cliff face

● The tide is high, propelling waves higher up the cliff face

● Heavy rainfall, percolation of water through permeable strata and surface runoff weakens the cliff

Debris from previous erosion has had time to be removed from the cliff foot, as debris can protect against waves

influence of lithology on erosion

· For abrasion to be effective, suitably loose sediment has to be available such as shingle. Softer sedimentary rocks are more vulnerable than hard igneous ones.

· Softer rocks are rapidly reduced in size by attrition

· Mainly affects limestone which is vulnerable to corrosion by weak acids

· Heavily jointed/fractured sedimentary rocks are vulnerable. In very hard igneous rocks hydraulic action on cooling cracks may be the only erosive process occurring.

what is beach morphology

· Beach morphology describes the profile of the beach (steep or shallow sloping). It is concerned with the movement of sediment caused by wave action.

· Seasonal changes may create summer and winter profiles or gradients, due to the variation of wave activity - sediment is dragged offshore by destructive waves during winter, and returned by constructive waves in the summer.

describe beach morphology in the winter vs summer

In summer

· Low energy constructive waves

· Steepening beach angle with larger shingle particles towards back of beach. ​

· Constructive waves build berm ridges, typically of gravel/shingle at high tide mark.

In winterterm-15

High-energy destructive waves

lowering angle of beach profile and spreading shingle over the whole beach.

Destructive waved form offshore ridges/bars and cause subsequent deposition of sand and shingle offshore

what is a berm

Berm is a bank of sand or gravel ridge (few cm high), parallel to the shoreline and is created by wave action

describe coastal deposition

· 80% of the earth's coasts are erosional whereas only 20% are depositional.

· Deposition occurs in low energy environments where the effects of waves, tides and currents are reduced.

· The water slows down, loses energy and therefore drops what it is carrying.

· The smallest sediment is deposited last (FLOCCULATION - clay particles stick together becoming heavy enough to sink. Eventually they settle out so the water becomes clear)

· The heavier largest sediment is deposited first (GRAVITY SETTLING)

how are erosion and deposition linked

Depositional coasts rely on sediment being transported in from erosional coasts.

Sediments are moved there by;

Waves - move up and down the beach in the direction of the wind (LONGSHORE DRIFT)

Tides - determine where erosion is taking place

Rip currents - take sediments from the foreshore to the nearshore area

Other currents - take sediments from offshore to onshore or vice versa

describe longshore (littoral) drift

· the movement of material along the shore by wave action. It happens when waves approach the beach at an angle.

· The swash (waves moving up the beach) carries material up and along the beach. The backwash (waves moving back down the beach) carries material back down the beach at right angles.

· This is the result of gravity. This process slowly moves material along the beach and provides a link between erosion and deposition.

● Two longshore drift directions are seen at Dungeness in Kent

Sediment is carried by: suspension (Holderness sea is muddy brown), dissolved, saltation (on a dry windy day seen 2-10cm above the beach surface) (dislodges more pebbles) and traction (heard at the beach) and drag.

factors affecting LSD

· Dominant wind

· Prevailing wind

· Geology (sediment type)

· Shape to the coastline - refraction

· Rivers

· Man made obstacles (sea defences)

· Tides

describe transportation and sediment transportation

· Greatest rates of LSD occurs when waves approach the coast at a 30 degree angle.

· Swash carries materials up the beach in the direction the wind is blowing.

· Backwash carries sediment back to the sea (at right angles to the sea due to gravity).

Sediment transportation

Marine sediment can be transported in different ways according to sediment size and composition.The faster the flow of water the more kinetic energy it has to move the load it is carrying.

Larger sediment can roll (traction) or be dragged

depositional landforms- beaches

Commonly found in bays where wave refraction creates a low-energy environment, causing deposition.

Consist of either sand or shingle, depending on factors like the nature of the sediment and the power of the waves.

what are the 2 types of beaches

Swash-aligned: wave crests approach parallel to the coast, so there is limited LSD eg lulworth cove

Drift-aligned: wave crests break at an angle to the coast, so there is consistent LSD and the generation of elongated depositional features.

depositional landforms- tombolo

A narrow ridge of sand or shingle that connects an island to the mainland..

● Form due to wave refraction around an offshore island which creates an area of calm water. Opposing longshore currents may play a role.

● e.g. Great Orne in Llandudno.

depositional landform- bar

● ridge of sand or shingle that connects two headlands, cutting off a bay.

Formation:

Formed when a spit extends across a bay due to continuous longshore drift and deposition.

● e.g. Slapton Ley, Devon or Chesil Beach, Dorset

depositional landforms- lagoon

●A shallow body of water separated from the sea by a barrier beach, bar, or spit.

Formation:

Occurs when deposition isolates part of the coastline. often brackish (mix of salt and fresh water).

