Rivers Flashcards
1
Q
What is a drainage basin?
A
an area of land drained by a river and its tributaries
2
Q
What is a confluence?
A
The point at which two rivers join
3
Q
What is a mouth?
A
Where the river flows into the sea or sometimes a lake.
4
Q
What is the source?
A
The upland area where the river begins.
5
Q
What is the watershed?
A
The boundary dividing one drainage basin from another - a ridge of high land.
6
Q
What is a tributary?
A
A river that joins a larger river.
7
Q
What is percolation?
A
The filtering of water downwards through soil and through the joints and pores of a permeable rock.
8
Q
What is soil moisture store?
A
water stored in the soil
9
Q
What does saturated mean?
A
The soil can’t hold any more water.
10
Q
What is the field capacity?
A
When a soil is drained, some water cannot drain as it is stuck to the outside of water particles. The volume of this water is known as the field capacity. However, it can be used by plant roots and is know as soil moisture utilisation. It is called field capacity as there is an electric field which sticks the water to the soil particles.
11
Q
What is the imput to a drainage basin system?
A
Precipitation.
12
Q
What are forms of storage in the drainage basin system?
A
Interception storage (water droplets on trees and plants) surface storage (puddles or lakes) soil moisture storage (water in the soil) groundwater storage (water stored in porous rocks)
13
Q
What are transfers of water in the drainage basin system?
A
Stemflow
Surface run off
Infiltration ( surface water soaking into soil)
Throughflow ( downhill through the soil layer)
Percolation (water moving downwards from soil into rocks)
Groundwater flow (slow movement through rock layers deep below surface, towards river.
14
Q
What are outputs of the drainage basin system?
A
Channel output ( water leaving the system in a river, to the sea)
Transpiration ( water vapour released by trees and plants)
Evapotranspiration (transpiration and surface water lost through evaporation)
15
Q
What is the water budget?
A
The relationship between inputs and outputs in a drainage basin system. Allows you to work out the amount of water in a drainage basin throughout the year. Can help predict flooding and water shortages.
P=Q+E+/- change in storage in soil or bedrock.
Where:
P= precipitation
Q= Run off
E = Evapotranspiration
16
Q
How is river discharge calculated?
A
By determining the discharge in each subsection of a channel cross section and summing the subsection discharges to obtain a total.
Area = Depth x width
Discharge = Area x Velocity
Discharge is measured in cumecs (cubic metres per second)
17
Q
What is a flood?
A
When a river bursts its banks and spills out onto the floodplain. Q>Qbf
discharge> bankfull storage
18
Q
What is the lag time?
A
The time gap between peak rainfall and peak discharge.
19
Q
What factors result in a short lag time?
A
- impermeable surfaces
- steep relief
- sparse vegetation
- shallow soil
- high drainage density
20
Q
Which factors result in a long lag time?
A
- gentle relief
- lots of vegetation
- permeable surfaces for infiltration
- deep soil that can hold more water
- low drainage density
21
Q
What is hydraulic radius and how is it calculated?
A
Hydraulic radius is the efficiency of a channel.
Hydraulic radius = cross sectional area/wetter perimeter
22
Q
How does channel shape affect river velocity?
A
Greater wetter perimeter means increased friction so slower velocity. A narrow deep channel has a smaller wetter perimeter so flows faster than a wide shallow channel.
23
Q
How does river gradient affect river velocity?
A
A steeper slope means a stronger gravitational force pulling the water downwards than there would be on a gentle gradient.
24
Q
How does channel roughness affect river velocity?
A
More rocks and boulders in the channel increase its roughness and create more friction, reducing the velocity. Channels with minimal roughness have faster velocities.
25
How does volume of water affect river velocity?
An increasing mass of water (at confluences) mean the water is more capable of erosion, resulting in wider deeper channels which allows water to flow faster as there is less resistance.
26
What is sinuosity?
The bendiness of a river channel. Sinuous index is calculated by:
Channel length/down valley length (direct route)
27
What is a river's capacity?
The total volume of sediment a river can transport?
28
What is river competence?
Largest sized sediment a river can transport.
29
What is abrasion?
When a river picks up material and then rubs it against its beds and banks. Erosion occurs by the process of abrasion and is most effective during times of flood.
30
What is attrition?
When boulders collide with one another as they move down the river and break into smaller pieces. The rocks become more smooth over time.
