Option A - Flooding and flood mitigation

Question 1

Storm/flood hydrograph

Answer 1

shows how a river channel responds to the key processes of the hydrological cycle; it measures the speed at which rain falling on a drainage basin reaches the river channel

Question 2

What are the factors that could influence storm hydrographs?

Answer 2

• drainage density - higher density means more overland flow
• vegetation cover - thick vegetation cover means precipitation is interrupted -> increases lag time -> decrease discharge because vegetation will absorb water
• soil and rock type -> unconsolidated soils allow water to infiltrate -> store, reduce the peak discharge
• shape - elongated basin will produce a lower peak flow and longer lag time
• weather and climate - more water = higher discharge
  snow = increases discharge because snow melts

Question 3

How does geology influence hydrographs?

Answer 3

• impermeable rocks: more overland flow -> shorter lag time and higher peak flow
• permeable rocks -> less overland flow -> longer lag time and lower peak flow

Question 4

What are the features of urban hydrographs?

Answer 4

• shorter lag time
• steeper rising limb
• steeper recessional limb
• higher peak discharge

Question 5

What are the factors that affect flood risk?

Answer 5

• precipitation type and intensity
• temperature and evapotranspiration
• antecedent moisture
• drainage basin size and shape
• drainage density
• porosity and impermeability of rocks and soils
• slopes
• vegetation type
• land use

Question 6

How does urbanisation increase the magnitude and frequency of floods?

Answer 6

• creation of highly impermeable surfaces i.e. roads, pavements, roofs
• smooth surfaces with a dense network of drains, gutters and underground sewers increase drainage density
• natural river channels are constricted by bridge supports which reduce carrying capacity

Question 7

How does urbanisation conflict with hydrological processes?

Answer 7

Removal of trees and vegetation

• Decreased evapotranspiration and interception
• Increased stream sedimentation

Initial construction of houses, streets, and culverts

• Decreased infiltration and lowered groundwater table
• Increased storm flows
• Decreased base flows during dry periods

Complete development of residential, commercial, and industrial areas

• Decreased porosity → reduced time of run-off concentration → increased peak discharges
• Increased volume of run-off and flood damage potential

Construction of storm drains and channel improvements
- Decreased overland flow → decreased risk of floods

Question 8

How does deforestation affect flood risk?

Answer 8

• increased surface run-off
• decreased interception and infiltration
• decreased evapotranspiration
• decreased in channel capacity (increased deposition)

Question 9

How does channel modification affect flood risk?

Answer 9

• channelisation (creating new channels): reduced time lags
• enlargement: higher peak flow
• straightening: speed up flow (reduced time lags), remove water from area quickly

Question 10

What is soft engineering?

Answer 10

techniques that work with the river and use natural processes to reduce coastal erosion and flooding

Question 11

What is hard engineering?

Answer 11

highly visible man-made structures used to stop or disrupt natural processes to prevent flooding and protect coasts

Question 12

What are reservoirs?

Answer 12

store excess water in the upper small drainage basin

Question 13

What is flood abatement?

Answer 13

involves decreasing the amount of run-off and reduces the flood peak in a drainage basin

Question 14

Channel modification ways

Answer 14

• raising the banks: hold more water
• straightening the river: remove water quickly and speed up flow
• create new channels: carry water

Question 15

What are the two types of flood mitigation? What does each type contain?

Answer 15

structural measures (dams, afforestation, channel modification and levee strengthening) and planning (personal insurance and flood preparation, and flood warning technology)

Question 16

Describe and name the advantages + disadvantages of dams.

Answer 16

Dams: barrier constructed to hold back water and raise its level, forming a reservoir used to generate electricity or as a water supply
Very expensive
Advantages: trap and store water so reduce surface runoff as water is released in controlled way, and the reservoir lane can be used for recreation
Disadvantages: change ecosystems and local residents may have to more if the land is flooded, and it traps sediment behind them that should flow; cause problems since the farmers rely on silt providing fertile soil after floods

Question 17

Describe and name the advantages + disadvantages of afforestation.

Answer 17

Afforestation: process of planting trees, or sowing seeds to create a forest
Increases the amount of interception and reduces the amount of overland flow → reduces the risk of floods

Question 18

Describe and name the advantages + disadvantages of channel modification.

Answer 18

Channel modification: process of raising banks to hold more water, straightening the river to speed up flow, and creating new channels to carry water
Advantages: make the river deeper so it can hold more water before it floods, and the rivers can flood without much damage
Disadvantages: do not last long

Question 19

Describe and name the advantages + disadvantages of levee strengthening.

Answer 19

Levee strengthening: increase of banks of the river in height so the river can carry more water and sediment
Increases the carrying capacity of the river
Advantages: make the river deeper so it can hold more water before it floods, and the rivers can flood without much damage

Question 20

Describe and name the advantages + disadvantages of the following schemes: personal insurance.

Answer 20

Using sandbags to protect homes against the risk of flooding
Sealing doors and windows, moving valuables upstairs, moving electrical generators to the top of buildings rather than having them in the basement
Designing homes so the electrical features are at a level higher than the expected flood level
Disadvantages: not all flood-prone households have insurance, many of those who are insured may be underinsured, and insurance is expensive for LICs

Question 21

Describe and name the advantages + disadvantages of the following schemes: flood warning technology.

Answer 21

Over 50% of all of unprotected dwellings in England and Wales have less than 6 hours of flood warning time
Bangladesh: authorities have about 72 hours warning, and flooding from monsoon rains may be predicted by the use of satellite imagery
Many people have access to television, smartphones, and computers, so making warnings available to people is easier
Disadvantages: poorest members of societies have least access to new technology, so they may be the least informed