● E.g. Slapton Lee Lagoon

depositional landforms- spit and the different types

straight or gently curved deposit of sand or shingle attached to the land at one end.

Formed by prevailing wind and longshore drift.

Often has a recurved end due to changes in wind or wave direction.

b) Recurved Spit

A spit with a hooked end caused by wave refraction or secondary wind directions.

c) Compound Spit

A spit that shows multiple recurved ridges, indicating periods of growth and stabilisation.

Shows complex depositional history.

Example for all spits: Spurn Head, Holderness Coast.

depositional landforms- cuspate forelands

Cuspate forelands are triangular beaches. They form due to longshore drift moving sediment in opposing directions.

Cuspate forelands form due to the positioning of the coast and their orientation to incoming tides, prevailing winds and longshore drift.

● E.g. Dungeness, Kent

o Dominant wind direction creases west-to-east longshore drift as well as longshore drift from the east as well as swell waves through the English Channel.

3000 years old and contains 40% of all Britain's coastal shingle

coastal landforms created by LSD and deposition- barrier beaches

· Long, narrow beaches or islands of sand or shingle parallel to the coast but detached from it.

Formation:

Created by longshore drift and wave deposition during rising sea levels (post-glacial).

Often form in low tidal range areas.

Example: Chesil Beach, Dorset.

oastal landforms created by LSD and deposition- offshore bars

·Submerged or partly exposed ridges of sand or coarse sediment formed offshore, parallel to the coastline.

Formation:

Created by wave action moving sediment offshore.

Common during storm conditions or when backwash > swash.

Function:

Can absorb wave energy and reduce coastal erosion.

coastal landforms created by LSD and deposition- barrier beaches and offshore bars and their value

Probably created by post glacial outwash at the end of the last ice age. The sediment is then swashed back inland by storm systems. Some accumulate vertically where there is an offshore obstacle like a reef or even a ship wreck. They are frequent feature in the USA and Netherlands eg chesil beach Dorset started as offshore bar but moved towards land by rising sea levels to become a barrier beach.

● Value:

o Natural habitat as no humans or prey

o Provide sediment for defences

o Cheaper + easier to put wind turbine as doesn't dig as deep e.g. Scroby Sands in Norfolk off the East Anhlian Coast

o Provide sand for building e.g. cement

dynamic challenge c to depositional landforms- vulnerability 1

Depositional landforms are made from unconsolidated sediment so are vulnerable to change (they are dynamic landforms).

Vulnerability 1: Change can result from major storms (especially when during spring tides). Huge volumes of sediment can be eroded during a single storm, leading to erosion and re-deposition elsewhere.

A major storm and tidal surge in Dec 2013 breached Spurn Head, Holderness Coast.

Chesil Beach was breached by a major storm in 1990; a storm in January 2014 dramatically altered the beach profile.

dynamic challenge c to depositional landforms- vulnerability 2

Vulnerability 2: Human actions can have large impacts. Human actions miles away along the coastline can change the availability of sediment and dramatically affect the volumes of deposition. Sea defences in particular can starve drift aligned beaches of sediment. When the defences end the sea tries to rebalance the sediment as so extra erosion happens: Terminal groyne syndrome

describe reliance: sediment transfer

· Depositional landforms depend on a continual supply of sediment and continuous deposition to balance any erosion that takes place

· This is especially true of spits, as the seaward end is constantly being eroded.

describe the need for stabilisation

● Vegetation stabilises depositional landforms

· Plant succession, in the form of salt marshes and sand dunes, bind the loose sediment together and encourage further deposition.

· Reducing Energy: As vegetation grows, its leaves, stems, and roots create surface roughness that slows down wind and water. This reduced energy means that these agents can no longer transport as much sediment, leading to deposition.

· Trapping Sediments: The physical barrier provided by plants, especially pioneer species, helps trap windblown sand or waterborne sediments. As the flow loses energy, sediments settle out, building up the landform.

· Stabilizing the Substrate: Root systems bind the soil or sand, making it less susceptible to erosion. This stabilization allows sediments to accumulate over time.

● Vulnerable to overgrazing + trampling from tourism and leisure activities

● Seaward end of spits constantly eroded

● E.g. Hurst Castle, Spit build military base in vulnerable location

describe the sediment cells concept

· Along a section of coastline, erosion, transport and depositional processes operate in a linked system.

· A sediment/littoral cell has sources, transfers and sinks which is important to understand the coast as a system.

· Each cell can be regarded as a closed system where there are large barriers between them with little transfer from one cell to another.

· But in reality, under certain conditions transfers between the main sediment cells will take place.

· In theory erosion in one place called the source will be balanced by deposition in another place called the sink and LSD and currents transfer sediment within the cell.

