River landscapes Flashcards
(90 cards)
1
Q
Describe how the long profile of a river changes downstream
A
Steepest gradient is at the source of the river. The gradient reduces as the river flows from uplands to lowlands and on to the sea. May be slight irregularities in the long profile due to geology. Bands of tough, resistant rock may form ‘steps’ in profile marked by waterfalls.
2
Q
Describe how the cross profile of a river changes downstream
A
River channel becomes wider and deeper with distance downstream.
3
Q
Explain why the cross profile of a river changes downstream
A
As river travels downstream it is joined by tributaries, increasing the volume of water, giving the water a higher velocity and therefore more erosive power. This allows it to cut a deeper channel as distance downstream increases. Channel becomes wider downstream because gradient decreases so there is less vertical erosion. By the time the river is in middle course, lateral erosion is dominant, eroding the river banks.
4
Q
Describe the river profile in upper course of a river
A
Narrow and steep-sided due to vertical erosion (hydraulic and abrasive) by the river as well as weathering and mass movement of valley slopes.
5
Q
Describe the river profile in middle course of a river
A
Wider valley floor formed by lateral erosion of a meandering river, gentler valley side-slopes.
6
Q
Describe the river profile in lower course of a river
A
Almost flat valley floor (wide floodplain and meandering river). Much of valley floor is sediment deposited by the river, accounting for its flatness.
7
Q
Describe erosion, transport and deposition in upper course
A
Erosion - Vertical erosion by hydraulic action
Transport - Mostly traction, large boulders moved
Deposition - Large boulders deposited
8
Q
Explain discharge and velocity in upper course
A
Low velocity and small discharge. Small channel means lots of friction between water and river banks, slowing down the river
9
Q
Describe erosion, transport and deposition in middle course
A
Erosion - Less vertical erosion, more lateral erosion
Transport - Mostly suspension, increased traction. Load becomes smaller and less angular
Deposition - More deposition, especially on inside bend of meanders
10
Q
Explain discharge and velocity in middle course
A
Fairly high discharge and velocity due to wider, deeper channel so less water in contact with bed and banks, less friction.
11
Q
Describe erosion, transport and deposition in lower course
A
Erosion - Very little erosion, only lateral erosion
Transport - Mostly suspension and solution. Small particles of load yet great quantity of it
Deposition - Main fluvial process. Fine material deposited
12
Q
Explain discharge and velocity in lower course
A
Highest discharge and velocity as very broad, deep channels means less water is in contact with bed and banks, velocity is much higher. Additional discharge from all the tributaries also boosts speed.
13
Q
State the 4 types of erosion in rivers
A
Hydraulic action
Abrasion
Solution
Attrition
14
Q
Explain the erosional process of hydraulic action
A
Sheer force of fast-flowing water hits river banks and beds, forcing water into the cracks which compresses the air inside them, eventually weakening the channel.
15
Q
Explain the erosional process of abrasion
A
Small boulders and stones carried by the river scrape and scratch the river’s bed and banks, acting as sandpaper. Stones which have recently fallen into river will be angular and have sharp, jagged edges, acting as perfect tools for abrasion.
16
Q
Explain the erosional process of attrition
A
Affects a river’s load rather than river bed and banks. When stones first fall into river, they are jagged and angular. As they are transported downstream, stones collide with each other and also the river’s bed and banks. Stone’s jagged edges are knocked off so they become smooth and rounded. Some collisions also cause stones to smash into several smaller stones.
17
Q
Explain the erosional process of solution
A
The dissolving of soluble chemicals (certain rocks) such as chalk and limestone as the river travels over them.
18
Q
In what course(s) does hydraulic action take place?
A
Responsible for vertical erosion in the upper course.
Responsible for lateral erosion of banks on outside bend of meander in lower course.
19
Q
In what course(s) does abrasion take place?
A
Responsible for both vertical and lateral erosion of channel throughout river course
20
Q
Describe vertical erosion and which course it occurs in
A
Deepening of the river bed, mostly by hydraulic action. Mostly happens in upper course where the little energy of river left over after overcoming friction is used to deepen its channel.
21
Q
Describe lateral erosion and which course it occurs in
A
‘Sideways’ erosion. Wears away the banks of the river. Happens in middle and lower courses of river.
