4.8 Flood Risk And Management

Question 1

Why is flood risk increasing?

Answer 1

Air temperature is rising due to climate change. It has warmed 1ºC in 100 years. Warm air holds more moisture (1ºC= 7% more moisture) this leads to more rain. 
Extreme storms (Desmond) are now 40% more likely de to these points. The location of the jet stream now sits over the UK due to Arctic warming - more storms are brought. 

Human activity
Building on flood plains:
-more properties are being built in high risk areas. Building on flood plains is increasing quicker 1.2% than safe areas.
- magnitude increases as we are developing and building expensive homes/possessions in flood prone areas.
- likelihood will increase because more impermeable surfaces so more surface run-off.

Destroying upper course wetlands/bogs:

• wetlands and bogs decrease surface run-off. When they are removed then the surface run-off is increased. Peaty soil can be up to 90% water.
• flood magnitude is increased because when a storm hits trees can act as a natural dam but when they are cut down then there is an increase of surface run-off.
• the roots of the vegetation loosen the soil a increase infiltration. When this vegetation is removed then the soil becomes more compact and therefore surface run-off is increased. Farming can increase the surface run-off.

Removing Meanders:

• removing meanders causes worse flooding in the lower course
• meanders allow the river to flood in certain areas, storing water in fields and released slowly after the danger has passed.

Flood defences:

• government is cutting budget on flood defences by 8%. This results in poor design quality and planning required to meet the expectations against floods.
• the government is spending its money on expensive, yet ineffective, flood defence schemes, such as Carlisle’s £38 million flood defence scheme.

Question 2

Compare how soft and hard engineering scheme s work?

Answer 2

Hard engineering is when the schemes work against nature. They usually use large amounts of man-made materials such as concrete. They stop natural processes. They typically involve large scale construction and disruption.
Soft engineering is when the schemes work with nature. They usually use natural materials such as sand or wood. They work with natural processes. They typically involve small scale construction but require more maintenance.

Question 3

Explain what each hard engineering scheme is and how they work (pros and cons)?

Answer 3

Flood walls:
High walls are built alongside a river. Increases the capacity of the river and therefore prevents flooding.
Relatively cheap, depends on the materials. Once built, they rarely need maintenance.
Useful for city centres where space is needed.

Levees
Similar to flood walls. Built further away from the river, increasing the river capacity.
Expensive, up to £1 million, depending on the materials used. Levees can fail, erosion, slumping or the water rises above the levees.
They allow people to live beside rivers or farm with reduced fear.

Dredging and flood relief channel
Sediment is removed from the bottom of the river, capacity of the river is increased. Can be lined with concrete to speed up water flow and allow flood to flow away quicker.
Concrete lining is expensive, cheap main. Dredging needs to be done every year due to sediment filling it back up. £50,000. Speeding up flow increases the flood risk downstream.
Reduces flooding in the area that was dredged.

Question 4

Explain what each soft engineering scheme is and how they work (pros and cons)?

Answer 4

Flood plain retention
Flood plains lowered, filled with shrubs and grassland. Vegetation soaks up water and releases it slowly into the river. £1.2 million in total for a 2km stretch. Increases the interception so a larger storage for flood water.

River channel restoration
Some meanders were rebuilt, slowing water down. Banks are lowered so it floods parks instead of towns. Concrete and other hard materials are removed. Sediment fills in. £1.2 million in total for a 2km stretch. Improves local ecology. It is more aesthetically pleasing.

Afforestation
Planting trees in the upper course of the river intercepts large volumes of water and allows for more evapotranspiration. Surface run-off is reduced as a result and the flood risk is reduced.
Trees (if bought largely grown) are expensive, and if they are saplings they take along time before they are effective. Natural ways of reducing flood risk which improves the view in an area. It increases natural habitats for wildlife too. There is low maintenance required.