· The amount of sediment gained from sources and lost to sinks can be quantified and a sediment budget calculated.

· These calculations can be useful in assessing coastal change and the effects of coastal management schemes.

xexamples of sources, sinks and transfers

Sources

Erosion of cliffs

Onshore currents supplying sediments to the shore

Land sediments eroded by rivers

Tides moving sediments

Subaerial processes (weathering and mass movement)

Wind-blown (aeolian) sediments from land

Wind along shore or offshore

Shells and remains of marine organisms

Transfers

Longshore drift

Wave transport through swash and backwash

Currents - localised movements in and out

Nearshore depositional landforms (e.g. bars)

Offshore sediment deposition to deep offshore waters (e.g. through undersea canyons)

Sinks

Backshore depositional landforms (e.g. sand dunes)

Foreshore depositional landforms (e.g. beaches)

Nearshore depositional landforms (e.g. bars)

Offshore sediment deposition (deep offshore waters e.g. through undersea canyons)

describe sediment cells in dynamic equilibrium

Each sediment cell system is in balance.

Storms can cause short term disruption but the equilibrium re-establishes quickly.

Negative feedback mechanisms rebalance the system.

E.g. A large landslide (erosion event) will then protect the cliff slowing down erosion.

E.g. Sand dune erosion will drag sediment offshore creating a barrier beach which protects the spit so deposition can occur and the spit rebuilds

describe the positive feedback loop

enhance or amplify changes; this tends to move a system away from its equilibrium state and make it more unstable

describe the negative feedback loop

tend to dampen or buffer changes; this tends to hold a system to some equilibrium state making it more stable

describe christchurch bay

· 16km stretch of coast

· Dominant waves from the south-west.

· Beaches comprise finer material in the sheltered western side, and coarser material to the east.

· Cliffs erode at a historic rate of ~1m/year.

· Over the past century, coastal management has been built which has affected the shape of the coast.

describe christchurch bay and its sources, sink and transfer

source

Erosion of cliffs (1m/yr)

Fluvial input (land sediment eroded by rivers)

Wind blown sediment

Coastal slope erosion

Estuarine sediment from the West Solent

Weathering and mass movement

Shells and marine organism remains

Transfer

LSD (littoral drift) - eastwards

Offshore sediment transport e.g. offshore winds

Estuarine sediment transport

Onshore to offshore transport

Wave driven nearshore and offshore zone transport

Tides

Local currents

Sink

Backshore deposition eg. Sand dunes

Hurst spit and Mudeford spit

Offshore bars e.g. Dolphin Sand

Beaches (foreshore deposition)

describe the positive feedback of christchurch bay

Positive feedback

A storm destroying sand dunes,then no new grass can grow and then more erosion of the sand dune occurs.

If the sediment budget falls, waves continue to transport sediment, and erosion may therefore increase in some areas as the sea has surplus energy

describe the negative feedback of christchurch bay

There could be a rock fall but then the debris protects the area so less erosion of the cliff occurs.

Spit erosion will drag sediment offshore creating an offshore bar which protects the spit so deposition can occur and the spit rebuilds

if the sediment budget increases, e.g. due to a storm, more deposition is likely. The sea corrects itself, because it can only carry so much, so surplus sediment is deposited

what are subaerial processes

vInclude weathering and mass movement.

These processes operate on the cliff face

to weaken it and provide material for

coastal erosion.

These are land processes and act along

with marine processes (erosion) to create

coastal landforms

definition of weathering and ersoion and mass movement

Weathering- breakdown of rocks in situ, at or near the Earth's surface, exploiting weaknesses in rocks over a long timescale. It does not involve any movement.

Erosion- the breakdown of rock due to the action of some external force which then transports the eroded material to a new location

the movement of weathered material

down slope, as a result of gravity.

effects of various factors on mass movement and weathering

● Mechanical weathering dominates in cold climate, while chemical reactions speed up in hot wet conditions.

● Even in a hot, wet climate basalt weather 1-2mm every 1000 years.

● Slip is more common where there is drier material like unconsolidated sands, where the cliff material tumbles to create a talus slope.

● Where there is plentiful water and a higher clay content, mud flows may result, which spill out over the foreshore as a lobe.

● Topple occurs where rock strata have a very steep seaward sip, undercutting by erosion will lead to instability and blocks of material toppling seaward.

sub aerial processes- flows- soil creep

rate- imperceptible

Process: The slow, downward movement of soil particles due to gravity.

• Mechanism: Particles expand when wet/freeze and contract when dry/thawed, slowly shifting downhill.

• Influence on coastline: Gradually degrades cliff slopes, contributing small amounts of material to the cliff base where it may be removed by marine processes.

sub aerial processes- flows- soilfluction

Occurs in periglacial (cold) environments.