31
What is hydraulic action?
The force of water that hits river banks and pushes water into cracks exploiting weakness in the bank and causing the river bed to in time collapse.
32
What is corrosion?
A continual process as a result of the chemical composition of the water.
33
What are the main factors leading to deposition?
- Low rainfall reducing precipitation
- Increase in load
- Water becoming shallower, energy dissipates
- River overflows its banks, depositing material on flood plain.
- A river entering a sea or lake, reducing velocity.
34
What type of channel does vertical erosion result in?
- Faster flowing rivers with energy to downcut
- Larger, more angular bedloads
- Steep sided valleys.
35
What does lateral erosion result in?
- Floodplain
| - Meanders
36
What is the Hjulstrom curve?
This illustrates the relationship between velocity and competence. It shows the velocities at which sediment will normally be eroded, transported or deposited, depending on its size.
37
How do interlocking spurs form?
When the river winds around protrusions, hills or ridges of land.
38
What factors does the shape of the valley depend on?
Climate- sufficient water needed for high discharge levels to produce more vertical erosion and move sediment.
Geology- the type of rock and its structure may tend to a steep or gentle relief.
Vegetation - more vegetation helps bind the soil leading to stable valley sides.
39
When do rapids form?
Rapids form where there is a sudden increase in the slope of the channel or where the river flows over a series of gently dipping harder bands of rock. Water becomes more turbulent so its erosive power is greater.
40
How do potholes form?
Potholes are round oval shaped holes in the bed of a river created where sediment accumulates within naturally occurring depressions on the rock surface on the river bed.
Turbulent flows swirl stones around in the depressions, widening and deepening them through abrasion.
Vertical erosion here is known as scouring.
41
What are braided rivers?
Rivers that have multiple channels with islands of sediment in between them.
They form in deltas and areas where the banks are made of less resistant material or areas of high sediment load where discharge varies.
Discharge is variable so when its high, the river can carry a lot of glacially eroded till, but when the river spreads out and loses energy, sediment is deposited. Sediment can build up into islands.
42
How do waterfalls occur?
Waterfalls occur where the long profile of a river is steep, due to an outcrop of harder rock, overlying softer rock. The softer rock is eroded more easily and a deep pool is created. As the pool deepens, water falls into it with more power, leading to even more erosion by hydraulic action, creating the plunge pool. The waterfall then becomes undercut as a result of upstream currents created in the plunge pool. Eventually the hard cap rock above collapses, resulting in the waterfall retreating up stream, creating a gorge.
43
What is an alluvial fan?
When sediment is deposited as the river loses energy when it spreads out.
44
What are levees?
Built up river banks, when sediment is deposited when a river floods.
45
What is a floodplain?
Flat, fertile land either side of a river.
46
What is flocculation?
Where clay particles in suspension join together and because of electrostatic forces. They form clumps and are then deposited.
47
How do meanders form?
Meanders form where alternating pools and riffles (deep and shallow water) develop at equally spaced intervals along a stretch of river. Where the river is deeper at pools, there is more energy so it has a greater erosive power, whereas energy is lost at riffles due to friction. The spacing between pools and riffles means the flow of the river becomes uneven and maximum flow is concentrated on one side of the river. Turbulence increases in and around pools as the water speeds up, so flow of water twists and coils causing helicoidal flow, which spirals from bank to bank, causing more erosion and deepening of the pools. Eroded material is deposited on the inside of the next bend where river loses energy.
The combination of erosion and deposition exaggerates the bends until large meanders are formed.
48
What is a flood management strategy?
Strategies that seek to reduce the effects of flooding on the environment - human and natural.
49
What is river basin management?
```
A form of soft engineering that aims to reduce harm done by floods.
Methods include:
-Flood Abatment
-Floodplain zoning
-Flood proofing
-Flood Prediction
```
50
What is flood abatment?
A form of river basin management that reduces the possibillity of flooding by managing land use upstream. For example afforestation and contour ploughing.
51
What is flood proofing?
Constructing new buildings with flood proof walls or temporary flood gates that can be installed at times of high risk.
52
What is floodplain zoning?
Using records of past floods to predict future patterns and using this to map zones of risk.
Zone A = Prohibitive zones. These are nearest to the channel and have a high risk of flooding so there will be little future development.
Zone B - Restrictive zones. These have flood proofed development e.g. carparks.
