River Landscapes of the UK

Question 1

What are the fluvial processes?

Answer 1

Erosion, transport and deposition. They happen in the river’s channel and valley, shaping it as it flows

Question 2

What are the main processes of erosion in a river?

Answer 2

Hydraulic action: water flows into cracks, compressing air in them. As it leaves pressure is released, weakening the rocks.
Abrasion: stones scratch and scrape their way down a river, wearing down its banks and bed.
Attrition: stones collide with each other, becoming smoother and more rounded or breaking up.
Solution: rocks like chalk and limestone dissolve as water travels over them.

Question 3

What is vertical erosion?

Answer 3

The deepening of the river bed, mostly by hydraulic action

Question 4

What is lateral erosion?

Answer 4

Sideways erosion, wearing away river banks

Question 5

How do rivers transport their load?

Answer 5

Solution: the smallest minerals are dissolved in the water and carried along as solute load.
Suspension: fine, light material like alluvium is held up and carried within the river’s flow, called suspended load.
Saltation: small pebbles and stones are bounced along the river bed, lifted then dropped in line with the rise and fall of the river’s velocity.
Traction: large boulders and rocks are rolled along the river bed, called bedload.

Question 6

Why do rivers deposit sediment?

Answer 6

When velocity falls, large boulders no longer have enough energy to keep moving, so they are dropped. As velocity falls further smaller and smaller sediment will be deposited until there is none less.

Question 7

What examples are there of where deposition will take place?

Answer 7

At the base of a waterfall
On the inside bend of a meander
Where the river enters the sea
Where the river enters a lake

Question 8

What is the Hjulstorm Curve?

Answer 8

A graph showing the different critical velocities and particle diameters at which erosion, transportation and deposition occur. Large particles 10mm in diameter are transported between velocities of 85 and 110cm/sec. Particles 0.1mm in diameter will be transported between velocities of 0.9 and 35cm/sec.

Question 9

What is the long profile of a river?

Answer 9

It shows the gradient of a river as it moves from source to mouth. It is a side view. The river tries to create a smooth gradient in order to reach its base level at the sea, called a graded long profile. The source is where it starts and the mouth is where it reaches the sea.

Question 10

How does the long profile of a river change downstream?

Answer 10

At the upper course there is steep relief. At the middle course it is lower down and there is a gentler gradient - it is hilly rather than mountainous. At the lower course the land is low lying and the relief is almost flat.

Question 11

Why does the long profile of a river change downstream?

Answer 11

Faster speeds mean it erodes to flatter ground. It flows naturally to lower ground from gravity. Vertical erosion happens more in the upper course.

Question 12

What is discharge?

Answer 12

The volume of water passing through a point in the river. It is measured in cumecs (m^3/s). It is calculated by velocity * cross sectional area

Question 13

Why do discharge and velocity increase downstream?

Answer 13

There is more friction in the upper course as more of the water is in contact with the bed and banks. More tributaries join the river downstream, increasing volume. Gravitational potential energy is converted into kinetic energy.

Question 14

What are the processes of erosion, transportation and deposition like in the upper, middle and lower courses of a river?

Answer 14

Upper: erosion - mostly vertical by hydraulic action transportation - mostly traction, large boulders moved deposition - large boulders deposited
Middle: erosion - less vertical, more lateral, attrition and abrasion, some solution transportation - mostly suspension, increased traction, load is smaller deposition - more deposition, especially on inside bends of meanders
Lower: erosion - very little, only lateral transportation - mostly suspension and solution, small particles of load, great quantity deposition - main fluvial process, fine material deposited

Question 15

How does the river valley cross profile change downstream?

Answer 15

Upper course: V shaped valley with steep sides. Middle course: valley floor is developing and the valley widens with less steep sides. Lower course: wide floodplains and levees either side of the channel. The valley floor is wide and gentle.

Question 16

Why does the river valley cross profile change downstream?

Answer 16

There is lots of vertical erosion and not much lateral erosion in the upper course. In the middle course there is more lateral erosion, widening the valley. In the lower course there is almost only lateral erosion so the valley is wider and the sides are lower. A build up of alluvium creates levees.

Question 17

How and why does the river channel cross profile change downstream?

Answer 17

In the upper course the channels are narrow and shallow from vertical erosion. There are large boulders, as velocity is too low to erode them. In the middle and lower courses the river performs more lateral erosion, so the channel is wider and smoother. Over time erosion smooths it out to make it more efficient.

Question 18

What are the characteristics and formation of river rapids?

