Topic 4: Rivers Flashcards
Discharge
The volume of water flowing in a river, measured in cubic metres per second.
Velocity
The speed of a river, measured in metres per second.
Load
Material carried by the river.
Upper course
• Resistant rocks
• Steep
• Narrow and allow
• Slow velocity and low energy
• Inefficient
• Low load
• Large load
• Traction and saltation in times of high flow (when energy increases)
• Narrow and V-shaped
• Waterfalls and V-shaped valleys
Middle course
• Becoming less resistant rocks
• Becoming more gentle
• Getting wider and deeper
• Increasing velocity and energy
• Becoming more efficient
• Increasing load
• Smaller size of load
• Less saltation and traction. More suspensions and solution.
• Getting wider and flatter
• Meanders and floodplains develop
Lower course
• Less resistant sedimentary rocks
• Almost flat land
• Wide and deep channel
• Very fast flowing (less friction)
• Very efficient
• Load is high
• Small size load
• Suspension
• Wide, flat floodplain
• Oxbow lakes, floodplains, levees, deltas
Why velocity increases going downstream?
It is more efficient because there is less friction so energy increases.
Named example: The River Seven
• The source is on the slopes of Plynlimon, Wales
• Runs through Shrewsbury and Gloucestershire
• Hard more resistant rock (sedimentary) to clay at the end
• The mouth is in the Bristol Channel.
River landforms:
Upper course
1) Waterfalls
2) V-shaped valleys
Waterfalls
• Bands of hard rock and soft rock, and overtime soft rock gets exposed to the water and eroded.
• The soft rock is being undercut and wears away.
• Overhang becomes unstable.
• Plunge pool is formed.
• Rock is eroded through hydraulic action and abrasion.
• This repeats and gets deeper.
V-shaped valleys
• Valleys are formed by vertical erosion when the river is in bank full conditions.
• This means it has lots of energy due to high discharge and therefore efficiency.
• The bedload transported by traction and saltation causes abrasion on the channel bed.
• The valley sides are not vertical due to weathering and mass movements.
Interlocking spurs
Fingers of land that potrude into the river and restrict the view down the valley. Forms when river doesn’t have enough energy to erode more resistant rocks so diverts around.
Meanders
• River velocity varies in the river channel.
• Erosion will be happening on the outside of the bend through hydraulic action and abrasion where it’s fastest and deepest.
• Deposition will happen on the inside of the bend where it’s slowest and friction is highest.
• On the outside of the bend river cliffs will form.
• On the inside of the bend a slip of slope/river beach will form.
• Meanders continue to grow
• River increases its sinuosity (bends/curves)
Oxbow lakes
• Meanders become so large and tight that they are nearly touching.
• In times of flood, the fast flowing water erodes a more efficient channel and breaks through the land between a meander.
• Sediment is then deposited between the new river channel and the meander.
• The meander is cut off and eventually drys up.
Levees
• In times of flood eroded material is deposited.
• Thickest and heaviest sediments is deposited first closest to the river channel.
• Thin and fine sediments deposited after,
• Over time deposited material builds up creating levees along the edges of the channel after a flood.
Deltas
• Rivers are forced to slow down when they meet the sea or a lake causing them to deposit material.
• This forms many channels (distributaries) if it’s not washed away.
Transportation types
1) Traction
2) Saltation
3) Suspension
4) Solution
Traction
Heavy boulders are rolled along the river bed.
Saltation
Smaller rocks are bounced along the river bed.
Suspension
Fine sediment is picked up, suspended (floating) and carried along the river course.
Solution
The finer sediment is dissolved as it travels in the river.
What factors increase surface runoff and mean a river is more likely to flood?
• Urban areas
• Deforestation
• Large amounts of rainfall
• Waterlogged soil and frozen soil
• Frozen ground
• Hard rock
• Steep mountains
• Narrow drainage basin
What factors encourage more infiltration and mean a river is less likely to flood?
• Rural areas
• Forested areas
• Little rainfall
• Dry soil
• Mild conditions makes ground permeable
• Soft less resistant rock
• Flat land
• Circle drainage basin
Hydrograph
Show how fast rainfall gets to the river.
Flashy hydrograph
• Short lag time
• High peak discharge
• Steep rising limb
• Rainfall reached river quickly
• More likely to cause flooding
Subdued hydrograph
• Long lag time
• Low peak discharge
• Gentle rising limb
• Rainfall reached river slowly
• Less likely to cause flooding
River Wey Woking:
Significance of situation
• Confluence with the Thames
• River Wey at Old Woking
• Confluence at Tilford to form the main river Wey
• 2 sources. Alton and Haselmere
Causing of flooding in Woking?
• Urbanisation
• Global climate change
• The Jet Stream moved, causing more rain (high level wind that blows west to east across Northern Hemisphere)
• Deforestation
Impacts of flooding in Woking?
• Damaged homes
• Money needed to repair damage
Why is flood risk increasing?
• Increase in storm frequency and severity
• Global warming - changes to weather patterns
• Urbanisation is causing urban sprawl which means less green space so more impermeable surfaces.
Hard engineering techniques to mitigate flood risk?
• Flood walls
• Flood barriers
• Embankments
Positives and negatives on flood walls?
+ One off cost
+ Increases channel capacity
- Disperse water quickly and so can cause flooding downstream
- Not natural looking
Positives and negatives on flood barriers?
+ Can protect large areas
+ Gates can open and close when a storm surge is forecasted
- High cost and need regular maintenance
Positives and negatives of embankments?
+ Stop flooding in places of high economic value
- Water may go over the top and get trapped
Soft engineering techniques to migrate against flood risk?
• River channel restoration
• Flood plain retention
Positives and negatives of river channel restoration?
+ More natural, improves ecology
+ Meanders being rebuilt, slows done water (reduces flooding)
+ Less risk of flooding downstream because discharge is reduced
- Can increase local flood risk
Positives and negatives of flood plain retention?
+ Increased ability to store flood water
+ Restored to grassland, so retain and release water slowly
- Restricts development
- Can’t be used in urban areas
