A.2.2 Factors affecting flood risk

Question 1

Why do floods occur?

Answer 1

Occurs when a river can no longer contain all the water it is transporting in its channel.

The water flows out of the channel onto a flood plain

Question 2

Difference between small floods and larger floods

Answer 2

Most small floods are contained on the flood plain close to the river

Larger floods, which occur less often, cover a larger part of the flood plain

Question 3

Recurrence interval

Answer 3

The frequency with which events of a certain size occur.

Very large events happen very infrequently, small events happen very frequently.

Question 4

Urbanisation impact on hydrographs

Answer 4

Urban hydrographs are different from rural ones because there are more impermeable surfaces and drainage channels.

Urban hydrographs have:
- A shorter lag time
- A steeper rising limb
- A higher peak flow (discharge)
- A steeper recessional limb

Question 5

Hydrological impact of urbanisation

Answer 5

Increases the magnitude and frequency of floods in at least three ways:
1. Creation of highly impermeable surfaces (e.g. roads, roofs, pavements)

2. Smooth surfaces served with a dense network of drains, gutters and underground sewers increase drainage density
3. Natural river channels are constricted by bridge supports or riverside facilities reducing their carrying capacity

Question 6

Urbanisation impacts on hydrological processes

Answer 6

Increased erosion due to more water getting into rivers

Increased speed of flow and transport of materials due to enlarged channels

Less erosion due to riverbank protection schemes

Question 7

Deforestation impact on hydrographs

Answer 7

Deforestation reduces interception, increases overland flow and reduces evapotranspiration.

Causes hydrographs to have shorter lag time and higher peak flows (discharge)

Question 8

Hydrological impact of deforestation

Answer 8

Cause of increased flood run-off and decrease in channel capacity.

Occurs due to an increase in deposition within the channel.

Question 9

Deforestation impacts on hydrological processes

Answer 9

1. More overland flow leading to more frequent erosion
2. Rivers transporting more sediment
3. Reduced evapotranspiration

Question 10

Channel modification impact on hydrographs

Answer 10

Channelisation may create new channels, which are likely to be quite straight. This speeds up water movement so time lags are likely to be reduced

Enlarging channels through levees enables rivers to carry more water, enabling rivers to carry more water. Thus the peak flow may be higher

Question 11

Hydrological impact of deforestation

Answer 11

Widening and deepening of the channel increases the amount of water the river can hold

Straightening of the channel speeds up the flow and remove water from the area quickly

Removing sediment from a channel’s bed allows the river to carry more water, reducing the risk of flooding