Answer 18

Turbulent flow of water, white water, uneven river bed, steep gradient
Bands of resistant rock and less resistant rock are on the bed from vertical bedding. Differential rates of erosion steepen the gradient and make the bed uneven, speeding up flow and increasing turbulence, resulting in turbulent flow. E.g. Jackfields Rapids

Question 19

What are the characteristics and formation of waterfalls?

Answer 19

White water flows into a plunge pool at the base, indentation in the bare softer rock below the overhang, plunge pool deepening by abrasion and hydraulic action.
Hard rock is found over soft rock, resulting in a steeper gradient. There is undercutting and the waterfall erodes a plunge pool in the soft rock. The overhang collapses and the waterfall retreats back upstream to create a gorge. E.g. The Severn Breaks its Neck

Question 20

What are the characteristics and formation of gorges?

Answer 20

Narrow valley, steep high valley sides, located downstream of a waterfall, river channel takes up most or all of the valley floor, turbulent fast flowing white water, many areas of bare rock on valley sides, boulders litter river bed.
As a waterfall retreats upstream it leaves a steep sided valley called a gorge downstream. Every time the overhang cap rock breaks off, the gorge retreats and grows larger. E.g. Ironbridge Gorge

Question 21

What are the characteristics and formation of interlocking spurs?

Answer 21

Steep gradient, convex slopes, spurs project from alternate sides of the valley, separated by narrow valley floor mostly taken up by river channel, sometimes wooded, may have scree slopes
In a V shaped valley freeze thaw weathering widens the valley a little. Spurs of high land enter the valley floor from alternate sides of the valley. Obstacles of harder rock in the river’s path make it take the easiest route, resulting in projections of high land entering the valley. E.g. By the Hafren Forest

Question 22

What are meanders?

Answer 22

Wide bends of a river found mainly in lowland areas. They are the most efficient channel for a river. They are constantly changing shape and position. E.g. at Shrewsbury

Question 23

What is a thalweg?

Answer 23

The line of fastest velocity in a river, swinging from side to side and causing erosion on the outside bend and deposition on the inside bend, causing meanders to migrate.

Question 24

What are riffles and pools?

Answer 24

Riffles are shallow, fast flowing section of a river. They have deposits of coarser gravel
Pools are deeper, slower moving sections of a river. They have finer deposits and develop close to the outside of the meander.

Question 25

What are the characteristics and formation of meanders?

Answer 25

Inside bank with gentle slip off slope, sediment and vegetation far from the water. Outside bank with a steep drop that can be several meters high and bare earth. Bend in a river
A small disruption in the river’s flow e.g. a rock can cause the river to take an easier path around it at laterally erode the outer bank. The fastest flow is on the outer bend as it is deeper, so it erodes it further, while the inner bend experiences deposition. This causes meanders to migrate

Question 26

What are the characteristics and formation of oxbow lakes?

Answer 26

Gentle slip off slope, marsh plants colonise the edges, stagnant water, deepest water and steep drop on outer bend.
Meanders may start to erode towards each other. Gradually the neck narrows until it is completely broke through to form a new straighter channel. The old meander loop is cut off by deposition to form an oxbow lake.

Question 27

What are the characteristics and formation of levees?

Answer 27

Raised banks (2-8m), composed of gravel, stones and alluvium, sediment grades - coarsest closest to river channel, steep sided but steeper on channel side, fairly flat top covered by grass.
During a flood velocity drops so the heaviest most coarse sediment deposited closest to river, while finest particles further away as easier to transport. With every flood the river banks are built a little higher. E.g. The Severn Way

Question 28

What are the characteristics and formation of floodplains?

Answer 28

Large flat area of land prone to flooding, area of higher land usually along edge (bluff)
When meanders migrate to the edge of the floodplain they erode the bluff, widening it. Over multiple floods layers build up, creating a thick deposit of fertile alluvium. E.g. by the River Severn at Gloucester

Question 29

What are estuaries?

Answer 29

They are found at river mouths. They are transitional zones between river and coastal environments. They are affected by wave action and river processes, mainly deposition during a rising tide. At low tide the fine deposits form mudflats, which develop into saltmarshes. They are very wide: the River Severn’s is about 3.2km wide.

Question 30

What is the Severn Bore?

Answer 30

An unusual activity, where a large surge wave travels up the Severn against the current, increasing the river’s lateral erosion

Question 31

What is flooding?

Answer 31

Where land that is not normally underwater becomes inundated. A river flood occurs when a river’s channel can no longer hold the amount of water flowing it. Water then overlaps the banks and floods the adjacent land - the floodplain.

Question 32

What is the drainage basin hydrological system?

Answer 32

Precipitation enters the system. Some is intercepted by trees. Some is stored as surface storage, while some infiltrates the soil and is stored as soil moisture. Some then percolates into rocks and is stored as groundwater. Some runs off the surface as surface runoff, some flows through the soil as throughflow, some flows through rocks as groundwater flow. Some leaves the system as evaporation, some as transpiration, some in a river carrying water to a lake or the sea.