In summer, surface layer thaws but remains saturated due to frozen ground below (permafrost), leading to flow of waterlogged soil downhill.

sub aerial processes- flows- earth and mudflows

Earthflow: Slower movement of fine-grained saturated soil.

Mudflow: Faster movement of water-saturated mud; common after heavy rainfall or on steep slopes.

Both leave lobe-shaped deposits at the base.

Can rapidly transport large volumes of material downslope, weakening cliff stability and increasing erosion rates at the base.

Subaerial processes- slides- rock fall

Sudden and rapid free fall of rock from a steep cliff.

Triggered by freeze-thaw, weathering, or undercutting.

Forms scree or talus slopes at the base.

Subaerial processes- slides- rock/debris slides

Large blocks of rock slide along a flat bedding plane.

Can occur suddenly after rainfall or earthquake.

Subaerial processes- slides- slumps

Curved slip plane movement, often in clay-rich coastlines.

Saturated material moves in a rotational manner, creating a stepped cliff face.

Common on Holderness Coast.

Creates terraced cliff profiles; increases sediment supply for beach formation.

Subaerial processes- mechanical- freeze thaw

Found on coasts, where the temperature changes daily, above and below freezing. Water creeps into the joints and cracks and when it freezes it expands by 9%, exerting pressure of 7-15kg/cm2 and forcing the rock apart. This causes a jagged cliff face and scree slopes at the base.

Relatively uncommon I the UK and salt spray reduces effect further. Contributes to rock fall and cliff retreat

Subaerial processes- mechanical-salt weathering

Waves break on cliffs and the water evaporates, leaving rhodium and magnesium salt compounds in the joints and cracks. The salt crystals grow and exert pressure to force the rock apart. Occurs also if sea water enters porous rock and grows into crystals itself. Creates scree slopes.

Occurs in porous and fractured rocks like sandstone, greater effect in hotter, drier climates were evaporation is more pronounced.

Subaerial processes- mechanical-wettying and drying

Since rock is a poor conductor, hot days earn the outer parks of a rock are much hotter than its interior. At night, loss of surface heat by radiation reverses the temperature gradient. This leads to differential expansion. This insufficient without water in the process. This is as wet rock expands and dry rock shrinks.

Weakens rock leading to slumping and erosion

sub aerial processes- biological- plants- seaweed acids

Some seaweed cells contain pockets of sulphuric acid when these cells break, in contact with rock the acid will dissolve some of the rock minerals. Rock minerals are no longer bonded together so rocks crumple and greater risk of erosion as they are weaker.

sub aerial processes- biological animal- boring molluscs

Many marine molluscs live on coastal rocks, scrapping away at the rock surface to get food or boing a hole in the rocks to make a core. One example is the Piddock, which has a shell with serrated cutting edges.

subaerial processes- chemical- carbonation

This involves the reaction of minerals with carbonic acid, which natural forms when carbon dioxide dissolves in moisture.

Gradually dissolves rock, forms solution features (e.g. caves), and weakens cliffs.

vsubaerial processes- chemical-hydrolysis

One or more minerals are split by chemical reaction with water. This is then leached away.

Occurs in igneous and metamorphic rocks containing feldspar and other silicate minerals.

Weakens granite and other igneous/metamorphic rocks; contributes to slope failure.

subaerial processes- chemical-oxidation

Oxygen combine with iron-based minerals in a rock, causing a chemical breakdown of the minerals, as shown by a red-oxygen rusty colour on a rock face. 4FeO+3H2O+O2→2Fe2O3*3H2O

Sandstones, siltstones and shales often contain iron compounds that can be oxidised.

example of subaerial processes

● April 2013 a large rockfall occurred in St Oswald's Bay Dorset, in the cretaceous chalk cliffs. Mass movement occurred overnight without warning and an 80-100m long

● Rotational slides occur.

describe coastal mass movement in norfolk

● the early part of the 21st century, erosion rates were greater than 10m per year in unprotected places with 36,000 tonnes of sediment being eroded from a 200m section of the cliffs made of glacial deposits.

● Rock fragments fall to the base of the slope and form talus scree slopes. These steep, fan-shaped mounds of angular material have larger boulders at their core and smaller material at the top. Slope angle 34-40°. Wave processes will work on the talus scree, gradually reducing it in size until it can be transported away.

● Rotational scars are a crescent-shaped scar left after rotational slumping occurs.

● Terraced cliff profiles

What are the 4 depositions processes

Traction: Larger rocks and pebbles are rolled along the seabed by wave and current action.

2. Saltation: Smaller particles (like sand and shingle) are bounced or skipped along the seabed by wave action.

3. Suspension: Lighter materials, like silt and clay, are carried in the water column by currents.

4. Solution: Some materials, such as chalk, are dissolved in the water and transported as dissolved ions.