Sustainable Management Of Rivers. Flashcards
Why do rivers flood?
Due to a mixture of physical and human factors.
Usually caused by sudden heavy rainfall, typically associated with an extreme weather event, such as a hurricane or tropical storm, or meltwater from ice.
What are the physical causes of flooding.
Heavy rainfall.
Excessive surface runoff- flash flood
Prolonged rainfall as in a monsoon climate.
Saturates the ground- slow, long flood.
Melting of a glacier or snow.
Creates high river discharge in spring- rivers overflow.
Underlying rock is impermeable or clay soil/ ground is frozen
Reduces infiltration, meaning precipitation is more likely to reach rivers through surface runoff- high risk area for flooding.
Surface runoff -> quicker transfer
What are the human causes of flooding?
Deforestation:
Less root support for soil, less water used by plants, big raindrops no longer broken up by leaves- more surface runoff.
Urbanisation:
More tarmac and concrete creates impermeable surface- more surface runoff.
Narrowing a river channel, Eg by bridge building:
River has a reduced capacity to hold water- more surface runoff.
Building of levees:
Provides short term protection, but…- more surface runoff.
(More surface runoff- more flooding)
What happed at Somerset levels?
In the winter of 2013-14 Somerset levels experienced the worst prolonged flooding in over a century. There was over 6880 hectares of flooded farmland, which caused animals to be moved and roads became impassable.
What were the physical causes of flooding in Somerset levels?
There was a combination of impermeable bed rock (clay) and low interception levels (grass).
England received 207mm of rainfall in January alone, twice the expected total for that month.
December 2013 was one of the stormiest Decembers on record.
High tides caused floodwater to back up along the rivers Tone and Parrett.
What were the human causes of flooding in Somerset levels?
The rivers Tone and Parett had not been dredged properly in 20 years. Locals believe if the rivers had been dredged they would have had more capacity to handle the floodwaters.
Extra water was being sent in from the rivers Taunton and Bridgewater.
What are the positive impacts of flooding on people?
Replenishes drinking water supplies.
Fertilises soil with silt.
Helps crops such as rice grow.
Encourage innovative solutions in building design.
What are the negative impacts of flooding on people?
Spreads waterborne diseases
People and animals become homeless or even drown.
Buildings and infrastructure damaged or destroyed.
Crops grown on floodplains can be washed away.
Can increase the cost of insurance or even make it impossible to get.
What are the positive impacts of flooding on the environment.
Fish breed in the standing floodwater.
In dry areas floods bring relief from drought.
Alluvial silt deposited adds to fertility.
Regular flooding can remove salts that build up in hot climates.
What are the negative impacts of flooding on the environment
Can wash chemicals or sewage into the local rivers and pollute them
Wild animals may drown or lose habitat
Banks can be washed away or change the rivers course.
What are the two main groups of engineering methods for controlling rivers?
Hard engineering
Soft engineering
What are hard engineering methods?
Large artificial structures design to control the river. These measures are not sustainable long term.
What are some types of hard engineering methods used for rivers?
Dams:
Trap and store water, and let it out in a controlled way. The reservoir can be used for leisure or hydroelectricity, but are hugely expensive
Levees:
Raised embankments, making the river deeper so it can hold more water. They protect the land around them, but are expensive and don’t look natural.
Flood walls:
Built around settlements and important factories or roads. Pretty effective at flood control, but are expensive and don’t look natural.
Straightening a river:
Speeds up the flow of a river, and allowing it to hold more water.
Storage areas. Allow excess water to be pumped out of rivers and into temporary lakes, and pumped back when the water level has gone back down. Effective, but requires a large price of land that is being used so it can be flooded.
What are soft engineering methods?
A strategy to control a natural hazard which blends into the environment, so it’s more sustainable.
What are the types of soft engineering methods for rivers?
Afforestation:
Replanting trees to increase interception. Also reduces the flow of water to a river from ground moisture thanks to deep roots.
Wash lands:
Parts of the floodplain that are allowed to flood temporarily. They can’t be built upon.
Sand use zoning.
Land has different building controls depending how far away from the river it is. Low risk areas like pasture or playing fields can be built closer, while housing or hospitals need to be built further.
