- Created by: ccoatesx
- Created on: 10-11-18 09:29
Types of Erosion
There are several types of erosion including, vertical and lateral.
Abrasion - when rocks scratch away at riverbeds and banks.
Attrition - When rocks bang into each other, causing them to become smoother, rounder and smaller.
Solution - Soluble particles dissolved in water, e.g. limestone.
Hydraulic action - The force of the river on the banks, causes air to be trapped in cracks - pressure weakens banks which erode.
Types of transportation
Traction - Large boulders which roll along the river bed.
Saltation - Smaller rocks that bounce on the water.
Suspension - Smaller rocks being carried in the river.
Load - All materials being carried.
Bedload - Large boulders moving along the river bed.
Solution - Soluble particles that have dissolved, e.g. limestone.
Drainage basin - Area which is drained by a river.
Source - Where the river begins.
Mouth - Where the river ends.
Tributary - Smaller rivers that join larger ones.
Confluence - Where two rivers join.
Watershed - The boundary between two drainage basins.
Deposition - Occurs when river velocity decreases.
Rivers long profile
RIVERS LONG PROFILE - Shows gradient of river as it flows from source to mouth.
Upper course - Vertical erosion, hydraulic action
Middle course - Lateral erosion
Lower course - Little lateral erosion
Upper course - Traction (large boulders)
Middle course - Small particles
Lower course - Suspension (Smaller particles)
Upper course - Large boulders deposited
Middle course - more deposition
Lower course - Fine materials deposited
Rivers long profile pt.2
Discharge increases downstream as tributaries join the main river, meaning there is more water flowing at a certain point.
Velocity increase down the long profile.
Velocity and discharge increase because the river gets wider and deeper.
Formations in the Upper Course
A valley that is v-shaped, caused by hydraulic action and freeze thaw.
Interlocking spurs -Land that interlocks, either side of the river. The water hasn’t got enough gravitational potential energy to erode laterally, so the river flows side to side, eroding vertically.
The formation of waterfalls
There is both soft and hard rock, so differential erosion takes place.
The soft rock beneath the hard rock gets eroded by abrasion and hydraulic action. Leading to undercutting, forming a cave.
An increase in velocity causes more soft rock to be eroded - forming a plunge pool.
Undercutting causes the hard rock to fall into the plunge pool.
The waterfall retreats backwards, called headward erosion.
Leading to the formation of a gorge.
Landforms in the middle and lower course - Meander
As the river erodes laterally, it forms bends - called meanders.
This is due to deposition and erosion.
The force of the water erodes and undercuts the river bank.
On the inside of the bend there is less energy, so material is deposited.
Overtime the meander becomes tighter - until the ends become very close together. As the river breaks through the loop is cut off from the main channel, and an oxbow lake is formed.
Thalweg - Deepest part of a river, high velocity.
Helicoidal flow - Corkscrew motion of water, eroding on sides and depositing on the next slip-off slope.
Landforms in the middle and lower course - Oxbow L
- In the lower course, the meander becomes even larger due to increased lateral erosion.
- Hydraulic action and abrasion erode the river cliff side due to increased velocity, as the water has to travel further and depose on the slip-off slope side where there is less energy and velocity.
- This process is aided by helicoidal flow.
- The erosion causes the meander neck to form at two adjacent river bends until eventually it breaks through. This takes place when the river has more energy, e.g. when it floods.
- The river now follows a straight path.
- After the meander has been cut through, the river seals off the bend by deposition and an ox-bow lake is formed.
- When it is dry, an ox-bow lake may dry up and form a meander scar.
Landforms in the lower course - Levee
Levees are raised banks, made up by transported materials, that follow the course of the river.
Alluvium - River deposits
River bluff - Where the river meets valley
Formation of a levee:
When a river exceeds its bankfull discharge, because of a storm or heavy rainfall, due to traction, large particles are deposited nearer the river channel, smaller particles (saltation) are deposited further away and even smaller particles (suspension) are deposited farthest away becoming flatter, (horizontal stratification).
Larger particles are deposited closest and smaller rocks further away - vertical stratification occurs when small rocks are lying over big boulders.
Process repeats again.
Landforms in the lower course - Floodplain
Formation of a floodplain:
As horizontal stratification occurs, the smaller particles (suspension) have been deposited further away from the river channel where it is much flatter than where the large boulders have been deposited.