22
Q
State the 4 types of transportation in rivers
A
Traction
Saltation
Suspension
Solution (different from erosional one)
23
Q
Explain the transportation process of traction
A
Large boulders and rocks are rolled along the river bed by force of water. Called bedload.
24
Q
Explain the transportation process of saltation
A
Small pebbles and stones are bounced along the river bed. Load is lifted and dropped as velocity of water rises and falls.
25
Explain the transportation process of suspension
Fine, light material is held up and carried within the river's flow. Called suspended load.
26
Explain the transportation process of solution
Minerals dissolved in the water. A chemical change affecting rocks such as limestone and chalk. Called solute load.
27
Describe when material is deposited by a river and what size of material
River sediment is deposited in low flow conditions, when velocity of river can no longer carry the sediment load. This can occur anywhere along a river's course when velocity falls. When velocity falls, large boulders are the first to be deposited and finest particles are deposited last. More material is deposited during a drought when discharge is low.
28
State the 3 landforms of the upper course of a river
Interlocking spurs
Gorges
Waterfalls
29
Explain how interlocking spurs are created
Winding path taken by river is due to obstacles of harder rock in its path, River takes easiest route over the land due to low velocity and low energy. Results in projections of high land entering the valley from alternate. Vertical rather than lateral erosion dominates in upper course of a river, deepening the valley but not eroding sideways, explaining why the river is forced to flow around harder rock.
30
Explain how waterfalls are created
Waterfall forms where there is a junction between hard rock upstream and soft rock downstream. Differential erosion means river erodes softer rock and water falls vertically from the hard rock to soft rock below. Water hits bottom of fall with great force, eroding a deep plunge pool by abrasion and hydraulic erosion. Splash back of water causes hydraulic action to weaken rocks behind waterfall. Continued undercutting of softer rock below hard rock means hard rock cap overhangs the drop. This hard rock cap eventually collapses. Fallen rock becomes trapped in bed, forming plunge pool by abrasion.
31
Explain how gorges are created from waterfalls
(After waterfall process)
Erosion (hydraulic action and abrasion) undercut hard rock, forming waterfall and creating an overhang of hard cap rock. This cap rock collapses into the plunge pool, causing waterfall to retreat upstream. This process of undercutting and collapsing repeat, and gorge grows further.
32
Explain how water 'rapids' are created from a waterfall
Differential erosion of hard rock and soft rock (soft rock eroded more easily) causes the river bed to be uneven, making the river's flow turbulent
33
Describe the characteristics of a gorge
Narrow valley with steep sides
Located immediately downstream of a waterfall
Turbulent, fast-flowing white water
River channel takes up most of valley floor
34
State the landforms in middle course of river
Meanders
Oxbow Lake
35
State the landforms in lower course of river
Levees
Floodplains
Estuaries
36
Explain how meanders are formed
Fast-flowing water on the outside bank causes lateral erosion (abrasion and hydraulic action), undercutting the bank and forming a river cliff. Corkscrew movement (called helicoidal flow). After top part of flow hits outside bank. eroding it, flow 'corkscrews' down to the next inside bend, depositing its load (sand and pebbles) as friction slows it down. Water moves slower on inside bend. This forms a gentle slip-off slope.
Fast flow causes vertical erosion on the outside bend, deepening the river bed and resulting in an asymmetric cross-profile.
37
Explain how an oxbow lake is formed
Meander loop becomes very large leaving only a narrow strip of land (meander neck) separating the river channel. Due to continued lateral erosion (abrasion and hydraulic action) the neck becomes increasingly narrow.
During high flow (flood) conditions the meander neck is broken through. River adopts shorter route, carving a new channel. Deposition occurs at edges of new banks of straight section, cutting off the meander. Marsh plants colonise the area widening the gap.
38
Explain how levees are formed
When a river bursts its banks, friction with land reduces velocity of water, causing deposition. Heavy sediment is deposited closest to the river. Size of sediment becomes progressively smaller with increased distance from the river . With each successive flood, banks are built up higher. River bed builds up bedload deposits over time as well, raising the level of the river and therefore increasing the probability of flooding.
39
Explain how flood plains are formed
During a flood, water containing large amounts of alluvium (river silt) pours over the flat valley floor. Water slowly soaks away, leaving behind deposited sediment. Over hundreds of years, repeated flooding forms a thick, alluvial deposit which is fertile and often used for farming.