Question 33

What are some physical causes of flooding?

Answer 33

Bands of depressions - more rain
Torrential storms especially after drought - large increase in surface runoff
Prolonged light rainfall and antecedent rainfall - saturates soil
Sudden snow melt - water quickly enters system
Impermeable soil - more surface runoff
Geology - more surface runoff and throughflow
Relief - steep slopes means water moves faster to the river
Vegetation - trees intercept water and slow its flow

Question 34

What are the urban human causes of river flooding?

Answer 34

New infrastructure - means a greater area covered by impermeable surfaces e.g. concrete. More surface runoff
Disappearing gardens - saves mowing the law or allows for more car parking, but it means more impermeable surfaces. More surface runoff
New homes - increase in impermeable surfaces and water flows quickly through the drainage system. Quick rise in river discharge

Question 35

What are the rural human causes of river flooding?

Answer 35

Deforestation - land is needed for farming and housing, so trees are cut down, reducing interception and roots taking water from the soil. After felling, soil gets saturated and runoff occurs. Soil can wash into rivers, building up beds and reducing channel capacity.
Disappearing fields - fields are used for housing and riding stables or sheds and yards for factory farming. Overgrazing pastures has compacted soil, resulting in muddy runoff into rivers. Polytunnels reduce fields soaking up water.
Farming - loss of hedges means less interception. Crop farming leaves bare soil in winter, so there is no vegetation. Fields ploughed up and downhill create channels for water to flow down easily. More soil enters rivers and raises their beds.

Question 36

What are the social, economic and environmental effects of flooding?

Answer 36

Homes damaged, people evacuated, loss of power or clean water, blocked roads, death and injury, trauma
Businesses shut down, insurance premiums go up, infrastructure damage - high cost to councils, farms damaged, reduction in mortgage offers
Damaged habitats, sewage on land, animals drowned or starved

Question 37

What is hard engineering?

Answer 37

Using heavy machinery to build artificial structures which work against nature to reduce flooding risk

Question 38

What hard engineering strategies can reduce flood risk?

Answer 38

Channel straightening, embankments, flood relief channels, dams and reservoirs, flood barriers, dredging

Question 39

What is channel straightening?

Answer 39

Engineering a meandering river section to widen, straighten and deepen it, increasing discharge by reducing friction.
It can improve navigation, increasing tourism and trade and changes the river’s ecosystem potentially for the better, but does not work with some towns on a meander like Shrewsbury and causes increased sedimentation therefore increased flooding risk downstream, so some schemes have been reversed. E.g. Tributaries of the River Severn in Gloucestershire

Question 40

What are embankments?

Answer 40

Artificially raised river banks that enable more water to be contained within the river channel, made of impermeable soil.
They can make attractive footpaths, can provide habitats and are cheap compared to other options, but deprive river access for boating and fishing, are not the most reliable method, take up lots of space, and if they are breached water lies on land for a long time. E.g. along the Severn Estuary in Gloucestershire

Question 41

What are flood relief channels?

Answer 41

Artificially made channels designed as backup channels for a river that frequently floods, working like a bypass and taking excess water.
Some channels include reed beds and grass for new habitats, they remove flooding risk for an area and allow for boating and canoeing, but people have to be moved, are the most expensive form of engineering, and increase flooding risk downstream. E.g. Jubilee River channel for River Thames in Maidenhead

Question 42

What are dams and reservoirs?

Answer 42

A large concrete barrier built along a river to impede its flow, causing the valley behind it to flood (forming a reservoir). The supply of water downstream is restricted and water can be released in a controlled manner through sluice gates.
It provides jobs, infrastructure, hydroelectric power, new habitats, boosts tourism and is highly effective, but displaces people, is expensive, interferes with the path of fish and makes soils downstream less fertile through lack of sediment from few floods. E.g. Clywedog Reservoir in the River Severn’s upper course

Question 43

What are flood barriers?

Answer 43

Metal barriers that can be erected quickly when flood warnings are issued, then removed again after the channel discharge has subsided. They stop water flowing out onto the floodplains by holding it back.
They are relatively inexpensive, can be put up in areas of little space, can be put up and down quickly and can be added to existing flood defences, but there needs to be the conditions for foundations, rely on accurate and quick flood warnings and are not a permanent solution. E.g. Bewdley, Worcestershire along the River Severn

Question 44

What is dredging?

