Unit 2.3 River Landscapes

Drainage Basin: It is a land drained by the river system. It acts like a funnel, collecting all the water within one area and then channelling it into a waterway.

• Mouth - where a river meets the sea
• Source - the start of a river
• Tributary - a stream that joins a large river
• Watershed - the boundary of a river basin
• Confluence - the point where two rivers meet

Hydraulic action: The pressure of the water being pushed against the banks and the bed of the river. Includes compression of air cracks when water gets into them resulting in pieces of rocks breaking off.

Corrasion: Particles (the load) carried along the river thrown against the river banks with considerable force

Corrosion: Chemical reaction between certain rock types and the river water. Especially evident on limestone where river eats through it and flow underground

Attrition: Wearing of rocks in the river. Rocks in the river's upper course roll around and knock away at each other forming smooth pebbles and sand

Weathering: The breaking up of rock, but they don't move

Physical: Involves the breakdown of rocks and solids, torn apart by physical force, through direct contact with heat, water ice and pressure

• Freeze-Thaw: Water gets into cracks and expands as it turns to ice when temperatures fall. This expansion puts pressure on the rock around it causing fragments of rock to break off.

Chemical: Breaking down the bonds holding the rocks together, causing them to fall apart, forming smaller and smaller pieces.

• Rainwater contains weak acids that react with certain rock types. Carbonates in limestone are dissolved by weak acids causing the rock to break up or disintegrate.

Biological: Caused by living organisms, most often plant roots or burrowing animals.

• Rain causes seedlings, that fall in cracks, to grow. Roots force their ways into to cracks and break up the rock as they grow. Burrowing animal are responsible for the further break-up.

Mass Movement: when material moves down a slope due to the pull of gravity

Soil Creep: The slowest downhill movement. Gravity pulls water in soil downhill. Soil particles move with water. Heavy rainfall causes faster soil creep. The slope appears to have ripples known as terracettes.

Slumping: Happens on river banks, where part of it slips into the river. Common where the river passes clay as the dry weather makes clay contract and crack. Therefore rainwater gets into the cracks and the soil becomes saturated. Due to the pull of gravity, the large piece of weakened rock slips into the river. It is said to have slipped on the slip plane of saturated rock.

Width/Depth: The river will become wider/deeper as the amount of water in it grows due to it being joined by tributaries as it moves towards the sea.

Velocity: As the river moves towards the sea and becomes deeper and wider, less water is in contact with the bed and banks, therefore, less friction occurs and the velocity increases.

Discharge: The amount of water in the river increases as the river moves towards the sea because of the increase in volume due to it being joined by tributaries.

Gradient: The slope will become less steep as the river moves out of the hills and into flatter areas on its way to the sea

• A band of more resistant rock lies on top of the less resistant rock

• The softer rock is eroded more quickly, undercutting the hard rock

• The hard rock overhangs until it eventually collapses 

• The collapse adds large blocks of rock to the base of the waterfall. These can increase erosion by abrasion

• The base of the waterfall is eroded to form a plunge pool by abrasion and hydraulic action

• The softer rock continues to be eroded and the overhang collapses again causing the waterfall to retreat

• The process is repeated again and again and the gradual retreat of the waterfall forms a steep-sided gorge of recession.

As the river meets the hard rock in the v-shaped valley it doesn't have enough energy to cut through it (erode) so it just winds around the spurs leaving them to be interlocked

On the outside of a meander, the water is deeper so there is less friction and the current flows faster. The force of the water erodes and undercuts the outside bend by abrasion, forming a steep bank called a river cliff.

On the inside bend, there is shallow water, therefore, more friction and the current is less strong, which encourages deposition. There is not enough energy to transport material so sand and small pebbles are deposited creating a gentle slip-off-slope.

The lateral erosion on the outside bend widens the valley floor and erodes away the ends of the interlocking spurs. A flat valley floor is created.

• Erosion by abrasion narrows the neck between the two meanders

• The river will usually cut through the neck during a flood

• The river then flows along the straight path which is the shortest route which takes less energy

• Alluvium deposition will seal off the ends and the cut-off becomes an oxbow lake

• A floodplain is a flat area of land either side of the river

• When a river holds too much water to stay in its channel, it will flood

• The water is shallower on the land than the river so it deposits the material it is carrying causing the formation of a flood plain

• Every time it floods deposition builds up the floodplain

• These are high banks of a river that build up over time.

