Processes of river erosion

erosion

the weathering down of the bank and bed, along with particles being carried

transportation

the movement of particles with the flow of the river

deposition

The dropping of particles which have been carried in the river. This occurs when the river loses energy.

hydraulic action/quarrying

The pressure of water breaks away rock particles from the river bed and banks. The force of the water hits river banks and then pushes water into cracks. Air becomes compressed, pressure increases and the riverbank may, in time collapse.

abrasion/corrasion

the sediment carried by a river scours the bed and banks, it rubs over the surface wearing it away.

attrition

rocks collide and break into smaller fragments as they hit against eachother when they are moved in the flow of water. the pieces of sediment become smaller, with smoother, rounder edges as they move downstream.

solution/corrosion

carbon dioxide dissolves in the river to form a weak acid. this dissolves rock by chemical processes. this process is common where carbonate rocks such as limestone and chalk are evident in the channel

transportation - solution

minerals are dissolved in the water. this is a chemical change affecting rocks such as limestone and chalk. load transported in this way is called solute load.

transportation - suspension

fine light material (such as alluvium) is held up and carried within the river’s flow. this is called suspended load.

transportation - traction

large boulders and rocks are rolled along the river bed. load carried in this way is called bedload

transportation - saltation

small pebbles and stones are bounced along the river bed. the load is alternately lifted then dropped in line with a local rise and fall in the velocity of the water.

why do river landforms change over time?

* fluvial erosion
* transportation
* deposition

name the four types of transportation

* suspension
* traction
* saltation
* solution

when and why does deposition occur?

* occurs because there is a decrease in the river’s level of energy, hence the speed of the flow of the river is too slow for it to carry load
* occurs when
* there is a reduction in the gradient of the river
* the discharge is reduced, such as during a dry spell of weather
* there is shallow water, for example on the inside of a meander, therefore the water on the inside of the bend is moving slowly and cannot transport the load
* there is an increase in the size of load
* at the mouth of the river, where the river water flows against the direction of the sea, losing all its energy

what happens when the river floods and overtops its banks?

there is a reduced velocity on the floodplain outside the main channel

why does the upper course of the river have lots of potential energy? what does this enable the river to do?

* it is so high above sea level
* however, there is also a lot of friction due to rocks,, this reduces energy available

which direction does the river erode in the upper course?

vertically (steep)

which direction does the river erode in the middle and lower course?

laterally (wide flood plains)

v-shaped valleys formation

* these are found in the upper course of the river valley
* the river has a lot of potential energy and erodes vertically in order to acheive base level
* the exposed upper sides of the valley are affected by erosion by wind and rain e.g. sediment carried in wind or rain abrading the surface
* freeze thaw weathering and biological action
* freeze thaw is more likely on upper slopes as the greater exposure will mean greater temperature fluctuations and there will be greater plant growth and animal action on the upper slopes which can cause the rock to break apart (e.g. plant root growth)
* causing them to retreat backwards at a greater rate than the lower slopes and a V-shape is created
* vertical erosion makes the channel cut deeper into the landscape, leaving steep valley sides
* stream has to go around interlocking spurs
* weathering material is carried into the river by gravity and washed away

in which two ways can waterfalls be formed?

* glacial erosion
* differential erosion

describe the process of glacial erosion which forms waterfalls.

where waterfalls have formed due to the erosive power of a glacier during the ice age. glaciars carved steep valleys into the landscape, often hanging above one another. once the glacier melted, water drains from the smaller valleys and falls into the larger ones

describe the process of differential erosion which forms waterfalls.

* a waterfall normally forms when a band of resistant rock sits over a band of less resistant rock
* the water undercuts the more resistant rock via abrasion and hydraulic action, causing an overhang to form
* when the overhang can no longer supports its own weight, it collapses, adding large blocks of water to the base of the waterfall
* the sheer force of the water falling to the base abrades the base into a deep plunge pool
* overtime, the process of undercutting and collapsing is repeated many times, causing the waterfall to retreat and the plunge pool to deepen, as well as creating a steep sided gorge

what is a gorge?

a steep-sided narrow valley with a river running along the bottom of it.

how is a gorge formed?

when a waterfall collapses and retreats upstream, and has characteristic vertical sides

where are meanders usually found?

middle and lower courses of a river valley

what is a slip off slope?

a bank of gently sloping deposited material found on the inside bend of a meander

how is a meander formed?

