Flooding Flashcards
How is flood frequency (flood recurrence interval) calculated?
- River discharge records
- collected for a large number of years
- ranked from highest to lowest
What flood recurrence interval does the environment agency recommend urban areas be protected against?
1 in 100 years.
What is the definition of natural hazard?
A natural hazard is any natural event that poses a risk to human life or property.
Why does flooding result from intense rainfall over a short period of time?
When rainfall is particularly intense, the grounds infiltration capacity maybe exceeded.
This means that rainfall cannot penetrate the soil quickly enough, leading to an increased flow of water into rivers than would otherwise occur.
How does flooding occur when less intense rainfall occurs over prolonged time?
Prolonged precipitation leads to soil saturation causing the water table to rise.
If the water table rises to the ground surface, increased overland flow occurs leading to an increased flow of water into rivers.
What are the physical causes of flooding?
- large drainage basin
- Low level of vegetation
- Geology, impermeable rock
- melting snow
- storm surges
- high drainage density
- urban land use
- intense heavy rainfall
What ways could climate change lead to increased flooding frequency?
- Increasing temperatures will lead to melting of the polar ice caps.
- Sea temperature rises may lead to increasing occurrence and size of tropical storms.
- Rainfall patterns will change - reduced rainfall in some regions, but increased and others.
- There will be an increase frequency of El Niño events.
What 4 ways could urbanisation increase the effects of flooding?
- Increases the number of people living on the flood susceptible floodplains.
- Use of tarmac and concrete to make roads and pavements means that much of the urban area is impermeable. Precipitation therefore cannot infiltrate slowly into the soil, meaning that more of the rainfall makes its way into rivers.
- Channelling water directly into drains and sewers means that precipitation reaches rivers quickly, decreasing lag time between peak rainfall and peak discharge.
- Bridges can constrict river channels, slowing down velocity and reducing the rivers carrying capacity.
How does deforestation add to flood risk?
When trees are removed during deforestation soil erosion increases.
This sediment finds its way into rivers and adds to the flood risk.
In addition trees take up water from the soil in forested areas, therefore deforestation leads to increased amounts of water reaching the river channel.
What is river management?
This is when engineering projects are undertaken within the river basin in an effort prevent flooding and ensure that an adequate water supply is maintained.
How can river management increase flood risk?
The engineering projects designed to prevent flooding in one region have accidental knock on effects, adding to flood risk in other regions, typically downstream regions.
