Physical Landscapes Flashcards
How can vertical erosion be slowed?
If the rock type beneath is very hard
What is alluvium?
The mud and sand that the river deposits
What is a river’s energy used for?
95% for overcoming friction with beds/banks
5% for erosion and transporting load
What type of erosion are V shaped valleys formed by?
Vertical erosion
What type of erosion are U shaped valleys formed by?
Glacial erosion
An example of a U shaped valley?
Nant Ffrancon, Snowdonia
What type of erosion causes valleys with wide floodplains?
Lateral erosion
What is scree?
Angular, broken pieces of rock due to freeze thaw weathering of the cliff/rock face
When do discordant landscapes occur?
When the landforms seem to have no relationship with the underlying geology e.g. rivers do not follow lines of weakness such as soft rock or fault lines
What is a plateau?
A high flat place e.g. flat topped hills or mountains which are usually harder than the surrounding rock
Difference between dykes and sills?
- Dykes are vertical intrusions with horizontal cooling cracks
- Sills are horizontal intrusions along the lines of bedding planes with vertical cooling cracks
How are fault scarps formed?
From movement along a fault bringing softer rock against harder rock - erosion at different rates so hard rock forms upland
Difference between erosion and weathering?
Erosion occurs due to the movement of rocks whereas writhing occurs when an object is in situ
How is material transported in a river?
- fine material rubs against bank, wearing it away
- large boulders rolled along by traction collide and break up
- smaller stones and pebbles bounced along river bed break down in size
- dissolved material carried along in solution
Landforms in the upper course of a river?
Small waterfalls, rapids, potholes, large boulders, uneven steep river bed, v shaped valley, interlocking spurs
Describe the valley profile in the upper course of a river
Steep sides
Erosion in the upper course of a river?
Hydraulic and attrition, mostly vertical
Transportation in the upper course of a river?
Mostly large boulders (ledload). Some in suspension and little in solution
Deposition in the upper course of a river?
Limited to the large bedload
Landforms in the middle course of the river?
Rapids, small meanders, small floodplain,
Describe the valley profile in the middle course of the river
Small floodplain, less steep sides than in upper course
Erosion in the middle course of the river?
Mostly attrition with a little hydraulic, less vertical erosion, lateral erosion begins
Transportation in the middle course of the river?
Smaller sized bed load moved by traction, suspension load increased. Little in solution
Deposition in the middle course of the river?
Coarser material builds up, deposition. On slip-off slopes, floodplain built up in times of foood
Landforms in the lower course of the river?
Large meanders, ox bow lakes, levées, floodplain
Describe the valley profile in the lower course of the river
Wide floodplain, with levées either side of the river
Erosion in the lower course of the river?
Erosion reduced - some lateral erosion on outside bends of meanders
Transportation in the lower course of the river?
Smaller sized bed load of peoples, gravel and sand form the load. Most transported by suspension
Deposition in the lower course of the river?
Mostly fine particles. Forms slip-off slopes, levées and floodplains
How are waterfalls formed?
- upper course - water erodes soft rock faster than hard rock
- hard rock is undercut and left over hanging
- overhang isn’t supported and collapses
- fallen rocks into plunge pool - created due to falling water and rocks
- fallen rocks erode rock (abrasion grinds at bedrock and attrition makes fallen rocks smaller)
- waterfall moves upstream and steep gorge is created
Where is the river tees?
In the north east of England
What direction does the tees flow in?
An Easterly direction
How tall do v shaped valleys tend be in the upper course of the tees?
575-675m
How are V shaped valleys formed?
- vertical erosion
- weather exposed rock
- mass movement
- transportation
What is the Hjulström curve?
A graph to show how the speed of a river flow affects the size of the particles in the water
What is along the top of a Hjulström curve graph?
The type of particles starting from the finest along to the least fine
What happens when the size of sediment goes below 0.1mm?
The grains are so small that they start to stick together and need a greater speed of water for them to be eroded
How are meanders formed?
- deep pools in the river have less friction so the river has more energy for erosion
- erosion is lateral
- lateral erosion moves river sideways so it bends
- shallow riffles stay put
- river deposits on inside of bend and erodes on outside of bend
How are ox bow lakes formed?
- river erodes outside of bend and deposits inside of bend
- neck of land between loop gets narrower, new shape of meander
- river cuts through narrow neck of land during flood
How do meanders migrate?
- as the river flows downstream it becomes deeper and wider
- meanders become larger and wider as they migrate downstream
- erosion of bend widens valley
- deposition creates valley floor
- erosion and deposition move downstream and create a line of river cliffs along the edge of the valley floor
Why does deposition happen?
- the velocity decreases such as the inside bend in a meander
- an obstruction e.g. river enters a lake and velocity falls
- a fall in the volume of water e.g. at times of low flow during a period of drought
What does a river deposit first when it floods?
The heaviest of the particles first
How are levées formed?
- before the levée, silt deposits lie over the bedrock on the floodplain
- during flood - fine particles carried further onto floodplain and heaviest sediment deposits closest to river
- after many floods - river bed builds up bed load deposits over time so river level is raised and increases probability of floods
What is an estuary?
