rivers Flashcards
the hydrological cycle in a drainage basin:
precipitation
|
interception ——> transpiration
|
stem flow
|
surface storage
|
infiltration
|
soil moisture ——> through flow
|
percolation
|
groundwater store
|
ground water flow
surface runoff ——> river carrying water to a lake or sea
|
evaporation
what does it mean when:
on a global scale, the hydrological cycle is a closed system
there are no inputs or outputs - nothing can enter the system or leave the system
what does it mean when:
on a local scales, within a drainage basin, the hydrological cycle is an open system
there are inputs (e.g. precipitation)
water table meaning
current upper level of saturate rock/soil where no more water can be absorbed
groundwater flow meaning
water flowing through the rock layer parallel to the surface
through flow meaning
water flowing through the soil layer parallel to the surface
surface runoff meaning
water flowing on top of the ground
evaporation meaning
water lost from ground/vegetation surface
transpiration meaning
water lost through pores in vegetation
ground water meaning
water stored in rock
percolation meaning
water seeping deeper through rock
soil moisture meaning
water help in the soil layer parallel
infiltration meaning
water sinking into the soil/rock form the ground surface
surface storage meaning
water held on the ground surface .e.g. puddles
interception meaning
water being prevented form reaching the surface/ground by vegetation or trees
precipitation meaning
any source of moisture reaching the ground .e.g rain, snow, frost
how might human activity impact the hydrological cycle in drainage basin
- deforestation would impact interception
-> less trees means water isn’t intercepted - deforestation would impact surface runoff
-> increases speed as less precipitation is intercepted
what is the source
the start of the river
what is the drainage basin
the area of land drained by a river and its tributaries
what is a confluence
where a tributary joins a larger river
what is the mouth
the end of a river (usually where the river meets the sea)
what is the water shed
the edge of the river basin
what is a tributary
a small stream that joins a larger river
how many ‘courses’ are there in a river
3 - upper, middle, lower
how does the long profile of a river change from source to mouth
the gradient decreases from source to mouth
upper - steep
middle - shallow
lower - fairly flat
what is the river valley like in the upper course
steep
what is the river valley like in the middle course
shallow
what is the river valley like in the lower course
flat
what is the river channel like in the upper course
narrow
what is the river valley like in the middle course
wider
what is the river channel like in the lower course
wide
what do we mean by ‘fluvial’
water
what do we mean by ‘processes’
things that a river does
examples of fluvial processes
erosion, transportation, deposition
erosion definition
wearing away of material
in the upper course of a river what process appears to be dominant
erosion
why does the river tend to erode vertically in the upper course
discharge is smaller and the river cuts down in a bid to get to sea level
rock is also resistant
why does a river tend to erode laterally in the middle course
material carried by the river can erode the banks
what tends to be the dominant process in the lower course of a river and why
deposition because the river slows down and uses energy
hydraulic action:
- occurs when the force of the river compresses air trapped in cracks in the banks
- the increased pressure weakens and gradually wears away the banks
abrasion:
- occurs when rocks carried by the river scrape and rub along the river bed and banks, wearing them down
solution:
- occurs where water is slightly acidic
- dissolves certain types of rock on the river bed and banks
attrition:
- occurs as rocks carried by the river collide with each other and break into smaller pieces
what is the ‘rivers load’
once material has been eroded by the river, it is often carried along in the river itself
- the materials that are carried by the river are known as the RIVERS LOAD
this load is different sizes and shapes and materials
what is the ‘rivers discharge’
amount of water in a river at any given time
what is traction
large particles (boulders and rocks) are rolled along the river bed by the force of the water at times of high discharge
what is saltation
small rocks and sand particles are bounced along the river bed by the flow of water
what is suspension
fine flat and sand particles are carried along (suspended in) within the water even at low discharges
what is solution
some minerals dissolved in water such as calcium carbonate. this require very little energy
deposition:
- when some or all of the material being transported by river is dropped (deposited)
- this usually occurs when the velocity of the water slows, thus decreasing the energy of the water
- the heaviest material, that takes the most energy to carry will be dropped first
- deposition will happen when a river bursts its banks at a floodplain, or when the river reaches the sea to from a delta
- when a river floods friction with the land slows the water down
- river meander - as it swings around the bend it loses speed on the inside of the bend
- river mouth - slows down as it hits the large body of water