Zone C = Warning Zone. This is higher value land with more development and lower risk.
53
What is flood Prediction?
Predicting when floods will occur to give people time to prepare and set up flood defences.
54
What are structural methods of flood management?
These methods involve protection from floods by engineering. They include:
- Flood walls, embankments and levees
- Channel Improvements
- Relief channels
- Flood interception schemes
- Channelisation
- Flood Storage reservoirs
55
What is channelisation?
An attempt to alter the natural geometry of a water course, helping to prevent flooding by increasing the channel capacity and preventing bank erosion.
Resectioning = widening and deepening a channel.
Realignment = straightening - shortens the channel by removing meanders.
Revetments = used to strengthen banks.
56
What are flood walls, embankments, levees and?
They are walls either side of the river that increase the height of the channel to prevent water from spilling onto the floodplain.
Embankments are made of earth with rubble fill.
Levees raise the level of the river bank.
57
What are channel improvements?
These attempt to reduces the effects of floods either by creating a smoother channel for a faster flow, or deepening/widening the channel.
58
What are flood interception schemes?
These include re-routing rivers, using new channels and flood embankments, to contain flooding away from settlements under threat. Areas of land with low value are deliberately flooded.
59
What are relief channels?
These are constructed to redirect excess water through an alternative route rather than through a settlement. Water reenters the main channel further downstream.
60
What are flood storage reservoirs?
These aim to store water in the upper reaches of the catchment, such as dams. However these are very expensive to construct and require large amounts of land.
61
What were some physical causes of the Carlisle Flood on the 8th Jan 2005?
- Carlisle is very close to the mouth of the river.
- It is located on a low lying flood plain, less than 20m
- Lack of vegetation due to urbanisation limits interception rates.
- There are many mountainous areas surrounding Carlisle, with steep gradients and shallow soil, reducing lag time.
- The local geology is skiddaw slate, which is very impermeable.
62
Where is Carlisle located?
Carlisle is located in Cumbria, NW England. It is a city situated on the floodplain of the River Eden. Three rivers meet in the city.
63
What were the meteorological causes of the Carlisle Flood on 8th Jan 2005?
- Heavy rainfall between 6th-8th Jan, during which 175 mm of rain fell in 36 hours.
- Relief rainfall from the Pennines as well as frontal rainfall from a low pressure system from the Atlantic.
- This rainfall followed a month of high rainfall for Carlisle so the ground was already saturated.
- Water levels rose rapidly and the river reached 1100 cumecs discharge.
64
What were some human causes of the Carlisle flood on the 8th Jan 2005?
- Homes had been built on the vulnerable flood plain.
- Impermeable surfaces due to urbanisation, increasing surface run off.
- Sewage works
- Inadequate hard management techniques
- Deforestation
- Recreational activities built on floodplain
65
What were the short term effects of the Carlisle flood?
- 3 deaths
- 120 flood related injuries
- Roads blocked and vehicles stranded
- 1900 properties flooded
- 300 businesses flooded
- Schools hospitals and police stations cut off and had to be closed.
66
What were some long term effects of the Carlisle flood?
- £250 million in total costs to repair damage.
- House prices fell as some properties were uninsurable.
- Some families were displaced for months.
- The flooded businesses were affected- 50% had to shut down or were relocated
- There was emotional trauma.
67
What were the short term responses to the flood in Carlisle?
- People were evacuated by helicopters and boats
| - Ambulance, fire and police were out offering assistance.
68
What were the long term responses to the flooding in carlisle?
- Raising existing defences
- Set existing defences further back from the channel to increase floodplain storage.
- Use removable flood barriers so fishermen and walkers still have access to the channel
- Increase the height of some bridges
- A retaining wall placed inside some parkland areas to trap floodwater.
- The scheme cost £25 million
- Financial support was also provided for properties damaged, to get flood proofing and a reduction in council tax for people who's houses had become uninhabitable on the ground floor.
69
What are the main causes of flooding in Bangladesh?
- Deforestation
- It's location on a delta
- Himalayas
- Monsoon
- Urbanisation
70
What are the climatic reasons for Bangladesh's flooding?
In summer, the northern hemisphere is tilted towards the sun, so the land is warmer than the sea, making low pressure, so the summer monsoon winds bring moist air from the sea.
71
What are the terrestrial reasons for Bangladesh's flooding?