- the lower course of a river valley which has been drowned by a rise in sea level/fall in land level
- river channel is wide with mud flat and salt marshes
- valley is low lying and flat
Examples of estuaries in Britain?
The Thames, the Mersey, the Humber, and the Tees
Geology to cause a slow response?
Permeable rock e.g. chalk limestone
Gradient to cause a slow response?
Gentle gradients
Basin size to cause a slow response?
Large basin- large areas without streams - take time for water to reach streams
Basin surface to cause a slow response?
Forested
Basin shape to cause a slow response?
Pinnate basin - long and thin
Rainfall intensity to cause a slow response?
Frontal precipitation - long duration, low intensity, nimbostratus
Geology to cause a flashy response?
Impermeable rock - surface runoff/overland flow
Gradient to cause a flashy response?
Steep gradients
Basin size to cause a flashy response?
Small basin
Basin surface to cause a flashy response?
Urban - roads, roofs, gutters, pavements
Basin shape to cause a flashy response?
Circular basin
Rainfall intensity to cause a flashy response?
Convective storm - short duration, high intensity, cumulonimbus
e.g. on a summer afternoon, or as relief rainfall in mountains
Natural causes of flooding?
- rock type - impermeable rock means no infiltration
- steep slopes - water will not have time to seep into rock means surface runoff
- drainage density - when there are more streams water will get into the river more easily
- snowmelt - water will fill soil
- long profile - steeper in the upper course
Human causes of flooding?
- house construction - houses are built close to rivers on floodplains because flat and large
- deforestation - less interception
- agriculture - farmers should plough sideways so there are ridges to hold the water
- ineffective flood management and warning systems
- intense and prolonged rainfall
How frequent are high magnitude floods?
- large magnitude floods don’t happen very often
* small magnitude floods are more frequent
Why do large floods usually occur?
Due to usually heavy or prolonged rainfall
Where do large floods still happen often?
In places where there is annual snowmelt or monsoon rains e.g. Bangladesh
How do hydrologists predict how often a flood of a certain magnitude can occur?
By keeping records (e.g. hydrographs) - which is called the flood recurrence interval (RI)
What is the flood recurrence interval?
When hydrologists use hydrographs to predict how often a flood of a certain magnitude can occur
e.g. a small flood may have an RI of 2 years and a large flood may have an RI of 10 years
What could be reducing the RIs of river?
Climate change
What quality of flood defence do the EA recommend to build?
Flood defences should be built to withstand a 1 in 50 year event
What is hard engineering?
Involves building structures to defend places from floodwater
Examples of hard engineering?
- build flood banks
- straightening and deepening the river
- dams and reservoirs
- flood walls
- storage areas
- barriers e.g. the thames barrier
How do flood banks work in terms of river management?
Raise the banks of a river, so it can hold more water
Comment on flood banks in terms of river management?
- relatively cheap, one-off costs
- looks unnatural
- water moves quickly and increases the flood risk downstream
How does straightening and deepening the river work in terms of river management?
- straighten the river channel to speed up the flow, or line it with concrete
- dredge the river to make it deeper and able to carry more water
Comment on straightening and deepening the river in terms of river management?
- dredging needs to be done every year
- lining with concrete is expensive
- speeding up the flow increases the flood risk downstream
How do dams and reservoirs work in terms of river management?
Trap and store water, and release it in a controlled way. Can be multi-purpose and generate electricity
Comment on dams and reservoirs in terms of river management?
- very expensive
- the creation of a reservoir means huge changes to the ecosystem
- dams trap sediment
How do flood walls work in terms of river management?
Built around settlements, industry or roads
Comment on flood walls in terms of river management?
- expensive and looks unnatural
* effective if the flood isn’t too extreme
How do storage areas work in terms of river management?
Water is pumped out of the river and stored in temporary lakes. Its then lumped back when the water level in the river has dropped
Comment on storage areas in terms of river management?
- effective
* needs large areas of spare land
How do barriers work in terms of river management?
The barrier is raised when a high tide or flood is forecast
Comment on barriers in terms of river management?
- very expensive
* Thames barrier - a new one may be needed in the future to protect London from higher flood surges
Case study for river realignment or channelisation?
- the jubilee river - bypass channel for the Thames
- Maidenhead to Windsor
- opened 2002, cost over £80m
- why? serious floods in 1947 - cope with sudden increase in discharge of the Thames
- stopped flooding of 5000 homes, 12500 people, 700 businesses and M4
- efficient, reduced flooding
- however water moves quicker so properties below point where Channel rejoins the Thames have now been flooded for the 1st time (Datchet)
- ^probably due to poor management of other channels
What is soft engineering?
Involves adapting to flood risks and allowing natural processes to deal with rainwater
Examples of soft engineering?
- flood abatement
- flood proofing
- flood plain zoning
- flood prediction and warning
- washlands
- flood tubes
How does flood abatement work in terms of river management?
Change the land use upstream e.g. by planting trees
Comment on flood abatement in terms of river management?
• slows down the flow of rainwater into rivers e.g. by increasing interception
How does flood proofing work in terms of river management?
