Flood Management - Hard and Soft Engineering Stratagies

Dams

How it works

• Huge wall built across river and a reservoir forms behind
• Flood water caught behind dam, preventing downstream floods. The water is released steadily over the year

Benefits

• Turbines can be built in dam,  to generate electricity
• Steady release of water allows irrigation of land below all year
• reservoir can be used for recreation (e.g fishing)

Disadvantages

• Expensive
• Reservoir creation = flooding => destryed farmland and people having to move
• May affect wildlife ( ie. salmon swimming upstream to breed)
• Sediment can get trapped => dam may fail AND can lead to erosion downstream

Channel straightening

How it works

• River is straightened (meanders removed)
• Artifical cut-throughs allow the water to flow faster
• Flooding reduced as water drains downstream faster.

Benefits

• Navigation is easier as river is straighter and shorter

Disadvantages

• Flooding can occur downstream
• More erosion downstream
• altering the channel disturbs wildlife habitats

Levees

How it works

• Embankments built along river
• Increase bankfull level so it is less likey to flood (discharge < bankfull level)

Benefits

• Allow flood plain to be built on

Disadvantages

• Quite expensive
• Risk of severe flooding if they are breached
• Can prevent flood water draining back into river if flooding does occur

Diversion spillways

How it works

• Channels that take water elsewhere if water level is too high (water diverted around important area or to another river)
• Prevent flooing as discharge it reduced
• Often have gates so the reelease of water can be controlled.

Disadvantages

• There is and increase of discharge when spillway rejoins main channel which can lead to flooding at this point in the river
• If spillways are overwhelmed areas not used to flooding can be flooded which can cause bigger problems

Land use management

How it works

• Planning retrictions prevent roads of buildingd being built on the flood plain and use of flood plain resricted to playing fields, allotments or parks
• More water can infiltratie = surface runoff drecreased = discharge reduced

Benefits

• No roads of buildings damaged by flooding = less damage if flooding occurs
• provides recreational oppurtunities

Disadvantages

• Restricts development (problematic in areas with housing shortages)
• Can't be used in areas already urbanised

Wetland and river bank conservation

How it works

• Wetlands store water and slow it down = reducing flooding downstream
• Conserving them = natural protection
• Planting trees and shrubs on banks increases interception and lag time and reduces discharge

Benefits

• Vegetation protects surface soil from erosion and provides habitats for wildlife

Disadvantages

• Less land available for farming

River restoration

How it works

• Making the river more natural (i.e. removing man-made levees) allowing river to flood natrually
• As flood plains flood, discharge in channel is reduced reducing flooding downstream.

Benefits

• Little maintenance needed
• Creates habitats

Disadvantages

• Local flood risks can increase

Alteration or urban surfaces 

How it works

• Building porous pavements or soakways increases infiltration and reduces runoff, which increases lag time and reduces discharge

Benefits

• Any pollutants in the water are filtered out by the soil before water eaches the channel

Disadvantages

• Expensive

Weather forecasts and flood warnings

How it works

• The Environment Agency monitiors weather forcasts, rainfall and river discharge
• They issue warnings using media (i.e. TV, radio, newspapers and the internet)
• Gives people time to prepare (i.e. evacuate, move possessions, use sand bags)

Disadvantages

• Some people may not be able to access communication network
• Flashfloods may happen to fast for warnings
• People may ignore the warnings if they have been in accurate in the past