4c Rivers Flashcards
What is a drainage basin?
An area of land drained by a river and its tributaries
What is weathering and what are the three types?
Breakdown of rock by natural processes
- Biological, chemical and physical
What is physical weathering
free-thaw
- rainwater enters cracks and freezes, water expands and breaks rocks into the soil
What is chemical weathering
Acid rain reacts with the weak minerals and decays the rocks causing it to dissolve
What is biological weathering
Plant roots grow into the rock cracks and cause them to split apart
What is mass movement
Downward movement due to gravity
What is soil creep
Individual soil particles move slowly down the slope under the force of gravity and collect at the bottom of the valley
The river may erode this
What is slumping
Bottom of the valley is eroded by the river and the slopes become steeper and the material above slides downwards, rotating as it does so
- triggered by heavy rain and saturates overlying rock and makes it heavy and liable to slide
What is river erosion?
Water wearing away rocks and soils on the valley bottoms and causes it to slide
What is hydraulic action
When high velocity flows causes it to hit the riverbed and wears it away
What is abrasion
Material carried in the river rubs against the bed and banks of the channels and causes it to wear away
- abrasion causes the most erosion
What is solution
River water can dissolve some rocks and minerals
- limestone and chalk are the most affected
What is attrition
Sediment particles collide with each other
- become rounder and smaller downstream
What are the 4 transportation of load?
Deposition
- river discharge decreases and heaviest material is deposited first
- needs the least energy
- traction, saltation, suspension, solution
What is traction?
Rolling of stones over the river bed
What is saltation?
Particles the size of sand grains bounce over each other
What is suspension
Silt sized particles carried in the river
What is solution
DIssolved minerals in the water
How do interlocking spurs form?
- near the source the rivers are small so low power and mainly erode downwards
- flow around the valley side slopes, called spurs
- spurs left interlock
How do waterfalls and gorges form?
- formed along the river when the band of hard, more resistant rock lies above the less resistant rock
- river erodes less resistant rock at a faster rate and gradually undercutting more resistant rock
- continued erosion of soft rock by abrasion and hydraulic action causes an overhang of hard rock
- eventually, hard rock collapses due to gravity and abrasion erodes the river bed creating a plunge pool
- soft rock keeps eroding and hard rock collapses and forms a steep sided gorge
Where to gorges mainly form?
- hard rocks where vertical erosion by rivers in dominant
How do meanders form?
- found in a river’s floodplain
- water flow swings side to side - max. velocity and force of water is on the outside of the bend which causes lateral erosion by undercutting and forms an outer steepbank creating a river cliff
- inside of the river bend has less velocity which leads of deposition and formation of gently sloping bank creating a slop off slope and the material deposited creates a point bar
Why is the cross section of a meander asymmterical?
The outisde is steep and the inside is gentle
How do oxbow lakes form?
- meander bends more and the neck becomes narrower as the river erodes
- deposition at the end of the neck seals off the bend
- leaves a horse shoe shaped lake called an oxbow lake
How do flood plains form
- flat area of land on either side of the river = formed
- lateral erosion on the outside bends means they erode into the valley sides which are wide and flat and with less energy so the river deposits fine sediment
How do levees form?
They are natural embankments of sediment on both sides of the river bank which carry load and occasionally flood
- flood water causes the sediment to come out as the river flows
- overflows = the river loses velocity and energy and deposits larger and heavier sediment first on its banks
- repeated flooding causes the banks to get even higher creating levees
How are deltas formed
River reaches another body of water which causes it to lose its velocity and deposit sediment
- river deposits cause coastal processes to erode it causing sediment to build up in layers called deltas
- river splits into distributaries and creates large wetland areas
What is a river’s long profile include?
The height and the distance downstream from the source of the mouth –> is in a curved shape and steeper near the source and flatter near the mouth
What is the upper course of a river like?
- steep gradient and mainly vertical erosion
- V shaped valleys so steep slopes so lots of soil creep and mass movement
- narrow channel and shallow
- high turbulence so low river velocity
- full of angular stones
What is the middle course of a river like?
- wider and deeper channel
- lateral and vertical erosion
- wide and flat valleys with gentle slopes creating floodplains
- medium velocity
- some deposition as less energy
- softer rocks
What is the lower course of a river like?
- wide and deep river channel
- highest river velocity due to low friction with the river bed
- lateral erosion and transportation as high sediment load
- very wide and almost a flat valley
What is vertical erosion?
It deepens the valley making it more V shaped
- dom in the upper course
- more turbulence as it has rough angular particles which scrape along the river bed
What is lateral erosion?
It widens the river valley/ channel and forms meanders
- dom in the lower and middle course
How do you identify the upper course of a river in OS maps?
- high nearby land
- lots of contour lines crossed in a short distance
- close contour lines
- narrow valley floor and waterfalls and cliff marked nearby
How do you identify the lower course of a river in OS maps?
- low nearby land
- river doesn’t cross many contour lines so gently sloping
- meanders across a large flat area = floodplain
- thick wide blue lines
- large meanders so oxbow lake may be formed
How does climate impact river landscape?
- wetter so more discharge
- more rate of erosion as more power
- more load so V shaped valleys in upper course
- wide flat floodplain so more transportation then more energy and weathering for sediment load
How does geology impact river landscape?