Flood plains become wider because of lateral erosion of meanders. When outside bend of meander meets the edge of river valley, erosion cuts into it (hydraulic action and abrasion), widening the valley. As meanders slowly migrate downstream, the entire length of the valley is eventually widened.
40
Explain how estuaries are formed
Incoming tides force seawater and sediment up the river channel, mixing with river water (also carrying large amounts of sediment) flowing in the opposite direction towards the sea. Where the incoming seawater meets the outgoing freshwater, velocity falls dramatically, causing lots of deposition of sediment. Over time, these muddy sediments break the water surface during low tide, forming extensive mudflats
Vegetation colonises the mudflats, forming saltmarshes.
41
Characteristics of estuaries
Mudflats and saltmarshes
Formed at river mouths (where river meets the sea)
42
Which landforms are formed by erosion alone?
Waterfalls
Gorges
Interlocking spurs
43
Which landforms are formed by both erosion and deposition?
Meanders
Ox-bow lakes
44
Which landforms are formed by deposition alone?
Levees
Estuaries
Flood plains
45
State the 3 physical causes of flooding
Precipitation
Geology
Relief
46
Explain how precipitation can increase flood risk
Bands of depression passing over the UK result in continuous heavy rain, saturating the soil. The soil can no longer store water so surface runoff increases. Rainwater therefore enters the river quicker resulting in higher discharge and floods.
Sudden bursts of rain often result in infiltration rate being too slow to cope. This may occur after a period of droughts that has baked the soil hard. Surface runoff occurs, discharge increases and flash floods occur.
Sudden snow melt results in a release of stored water that flows over the ground as surface runoff.
47
Explain how geology can increase flood risk
Mountain rock is usually impermeable with thin soils and little vegetation to intercept the rain, resulting in rapid overland flow.
Low-lying areas often have impermeable clay soil, although it is usually vegetated. This compacted soil makes it difficult for infiltration to occur.
Flood risk is reduced in areas of permeable rock such as chalk.
48
Explain how relief can affect flood risk
Steep slopes mean surface runoff occurs before rain has had time to infiltrate the soils, increasing flood risk.
Low-lying, flat flood plains also have a high flood risk as there isn't enough gradient to remove the water, This flood risk is further increased by impermeable clay soils.
49
State the main human factor that affects flood risk (umbrella factor)
Land use (farming, urbanisation, deforestation)
50
Explain how farming as a land use can affect flood risk
Farming - One crops have been harvested, they leave soil bare in winter, leaving no vegetation to intercept rainfall, increasing surface runoff.
When fields are ploughed, the furrows create channels for water to flow down easily. More soil is also transported into rivers, raising river beds and increasing flood risk.
51
Explain how urbanisation as a land use can affect flood risk
Urbanisation - Growth of towns and cities means greater area is covered by buildings and roads (impermeable surfaces), increasing flood risk. Increased demand for houses means new houses are built on greenfield sites, the reduction of vegetation means little rainwater absorption and evaporation. Water runs off through gutters and drains, quickly increasing a river's discharge.
52
Explain how deforestation (land use) can affect flood risk
Deforestation - Felling trees reduces interception and roots no longer take water from the soil. After felling, the soil becomes saturated, runoff occurs, river discharge increases quickly and so the risk of flooding increases. Felling trees also causes exposed soil to wash into rivers, building up their beds. This reduces the capacity of the channels, so rivers are more likely to flood.
53
What are the causes of a 'flashy' rapid response hydrograph, demonstrating high flood risk?
Steep slopes resulting in rapid runoff
Impermeable rocks encouraging rapid overland flow.
Heavy/prolonger rainfall.
Saturated or frozen soils
Deforestation encourages rapid transfer of water to rivers.
Urbanisation - impermeable surfaces encourage rapid overland flow.
54
What are the causes of a 'flat' slow response hydrograph posing a low flood risk?
Gentle slopes slow down water transfer
Permeable rocks allow water to soak into rocks (slow transfer)
Drizzle rain
Deep, dry soils to absorb water
Afforestation results in water being intercepted and evaporated.
55
What is the rising limb of a hydrograph?
Shows how quickly the discharge rises after a rain storm (first part of line graph)
56
What is the lag time of a hydrograph?
The time difference between peak rainfall and peak discharge (measure horizontal distance between top rainfall bar to top discharge line)
57
What is falling limb of a hydrograph?
Shows the reduced discharge once the effect of runoff has passed (last part of line graph)
58
What is bankfull discharge of a hydrograph?