characteristics of the river channel and river valley in the upper course of the River severn
valley is steep
channel is narrow
characteristics of the river channel and river valley in the middle course of the river severn
valley is wider and less steep
channel is wider and deeper
characteristics of river channel and river valley in lower course of river severn
valley is flat and wide
channel is deep and wide
landforms in the upper course of the river severn
waterfalls
v-shaped valleys
interlocking spurs
landforms in the middle course of the river severn
meanders
oxbow lakes
landforms in the lower course of the river severn
leeves
floodplain
estuary
mudflats
processes/size of material in the upper course of the river severn
mainly erosion
load is large and angular
processes/size of material in middle course of river severn
erosion and deposition
load is slightly smaller and rounded
processes/size of material in lower course of river severn
transportation and deposition
load is small and round
what do we mean by ‘landforms’
a recognisable, naturally formed feature on the earth’s surface
interlocking spurs:
- hillsides which interlock with each other as the river winds around them
- vertical erosion in the upper course creates steep sided, v-shaped valleys
- rivers lack lateral erosion so they have to wind around hillsides
formation of a waterfall:
1) water runs over alternating bands of hard rock on the top and soft rock beneath it
2) erosion will start to break down rocks through the processes of hydraulic action, abrasion and solution
3) hydraulic action wears away the soft rock as water runs over it. This is because it is less resistant than the hard rock above it
4) overtime the soft rock starts to undercut the hard rock
5) the hard rock left unsupported and as a result of this and weight it collapses into the river bed
6) rocks that have fallen are either transported down the river or scrape away at the base of the river bed through the processes of abrasion. this coupled with continuous friction of water, a dent in the river bed starts to form - this is known as a plunge pool
7) the erosional processes repeat and overtime the waterfall retreats backwards/upstream
8) once a waterfall retreats, a steep sided gorge is left behind
where are the areas of deposition and erosion on a meander and why
deposition - inside bend
erosion - outside bend
because fastest flow of water tends to be on outside bend
how do meanders and ox bow lakes form
1) water flows over land, water swings to the outside of the river where there is less friction
2) as the river erodes the river bank, a bend starts to occur, this is known as a meander
3) on the inside of the bend, the flow is slower so we get deposits of sediment
4) the neck of the river grows narrower
5) when the neck of the river breaks through, the river will take the shortest route and take on a straight course
6) the look becomes sealed off altogether as a result of deposition - this is known as an oxbow lake
what is a levee
a raised bank formed on the banks of a river, in the lower course
how is a levee formed
1) before flood - water is held within the channel
2) during flood - water flows over the banks and deposits silts as the water hasn’t got enough energy to carry the materials
3) the levee gradually gets higher after successive floods
what is a floodplain
the area around a river that is covered in times of flood. this is a large, flat area of land that is formed by erosion and deposition
how is a floodplain formed
- when a river floods, it deposits the heavier particles first -> forms the levees
- as the water continues to flood further away from the river banks, the sands are deposited next, then the silts and finally the lightest clays
- this buildup of sand, silts and clay leads to the formation of a flat piece of land (floodplain) either side of the river - just after the levees
- every time the river floods, deposition builds up the floodplain
what is an estuary
where the river meets the sea
what factors increase the risk of flooding
- climate change
- historical flooding
- deforestation
- impermeable rocks or road (concrete)
- heavy rainfall
- urbanisation
- flat land
flood definition
flooding occurs when the amount of water in a river exceeds the capacity of the channel causing it to burst its banks onto land around it
discharge definition
the volume of water flowing through a river channel; measured at any given point in cubic metres per second (CUMECS)
causes of flooding
- urbanisation
- agriculture
- deforestation
- rock type
- basin size
- soil saturation
- relief
- precipitation
causes of flooding:
urbanisation definition
building in areas at risk of flooding. areas are made from impermeable rock (tarmac) which means water runs over the roads into drains and then into the river channel causing
causes of flooding:
agriculture definition
in arable farming, soil is left unused and exposed to the elements for periods of time. this can lead to more surface runoff
causes of flooding:
deforestation definition
trees take up water, therefore if we cut down trees, areas will have less trees to uptake water. therefore it is rapidly transferred into the river channels
causes of flooding:
rock type definition
impermeable rocks (eg. clay) do not allow water to pass through them. therefore a river is more likely to flood. this is because water will not seep into the rock.