Design new buildings, or alter existing ones, to reduce the flood risk
Comment on flood proofing in terms of river management?
Expensive to alter existing buildings
How does flood plain zoning work in terms of river management?
Different uses are allowed, depending on the distance from the river
grazing for animals - playing fields - industry - housing - hospitals
Comment on flood plain zoning in terms of river management?
- land close to river may only be used for grazing
* land furthest from the river is used for hospitals, old people’s homes etc
How does flood prediction and warning work in terms of river management?
The EA monitors river levels and rainfall. They use this information, plus weather forecasts, to predict flooding
Comment on flooding prediction and warning in terms of river management?
They issue warnings and produce flood maps which show areas at risk from flooding
How do wash lands work in terms of river management?
These are parts of the flood plain that are allowed to flood
Comment on wash lands in terms of river management?
Wash lands can’t be built on. They’re usually used for sports pitches or nature reserves
How do flood tubes work in terms of river management?
Blown up to form flood defences, weighted at the bottom
Why does the river tees need a catchment flood management plan in the upper course?
- 1% of residential and 2% of commercial properties are at risk with a 1% chance of occurring in any year
- after heavy rainfall or snowmelt river level rises and can cause ‘tees roll’ - a 1m height in 15 mins
- rainfall, steep slopes and little infiltration
Why does the river tees need a catchment flood management plan in the middle catchment?
- gentle gradient and valley sides open to reveal wide, flat floodplains
- Croft-on-Tees - about 390 people at risk, 6% of residential and 13% of commercial properties
- A1 and A66
- electricity stations
Why does the river tees need a catchment flood management plan in the lower course?
- meanders - wide flood plains
- major population centres
- yarm- jan 1995 & neasham 1771 = regular flood events surrounding meanders
- small communities at risk - 4% of residential and 28% of commercial properties (£2 mil property damage)
- 2012 & 2015 serious floods in Stockton - 3m deep, 240 properties, 3% of homes, 5% of businesses at risk
Management schemes used in the upper course of the river tees?
- informing people, flood watch (soft engineering)
- building dams like Cow Green, to hold back flow in reservoir (hard engineering)
- in Barnard Castle weirs slow down the flow (hard engineering)
- tree planting in Barnard Castle (soft engineering)
Management schemes used in the middle catchment of the river tees?
- avoid building on a flood plain (soft engineering)
- install temperature flood walls for Crofton-on-Tees (hard engineering)
- river Skerne near Darlington - tributary originally built concrete walls and deepened river but this caused flooding downstream - concrete was removed and the floodplain was put back with natural wasteland (hard engineering)
Cost of management schemes in the middle catchment of the river tees?
temp. flood walls - £1 mil
Impact of management schemes used in the middle catchment of the river tees?
- some houses may not be protected because of cost e.g. Croft-on-Tees so need sandbags
- now have a great place for recreation - close to residential area in Darlington
Management schemes used in the lower course of the river tees?
• Lustrum Beck which flows into Tees near Stockton, and Yarm:
- reinforced concrete walls with flood gates (hard engineering)
- install levées to contain river and placed gabions (hard engineering)
- planting scheme (soft engineering)
• Tees Barrage - stop incoming tide from moving up the Tees and preventing storm flow, regulate river flow
Cost of management schemes in the lower course of the river tees?
- new flood scheme at Lustrum Beck £1.2-2.1 mil
* Tees barrage - £54 mil
Impact of new flood scheme used in the lower course of the river tees?
- stop the £2 mil property damage, saved up to 150 properties
- provides access for people and vehicles
- gabions protect walls and embankments from erosion
- surrounding wildlife can thrive
- reduced flood risk to Yarm
Impact of the Tees barrage in the lower course of the river tees?
- water is fresher and cleaner - doesn’t mix with tidal salt water in lower estuary
- reduces risk of flooding at high tide/during storm surge
- £500 mil investment in offices, educational, leisure and shopping facilities - attracted jobs
- regeneration of derelict land along river side
- recreation - international canoeing location
Which strategy (hard v soft) dominates down the tees? Why?
Hard engineering dominated because the EA need quick solutions to stop further property damage due to flooding
Are the EA getting it right in terms of the river tees catchment flood management plan?
- they are effectively reducing flooding of the tees
- need to look at more soft methods - longer term and less expensive
- if government cut EA’s budget then hey will not be able to maintain and build more structural defences
How do tributaries affect erosion downstream?
- increase the volume of water which increases kinetic energy, velocity and thus erosive power
- cuts a much deeper channel with increased distance downstream
Why does a channel become wider downstream?
Less steep gradient, so less vertical erosion
How are interlocking spurs formed?
Fluvial erosion
What is lag time?
Time difference between peak rainfall and peak discharge
What is the rising limb?
Shows how quickly discharge rises after a rainstorm
What is the falling limb?
Shows reduced discharge after the main effect of runoff has passed
What is bank full discharge?
the level of discharge above which flooding
will occur as the river will burst its banks
What is a flood-relief/bypass channel?
Back up channel for a river that frequently floods