A horizontal line marking level of discharge above which flooding will occur as river bursts its banks.
59
What is base flow of a hydrograph?
Normal flow of a river when its water level is sustained by groundwater flow (shows by a separate line)
60
What are the differences between the appearance of a slow response hydrograph and flashy response hydrograph?
Slow response - Less steep rising limb. Peak discharge is lower and lag time is longer. Low flood risk.
Flashy response - Steep rising limb. High peak discharge, shorter lag time. High flood risk.
61
State the 4 hard engineering strategies
Dams and reservoirs
Channel straightening
Embankments
Flood relief channels
62
State the 4 soft engineering strategies
Flood warnings and preparation
Floodplain zoning
Planting trees (afforestation)
River restoration
63
What are the benefits of dams and reservoirs
Provides HEP power - Benefits industry and meets domestic needs. Kielder dam provides electricity for 10,000 people.
Boosts tourism - Opportunities for sailing, fishing, walking. Kielder dam attracts 300,000 tourists annually, boosting economy by £6 million.
Highly effective against floods - Releases of water are controlled, controlling discharge, ensuring no flooding occurs downstream.
Environmental benefits - creation of wetland habitats, benefitting marine biodiversity. Forestry around reservoir. 150 million trees planted at Kielder.
64
What are the costs of dams and reservoirs?
Displacement of people - Causes distress and breaks up communities. Kielder reservoir led to relocation of 58 families.
High economic costs - Kielder dam cost £167 million
Effects on farmers - Soils downstream become less fertile due to lack of sediment from floods, reducing crop yields. Farmland is lost by flooding for reservoir.
Environmental effects - Concrete dam interferes with path of migrating fish. At Kielder, 1.5 million trees were lost to make room for reservoir.
64
What are the benefits of channel straightening?
Insurance premiums are reduced for homeowners due to lower flood risk.
Straightened course of river can improve navigation such as Portrack cut on River Tees.
Effective in speeding up water flow along a short stretch of river as there is less friction with bed and banks. Fast-flowing water also removes sediment that would have built up height of river bed and caused flooding.
65
What are the costs of channel straightening?
Straightening speeds up water flow along a river, increasing risk of flooding along meandering sections downstream as sediment will increase downstream.
Dredging a river to remove silt downstream (as a result of straightened channel) is very expensive.
Aquatic habitats can be harmed by changes in velocity of the river, changing ecosystem of river.
Some straightened rivers have a concrete lining, making it unattractive as well as preventing river bank animals from burrowing.
66
What are the benefits of embankments?
Reduces flood risk effectively as river channel has an increased carrying capacity and is less likely to burst its raised banks.
Embankments are often used as attractive river-side walkways for local people.
Earthen embankments provide habitats for riverbank animals such as otters.
67
What are the costs of embankments?
Higher maintenance costs than other management strategies as they need constant monitoring and repair.
Earthen embankments are prone to erosion, increasing sedimentation downstream which will incur a dredging cost to avoid flooding.
If the embankment is breached, water lies on land for a long time as it has a restricted route back to the river, leading to more serious damages.
Concrete linings prevent riverbank animals from burrowing and can look unattractive.
68
What are the benefits of flood relief channels?
Effective in reducing flood risk in high-value areas as relief channel takes excess water during times of high discharge.
New channel provides recreational opportunities; footpaths and cycle tracks built, calm water provides areas for canoeing.
Some relief channels include artificial reed beds, used to create new habitats.
Insurance premiums are reduced for local people and value of homes increases. Jubliee River reduced flood risk for 3000 properties.
69
Costs of flood relief channel
Can be very expensive if constructed across high-value land. Jubilee River cost £110 million.
Regular, expensive maintenance is required.
Settlements downstream of relief channel suffer from increased flooding as the merging of water from relief and main river swells this part of the channel.
Relief channels look unattractive in times of low flow as concrete lining is exposed.
70
Case study for flood management scheme
Jubilee River flood-relief channel
71
Why was the Jubilee River flood-relief channel required?
This particular area of the Thames flood plain is low-lying and prone to flooding. Area contains royal settlement of Windsor, attracting many tourists, as well as Eton, home to the prestigious public school. High-value property in the area. Due to impermeable surfaces of the built-up area, this area had experienced flooding in the past after heavy rainfall.
