The Drainage Basin
The Drainage basin- An area of land drained by a river and its tributaries, an open System.
Water shed- High ridges which seperate individual drainage basins
The water reaches the river in the drainage basin by:
- Outputs- Evaporation, transpiration from plants, runoff in the sea
- Stores- Puddles, rivers, lakes, soul storage, water stored on vegetation
- Transfers or flows- Infiltration, percolation, Surface runoff, throughflow, groundwater flow
Infiltration- Movement of water in to the soil
Percolation- Flow of water due to gravity through soil
Groundwater flow- Movement of deeper water underground through underlying rock strata.
Three river processes:
- Erosion- Hydraulic action, Abrasion, Attrition, Solution
- Transportation- Solution, Suspension, Saltation, Traction
Hydraulic Action- The turbulent flow of water lifts loose sediment when the river has a high velocity.
Abrasion- The rubbing of the bed and banks by material carried along by the river.
Attrition- The reduction in size of the sediment particlesas the collide with each other, the bed and the banks.
Solution- Where the rocks dissolve into the water and are corried. Most commonwhen the bed is made of carbonate rocks like limestone.
Deposition- When the river drops its load due to it no longer being competent or no longer having the capacity to carry all its load
Depostion frequently occurs when:
- there is a sudden reduction in gradient
- the river slows down, enters a body of water
- discharge is reduced due to low rainfall
- there is a sudden increasein the volume of sediment available like at a confluence.
The Hjulstrom curve- Image Card
The Hjulstrom curve
The Hjulstrom curve- The relationship between velocity and the maximum size load the river is able to carry(competence). Shows the velocities that sediment will nomaly be eroded, transported or deposited.
Two key points can be made from The Hjulstrom curve:
- Fine particles requirer a higher velocity to erode like clay and sand.
- Less velocity is needed to keep fine particles in suspension then to be eroded. fine particles can stay in suspension even when the river is vertually still.
Competence- The maximum size load a river can carry.
Capacity- The maximum amount of load a river can carry.
The long profile of a river- Image Card
The long profile of a river
Long profile of the river- reprsents the height of the river floor from source to mouth. It is an idealised profile which is a concave pattern which is steeper towards the source. The profile is never smooth due to waterfalls and knick points.
Changes in the long profile of a river downstream:
- Upper course- gradient is steep as river is close to the sourceand high above sea level= High potential energy. This means vertical erosion from abrasion.
- Middle course- river leaves the montains and hills, receives more discharge. Knetic energy increases causing lateral erosion from hydraulic action. Decreasing potential energy means less vertical erosion and gentler gradient.
- Lower course- Gradient is almost flat. River now carries lots of sediment using up the rivers energy meanin the river does not have much energy left for erosion.
A graded profile- a long profile of a river that has eventually balanced out the createa smooth concave curve, sometimes called dynamic equilibrium.
This means that the deposition and the erosion in the river is balanced and the irregularities in the river are eventually smoothed out.
Factors that interrupt this pattern:
Human- Damaging the river in the upper course can lead to abnormal bits of flat gradient as sediment is depsited. Also human intervention of the river as the river is used to drive factories and held in docks can also interupt the smootness of the river.
Physical- Rejuvination can vertically erode the river close to the source leading to steps in the river valley lick nick points and waterfalls.
Fluvial Landforms (Erosional)
Rapids- Fast flowing water that tumbles over rocky shallow riverbeds. White water is when air mixes with the flow of the river.
Formed by the steepening of the river gradient where potential energy increases vertical erosion.Formed where there is alternating stripes of resistant and soft rock exposed on the river bed.The softer rock erodes deeper than the hard rock leaving a series of hard rock bands which the water tumbles over and breaks up creating turbulence and white water.
Waterfall- A sudden fall of water that occurs where there are rapid changes in the gradient of the river's course.
Formed where there is a layer of resitant rock over a layer of softer rock. The softer rock is undercut and forms and over hang of the resitant rock.The plunge pool at the foot of the falls experiences the force of the falling water and the swirling action leads to increased erosion and the formation of a deep pool.
When the over hang collapses the process starts again, meaning the waterfall retreats leaving agorge behind
Fluvial Landforms (Depositional)
Floodplains- Flat areas of land, on the valley floor at each side of a river channel where sediment builds up called alluvium. Middle course of the river and widen further down stream.
Formed by deposition when the river breakes its banks and looses energy and competence due to friction on the valley floor. This means the river slows down and deposits sediment. The finner the material the further away from the river the sediment is deposited.