• When a river floods it deposits the heaviest of particles it is carrying first

• The larger particles, often pebble sized, form the levees

• Sand, silt and clay are lighter so they can be carried further due to not much energy being needed by the river. Sand and silt are deposited first and clay is last

• If a valley has steep slopes, water will move into the river more quickly causing frequent floods

• Area formed from impermeable rock is more likely to flood more often

• Rain for a number of days causes ground saturation. Meaning excess water will then flow straight into rivers causing them to flood

• A sudden rise in temperature after heavy snowfall causes rapid thaw. Rivers will be unable to cope with the amount of water and will flood

• Vegetation removed from slopes causes less interception. Meaning water will move to the river more quickly

• A dam could burst to lead to more water in the river channel and large areas close to the river would be flooded

• Large areas of tarmac are found in urban areas. This stops rainwater from soaking into the ground so it runs into the drains which allow it to move into the river at a greater speed. Therefore it is more likely to flood

• If farmers plough up and down slopes instead of around the hillside, the channels created by the plough allow rainwater to travel faster to the river

2007 Tabasco Flood - Mexico

• 20,000 people forced to leave homes
• Destroyed agricultural cocoa production plants. 30,000 families earned their living through this were affected
• A large number of farm and companion animals were trapped by flood water

2008 Bihar Flood - India

• Killed 260 people. 300,000 houses destroyed.
• Villagers in Bihar forced to eat raw rice and flour mixed with polluted water
• Koshi Wildlife Reserve was severely impacted, including its wildlife and biodiversity

Forecasting

• Met office predicts the likelihood of a flood on the TV, radio and internet
• Householders advised to ring flood hotline or go to Environment Agency website to check for likelihood of flood in their area

Building Design

• Electricity sockets are placed half way up the walls
• Concrete floors laid instead of wooden ones so that they don't rot if wet
• Property built on stilts

Planning

• Flood risk assessment carried out before planning permission granted
• DEFRA decides which areas are defended against river flooding. It also provides most funding and Environment Agency build and maintain.

Education

• Governments give advice on how to protect homes on the internet
• System of warning codes to learn: flood watch, flood warning, severe flood warning and all clear. Gives people info on what to expect and how to react

Embankments - raised banks along the river

• Used as pedestrian path. Effective at stopping bank erosion.
• Often not built high enough. Ugly and spoil view

Channelisation - widened or deepened to carry more water. Straightened so water can travel faster

• Effectively protects immediate area as water is moved quickly. Long lasting.
• May lead to greater risk of flooding downstream as water is carried there faster. Looks unnatural.

Flood relief channels - channel course of river altered to divert flood waters away from settlements

• People living near main river are safe. Can be used for water sports
• Requires a lot of land, sometimes difficult to purchase. Extremely expensive

Dams - holds back water and releases in controlled way to control discharge

• Stored water used for hydroelectric power
• Expensive. Settlements lost when the river is flooded to form a reservoir

Flood walls - vertical barrier usually made of concrete

• Used in areas of limited space. Easily erected in pre-constructed sections
• Must be assembled well so water doesn't get through joints

Storage areas - close to river and fills with water when river overflows

• Natural looking. Doesn't damage environment
• Need large are of land. Only useful when river is being flooded

Floodplain zones - Introduced policies, by national government, to control urban development to or on a floodplain

• Cheap. Surface runoff less likely to happen as it hasn't been built on
• Resistance to laws due to shortage of housing. Hard to regulate in LICs

Washlands - river is allowed to flood naturally in wasteland areas to prevent flooding in other areas

• Nothing is built = very cost effective. Provides wetland sites for plants and birds.
• Cannot be built on after. Productive land could turn into marshland

Warning systems - Network of sirens giving people early warning of possible flooding

• Very cheap. Electronic communication is effective
• Sirens could be vandalised. Might not give enough time for residents to prepare

Afforestation - Trees planted to intercept rainfall which slows down flow of water to the river

• Improves environmental quality. Very sustainable
• Tree type that is often planted makes soil acidic. Increases fire risk because of leisure activities in forest 

2002

• Level of land raised by 10mm by building an embankment across the river valley. Water would be stored behind the embankment in times of flood

2003 

• Flood warning systems improved. Residents in Far Cotton given 2 hours notice.

2007 

• Washland created in Upton where 1.2 mil cubic metres of water could be stored during a flood.
• Major roads in the area such as the A45 are on embankments up to 6m high
• Railway station protected by building 4m floodwalls at Foot Meadow