IGCSE flooding

45. What causes river flooding?

Flooding occurs when the amount of water moving down a river exceeds the capacity of the river’s channel


- Heavy rainfalls

- Some rain runs off surface

- Some rain infiltrates into the ground

- Runoff reaches river quickly

- River rises

- Water reaches higher level and floods



Physical and human causes of flooding

  • Geology – Flooding is much more likely in places where the rock or soil are less permeable because water can’t get into the ground but has to flow over it and into rivers making them rise

  • Relief – The height and slope of the land can mean rain runs off more easily. A valley floor with steep sides is a large flood risk. Low-lying flat land has high infiltration and then becomes saturated, and any further rain causes flooding

  • Weather events – Heavy rain may be too fast for infiltration to happen and so it leads to surface runoff. Prolonged previous rain may also lead to saturated soils and surface runoff

  • Forestry/deforestation – Felling trees stops them from absorbing water through roots and stops interception. If there are no roots less water can enter the ground as roots are good at breaking up the soil.

  • Monsoon rain – In Asia heavy rains fall in the monsoon causing large scale flooding as there is so much water in a short time that it can’t infiltrate

  • Snow melt – Sudden snow melt causes a release of stored water and a rise in river level

  • Urbanisation – New housing, pavements and car parks are being built with impermeable surfaces and so rain cannot infiltrate into the ground but runs off into rivers making them rise and flood



46. What does hard engineering flood management mean?

Hard engineering: Uses heavy machinery to build artificial structures which work against nature to reduce the risk of flooding



47. What does soft engineering flood management mean?

Soft engineering: Adapts to a river and learns to live with it. These strategies are cheaper but can be less effective



48. How does hard engineering schemes solve flooding?

1. Dams and reservoirs


- It is hard engineering because it is a large, man-made structure to trap water and create a reservoir

- Dams work to trap water and build up behind it - this forms reservoirs. Water can then be released in a controlled way. This aims to control flooding and reduce effects of a water shortage



Advantages

  • Can be used to produce electricity by passing the water through a turbine within the dam

  • Reservoirs can attract tourists



Disadvantages

  • Very expensive and displaces people to make room for the dam, leading to the displacement of people

  • Dams trap sediment which means the reservoir can hold less water

  • Habitats are flooded - this leads to rotting vegetation which releases methane



2. Channel straightening


- This is hard engineering because this involves physically changing the river’s course using human-made engineering

- The aim of this is to increase speed of river flow so that water is transported downstream more quickly, reducing risk of flooding



Advantages

More water can be held in the channel which can reduce flood-risk in built up areas

Disadvantages

Speeding up river increases flood risk downstream

Dredging also needs to be done frequently



3. Embankments


- These are hard engineering because humans physically raise the banks of a river, usually using clay/earth

- This means that rivers can hold more water which reduces risk of flooding



Advantages

- Cheap with a one-off cost

- Allows for flood water to be contained within the river



Disadvantages

- Water speeds up and can increase flood risk downstream



4. Flood relief channels


- This is hard engineering because humans construct a new artificial channel to control where floodwater flows

- The man-made channel is parallel to the main river - in times of high discharge a gate will automatically close off the main river and divert water through the artificial spillaway to reduce risk of the main river flooding - works like a bypass

→ taken to an area where it can be absorbed, or re-enter the river further down its course



Advantages

Reduces flooding



Disadvantages 

- Expensive to build

- If water levels continue to rise, the relief channel may also flood



49. How does soft engineering schemes solve flooding?

1. Floodplain zoning


- Allowing only certain land uses on the floodplain reduces the risk of flooding to houses and important buildings 

- This is soft engineering because it works with the river’s natural behaviour instead of physically controlling it



Advantages

- More land use is further away from the river - less flood risk

- Less damage is caused, leading to fewer insurance claims



Disadvantages

- Not always possible to change existing land uses

- Planners have to decide what type of flood to plan for




2. Afforestation

- Afforestation refers to planting trees in the drainage basin to increase interception and infiltration

- It is soft engineering because it works with natural processes rather than physically controlling the river



