Flooding: Causes and Impacts

Question 1

What are the main causes of flooding?

Answer 1

Prolonged rainfall and heavy rainfall.

Question 2

When does flooding occur?

Answer 2

When the discharge of a river is so high that the river spills over its banks onto the flood plain.

Question 3

Explain prolonged rainfall.

Answer 3

A major cause of flooding. After a long period of rain, the ground becomes saturated so any further water can’t infiltrate, which increases surface runoff and hence increases discharge.

Question 4

Explain heavy rainfall.

Answer 4

Heavy rainfall can lead to rapid surface run off if the rainfall is too intense for infiltration to occur. This can lead to a sharp rise in rivers discharge called a flash flood.

Question 5

What can melting snow and ice lead too?

Answer 5

A huge increase in the rivers discharge. For example melting sow in the Himalayas contributes to the annual summer flooding of the River Ganges, Bangladesh.

Question 6

What are the physical factors that increase the risk of flooding?

Answer 6

Sparse vegetation, deciduous trees, impermeable ground, circular drainage basins, high drainage density and steep slopes.

Question 7

Sparse vegetation…..

Answer 7

Means little rainfall is intercepted, so more rain reaches the ground. This increases the volume of water reaching the river, which increases discharge.

Question 8

Deciduous trees…

Answer 8

Have no leaves in the winter, and hence as the same affect as sparse vegetation and little rainfall is intercepted.

Question 9

Impermeable ground…

Answer 9

Clay soils and some rocks, such as granite and shale, are impermeable- they don’t allow infiltration of surface water. Therefore this increases surface run off and discharge. Secondly if the ground has been baked hard by the heat of the summer, or its frozen, water can’t infiltrate so surface run off and discharge are increased.

Question 10

Circular drainage basins….

Answer 10

Water draining into the main river channel will all arrive in a short space of time because all points in the basin are a similar distance from the river, this therefore increases discharge.

Question 11

High drainage density…

Answer 11

Means the drainage basins drains quickly so have short lag times. Lots of water flows from the streams into the main river in a short space of time, increasing discharge.

Question 12

Steep slopes….

Answer 12

If the drainage basin has steep-sided valleys, water will reach the river channel much faster because water flows more quickly on steeper slopes, this increases discharge.

Question 13

What are the human factors that increase the risk of flooding?

Answer 13

Urbanisation, deforestation, agriculture, climate change and flood management strategies.

Question 14

Urbanisation…

Answer 14

Urban areas have large areas of impermeable tarmac and concrete, so when it rains surface run off is very rapid and secondly, gutters and drains quickly take run off to rivers. Both of these factors reduce lag time and so increase discharge.

Question 15

Deforestation…

Answer 15

Clearing trees and plants reduces interception and evapotranspiration, which increases the volume of water that reaches the channel which increases discharge. Deforestation also leaves the soil loose, as the soil is eroded by rainwater and carried to the river which raises the river bed. This reduces the channel capacity, so it takes less water for the river to flood.

Question 16

Flood management strategies…

Answer 16

Can end up making flooding worse. For example if a dam fails they release huge volume of water all at once, giving a huge increase in discharge.

Question 17

Agriculture…

Answer 17

Overgrazing leaves areas with less vegetation, so the same effect of deforestation occurs. Both overgrazing and ploughing also increase soil erosion.

Question 18

Climate change…

Answer 18

Could cause an increase in rainfall and more storms in some areas, which could therefore increase flooding.

Question 19

What are the social impacts of flooding?

Answer 19

1. People and animals can be killed.
2. There can be a lack of clean drinking water as floodwater is often contaminated with sewage. This can also put people at risk of diseases such as diarrhoea.
3. Possessions can de damaged or lost.
4. People can be made homeless as their properties are inundated or damaged.

Question 20

What are the economic impacts?

Answer 20

1. Businesses often have to be shut down as premises are inundated and power supplies are affected.
2. Rescues and repairs are usually costly.
3. Insurance premiums will go up after floods.
4. Unemployment levels will often rise as a result of the businesses shutting down.
5. Public transport, roads and bridges can be destroyed.
6. Crops can be destroyed, which can lead to an increase in the price of food.

Question 21

What are the negative environmental impacts?

Answer 21

1. Contaminated floodwater, with sewage and rubbish, can pollute rivers.
2. Rivers banks can be eroded.

Question 22

What are the positive environmental impacts?

Answer 22

1. River sediment is deposited on the flood plain which makes the land more fertile.
2. Wetlands can be created, e.g. marshes and ponds, which are habitats for many species.

Question 23

Explain the social impact.

Answer 23

The social impact is usually higher in poorer countries because flood defences are poorer, people are less able to evacuate, sanitation systems aren’t as good and buildings are of a poorer quality.

Question 24

Explain the absolute economic impact.

Answer 24

It is usually higher in richer countries as they have higher value buildings and infrastructure.

Question 25

Explain the relative economic impact.

Answer 25

Usually higher in poorer countries as even though the buildings and crops are worth less, it affects the economy more as they have less money to recover from it.

Question 26

What is the Flood return interval (RI)?

Answer 26

The prediction of how often a flood of a certain magnitude may occur.