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River Landscapes in the UK (Erosion, Transportation and Deposition…
River Landscapes in the UK
The River Valley
Long Profile and Cross Profile
Path of a river is called its course
Rivers have an upper, middle and lower course
Rivers form channels and valleys as they flow downhill
They erode the landscape then transport it somewhere else and deposit it
The shape of the valley and channel changes along the river depending on whether erosion or deposition is having the most impact
Long profile - how the gradient changes
Cross profile - cross-section of the river looks like
In upper course the gradient is steep and the valley is V-shaped with steep sides, the channel is narrow and shallow
In lower course the gradient is gentle and the valley very wide, almost flat, the channel is very wide and deep
Vertical and Lateral Erosion change the Cross Profile of a River
Vertical erosion - deepens the river valley and channel, making it V-shaped, it's dominant in the upper course of the river. High turbulence causes the rough, angular particles to be scraped along the river bed, causing intense downwards erosion
Lateral erosion - widens the river channel during the formation of meanders, dominant in the middle and lower course
Erosion, Transportation and Deposition
Four Processes of Erosion
Hydraulic action - breaks rock particles away from river channel
Abrasion - eroded rocks picked up by the river scrape and rub against the channel, wearing it away
Attrition - eroded rocks picked up by the river smash into each other and break into smaller fragments. Their edges also get rounded off as they rub together - the further material travels, the more eroded it gets
Solution - river water dissolves some types of rock
Transportation is Movement of Eroded Material
Traction
Saltation
Suspension
Solution
Deposition
River drops the eroded material it's transporting
Happens when river slows down, can happen when:
Volume of water in river falls
Amount of eroded material in water increases
Water is shallower
River reaches its mouth
River Landforms - Erosion
Waterfalls and Gorges
Waterfalls form where a river flows over an area of hard rock followed by an area of softer rock
Softer rock is eroded more than hard rock, creating a 'step' in the river
As water goes over the step it erodes more and more of the softer rock
Eventually a steep drop is made - a waterfall
Hard rock is eventually undercut by erosion, becomes unsupported and collapses
Collapsed rocks are swirled around at the foot of the waterfall where they erode the softer rock by abrasion, creating a plunge pool
Over time, more undercutting causes more collapses, the water fall will retreat, leaving a steep-sided gorge
Rivers wind around interlocking spurs
In upper course of a river most of the erosion is vertically downwards. Creates steep-sided, V-shaped valleys
Rivers aren't powerful enough to erode laterally - they have to wind around the high hillsides
Hillsides that interlock with each other as the river winds around them are called interlocking spurs
River Landforms - Meanders
Meanders
Rivers develop large bends called meanders in their middle and lower courses, in areas where there and both shallow and deep section sin the channel
The current is faster on the outside of the bend because the river channel is deeper
More erosion takes place on the outside of the bend, forming river cliffs
Current is slower on inside of the bend because river channel is shallower
Eroded material is deposited on the inside of the bend, forming slip-off slopes
Ox-bow lakes
Meanders eventually turn into an ox-bow lake:
Erosion causes the outside bends to get closer until there's only a small bit of land left between the bends
River breaks through this land, usually during a flood
And river takes shortest corse
Deposition eventually cuts of meander forming an ox-bow lake
Identifying River Landforms
Contour Lines tell you the Direction of a River
They tell you the height of the land and the steepness of the land
Using the contours you can work out which way the current is flowing - from high to low
Maps contain evidence for River Landforms
Evidence for the upper course - nearby land high, river crosses lots of contour lines in a short distance - so steep, river's narrow, contour lines very close together around it so valley is steep V-shaped
Evidence for lower course - nearby land low, river meanders across a large flat area, river doesn't cross any contour lines so gently sloping
River Landforms - Deposition
Flood Plains
Wide valley floor on either side of a river which is occasionally flooded
When a river floods onto the flood plain, the water slows down and deposits the eroded material that its transporting, this builds up the flood plain
Meanders migrate across the flood plain, making it wider
Meanders migrate downstream, flattening out the valley floor
The deposition that happens on the slip-off slopes of meanders also builds up the flood plain
Levees
Natural embankments along the edges of a river channel
During a flood the eroded material is deposited over the whole flood plain
The heaviest material Is deposited closest to the river channel, because it gets dropped first when the river slows down
Over time, the deposited material build up, creating levees along the edges of the channel
Estuaries
Found at the mouth of a river - where it meets the sea
The water is tidal
The water floods over the banks of the river carrying silt and sand onto the valley floor
As the tide reaches its highest point, the water is moving slowly so the sediment is deposited
Over time, more and more mud builds up, creating large areas of mudflats
UK River Landscape
The River Clyde is about 160 km long
Source in Southern Uplands region of Scotland and river flows north-west through Motherwell and Glasgow
Some features and landforms in the valley that it flows through