* where the river is flowing faster (the thalweg), it will have more energy. this means it has greater power to erode via abrasion and hydraulic action
* the river is deepened and the bank is steepened
* a feature called a river cliff is created
* meanwhile, on the inside bend the river is slower and has less energy, and so sediment is deposited
* this creates the build-up of sediment called a slip off slope
* the river flow is forced outwards as more and more sediment builds up on the inner bank, the outer bank is eroded away further
* there is constant migration of the river channel

how can waterfalls change overtime?

* retreat backwards
* plunge pool can get deeper
* as the resistant top rock erodes away (very slowly) the height the water falls may decrease
* however, it can also increase if the river channel at the base erodes downwards at a greater rate

describe the formation of an oxbow lake.

* the thalweg is the path of fastest flow in the river and can be found on the outer bends of the meander
* the thalweg has a lot of energy, so will erode the outer edges of the meander via hydraulic action
* as a result, a river cliff may form, due to the undercutting of the bank on the outside bend
* simutaneously, the inner bend of the meander has very little energy, so nuch sediment is deposited on the inner banks of the meander
* this process often forms slip off slopes
* in the event of a flood, the meander neck will be cut through via erosion. a new path of least resistance for the water has opened up and the river will now flow straight and not through the meander (only a very small volume of water will flow through the meander)
* eventually new deposition carried by the river will seal the meander neck off, and warn the meander neck away. thus, an oxbow lake is formed

why is there large scale deposition at the mouth of the river?

as the river flows into the sea it loses all its energy

how much energy does the lower course of a river have and why?

* a lot of energy
* this is because many tributaries have joined and the water has eroded a wide and deep channel; the shape of the river is very efficient
* the proportion of water that touches the bed and banks is low compared to the cross sectional area and this means little friction impedes the flow
* this means energy is available to be used for transportation and deposition

where are wide floodplains found?

in the middle and lower courses of the river

why are meanders commonly found on floodplains?

as the river is no longer restricted by valley sided

describe the formation of a floodplain

* rivers flood on a regular basis. the area over which they flood is known as a floodplain
* when they flood velocity is slowed and deposition of any sediment being transported occurs
* the deposition leaves a layer of sediment (silt and alluvium) across the whole floodplain
* after multiple floods (over thousands of years) there are many layers of sediment on the area surrounding the river and this creates a wide flat area of land known as a floodplain

how are levees formed?

* energy is lost when the river floods due to friction with vegetation on the floodplain
* during flood, thick and coarsest sediments deposited at channel edges
* thin and fine sediments deposited over outer parts of floodplain
* largest sediment is deposited first because it takes more energy to transport it
* after many floods , natural levees built up by many floods

name 3 slope processes which result in cliff retreat

* weathering
* rockfalls
* landslides

process of freeze-thaw weathering

* physical
* rainwater enter crack on cliff face
* at night, temperature drops and water freezes and expands, making the crack bigger
* during the day the ice melts and more water is added to the crack
* process repeats until the rock falls off

process of salt crystal growth

* physical
* salt crystal growth
* seawater left on rock
* water evaporates, leaving salt behind
* salt crystals grow and exert pressure on the rock
* rocks broken apart

process of biological action

* plant roots
* plants grow on top of cliff
* roots push into cracks in the rock
* rocks broken apart
* burrowing animals
* small animals borrow through soil and into cracks in the rock
* rock is broken apart

process of chemical weathering

* carbonation
* rain water enters cracks on the cliff face
* the weak acid reacts with carbonates in the limestone
* cracks get bigger

weathering

the breakdown of rocks in situ by elements of weather

types of mass movement

* rockfalls
* landslides

mass movement

when soils, rocks and stone move down a slope

what happens once rock has been eroded from the upper part of a cliff?

* material moves down towards the beach resulting in mass movement, e.g. rockfalls and landslides
* parts of the cliff have been loosened by weathering processes
* evidence of rockfall at the base of the cliff - angular rocks
* waves hitting the base of the cliff can cause a rockfall

state 3 types of coastal erosion

* hydraulic action
* abrasion
* solution

hydraulic action at the coast

the force of waves crashing into cliffs. air trapped in the cracks is compressed, which breaks up the rock.

abrasion at the coast

waves hurl sand and pebbles against the cliff, which wears the land away. when pebbles grind along a platform, much like sand paper. over time the rock becomes smooth.

