REJUVENATION- Causes of flooding

Causes of flooding

Flooding = occurs when a river's dicharge exceeds the capacity of its channel to carry that discharge = the river overflows its banks. 

Flooding may be caused by a number of natural causes or physical factors:

Excessive levels of precipitation occurring over a prolonged period of time - leads to saturation of the soil. 

When the water table reaches the ground surface = increased overland flow or runoff

Intensive precipitation over a short period of time particularly when the ground surface is baked hard after a long period without rainfall = the infiltration capacity is such that the ground cannot soak up the rainfall quickly enough, so more water reaches the river than would normally be the case

The melting of snow particularly when the subsoil is still frozen = infiltration capacity is reduced

Climatic hazards such as cyclones in Bangladesh, hurricanes in the Gulf of Mexico or deep low-pressure weather systems in mid-latitudes = bring abnormally large amounts of precipitation

The nature of the drainage basin has an influence on the  likelihood of flooding. Some drainage basins are more likely to flood than others. Relief, vegetation, soil type and geology all have a part to play. In areas of the world vegetated by dense forest, interception and uptake by plants reduce the risk of flooding during time of heavy rainfall.

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REJUVENATION- Incised meanders

Incised meanders

If a rejuvenated river occupies a valley with well-developed meanders = renewed energy= they become incised + deepnd. 

Incised streams + rivers have cut deeply into the landscape in many parts of the British Isles.

 The nature of the landforms created is largely a result of the rate at which vertical ersion has taken place. 

When incision is slow + lateral erosion is occurring = an ingrown meander may be produced.

 The valley becomes asymmetrical, with steep cliffs on the outer bends and more gentle slip-off slopes on the inner bends. 

With rapid incision, where downcutting or vertical erosion dominates = the valley is more symmetrical, with steep sides and a gorge-like appearance, These are described as entrenched meanders.

FLOODING- Hard and soft engineering

Hard and Soft engineering: which is the better option?

Hard engineering strategies involve the use of technology - to control rivers
Soft engineering, adopts a less intrusive form of management, seeking to work alongside natural processes.

 Hard engineering = immediate results + are expensive. However, in the future, they may make problems worse or create unforeseen ones. 

Soft engineering = much cheaper + more sustainable = it does not interfere directly with the river’s flow.
 
What’s  more important?
In the aftermath of the 2009 Cumbrian floods, local people were angry that more hadn’t been done to prevent them. 

They accused the authorities of ‘putting salmon before people’ after their earlier request to lower the river bed by 3 metres in Cockermouth had been turned down because it might harm fish stocks.

The cost of protection
Professor Samuels advises the government on managing rivers. He said ‘It is technically possible to defend places like Cockermouth against extreme events, but only by building huge walls and embankments along the river, which would cost billions and alter the character of the town. 

For most people, that would be unacceptable as the floods.’

Hard Engineering

Hard engineering involves building structures to defend places from floodwater.

 Dams and reservoirs exert a huge degree of control over a river.

 The natural flow of water = prevented by a dam (often a concrete barrier across the valley), water fills the area behind it and is released or held depending on circumstances such as current and expected rainfall. 

Dams and reservoirs = normally constructed as part of a multi-purpose project rather than with just a single aim in mind.

Soft engineering

Soft engineering involves adapting to flood risks, and allowing natural processes to deal with the rainwater. 

It is a strategy that accepts the natural processes of the river + seeks to work with it to reduce the effects of folding rather than attempting to gain control of it. 

A conscious decision can be made to ‘do nothing’ but simply to allow natural events to happen, even if this involves the risk of flooding. In some poorer areas of the world, this is a necessary approach.

 In richer areas - money is set aside in years when flooding does not occur to provide relief after the event. 

However, there are many more positive approaches that can be adopted to reduce the risk of flooding without exerting a major force over the river and its processes.

FLOODING- Impacts

The impact of human activities on flooding

Urbanisation

More people are living in towns and cities

Population growth + urbanisation= demand for land to build on - floodplains are flat and are food for housing

Concrete and tarmac = impermeable = preciptation cannot infiltrate = into the river much more quickly = reaches bank full discharge = floods

Less interception - trees and plant matter is removed = precipitation gets into the river much more quickly = reaches bank full discharge = floods

Surface water = channelled directly into drains and sewers = precipitation reaches the river much more quickly.

Bridges over rivers can constrict rivers, slow discharge and reduce the carrying capacity of the river.

Deforestation

In poor countries = rapid deforetation has taken place.

Land = used for framing, settlement and mining etc.

No trees =  greater risk of soil erosion =  the preciptation is not intercepted.

Flood damage =  greatest near the mouth of a river - wide,flat floodplains = most susceptible to damage.

The volume of water is greatest here because many tributaires have joined the river.

River Management

River management = reduce the likelihood of flooding.

However, in some circumstances it can actually increase the risk:

Bangladesh: flood embankments have built along some river channels = increase river capacity but at times have prevented floodwater draining back into the rivers

Farakka Dam, India: Lots of rainfall = lake behind the dam could burst = floodgates of the dam were opened = stopped the dam from bursting = increased the discharge of the river in Bangladesh.

This coincided with the normal floods and made the severity much worse

FLOODING- Flood prediction

Flood prediction

Hydrologists = try to forecast the likelihood of futrue flood events using past records. 

Data = river discharge records - precipitation + flood recurrence interval graphs.

These graphs = calculate statistically the probability of flooding in the future based on past records.

The further back flood records go = more accurate the prediction.

Records of a river's discharge are ranked - longest period available,from highest peak discharge to the lowest recorded. 

The following formula is used to calculate the recurrence interval:

recurrence interval (years) = number of years on record +1 / ranking of flood being considered

When the recurrence interval = plotted against discharge as a scatter graph on semi-logarithmic graph paper,

= possible to use the line of best fit to predict when the next flood of a particular magnitude might occur

= flood return period. 

The Department for Environment Food and Rural affaris (DEFRA) and the Environment Agency 

=  main organisations responsible for flood management of major rivers

E.G Thames and the Severn in the British Isles 

Use flood recurrence interval graphs to plan flood defence strategies.

Densely populated urban areas = sufficiently protected against a 1 in a 100 year flood events 

Grassland and low productivity agricultural land should not be protected at all.

Hydrologists use past data records showing: 

The regime or yearly pattern of discharge in relation to annual precipitation patterns

= likelihood of seasonal flooding can be assessed. 

RIVERS- KEY TERMS

Key terms

Hazard: a natural event that threatens life and property. A disaster = realisation of the hazard. Flooding is an exmaple of a natural hazard

River management: River basins = subject to strategies desigined to prevent flooding and to ensure that there is an adequate supply of water

REJUVENATION- River terraces

River terraces

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A river terrace - remnant of a former floodplain - has been left at a higher level after rejuvenation of the river. 

Where a river renews its downcutting- it sinks its new channel into the former flooodplianm leaving the old floodplain above the lovel of the present river. 

There terraces are cut back as the new valley is widened by lateral erosion.

 If renewed rejuvenation takes place-  the process is repeated + a new pair of terraces is formed beneath the original ones. 

The River Thames has created terraces in its lower course by several stages of rejuvenation. 

Terraces =  useful shelter from floods in a lower-course river valley + natural routeways for roads and railways. 

The built-up areas of Oxford + London are mainly located along the terraces of the River Thames.