Waterfall and gorge of recession formation
When a river runs over alternating layers of hard and soft rock, rapids and waterfalls may form.
A river flows over a fault line. The cap rock is a hard resistant rock whilst the underlying rock is softer.
When the water flows over the fault line, the river erodes a plunge pool by abrasion (where the rocks carried by the river hit the bed, eroding it); by solution (where salts in the rock are dissolved by the water) and by hydaulic power (where the sheer force of the water erodes the rock).
Over time, the cap rock collapses into the plunge pool because it is not supported underneath
This happens again and again so a gorge of recession is formed.
At High Force, a waterfall on the River Tees, the cap rock is Whinstone
A meander is a bend in a river
On the outside of the bend the river's velocity is fastest which causes the river to erode a river cliff
The erosion is by abrasion, saltation and hydraulic power
On the inside of the bend, the river's velocity is slower
This means there is more deposition so a slip-off slope is formed
There are meanders on the River Tees near Yarm
In the middle course the river has more energy and a high volume of water. The gradient here is gentle and lateral (sideways) erosion has widened the river channel. The river channel has also deepened. A larger river channel means there is less friction, so the water flows faster
An oxbow lake is a crescent-shaped body of water.
It occurs where a meander is cut off at the neck.
The velocity of the river is fastest here and as the water swings around the neck it erodes it, making it narrower over time
When a storm occurs, the water rushes through the neck
Deposition occurs closing the old meander.
The river erodes the neck by abrasion, solution and hydraulic power
There are many ox-bow lakes along the Mississippi near St Louis
Deltas are found at the mouths of large rivers
When the water reaches the sea, it loses a lot of its energy so deposits its load
A delta occurs when the sea cannot remove the sediment as fast as the river deposits it
There are three main types of delta:
- Fan-shaped or arcuate - There are many distributaries and it has a rounded shape (e.g. the Nile delta
- Cuspate - has a triangular shape and has few distributaries e.g. the Tiber delta
- Bird's foot - each part of the river juts out into the sea e.g. the Mississippi delta
A levee is a rasied embankement that runs either side of a river
It is a depositional land form
In times of a flood, when a river is laden with sediment, it may burst its banks
Then the velocity of a river falls so it cannot carry large bolders and stones
These are deposited first and build up the levees
Sometimes the levee gets so high that the river flows higher than the flood plain
Levees occur on the River Rhine in the Netherlands
A flood plain stretches either side of a river and is made by erosion and deposition.
The meanders migrate down stream eroding into the valley sides making the valley wider
The erosion processes are abrasion, hydraulic power and solution
Deposition occurs when the river which is laden with sediment, bursts its banks
The sediment is spread over the valley floor
There is a flood plain either side of the River Thames
Case study: River Clyde
The River Clyde is about 160km long
its source is in the Southern Uplands region of Scotland and the river flows north-west through Glasgow
the mouth of the river is an estuary on the west coast of Scotland
- The source is in the Lowther Hills where two tributaries met
- There are two interlocking spurs at Crawford. The spurs are between 300 and 500 m high
- The Falls of Clyde are four waterfalls near Lankark. The highest fall is Corra Linn - its about 27 metres high. There is also a gorge formed by the waterfall retreating
- There is an oxbow lake forming near the village of Uddingston
- The river meanders between Motherwell and Glasgow
- The estuary is 34 km west of glasgow where the river joins the firth of Clyde and meets the Irish sea
Causes of river flooding:
The UK floods in 2007 where caused by physical and human factors.
June had doubled the average amount of rainfall.
Previous months had also been wet - it was the wettest period from May to June since records began in 1776.
This meant the ground was saturated and could not absorb anymore water
Hull recieved 100mm of rainfall in 24 hours; this was more than the amount it usually recieved in a month
The rain was due to a series of depressions that crossed the UK from the Atlantic Ocean
Human factors made the situation worse:
- People are increasingly building on the land which creates an impermeabke surface - for instance people in Bedford have covered drive ways with paving
- Wetland areas in the Yorkshire Moors have also been drained increasing surface run-off
- People have built homes in flood plains a problem in Maidenhead particularly
Causes of river flooding
In 2007, the rivers Brahmutra and Ganges flooded Bangladesh.
Physical factors caused the flooding:
Bangladesh is a very low-lying area - most of the land is less than 1 m above sea level as it is a rivre delta.
