Ch 2.4b How do humans influence and manage river environments?

Question 1

What are the hard strategies to tackle flood problems in Hong Kong?

Answer 1

Channelisation.
Dredging.
Dykes/ Embankments.
Bypass floodway.
Village Flood Pumping Scheme.
Gabions.

Question 2

Name the explanation and benefits of channelisation.

Answer 2

1. Deepening and widening the channel -> Water-holding capacity Increases
2. Straightening the channel -> River velocity increases.
3. Concrete lining -> Bed and bank erosion decrease -> River velocity increase as friction is reduced -> Deposition decrease

Question 3

What are the limitation and disadvantages of channelisation?

Answer 3

1. Expensive.
2. Artificial structure may be an eyesore.
3. Concrete lining destroy river habitats/ degrade the riparian ecology and waterway’s health.
4. Loss of natural river banks.

Question 4

Name the explanation and benefits of dredging.

Answer 4

1. Deepening the river channel -> Water holding capacity of the channel increases.
2. Less sediment -> Channel roughness decreases -> river velocity increases
3. Reduce unpleasant odours.

Question 5

What are the limitation and disadvantages of dredging?

Answer 5

1. Only suitable for small river sections as it is costly and time-consuming.
2. Needs to be carried out periodically.
3. Stirs up sediment on the river bed -> Disrupts the aquatic life.
4. Cannot prevent large scale flooding.

Question 6

What is dykes?

Answer 6

It is an elongated structure to increase the height of the river banks.

Question 7

Name the explanation and benefits of dykes.

Answer 7

1. Raising the height of the river banks which increases the water holding capacity.
2. Embankments can be used as cycling or walking trails to provide recreational space.

Question 8

What are the limitation and disadvantages of embankments?

Answer 8

1. Quite expensive
2. Looks unnatural.
3. Collapse of dykes may lead to flooding.

Question 9

What is a bypass floodway?

Answer 9

Divert water from the channel that flows to a populated area.

Question 10

Name the explanation and benefits of bypass floodway.

Answer 10

Catches run-off and diverts the water to the downstream section of the main river without passing through populated areas -> Reduces discharge in the main river.

Question 11

What are the limitation and disadvantages of bypass floodway?

Answer 11

1. Expensive
2. Needs a large area of land to build
3. Suitable area is not always available
4. Damages the natural habitats along the floodway
5. Flooding may occur in other areas where the floodway passes.

Question 12

What is a village flood pumping scheme?

Answer 12

Build embankments and flood storage ponds.

Question 13

Name the explanation and benefits of village flood pumping scheme.

Answer 13

1. Prevents floodwater flowing into the village.
2. Building a stormwater drainage system to convey rainwater to a flood storage pond. Stormwater is stored in the pond and is later released to the nearby stream -> regulates discharge and increase time lag.

Question 14

What are the limitation and disadvantages of village flood pumping scheme?

Answer 14

1. Involves building and maintenance costs
2. Needs a large area to build the pond and embankment
3. Flood storage ponds can be a drowning hazard for children.

Question 15

Name the explanation and benefits of gabions.

Answer 15

1. Stabilise river banks and reduces bank erosion -> reduces load supply and therefore silting -> Minimise the risk of flooding.
2. Relatively cheap.
3. Gaps between rocks may filled with soil and vegetation as time passes making it blend with the environment and further reinforce the structure.

Question 16

What are the limitations and disadvantages of gabions?

Answer 16

1. Cannot prevent flooding.
2. Looks unnatural.
3. Cages will rust and break over time, causing injury risk.
4. Needs regular maintenance.
5. High velocity water will damage the gabions.

Question 17

What are the soft strategies which works with nature to manage flooding?

Answer 17

1. Land use management.
2. Afforestation.
3. Flood warning systems.

Question 18

Name the explanation and benefits of land use management?

Answer 18

1. Restricts development in flood-prone areas like only allows recreational land use -> Reduces potential damages and losses when flooding occurs.
2. Regulates the reclamation of fish ponds and wetlands on floodplains -> restores the ability of wetland to absorb flood water
3. Assesses the impact of development on drainage -> Prevent possible flooding during heavy rainstorms.

Question 19

What are the limitation and disadvantages of land use management?

Answer 19

1. Difficult to restrict development in areas with a shortage of land.
2. In areas with complex land ownership, enforcement of land use control is difficult.

Question 20

Name the explanation and benefits of afforestration.

Answer 20

1. Increases interception -> reduces overland flow.
2. Reduces soil erosion and mass wasting as trees can hold the soil -> reduces the supply of load and therefore silting.
3. Relatively low cost.
4. Creates wildlife habitats.
5. Provides recreational grounds.

Question 21

What are the limitations and disadvantages of afforestation?

Answer 21

1. Needs time for trees to grow.
2. Young trees may not have the desired effect.
3. Cannot prevent serious flooding.
4. Needs large space to plant trees.
5. Needs cost for afforestation and management.

Question 22

Name the explanation and benefits of flood warning systems.

Answer 22

1. Monitoring water levels and issuing flood warnings -> gives time for people to evacuate and take precautionary measures.
2. Increases people’s awareness of flooding.
3. Reduces damage or loss caused by flooding.

Question 23

What are the limitations and disadvantages of flood warning systems?

Answer 23

1. Cannot prevent flooding.
2. Effectiveness depends on the people’s awareness and the accuracy of the warning system.

Question 23

What are the measures to maintain river ecology?

Answer 23

1. Growing vegetation along river banks.
2. Providing holes in the river wall.
3. Build fish ladders.

Question 24

Explain on growing vegetation along river banks.

Answer 24

1. Provide habitats for wildlife.
2. Increase biodiversity.
3. Improve the appearance.

Question 25

Explain on providing holes in the river wall.

Answer 25

Provide shelters to aquatic animals.

Question 26

Explain on building fish ladders.

Answer 26

Provide passage of fish and migrating animals.