72
State general facts about Jubilee River flood-relief channel
Funded by Environment Agency and cost £110 million. It opened in 2002 and is 11.7km long. It has 5 weirs (large dams) along its course.
73
Describe how the Jubilee River flood-relief channel is effective
When discharge is high (following high rainfall), Jubilee River effectively diverts water from the River Thames, preventing the River Thames from overflowing its banks. This reduces the flood risk in Maidenhead, Eton and Windsor.
Under normal conditions, level of water in Jubilee River is low.
74
Describe the social issues of the Jubilee River flood-relief channel
Protection of wealthy properties in Maidenhead and Eton at the expense of less affluent areas further downstream at Wraysbury, below the confluence of the Jubilee River. These settlements are more prone to flooding now due to the merging of water from relief and main river which swells this part of the channel.
Paddleboarders were promised a navigable river yet are disrupted by weirs.
75
Describe the economic issues of the Jubilee River flood-relief channel
Most expensive flood-relief scheme in the UK with a total cost of £110 million. One year after its completion, weirs were damaged by floods. Maintaining the channel is a huge economic burden.
Additional flood relief measures are required downstream however the EA doesn't have enough money to fund them.
Insurance costs in downstream areas still affected by flooding are high. In 2014, business insurance costs for Wraysbury were around £500 million.
76
Describe the environmental issues of the Jubilee River flood-relief channel
Extensive flooding occured in 2014 downstream from where the flood-relief channel re-joined the Thames. Resulted in flooded roads, buildings, fields and disturbed habitats.
Concrete weirs are unattractive, especially during normal flow conditions when the concrete is more exposed
Algae collects behind the weirs, affecting the aquatic habitat through its oxygen consumption.
77
Benefits of flood warnings and preparation
Cheap way of protecting people and property without high costs of hard engineering.
If people are warned of a flood in advance, they protect their valuables earlier.
78
Costs of flood warnings and preparation
Need for monitoring equipment, scientific expertise and computer modelling.
People may not respond appropriately, particularly if warnings turn out to be false alarms. Only effective if people take action and listen.
79
Benefits of floodplain zoning
Reduces additional impermeable surface coverage of floodplain, reducing risk of flooding.
Low-cost, only administration costs are involved.
Traditional water meadows and habitats are protected from development and can also be used for recreational activities.
Reduces insurance costs when property is flooded.
80
Costs of floodplain zoning
Has limited impact as many cities have already sprawled over active floodplain.
Restricts economic development in an area as certain land uses are prohibited and difficult to get planning permission.
Current housing shortages will continue if land cannot be used for building. House prices will inflate.
Habitats may be destroyed by building elsewhere (on greenfield sites)
81
Benefits of planting trees/afforestation for flood management
Benefits wildlife by adding new habitats and creating more biodiversity.
Reduces water flowing downstream as trees reduce surface run-off by interception.
Helps to absorb and store carbon dioxide.
82
Costs of planting trees/afforestation for flood management
Loss of potential grazing land for farming
Can reduce habitat and species when hillsides are covered in trees.
Can lead to increased acidity in soils.
83
Benefits of river restoration
Recreates a natural, attractive fluvial environment, increases habitats and biodiversity.
Increased water storage areas reduce risk of flooding downstream. River Quaggy scheme protected 600 homes and businesses.
84
Costs of river restoration
Can be expensive to construct new channels and maintain them. River Quaggy cost £1.1 million
Some flooding will still occur in the area, creating inconvenience for local people using floodplain.
Change in land use and potential loss of agricultural land
85
Describe the process of river restoration
Restoring straightened channels to their previous meandering pattern to slow down water flow, reducing flood risk downstream although small floods occur in restored area.
86
Describe the process of planting trees
Increases interception of precipitation, reducing amount of water that would flow into the river. Trees encourage water to percolate into soils, storing it
87
Describe the process of floodplain zoning
Local planning to restrict land uses in high-risk areas, so that high-value land uses are far away from flood-prone areas.
88
Describe the process of flood warnings and preparation
Rivers are constantly monitored and computer programs enable scientists to predict possible flooding, issuing warning and evacuating people.
89
Major landforms of erosion and deposition in the UK example River Severn
Severn estuary (where river meets Bristol channel)
Levees at Ministerworth near Gloucester
Floodplain at Tewkesbury, Gloucestershire
Meander in Shrewsbury
Water Break-its-Neck waterfall