- Nearly all of Bangladesh is a floodplain and delta of the river Ganges and Brahmaputra.
- Melt water from glaciers in the Himalayas in the summer.
72
What are the advantages of flooding in Bangladesh?
Fertile land that is good for growing crops such as rice.
73
What human factors contribute to flooding in Bangladesh?
- Extraction of groundwater for irrigation in dry months had lowered the water table and caused the land to sink by 25 metres.
- Population growth in Himalayan countries, puts pressure on to produce more food and land is deforested because of this.
- Deforestation in Himalayas increases surface run off. Slopes are destabilised as there are no roots to bind them together so there are landslides, which can reduces channel capacity.
- Urbanisation = impermeable surfaces.
74
What are some negative effects of flooding in Bangladesh?
- Easy transmission of water borne diseases such as cholera.
- Many deaths
- Contaminated water supplies.
- People's homes destroyed (more than 150000)
- Food shortages
- Schools closed
- Manufacturing slows
75
What are the long term responses to flooding in Bangladesh?
- Better education such as teaching children how to swim and purifying water.
- Houses on stilts, in cluster villages.
- Flood protection shelters
- Improved flood warning systems.
- Slope protection to reduce erosion of embankments.
- Improving international relations, with India and Nepal.
76
What are the 4 ways a river can transport it's load?
- Traction (rolling along river bed)
- Saltation (larger particles like pebbles or gravel bounce along river bed)
- Suspension (fine material carried along in water
- Solution (dissolved in water)
77
What is rejuvenation?
When a river's base level is lowered, by the ground level rising or sea level dropping. A drop in base level gives the river a greater potential energy, increasing its vertical erosion potential, leaving knick points. The river's long profile is altered. Terraces are also created
78
What are some landforms associated with rejuvenation?
River terraces are floodplains, which have been left above the level of present day flooding, following increased vertical erosion.
Incised meanders are formed when a river keeps its meandering course as vertical erosion increases, resulting in a deep winding valley with steep sides.
79
What is helicoidal flow?
Water flow pattern where the fastest current spirals across the channel and downstream in a corkscrew motion.
80
What is magnitude?
The size of a flood.
81
What physical factors affect flood frequency?
- Size and shape of drainage basin dictates how much precipitation the river can receive and how quickly it will arrive.
- Permeability of the soil and rock / how saturated it is already.
- Vegetation cover, more dense vegetation intercepts precipitation and stores it.
- Relief of the basin
- Number of tributaries flowing into a river, more tributaries mean higher discharge.
82
What human factors can affect the frequency of flooding.
- Urbanisation resulting in impermeable surfaces
- Deforestation, reducing infiltration and increases surface run off.
- Storm drains send water to rivers.
- Climate change potentially.
83
What is hard engineering?
Hard engineering projects are ones that involve the construction of artificial structures that, through a combination of science, technology and a bit of brute force, prevent a river from flooding.
84
What is soft engineering?
Soft engineering uses natural resources and local people’s knowledge of the river to reduce the risk posed by a flood.
85
What are advantages of soft engineering compared to hard engineering?
- Cheap, more suitable for LEDCs
- More sustainable
- Low maintainence
- Don't disturb the natural processes and ecological system in a river basin.
86
What are the disadvantages of hard engineering?
Hard engineering projects are generally very successful but:
- the effects of a hard engineering project can disrupt ecological systems in the drainage basin.
- Hard engineering techniques generally involve the containment of large volumes of water so if they were to fail for some reason, the impacts could be many times worse than if the river had been allowed to flood naturally.
- There’s also the high cost, technological requirements & maintenance of hard engineering projects that makes them unfeasible in countries without significant economic resources.
87
What are dams?
A hard engineering flood management strategy. A giant wall built across a river's channel to slow its flow and form a reservoir that can be drained out at a controlled rate, keeping discharge of the river low.
- Dams can also be used to generate HEP, economically benefitting an area.
- Reservoir can be used as a drinking source or for leisure activities.
- VERY expensive
- Large area needs to be flooded, destroying habitats.
- Prevent sediment being transported downstream
- Can destroy landforms such as deltas
- Sediment trapped behind a dam can change chemical composition of water, killing aquatic animals living upstream.
88
What are artificial levees?
Act as embankments, extending the channel's height and increasing its bank full discharge.
- Larger than natural levees and constructed from material resistant to erosion.
- Allows floodplain to be built on, which can increase risk of flooding.