Advantages

→ Trees reduce surface runoff, lowering the risk of flooding

→ Relatively cheap compared with hard-engineering methods (e.g. dams)



Disadvantages

→ Trees take years to grow so flood protection is not immediate

→ Requires large areas of land which could be used for farming or development



3. Flood warnings and preparation

- The environment agency monitors rivers and issues warnings via newspapers, TV, radio and the internet when they are likely to flood so people can prepare

- This is neither soft nor hard engineering as it reduces the impacts of flooding on people rather than altering the river or necessarily managing the river itself



Advantages

→ People have time to protect their properties (e.g. with sandbags)

→ Many possessions can be saved, resulting in fewer insurance claims



Disadvantages

→ Flash floods may happen too quickly for a warning to be effective

→ They do not stop land from flooding – they just warn people that a flood is likely

→ Some people may not be able to access the warnings



50a. What causes flooding along the River Thames?

Why does the Thames flood?

- Built on a floodplain (flat area of land)

- Heavy rainfall

- A lot of the Thames valley is urban (urbanisation)



The flooding was due to a potent mixture of groundwater and high surface runoff across the catchment

Impacts of flooding

Social:

  • Loss of property + people had to be evacuated (approx 1,000 houses flooded)

  • Schools and services disrupted

  • Communities were disrupted - families were temporarily displaced and normal daily life was interrupted



Economic:

  • Businesses lost income (e.g. on Chertsey and Staines were affected when floodwater entered commercial properties)

  • £320,000 estimated residential property damage



Environmental:

  • Floodwater covered farmland, damaging vegetation & crlops - example of water pollution

  • Floodwater can contaminate other water sources.rivers



50b. How has the River Thames been managed?


Flood relief channels

→ New controlled route in 2 areas

→ 5 miles of new relief channels and more

→ Protecting over 11,000 homes in London + Surrey (transport infrastructures)

Weirs → increased capacity

51a. What were the causes of the Bangladesh floods?


Population: 170 million

Population density: 1342 per square meter

Low income country

Warm, humid climate. Experiences monsoon weather and tropical storms

Low-lying flat topography

Floodplains make up 80% of the land



The rivers are divided into five systems



What were the causes of the Bangladesh flood?


- Monsoon climate - June-October

- Rainfall intensity/weather events

- Relief - low-lying flat topography

- Forestry - felling of trees in the foothills of the Himalayas

- Snow melt - accelerated by climate change



What were the impacts of the Bangladesh floods?

Social:

- 144 deaths - nearly 20,000 cases of waterborne diseases (secondary impact)

- People were displaced - 472,856 people evacuated to 1,605 emergency shelters



Economic:

- US $722 million worth of damage

- $12.5 million of crops damaged



Environmental:

- 185,000 acres of paddy destroyed

- Water polluted - floodwater mixed with sewage



51b. How has the river been managed?

1. Plant forests/mangroves

- It is a shrub or tree that grows mainly in coastal saline or brackish water

- Mangroves reduce risks by protecting coastlines against flooding from waves and storm surge - they reduce the direct wave impact and surge water levels



2. Levees

- It is an elevated ridge alongside the banks of a river - it is a type of embankment

- It protects against flooding of the area adjoining the river by providing a physical wall/barrier through which water cannot permeate in the event of a flood



3. Building dams

- This aims to prevent flooding by staring floodwater into reservoirs which can be released in a controlled manner over a period of time - Bangladesh only has 1 because it is on low-lying land



4. Channel straightening

- This increases the speed of the river’s flow so that water is transported downstream quicker

5. Dredging the river bed

- Removing sediment and debris from the bottom of lakes, rivers, harbours and other water bodies

- This increases the rivers’ capacity which allows a greater volume of water to be contained within the river, reducing the likelihood of water overflowing the banks and flooding nearby areas



6. Let the river flood

- The country has a huge naturally flood-prone delta

- Water is given somewhere to go - floodwaer is allowed onto designated floodplains

- This reduces the pressure in the river as less water remains concentrated in the river channel