Answer 44

Removing silt from the river’s bed using an excavator or vacuum pump, increasing the river’s capacity to carry water downstream, so it has a higher discharge.
They are often combined with other defences and are not too expensive, but cannot prevent flooding on their own, machinery can disrupt locals, can weaken riverbanks and after major floods silt accumulates and it may need to be done again. E.g. Farmers in Somerset claimed a lack of dredging worsened flooding impact

Question 45

What is a flood hydrograph?

Answer 45

A graph showing how a river’s discharge changes in response to a precipitation event. Vertical axis measures precipitation (mm) and discharge (cumecs). Horizontal axis represents time (usually hours or days). Bars represent rainfall and line graph shows discharge. The lag time is the time between peak rainfall and peak discharge. If it is shorter, this means a flood is more likely to occur as the river cannot cope with so much precipitation in a short amount of time.

Question 46

What are some key terms for a flood hydrograph?

Answer 46

Peak rainfall - highest amount of rainfall
Rising limb - how quickly discharge rises after rain
Peak discharge - highest recorded discharge following rain event
Lag time - time between peak rainfall and peak discharge
Falling limb - shows the reduced discharge once main effect of runoff has passed
Base flow - normal flow of river when its water level is being sustained by groundwater flow

Question 47

What might cause a flashy hydrograph rather than a low, flat hydrograph?

Answer 47

Flashy: small banks often lead to rapid water transfer, high drainage density speeds up water transfer, impermeable rock, urbanisation, steep slopes, saturated soil, heavy rain
Low, flat: large basins, low density, permeable rocks, forests, gentle slopes, dry soil

Question 48

What is soft engineering?

Answer 48

Working with natural river processes to reduce the risk of flooding. It does not try to stop these processes, rather it aims to manage and slow the movement of water into a river channel to help prevent flooding

Question 49

What soft engineering strategies can reduce flood risk?

Answer 49

Afforestation, wetlands and flood storage areas, floodplain zoning, river restoration

Question 50

What is afforestation?

Answer 50

Planting shelter belts of trees across slopes and woodland in floodplains to increase interception and reduce discharge quickly reaching rivers.
They also absorb CO2 and other pollutants, add variety to landscapes, increase biodiversity and are relatively inexpensive as there are EU grants, but could be artificial looking, means you lose potential grazing land, and take up lots of space.

Question 51

What are wetlands and flood storage areas?

Answer 51

On river floodplains wetlands can store water very efficiently, so are deliberately allowed to flood to form flood storage areas. Water can be stored to reduce risk of flooding.
They can contain vegetation, which can increase interception, and require a space between rivers and settlements, reducing flood risk, but many towns can only release a small amount or land, which may be expensive to leave empty. E.g. Worcester along the River Severn

Question 52

What is floodplain zoning?

Answer 52

Where land in a major valley is used to minimise flooding impact. Authorities use Environment Agency maps on flood risk zones to guide decisions on building applications. Land next to rivers is left empty, while settlements are built further away.
It is low cost and requires little effort apart from administration, traditional water meadows are protected, providing habitats and green spaces are available, but many towns have already sprawled over floodplains and there is already a housing shortage.

Question 53

What is river restoration?

Answer 53

Where a river that has previously been hard engineered is restored to a natural channel. It uses the natural processes and features of a river such as meanders and wetlands to slow down river flow.
It creates new wetland habitats and increases biodiversity, increased water storage areas reduce risk of flooding and it is aesthetically pleasing, but there is a possible loss of agricultural land and flooding of crops near the river, can be expensive and is not always the most effective or practical strategy. E.g. the River Quaggy in Greenwich

Question 54

Why was the Jubilee flood relief channel needed?

Answer 54

It is a low lying part of the Thames flood plain so is prone to flooding
Windsor is a popular tourist attraction and Eton has a famous private school
There are many built up and so impermeable areas prone to flash flooding

Question 55

What are the main features of the Jubilee River scheme?

Answer 55

It is a relief channel for the Thames in south-east England, starting by Maidenhead and rejoining after Eton
It cost £110m, opening in 2002
It has meanders and reed beds to look more natural

Question 56

What are the social issues of the Jubilee scheme?

Answer 56

The scheme rejoins near Old Windsor, where people are less wealthy - in 2014 they suffered the worst flooding in 70 years
Paddle boaters were promised a navigable river, but some sections are too dangerous to cross

Question 57

What are the economic issues of the Jubilee scheme?

Answer 57

It is the most expensive flood relief scheme in the UK
The Environmental Agency ran short on funds so local councils and businesses had to make up the shortfall
Upkeep of the weirs is expensive

Question 58

What are the environmental issues of the Jubilee scheme?

Answer 58

Concrete weirs are ugly and algae collects behind them, disrupting the ecosystem
In 2014 there was flooding in Old Windsor