rivers

What are the four types of erosion?

Hydraulic Action: The force of the water hits the river bank/bed, forcing them to crack open.

Attrition: Rocks collide with each other and became smoother and rounder.

Abrasion: Sediment scratches against the river bank/bed.

Solution: Soluble rocks (limestone) are dissolved and carried away.

What are the four types of transportation?

Suspension: Very fine material is carried along the river.

Traction: Large boulders are rolled along the bed.

Saltation: Small pebbles bounce along the river bed.

Solution: Dissolved material is carried along the river channel.

What is deposition?

The dropping of sediment onto the riverbed determined by the velocity of the water.

What is the river you should ALWAYS refer to? Where is it?

River Tees, England.

What shape is the valley at the upper course?

V-shaped valley.

What shape is the valley at the middle course?

U-shaped valley.

What shape is the valley at the lower course?

Flat valley

What landforms are found in the upper course?

River rapids, waterfalls, gorges, interlocking spurs.

What landforms are found in the middle course?

Meanders and ox-bow lakes.

What landforms are found in the lower course?

Estuaries and levees.

What is a gorge?

A landform formed when water retreats upstream.

How are waterfalls formed?

• The river flows over hard rock that lies over soft rock.

• The soft rock is eroded more rapidly due to hydraulic action.

• This leads to undercutting and an overhang forms.

• Eventually the overhang breaks due to lack of support and falls into the plunge pool formed at the bottom of the waterfall.

• This deepens the pool and increases abrasion.

• Over time, the water will retreat upstream and form a gorge.

How are meanders formed?

• Water travels in a corkscrew.

• This creates bends on the outer edge due to erosion of the bank - river cliff.

• Inside is slower and shallower, leading to deposition.

• Meanders will move along the flood plain.

How are ox-bow lakes formed?

• Requires flooding (bursting of a river’s banks).

• River cuts through the meander neck to form a straighter course.

• The meander gets cut off, leading to an ox-bow lake.

• This area can become a marsh/meander scar.

What does a long profile of a river show?

Shows the changes in the river gradient from source to mouth.

What does a cross profile of a river show?

Shows the changes in the river channel and valley.

What are levees and how are they formed?

Levees are raised embankments alongside a river channel.

• Formed in flood conditions.

• Friction between the water and the bed increases, leading to less velocity and therefore deposition.

• Larger sediment is deposited first.

• This sediment increases the height of the floodplains.

• More floods lead to larger levees.

What are estuaries and how are they formed?

An estuary is where the river meets the sea.

• Form in calm tidal water.

• Incoming tides bring salt water and sediment.

• When this meets the freshwater of the river, river velocity decreases and deposition occurs.

• This forms mudflats which rise up.

• Vegetation can grow there forming salt marshes.

What river has a famous large estuary?

The River Severn.

What human factors affect flood risk?

Deforestation: Lack of trees reduces interception so soils become over-saturated.

Urbanisation: Impermeable rock leads to surface runoff.

Agriculture: Ploughing increases overland flow.

Climate Change: Rising temperatures can increase storm frequency and intensity.

What physical factors affect flood risk?

Geology: Impermeable rock reduces percolation and leads to surface runoff.

Oversaturated soils: Reduces infiltration and increases overland flow — e.g. clay.

Steep Slopes (relief): Rapid surface runoff.

Precipitation: Prolonged rainfall means that infiltration isn’t occurring at a suitable rate, leading to surface runoff.

Define river discharge.

The volume of water flowing through a river channel per second.

What representation of data can we use to predict flood risk, and what does it show?

A hydrograph. Shows river discharge over-time.

Define base flow.

The ‘normal’ level of discharge.

What is lag time?

The time difference between peak rainfall and peak discharge.

Define rising limb.

The increase in river discharge.

Define falling limb.

The river returning to base flow.

What is the difference between hard and soft engineering?

Soft engineering aims to reduce the impacts of a flood, while hard engineering aims to stop the flood entirely.

What are some hard engineering strategies and how do they manage the river?

Channel straightening: Helps the water flow quicker to the sea and move away from the area at risk. Quite expensive and affects river habitats (e.g. the River Quaggy, Lewisham)

Embankments: Increases the volume of discharge that can be held. Cheaper than other methods.

Dams & Reservoirs: Controls the flow of the river and can create HEP, which is green clean and sustainable. Can be used to help the economy, but quite costly and environmentally damaging.

What are some soft engineering strategies and how do they manage the river?

Planting trees: Leaves intercept rainfall, but this is a lengthy process.

Flood Plain Zoning: Houses people further away from the risk, but has to be planned before urbanisation. Low value land contains agriculture and parks, med value land is usually leisure, and high value land is housing, hospitals, and education.

Warnings: Prepares people located in high-risk areas, such as a weather report. Cheap solution but doesn’t really prevent anything.

What are considered the two most effective solutions?

Flood plain zoning & dams/reservoirs.