solution at the coast

salt water dissolves rocks made of calcium carbonate; chalk and limestone are prone to this type of erosion

attrition at the coast

pebbles are rolled back and forth. they collide with eachother which makes them smaller and rounder, eventually turning into sand

rockfall

* mass movement
* bits or rock fall off the cliff face, usually due to freeze-thaw weathering
* the movement of a mass of rocks off a cliff

mudflow

saturated soil (soil filled with water) flows down a slope

rotational slip

saturated soil slumps down a curved surface

process of longshore drift which creates spits

* a beach which juts out from the main land
* when swash is stronger than backwash and deposition occurs
* prevailing wind approaches beach at an angle
* swash pushes sediment up the beach at an angle
* the backwash pull sediment straight back down (at a 45 degree angle) to the sea due to gravity
* when the coast changes direction sediment continues to be moved by longshore drift and is deposited when the sea loses energy
* this process continues and sediment is moved along the beach by longshore drift
* the area behind the spit is sheltered from the power of the waves so silt is deposited and seeds begin to germinate forming salt marshes
* if the wind changes direction for a period of time the direction of sediment movement will change and a recurved end will form

formation of headlands and bays

* differential erosion
* less resistant rock is eroded at a greater rate than more resistant rock, creating bays and the more resistant rock sticks out as headlands
* headland juts into the sea
* beaches often form in sheltered bays

how are cliffs and wave cut platforms formed?

* cliff base is undercut vie hydraulic action and abrasion, creating a wave cut notch, making the cliff vulnerable to collapse. as this process continues the cliff retreats.
* with continued erosion at every high tide, the wave cut notch will make the cliff unstable and the overhang will collapse due to its own weight, as it is not being supported
* the material from the cliff will then be moved by the sea, and in doing so abrasion will smooth the surface of the wave-cut platform left behind
* if the cliff is made from well-jointed sedimentary rocks, then the wave-cut notch will often occur along the bedding planes, as these are a weak point and will erode much more quickly
* the wave cut platform is what’s left behiind as the cliffs retreat. rock pools are found on the wave cut platform due to differential erosion

what are bedding planes

gaps formed by hydraulic action/erosion

formation of arches

* form in headlands made from rock which is relatively resistant to erosion
* arches form when two caves are created on either side of a headland
* cracks at the base of the headland within the inter-tidal zone become exposed through hydraulic action, which presssurises air, forcing the crack to widen
* cracks are futher widened by weathering processes such as salt crystallisation and wet and dry weathering that affects chalk
* over time the cracks widen and develop as wave cut notches. further processes od hydraulic action and abrasion will deepen the notch to form caves
* as a result of wave refraction, which distorts the wave direction, destructive waves concentrate their energy on the sides. this deepens the caves
* wave refraction affects all three sides of the headland. if two caves are aligned the waves may cut through to form an arch. wave cut notches widen the base of the arch

landfall

a collapse of a mass of earth or rock from a cliff

formation of stacks

* formed from an arch
* over time, the sea is able to cut through the back wall by process of abrasion and hydraulic action
* once the sea has broken right through and water can pass underneath the rock, weathering will erode the roof of the arch so that the arch becomes higher and the roof becomes thinner
* at the same time, wave cut notches form on the base of the arch to widen it further
* eventually, the arch roof will become thinner until it eventually collapses (under its own weight), leaving a single pillar of rock called a stack

formation of beaches

* beaches form when swash is stronger than the backwash and deposition occurs
* a beach is a build-up of sand, shingle and pebbles deposited by waves
* sediment is deposited when there is not enough energy to transport it so is common in bays. pebble beaches are more common where cliffs are being eroded
* ridges or berms (lump/mound at back of beach) are common characteristics of a beach. they are small mounds coinciding with the high tide and storm tide lines (the furthest point in land where waves break). they form from sediment deposited by waves during high tide or storms respectively
* the exact profile and shape of a beach will depend on the geology and wave energy in the area. beaches help protect cliffs from erosion because they slow down waves and absorb some of the energy before waves break against cliffs. sand dunes develop on some beaches.

the impact of geology on coastal landforms concordant and discordant coastlines

* discordant coastlines - rocks of different geologies perpendicular to the coastline
* concordant geology - rocks of the same geology running parallel to the coastline
* headlands and bays are commont at discordant coastlines because the less resistant rock will erode at a greater rate than the more resistant geology

the impact of geology on coastal landforms rock type

* rock type is important in the formation of coastal landforms as less resistant rocks, such as clay will erode at a greater rate than more resistant rocks such as limestone
* differential rates of erosion are important in the formation of headlands and bays
* rock types are also important in the sediment budget of an area. If the local geology is weak more sediment will be added to the system through the erosion of cliffs. depending on energy and local currents (e.g. longshore drift), this sediment may be transported and deposited to form beaches.