causes of flooding:
basin size definition
when a river basin is large it is able to store more water, however if it is small it has an increased risk of flooding as it cannot carry as much so will overflow
causes of flooding:
soil saturation definition
when soil can no longer absorb any more water it becomes over saturated and causes flow of water over land
causes of flooding:
relief definition
steep slopes allow for water to run over the land quickly, leaving little time for water to seep into the soil. therefore more water enters the river channels
causes of flooding:
precipitation definition
torrential rainstorms can lead to sudden flash floods as river channels cannot contain the volume of water falling into them
what is the key factor that leads to flooding
precipitation and how intense / how frequent
how might human factors worsen flood risk
due to urbanisation, impermeable flat surfaces prevent rainfall being absorbed, meaning surfaces flood faster. increase surface runoff
hard engineering strategies
- reservoirs
- afforestation
- dredging
- land-use zoning
- controlled flooding
- river bank conservation -> tree planting
- channel straightening
- embankment
how do dams work
- large concrete barrier built across a river to hold back its flow
- causes the valley behind the dam to flood, creating an artificial lake called a reservoir
- this controls the amount of water downstream
- water is released in a controlled manner through gates in the dam
- if water releases are carefully controlled, there should be no flooding downstream
how does channels straightening work
- when a meandering section of a river is engineered to create a widened, straightened and deepened course
- by doing this it can carry more water and reduce flood risk
- this usually happens on low-lying areas that are highly at risk of flooding
how does embankments work
- embankments are raised river banks
- by raising these banks, more water is contained in the channel
- this reduces the flood risk
- an embankment is made by bulldozers moving huge mounds of impermeable soil on to the river banks to build up their height
- some embankments are lined with concrete
how does flood relief work
- flood relief is when another channel is built to deliberately move water away from human settlements during times of high discharge
- it is essentially a back-up channel for a river that frequently floods
flood water description
flooding of low-lying land and roads is expected. people should be prepared and watch river levels
flood warning description
there is a threat to homes and businesses. people should move items of value to upper floods and to turn off electricity and water
severe flood warning description
extreme danger to life and property is expected. people should stay in the upper floors and turn off electricity and water. people are also informed to leave property
soft engineering strategies for flooding
- flood warnings and preparations
- flood plain zoning
- planting trees
- river restoration
what is the river management case study
Jubilee river flood-relief channel
cost of the jubilee river
£110 million
length of the jubilee river
11.7km
cost of repairing the channel of the Jubilee river after an episode of flooding
£680,000
organisation funding the Jubilee river
EA (environmental agency)
year of completion of Jubilee river
2002
width of jubilee river
50m
year of worst flooding in Thames since 1947
2014
how many homes does the flood relief channel in the Jubilee river protect
3200
flood relief channel in Jubilee River:
the Jubilee river acts as a flood relief channel for the river Thames, allowing water levels to be controlled and diverted from the Thames during times of high flow
Jubilee river:
socioeconomic negatives:
- small businesses and shops are under pressure as they cannot open when premises are flooded and insurance costs are incredibly high. business insurance costs for Wraysbury alone was £500 million in 2014. this causes future insurance premiums to increase too
- paddle boaters had been promised a navigable river. however on two of the fiver weirs they have to carry boats over with them and Taplow Weir is too dangerous to cross
Jubilee river:
environmental negatives:
- ethics - some people are protected but not others. 3000 properties in affluent Eton and Windsor are protected, but less wealthy settlements in Wraysbury and Old Windsor are not
- the Thames now experiences a much higher discharge due to the merging of the two channels upstream. this problem came to light in 2014 when the river experienced serious flooding
- in 2014, extensive flooding meant the built environment suffered from flooded roads, buildings, fields and habitats
- the natural ecosystem is affected as algae collects and builds behind the weirs where the flow of the river is slowed or stopped
