Rivers, floods and management

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  • Created by: LauraRuff
  • Created on: 03-01-15 13:39
What is the water balance equation?
precipitation= total runoff + evapotranspiration + change in storage (in soil and rock)
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What is the equation for river discharge?
discharge= cross sectional area x velocity
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Describe a factor that affects river discharge...
Distance downsteam: due to the addition of water to the river channels as smaller tributaries join the main river
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Describe a factor that affects river discharge...
Climatic Characteristics: river discharge reflects precipitation both in its volume and seasonal patterns. Snowfall results in time delay so when it melts there is a surge in discharge. Temperature also affects evaporation rates and vegetation growt
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Describe a factor that affects river discharge...
Land use: afforestation reduces the discharge and makes it more constant. Urbanisation increases discharge because water is transferred rapidlyover impermeable surfaces such as tarmac and concrete and through pipes and sewers.
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Describe a factor that affects river discharge...
Water abstraction (removal): the abstraction fo water for dmestic use and the irrigation of crops reduces discharge. This can be seasonal , with greater demands for water in the summer months.
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Describe a factor that affects river discharge...
Channel modifications: contrsucting a reservoir regultes the discharge and makes it more constant. If the river channels itself is modified to reduce flooding such as channel straightening and enlargement, this can increase discharge.
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Describe a factor that affects river discharge... (continued)
Channel modifications: River restorstion adn the creation of flood storage areas can reduce discharge because they encourage small scale flooding in a rivers headwaters.
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Describe the characteristics of a 'flashy' hydrograph...
Short lag time and a high peak discharge with steeper limbs
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Describe the characteristics of a 'subdued' hydrograph...
Low, flat hyrdograph with a lower peak, gentle limbs and a longer lag time
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What drainage basin and precipitation characteristics would you associate with a 'flashy' hydrograph?
Small basin, high drainage density, impermeable rock, urban area, steep slopes, saturated soil and heavy rain.
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What drainage basin and precipitation characteristics would you associate with a 'subdued' hydrograph?
Large basin, low drainage density, permeable rocks, forests, gentle slopes, dry soil and light rain.
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The amount of energy availiable in a river to do work depends on...
The height the water has to descend(and its steepness) and the amount/mass of water avaliable.
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What parts of water have potential energy?
A still body of water at any point above sea level.
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When is kinetic energy created in a river?
When water starts to move downhill
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Why is the amount of kinetic energy generated less that the amount of potential energy?
Some energy is lost overcoming friction
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Why do fast flowing, large rivers tranpsort more sediment and carry out more erosion than small, slow-moving rivers?
Because they have a high amount of energy due to the steep gradient and vast amounts of water in the channel
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When does erosiom and transportation occur?
When a river has surplus energy.
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What factors determine the type and amount of sediment transported by a river?
The flow of the river, the nature of the riverbed/banks (loose material is more likely to be picked up rather than solid rock), human intervention (lining the riverbanks with concrete reduces the amount of avaliable sediment for transport)
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What causes deposition to occur?
When the river no longer has enough energy to transport its load e.g in slow flow. Friction witht he riverbed/banks leads to deposition and deposition also occurs when the river enters the sea or a lake where the rate of flow rapidly drops
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What is the main factor that controls erosion, deposition and transportation?
Velocity of the river
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In general, what changes in a river as you go downstream?
Upper course: river flowing down a steep gradient, rapids and waterfalls, steep-sided narrow mountain valley. Lower course: flatter valley, land used for farming and settlements, meanders, less white-water
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What accounts for the changes between the upper and lower course of a river?
The gradient of the land
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How does the cross profile of a river change downstream?
Close to the source: narrow, steep-sided, v-shaped valley Downstream: wider and less steep valley. River increases in size Close to the mouth: very flat landscape and extensive flood plain.
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Why does a river increase in size downstream?
As it is carrying more water so has more energy.
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How do velocity and discharge change downstream? Why does it change?
Velocity and discharge increase downstream as increasing mass of water, smoother river bed due to deposited sediments forming a smoother lining Deeper, narrower channel = more efficient wetted perimeter as less energy lost from friction.
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What factors affect the shape or profile of a river channel?
Resistant rock may cause a river to adpot a narrow, deep channel. Human engineering (e.g. dredging or lining with concrete). Farm animals such as cattle may trample riverbanks and alter the channel profile.
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How are potholes formed?
When rock fragments are drilled into hole and cavities by the tubulent whisking-like action of the river. Formed by corrasion.
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How are waterfalls formed?
Hard rock on top of soft rock. Soft rock erodes and river cuts down vertically making a cliff.
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How does a river become rejuvenated?
A drop in base level means that there is an increase in gradient giving the river more energy. This gives the river the energy to actively erode any irregularities in the long profile.
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What are the two main causes of rejuvenation?
Sea level change: if the sea level falls faster than the rate of vertical eroison then a nick point will be formed close to the coast.
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What are the two main causes of rejuvenation?
River capture: headward erosion can cause a river to break into an adjacent valley and capture the tributaries of a nearby river forming a waterfall due to the height difference. This is a nick point.
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What factors contribute towards river floods?
Rock type, steep slopes, long profile, drainage density, snowmelt, intense prolonged rainfall, building construction, deforestation, agriculture, ineffective flood management and warning systems
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Name the hard-engineering aproaches to flooding...
Dams/reservoirs, river straightening, raising embankments and diversion spillways
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Name the soft-engineering aproaches to flooding...
Afforestation, establishing wetlands, riverbank conservation, river restoration and land-use floodplain zoning.
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discharge= cross sectional area x velocity

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Card 4

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Card 5

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