Rivers estuary is about 34 km west of Glasgow - the estuary is 3 km wide. Areas next to the river channel are mudflats, which are exposed at low tide
Glasgow is built on the flood plain of the River Clyde, the land is about 5m above sea level on either side of the river
The river meanders between Motherwell and Glasgow
Interlocking spurs at Crawford, 300 to 500m high
Ox-bow lake forming from a meander in the New Lanark are
River Discharge and Flooding
River Discharge
Volume of water that flows in a river per second
Measured in cumecs - cubic metres per second
Hydrographs show how the discharge at a certain point in a river changes over time to rainfall
Causes of Rivers Flooding
Prolonged Rainfall - soil becomes saturated, any further rainfall cant infiltrate, which increases runoff into rivers, this increases discharge quickly, so flooding more likely
Heavy Rainfall - water arrives too rapidly for infiltration, so lot of runoff. Increases discharge quickly, increasing risk of a flood
Geology - clay soils and some rocks are impermeable so runoff is increased
Relief - if a river is in a steep-sided valley, water will reach the river channel much faster because water flows more quickly on steeper slopes. Discharge increases rapidly, increasing flood risk
Land Use - buildings are often made from impermeable materials and surrounded by roads made from tarmac - increases runoff and discharge increases quickly, so there's a greater risk of flooding
Hard vs Soft Engineering
Hard Engineering
Dams and reservoirs
Dams are built across the rivers, usually in upper course - reservoir is formed behind the dam
Reservoirs store water, reducing the risk of flooding. The water in it can be used as drinking and to generate hydroelectric
Very expensive to build. It can flood existing settlements. Eroded material is deposited in the reservoir and not along the river's natural course so farmland downstream can become less fertile
Channel straightening
River's course is straightened, meanders are cut out by building artificial straight channels
Water moves out of the area more quickly because it doesn't travel as far
Flooding may happen downstream and more erosion downstream
Embankments
Raised walls are built along the river banks
River can hold more water so it will flood less frequently, protecting buildings on the flood plain
Quite expensive and risk of severe flooding if water rises above the embankment or if they break
Flood relief channels
Channels are built that divert the water around important areas or take it elsewhere if the water level in the river gets too high
Flooding is prevented because river discharge is reduced - gates on the flood relief channels mean that the release of water can be controlled
Increased discharge where the relief channel rejoins the river which could cause flooding in that area
Soft Engineering
Flood warnings
Environment Agency warns people about possible flooding
Impact of flooding is reduced
Warnings don't stop a flood from happening and people may not hear or have access to them
Preparation
Buildings are modified to reduce the amount of damage a flood could cause. People make plans for what to do in a flood
Impact of flooding are reduced
Preparation doesn't guarantee safety and expensive to modify homes and businesses
Flood plain zoning
Restrictions prevent building on parts of a flood plain that are likely to be flooded
Risk of flooding is reduced and the impact of flooding is reduced
Expansion of an urban area is limited if there aren't any other suitable building sites
Planting trees
Planting trees in the river valley increases interception of rainwater and also increases the lag time
Discharge and flood risk reduced, vegetation reduces soil erosion in the valley and provides habitats for wildlife
Less life available for farming
River restoration
Making a river more natural - removing man made levees
Less risk of flooding downstream because discharge is reduced, little maintenance is needed as the river is left in its natural state and better habitats for wildlife
Local flood risk can increase, especially if nothing's done to prevent major flooding
Flood Management
Flash Floods showed need for Flood Defences in Boscastle
Village of Boscastle on the north coast of Cornwall was devastated by a flash flood on 16th August 2004, which caused millions of pounds worth of damage
Village is surrounded by steep valley sides, and land upstream of the village has been cleared of trees and vegetation, this increases surface runoff and means that during periods of heavy rain, river discharge increases quickly
Old bridge in the village had a low arch over a very narrow river channel, flooding in 2004 was made worse because trees and vehicles in the floodwater became trapped under the bridge, forming a dam
Village is a popular tourist destination and 90% of the local economy relied on tourism, after 2004, the number of tourists dropped significantly, increasing demand for protection
Flood management scheme in place
Hard and soft engineering
Old bridge replaced with one with higher arch
River channel widened and deepened
River embankment strengthened
New flood wall built
Gauge introduced to monitor water levels and improve flood prediction
Dead trees and vegetation removed to stop them blocking the river during floods
Issues
Social
Lives disrupted for years by rebuilding projects
New defences made Boscastle a safer place
Only protect in a 1 in 75 year flood - won't protect flooding of the same size as the 2004 flood - to protect against that would spoil the character of the village
Economic
Home and businesses are now less at risk of flooding - less risk of expensive damage, loss of stock and business and rising insurance costs
Flood management scheme cost over £4 million but could be better but those considered too expensive
Environmental
Vegetation and river habitats in the area are now continuously managed, biodiversity and river habitats have been improved
River channel engineered to look natural and to function as a normal river