example of concordant coast line

the north coast of the Llyn Peninsula is a concordant coastline, where layers of different rock types run parallel to the coastline. The metamorphic rock erodes at the same rate so the coast has few headlands and bays.

example of discordant coast line

the coastline between the Trwyn Llanbedrog headland and Abersoch Bay is a discordant coastline. The headland consists of more resistant igneous rock (erodes slowly) and the neighbouring Abersoch Bay of less resistant mudstone and shale (erodes faster). The different erosion rates result in the formation of headlands and bays

the effect of geology on coastland landscapes unconsolidated geologies

Unconsolidated geologies, such as clay, are easilv eroded. Waves erode the base of the cliff and a notch develops the upper material has nothing to support it and will collapse. This can happen very rapidly and the shape of the coast change in short periods of time. Due to the lack of internal strength features such as arches and caves won't be found on clay coastlines. The shape could never be held.

the effect of geology on coastal landscapes rock structure

* The number of bedding planes and joints within rocks determine how susceptible the coastline will be to erosion because they provide a place the sea can attack the rock. Sedimentary rocks have many more joints and bedding planes than igneous or metamorphic rocks because they are created in layers.
* Caves often start to form where the sea erodes a joint into a large space. Rock structure also determines the profile of cliffs and how stable they are.
* Horizontal and vertical bedding planes will produce cliffs with a very steep profile. As erosion creates a wave cut notch and material above it is unsupported and it will eventually fall. When bedding planes slope inland or towards the coast the profile of a cliff will resemble this.
* A cliff with bedding planes that dip inland is more stable than a cliff with bedding planes dipping towards the sea. When the bedding planes dip towards the land the rock is anchored into the land and given support and so is less likely to fall. Very weak geologies, such as clay, have little internal strength and will regularly slump. Notches created will last very short periods of time before gravity cause the cliff above to fall down. These cliffs change shape and retreat very quickly.

what happens if a coastline is made of boulder clay?

the foot is easily eaten away by waves, causing rotational slip, causing slumped profile

why are many cliffs on the South Coast of Dorsey vertical?

* made of sedimentary rock (chalk and limestone)
* horizontal bedding planes lead to vertical cliffs
* steep cliff profile
* more resistant to erosion

how important has rock structure been in the formation of Durdle Door, an arch in Dorset

* discordant coastline
* if it was concordant and only weak rock, cliffs would only retreat
* intricacies of bedding planes (pattern) may result in more/less jagged rocks
* weathering e.g. freeze-thaw and biological action shapes arch

how important has rock structure been in the formation of Old Harry, a stack in Dorset

* fissures in bedding planes, water can attack and expand via hydraulic action
* geology - solution (chemical breakdown) only chalk/limestone, certain rock types
* Old Harry’s Location - receives a lot of wave energy as he juts out from peninsula

the importance of prevailing wind direction on coastal processes

the prevailing wind affects the angle at which the waves break on to the coastline, and therefore the direction of erosion and transportation. the waves break on to the beach at this angle, pushing material up and across the beach. therefore, the wind direction determines where depositional features form (for example a spit).

the effect of climate on seasonal variation in river discharge

the more water that is flowing in a river, the higher the erosion rates will be. the highest erosion rates in UK rivers are found during the winter months when there is more rainfall.

the effect of extreme weather events on rates of landform change in both river and coastal environments

Extreme weather events may also alter the landscape. A powerful storm can change the appearance of a coastline overnight. The more severe the storm, the more destructive waves it creates. This is due not only to the increased wind speed, but also to the fetch - the distance the wave has travelled before breaking onshore. As a result, the most powerful and destructive storms that hit the UK are usually from the south-west. This is due to the large distance of open water that the Atlantic Ocean provides, which increases the fetch of the waves.

what type of waves are formed in calm weather?

* constructive waves
* which have a stronger swash than backwash
* this means they deposit sediment up the beach creating a berm

what type of waves are formed in stormy weather? 

* more likely in the winter
* destructive waves are formed
* stronger backwash than swash
* sediment is eroded and dragged from the beach and deposited a little way out to sea (when energy is lost so it can’t be transported) and a bar forms

how does the energy of the waves influence cliff retreat?