Successive floods add to the alluvium and increase fertility of the land. As rivers migrate over time and leave behind slip off slopes(point bars) this adds to the flood plain.
Levees- Parallel banks of sediment that run next to the river channel, natural levees can be upto 2m high. Can be heightened by people for flood defences.
Formed by depostion during flooding when the river looses energy and competence and deposits its sediment on to the floodplain. The larger coarstest sediment gets deposited first which builds up over time creating a natural embankment.
Fluvial Landforms (Depositional)
River Braiding- When a river channel is split into several channels that are seperated by islands of deposited load called sand bars. Occur in river with a high range of discharge like in semi arid enviroment-ephemeral waddies after a storm.
Are formed when the gradient is steep and there is plenty of load in the river and the discharge fluctuates daily or annually. When the river has high discharge, energy and competence that then decreases the river drops its load which chokes up the channel splitting the river into smaller channels.
Deltas- Sediment deposited at the mouth of a river when it enters a slow moving body of water such as a lake or the sea.Highly changeable landforms as they are made of unconsolidated sediment.
Oxbow Lakes- The curve of the meander taht gets cut off from the river channel by deposition that eventually drys up causing an oxbow scar.
The sinuosity os the meander (the curving nature of the meander) increases with more erosion the inner banks become closer tohether closing off the meander neck. At times of flood the neck can be eroded away giving the river channel a straighter,route. This forms a looped lake that eventually gets cut off by deposition forming an ox bow lake.
Flooding- Causes MEDC- Boscastle Flood 16th August
Was a flash flood, Totally unexpected, Never experienced such a flood in that area suggesting that it a was result of exceptional factors.
Human-Not caused by human factors but damage was increased by human factors:
- No flood control- The flood was a freak event so residebts of bosactledid not prepare.
- Sewer/drainage systems- Old and had a small capacity ment the large amount of sudden water broke the system.
- Brigdes- Cars and vegetation got stuck under the village bridge obstructing some of the flow of the river.
- Location- Very steep valley lead to accelerated rainwater and concentrated rainwater in a very narrow space which ment a lot of dischage in the river.
- Weather- Haevy rain fall a few days before lead to the ground being saturated. A clear warm day lead to large stom coulds being created and with in a few hours 5 inches of rain had fallen.
Flooding- Causes LEDC- Bangladesh floods 2004
- Urbanisation- increased surface runoff particullarly in the capital Dhaka.
- Deforestion- increased deforestation in the himalayas means less interception- increasing the speed precipitaion reaches the river.
- Low lying country- Most of which lies on the delta land of the three major rivers (The ganges, Brahmaputra and meghna).
- Snowmelt- The source for all three rivers are in the himalayas so adds to their discharge in spring.
- Monsoon season- Between may and september. This monsoon season causes cyclones which affects the dicharge of all three rivers causing floods.
Flooding- Impacts MEDC
- Transport- Roads were blocked making it hard for emergency services. Only two helicoptors could fit in the valley.
- Property- Many properties were damaged and unusable as sewage had filled the whole of most of the ground floors of the houses.
- People trapped- People were homless for a few days fourced to find refuge on rooves and emergency accomodation.
- Repairs- People had lots of repair work to do on homes and bussinesses which was time consuming and costly
- Home insurance- Was set to increase in the are due to the flooding.
- Tourism industry- Was a main industry in boscastle and the town was closed to tourism while repair work was done which was a massive loss to the town.
Flooding- Impacts LEDC
- 38% of land was flooded including most of the agriculteral land and the capital city Dhaka.
- 36 million people homeless (almost a quater of the population).
- By september death toll had risen to 800 many as a result of disease.
- Seriouse damage to the countries buildings- roads, brdges, railway lines and irrigation systems.
- Huge costs of damage- $2.2 billion
- Poor seriously effected- rural and agricultural areas cut off from aid.
- Enviromental impacts- river bank erosion, soil erosion, watter logging and water contamination.
Soft engineering: Methods that control river processes with manmade structures.
- Forecasts and warnings
- Landuse and management on the floodplain- floodplain zoning
- Wetland and river banks conservation
- River restoration- returning a river to its normal state so natural processes can continue.
Soft engineering is not very effective and is more about reducing the impacts however it is cheaper, sistainable and protects valuable habitats.
Hard engineering:Ecological approaches that work with nature to reduce flooding risks
- Meander strainghtening
- Raising Levees
- Diversion channels
Hard engineering is overall more effective at reducing flooding however is expensive to build and and maintain and is unatural to the surroundings and habitat.