- rivers through resistant rock so less rate of erosion and as more resistant means there is low sediment load = steep valley sides
- softer rock = more erosion and sediment load (gently sloping valley sides)
- waterfalls = hard rock on top of softer rock
- interlocking spurs = softer rock eroded first leaving areas of hard rock sticking out
How do slope processes impact river landscape?
- vertical erosion means that steep valleys means there are more mass movement
- mass movement means that there is more material (more likely in cold due to freeze thaw) and more rainfall makes the ground less stable
- soil creep = cause by expansion and contraction of soil (water adds weight to soil)= adds fine material to load
What do hydrographs show?
- peak discharge = highest (in cumecs)
- lag time = delay between peak rainfall and discharge
- rising limb = increases in discharge
- falling limb = decrease in discharge as river returns to normal
How can precipitation affect flooding?
- more rain
- more discharge and lower lag time
How can geology affect flooding?
- impermeable rocks can’t absorb water
- there is more run off
How can drainage basin type affect flooding?
- small then rainfall reaches quicker and narrow basin water takes a long time to reach the channel
- circular basins have more discharge and shorter lag time
How can soil affect flooding?
- more impermeable soil (clays) can’t absrob much
- more run off
- thin soil so saturated quicker or as if frozen soil so holds a little water
How can sloped affect flooding?
- if the soil is sleep = rapid run off as less infiltration
How can vegetation affect flooding?
- less vegetation means that there is run off as less interception to infiltrate water into soil
- more flood if more deforestation
How can urbanisation affect flooding?
- water can infiltrate into the tarmac
- more runoff and gutters which drains quickly run off into rivers so there is more discharge
How can antedecent conditions affect flooding?
- wet/cold weather means mroe run off as frozen soil
- saturated soils?
How long is the river Severn?
Longest river in the UK
- 354 km long
What is the upper course of the river severn like?
- source from the Welsh mountains
- avg rainfall is 2500mm = high runoff
- Mountains have hard impermeable shales and grits
- narrow, shallow channel, angular stones, slow velocity, vertical erosion = V-shaped valleys with steep sided slopes
- soil creep and mass movement happen
What is the middle course of the river severn like?
- towards Shrewbury
- rainfall of avg 700mm
- softer rocks have more permeable - sandstones, conlomerates and marts
- wide and deep channel with vertical and lateral erosion = floodplain
- deposition on the inside bends of meanders and flood plain when flooding
- rounded and smaller sediments and more tributaries causes more discharge
What is the lower course of the river severn like?
- soft geology - mudstones and lias
- wide channels in Tewsbury = 70m wide and deep
- less friction and high velocity
- more discharge and more sediment load means that there is more deposition as the river mouth
Why did the river severn flooding happen?
- settlements around such as Shrewsbury, Bewdley and Gloucester and resistant Welsh Mountains
- July 2017 - highest rainfall in 100 yrs - 140mm in a few hours
- series of depression across the UK due to the strong jet stream –> went south
- confluence so lots of tributaries and more discharge and lots of V-shaped valleys
How were bridging point settlemtns affected?
- damage of £140,000 of local council
- £20-£30,000 for each house
- 48000 houses affected
- £3.2bn in total taken
- more population means there is more building ons the floodplain therefore more impereable surface
Why is the flood risk increasing?
- more population, land use change, climate change (changing weather patterns and jet steam bought more intense storms)
- environment agency = reduces flood chance = land use, controlling development in flood plains and defences/ reduces impact = preparation and flood warning
How is flood risk figured out in the river severn?
- 1% annual probablity that flood puts 60,000 people and 29,000 businesses at risk
- rural areas reduce run off by increasing floodwater storage
- middle and lower = unsuitable development
- urban areas = improve defence and protect buildings
What are some ways of hard engineering?
- Hard engineering involves building artificial structures which try to control natural processes
- embankments (levees)
- floodwalls
- demountable
- flood barriers/ storm surge barriers
What are the advantages and disadvantages of Embankments?
- high banks on river banks (low cost)
- stop water spreading into settlements
- earth and grass banks (blend w environment)
- BUT - floodwater goes over top and is trapped behind and bursts under Pa = even more damage
What are the advantages and disadvantages of Floodwalls?
- artifical barriers to raise height of river banks to gold more water (more cost)
- stops water from spreading into high impact areas
- BUT expensive, may cause flooding downstream, not natural and may spoil the view
What are the advantages and disadvantages of Demoutable flood barriers (temp)?
- used where a permanent flood wall could act ugly and costs less
- only used in specific locations and may be risks if not installed on time
What are the advantages and disadvantages of Flood barriers or storm surge barriers?
- floodgates near the mouth to prevent storm surge
- closed in tide/ surge in a forecast
- protects like areas from conditions of storm surges
- high cost and regular maintenance
What is soft engineering?
- Soft engineering does not involve building artificial structures, but takes a more sustainable and natural approach to manage the coast
- Flood plain retention and river restoration
What are the advantages and disadvantages of flood plain retention?
- restores the original floodplain (low cost)
- restores soil structure floodplain
BUT change of land use (farmland)
What are the advantages and disadvantages of river restoration?
- using a variety of strategies to restore the original course
- more attractive and creates natural habitats and benefits the wildlife
- BUT floodbanks are still needed but change in land use causes disadvantages