* cliff retreat will be greater in areas where there are lots of waves
* as cliffs retreat a wide wave cut platform will be formed
* powerful waves will also ate the formation of landform, such as arches, stacks and stumps as erosion rates will be higher

effect of extreme weather on Jurassic Coast cliff - Dorset beach

* rockfalls when collapsed
* collapses due to undercut being extremely eroded due to extreme weather, overhang’s weight cannot be supported then cliff collapses

effect of extreme weather on Portland Dorset, Pom Pom rock

* storm demolished stack, broken to pieces and washed away

effect of extreme weather on Cornwall Beaches

* Storm Eleanor changes shape of Cornwall Beaches
* sand cliffs are created as a result

effect of extreme weather on Cornwall Beach

* high tides and strong winds demolished arches
* storm broke flood defences and destroyed landmarks/arches

why do we manage coastal areas?

* tourism (sea side houses)
* heritage sites
* to protect infrastructure, people’s homes, habitats, trade area

how do coastal defences work?

they reflect the sea’s energy back out to the ocean or absorb the sea’s energy so less is transferred to the cliffs

state some hard engineering methods (coastal)

* groynes
* concrete sea walls and revetements
* rock armour
* gabions

state some soft engineering methods (coastal).

* beach nourishment
* managed realignment
* planting marram grass and osier hedges

effect of groynes and pros and cons

* repeated zigzag of swash and backwash piles sediment up against the windward side of each groyne
* highly effective in a local context (may deprive other areas of sediment)
* although relatively cost-effective to construct, require ongoing maintenance
* aesthetically questionable
* need for expensive hardwood timbers raises question of sustainability

effect of concrete sea walls and revetements and pros and cons

* massive barriers both absorb and reflect wave energy - recuvred wall designs turn waves back on themselves
* effective property defence in high-risk locations
* very expensive to both build and maintain
* by deflecting waves, rather than dissipitating their energy, concrete sea walls most notably, whether recurved or flat faced, are prone to relatively rapid erosion

effect of rock armour and pros and cons

* blocks of hard resistant rock (such as granite or other igneous rock) are laid at the foot of vulnerable cliffs and used to dissipitate wave energy
* relatively cheaper than concrete constructions
* rock armour traps both rotsam and jetsam, leading to both smell and rat infestation
* presents a potential public safety issue as a tripping and trapping hazard

effect of gabions and pros and cons

* metal cages filled with beach cobbles and pebbles both reflect and absorb wave energy
* cheaper again
* unsightly and prone to ch=age weathering (rusting)

effect of beach nourishment and pros and cons

* sand, shingle and coastal sediments are added to the beach from elsewhere. pumping from the offshore zone is not uncommon
* aesthetically pleasing
* arguably essential in supporting the tourism industry
* requires expensive annual replenishment to compensate for continued erosion and drifting

effect of managed realignment and pros and cons

* controlled erosion of the coastline is allowed to occur; does not always lead to erosion e.g. inundation and accretion (such as saltmarsh creation) can decrease erosion of defences
* potential for estuary tidal reduction has been noted but not proven
* politicaly sensitive, given loss of land and/or property

effect of planting marram grass and osier hedges and pros and cons

* roots help to both stabilise and hold soil together
* aesthetically pleasing
* requires protection from trampling until established fully

what factors determine the speed of coastal erosion?

* geology
* power of waves
* eroded material

how is the coastline being eroded

* force of waves crashing against rocks, pockets of air, small explosions - this is hydraulic action
* rotational slip - when weight of cliff can’t be held by itself due to wave cut notch and collapses
* abrasion - sand paper action on rock

why is Holderness the fastest retreating coastline in Europe, with an average of 2m of cliff lost per year

* weak geology made of weak boulder clay
* foot is easily eaten away by waves, casuing rotational slip, causing slumped profile
* unconsolidated material

why are many cliffs on the South Coast of Dorset vertical?

* made of sedimentaru rock (chalk and limestone)
* horizontal bedding planes lead to vertical cliffs

how is it decided where a coastline is protected?

* species habitat - bird sports
* natural beauty - tourism
* economic loss vs cost of defences
* cost benefit analysis

what does interrupting patterns of longshore drift lead to ?

* accelerated erosion

state 4 types of stores in drainage basins.

* interception store
* surface store
* soil mosture store
* groundwater store