- If they fail (like the embankments along the Missisippi in 1927) flood damage would be worsened.
89
What is channel straightening?
Blocking off meanders and constructing straighter routes, means the river flows faster, preventing it from pooling and reducing risk of a flood.
-Downstream, flooding can become more likely and erosion is stronger due to more kinetic energy.
90
What is floodplain zoning?
A soft engineering technique involving placing restrictions on land usage in areas surrounding a river/
- Floodplain isn't urbanised, so there's a longer lag time due to infiltration.
- Damage if a river does flood is reduced.
- Only can be done in certain areas, others have already been developed on.
91
What is afforestation?
Planting of trees in a drainage basin to increase interception and storage while reducing surface run off, reducing river's discharge making it less likely to flood.
Also creates new habitats for animals + improves water quality.
-Requires a lot of space to be effective.
92
What is wetland restoration?
Soft engineering that involves creating conditions that are favourable for the development of weltands (marshes or swamps). They can store large volumes of water reducing discharge in the river. They also create new habitats for animals, increasing biodiversity.
-Reduce land avaliable to farmers for growing crops.
93
What is river restoration / naturalisation?
-Restoring a river back to its original course, by un-straightening, removing hard engineering etc. If land is no longer valued, this can reduce risk of flooding downstream.
If land is allowed to flood naturally it acts like a sponge and soaks up much extra water before it reaches the main channel.
It can involve re routing the river into meandering channels.
94
What is channelisation?
An attempt to alter the natural geometry of a water course. Can help prevent flooding by increasing channel capacity + preventing bank erosion.
Resectioning - widening and deepening a channel to improve its hydraulic efficiency. This increases capacity and moves water out of an area of high risk quickly.
Realignment - involves shortening the river course by removing meanders so floodwaters move away more quickly and navigation is improved.
Both methods are expensive and offer short term advantages with high maintainence costs and can worsen flooding downstream.
95
How was flooding manage in Carlisle after the 2005 floods?
-Something had to be done to protect the economically important city.
-A plan for upstream storage was rejected because of environmental impact and high costs
The scheme:
-Raise existing defences so a higher flood capacity can be contained.
-Where possible set defences further away from the channel to increase floodplain storage
-Increase the height of some bridges to avoid debris getting trapped.
-A retaining wall was placed inside parkland areas to trap flood water.
-The scheme cost £25 million and should protect against 1 in 200 year storm.
-Proved financial support:
-Help people floodproof their porperties
-Money for removing silt vegetating and repair roads damaged by flood water
-25% reduction in council tax for people who's ground floor had become inhabital because of flood damage.
96
Where is the River Quaggy?
17km urban river located in south East London, flowing through parks and playing fields.
97
What happened to the river Quaggy?
```
Initially it was channelised as the urban environment developed around it, to prevent flooding, and create a straight channel so water moves away rapidly, however this idea only made flooding worse downstream.
The QWAG (Quaggy Waterways Action Group) formed to fight flood alleviation proposals which would have destroyed natural parts of the river, and in May 2007 QWAG completed their proposed scheme for restoration of the Quaggy. In areas, concrete channels were bulldozed away and the river left to meadner across Chinbrook Meadows, allowing plants to colonise naturally and increase biodiversity in the area. The regeneration cost £1.1 million.
In parks, QWAG stated channelised rivers are ugly as well as dangerous. Increasing the risk of flooding, vertical sides make it easier for children to fall in and rescue more diffucult so rivers were often fenced off losing space. QWAG campaigned to naturalise many areas of the river.
```
98
What were the long term responses to flooding in Bangladesh?
- Highly dependent on foreign aid and little funding for expensive hard engineering projects.
- Previously huge embankments had been constructed by the Flood Action Plan (FAP) supported by several wealthy countries.
- However these embankments were detrimental to farmers in rural areas as they rely on flooding for their fertile soil.
- Embankment erosion and failure in some places meant costs of maintenance were high.
- Reducing deforestation in Nepal is important to increase interception. There have been logging bans enforced in some areas and more efficient cooking stoves provided, afforestation promoted, as well as ecotourism.
- Bangladesh is working to improve its disaster preparedness, with water level sensors to give warnings of flash floods to villagers in rural areas who can then head to the nearest flood shelter.
- Education is also important to reduce deaths, by teaching children to swim and teaching people about diseases like cholera.