The smaller particles form the floodplain.
Abrasion and hydraulic action make the floodplain bigger on one side (lateral erosion).
An estuary is the tidal part of a river where the channel broadens out as it reaches the sea, in a particularly enclosed area.
How are estuary mudflats formed?
Mudflats form in sheltered areas where tidal water flows slowly.
As the river transports alluvium down to the sea, incoming tide transports sand and marine silt up the estuary.
Where the waters meet, velocity is reduced, causing deposition.
Built up layers of mud form, called mud flats.
When freshwater meets salty water, flocculation occurs. Bits of mud stick to the salt, making the particle size bigger.
Tidal Bore - Huge waves that funnel up the river, differences between high and low tide.
How can we describe and contrast hydrographs?
Lag Time = Peak discharge - peak rainfall
Hydrographs show the relationship between precipitation and discharge.
How can human activity increase the risk of floodi
Building on a floodplain:
Building on the surface now makes the ground impermeable, whereas it used to be permeable.
How does human activity affect interception, infiltration and evapotranspiration?
Interception would decrease as trees are cut down, less root uptake and less evapotranspiration.
Infiltration would decrease as water will not be able to get into the soil. More surface runoff as less interception, and more water flowing into nearest river, meaning more flooding.
How does human activity affect how much water reaches the river?
More surface run-off, more water reaching the river in a quicker time, creating a flashy hydrograph with a high peak discharge.
Rural land use can increase the risk of flooding if farmers don’t plow their fields along contour lines, and there could be a build up of aggradation.
Hard and Soft Engineering
Hard engineering = Prevention of flooding by stopping natural processes
Soft engineering = Management of flooding by mimicking natural processes, e.g. river straightening.
To increase capacity, widen or deepen channel.
To increase velocity, straighten or concrete channel.
To reduce discharge, build dams or plant vegetation.
Channel straightening - Removing meanders from a river, making it straight, so it can carry more water quickly downstream.
Dams and reservoirs - Acting as barriers, interrupting the river flow to create a man-made lake, controlling discharge.
Flood-relief channels - hard engineering
Social: - Removes the risk of flooding
- Footpaths are built along new channels which can be used for cycling and dog walking, attracting visitors.
Economic: - Insurance costs are lower
- Houses are easier to sell
Environmental: -Provides new habitats
- When full of water they provide a tranquil setting
Social: - People living in the path of the relief channel have to be moved
- Settlements downstream of a relief channel suffer from increased flooding
Environmental: -Habitats are disturbed
- Look unattractive when there is a low flow
Embankments - Hard Engineering
Social: - Attractive walkway for local people
Economic: - Cheaper compared to other methods of hard engineering
Habitats are provided for riverbank animals
Safer from flooding as the channel now has an increased carrying capacity
Deprive people of easy access to the river for fishing
Not as reliable as other hard engineering methods
Higher maintenance costs, and need constant repair
If the embankment is breached, then water lies on the land for a long time
Dams and reservoirs - Hard engineering
Social: - Relaxing dog walks
Economic: - Boosts tourism
- More hydroelectric power
Environmental: - Areas around reservoirs may be planted with forests, over 150 million trees were planted at the Kielder Dam
Social: - Flooding displaces people, usually farmers
Breaks up communities
Economic: - Expensive - around £167 million
Reduces crop yield for local farmers
Environmental: - Can trigger earthquakes
Algae collects behind dams, which deoxygenates the water
Channel straightening - Hard engineering
Social: - Reduces flood risk by moving water out of the area quickly
Less people are now at risk of flooding
Economic: - Homeowners can invest in their property as there is no longer a risk of flooding
Environmental: - Stops the area getting flooded which is bad for animal habitats
Social: - People downstream may suffer from flooding
Economic: - Expensive
- May have an unattractive concrete lining
Increased pollution on the land
Changes in hydrology and flooding downstream
Flood warning - Provides reliable advanced information about possible floods
Flood plain zoning - Organised flood defences
River restoration - The process of managing rivers to reinstate natural processes to restore natural biodiversity
Flood alert - Possible flooding
Flood warning - Expected flooding
Severe flood warning - Danger to life
Trees are planted near rivers to increase interception
Low cost, and environmentally friendly