Cylone Yemyin brought heavy rain to the area and it rained for about 20 days
The monsoon rains in the Himalayas were also very heavy
Human factors made the situation worse:
There had been deforestation in Tibet and Nepal, this means the water is not stored in trees and flows into rivers
there had been urbanisation in India, ecspecially in Delhi meaning the rainwater flowed into rivers rather than the ground
Embankments built to stop the flooding forced it down stream cauing more flooding instead
Case study: Boscastle floods
The Boscastle floods, 16th August, 2004
- Rain - 60mm in two hours, remnants of hurricane Alex, tall convectional clouds, hot day
- Valley Shape - Steep sided valleys of R.Valency and R.Jordan, the valleys funnel the water into Boscastle
- Geology - impermeable rocks + urbanisation
- Human - built on a flood plain at the end of two narrow valleys
- Effected 1000 residents however there were no deaths and only one minor casualty
- 7 helicopters airlifted 100 people (including 6 fire fighters)
- 29 of the 31 Cornwall County Fire Brigade stations were involved for 7 days
- 58 properties were flooded, 25 businesses (40 further in other villages)
- 84 wrecked cars were found
- Eventual cost with management: £15million
To prevent it happenning again a new culvert is being built on the River Jordan
Effects of Bangladesh floods
There were short term consequences;
Nearly a third of the country was flooded
Over 400 people were killed and 7.5million people fled their homes
The floods cost over $0.5 billion
The crops in the fields were destroyed and the farm animals killed
In the long term, the government has had to spend money on rebuilding rather than educating, furthering poverty
As the crops were damaged people went hungry and suffered from malnutrition
There was very little clean drinking water so over 50,000 people got diarrhoea meaning they couldn't work and earnt even less money
People's seed were destroyed so couldn't plant crops the next year
However the fields were fertilised by the alluvium from the river
Responses to Bangladesh flood
The area recieved aid from organisations such as UNICEF and Care International.
The Bangladesh government has tried to reinforce flood defences around Dhaka but this can increase flooding elsewhere
There is a debate whether to protect rural or urban areas
The government is trying to build more cyclone shelters as over 400 people were killed in 2007
However it is a very poor country with a GDP per person of less than $1,400 so money is rare
To warn people of the floods, volunteers cylce around with megaphones urging movement to safety
The government wants to improve the system
Responses to Boscastle floods
A NUMBER OF CONTRASTING STRATEGIES WERE IMPLEMENTED
Braiding was encouraged as deposition on the river bed slows down the flow and removes material
Debris Screen - wire mesh to collect the load (500 tonnes collected in a test)
Trees felled - Traps debris
Raised and widened car park, re-enforcing the grass as a permeable surface
Widened and deepened channel
Bridge removed as it created a constriction
A culvert was introduced ensuring the Jordan flowed into the Valency more easily
Rich vs. Poor impacts of flooding
In 2007 there were floods in the UK and in Bangladesh
The Bangladesh floods caused $0.5billion of damage whilst in the UK it was $2billion
This is because people in Bangladesh are not as rich and have fewer possessions to lose
In Bangladesh 400 people died compared to only 13 in the UK
This is because the UK has a smaller population density so fewer people were in danger
The UK is a rich country and most could get away in their cars on the motorways, whilst in Bangladesh there are not enough Cyclone shelters and some don't have radios or TV
In the UK people can easily check the weather using the Environment Agency Flood Warning website
Hard and soft engineering
Hard engineering involves the use of concrete whilst soft engineering involves working with nature.
Hard strategies are expensive and involve engineering the river - cutting off meanders and building dams.
However soft strategies are cheaper; they involve putting in meanders and reed beds encouraging the river to flow down its natural course
Soft engineering is relatively sustainable and works with nature.
It is cheap because it is easy to construct and maintain. It involves understanding the river and how it will flow in the future.
Reed beds absorb water so reduce flooding and putting in meanders gives the river more space so it won't flood a built up area
Wales has a surplus of water with a rainfall level of over 1500mm per year. This is relief water as the land in Wales is mountainous. Snowdon is over 1,000 metres high
It has many large rivers such as the river Usk. It has a population density of less than 50 people per km squared so there is relatively little demand for water
The steep sided valleys in Wales are easily dammed meaning the water can be stored.
The situation is very different in Greater London, which is an area of deficit. The area has less than 60mm of rain per year. The major river is the Thames but this is salty when passing through London so cannot be used for drinking.
The population for the whole of Wales is only 3 million whislt Greater London has a population of 7million. Some parts of London have a population density of 5,000 people per km squared so there is a very high demand for water
The Thames basin is less than 30 m above sea level so it is not suitable for large resevoirs meaning the water is very expensive to store and collect unlike in Wales