Floods (Flood Hydrograph, Flood Risk, Flood Mitigation & Prediction)

Question 1

What is on the axis of a flood hydrograph?

Answer 1

Discharge & hours from the start of rainstorm

Question 2

What is lag time?

Answer 2

Period of time between peak rainfall & peak discharge

Question 3

Factors causing flood hydrographs to differ

Answer 3

• geology
• land use
• climate
• amount of vegetation
• relief
• area shape
• soil
• drainage density

Question 4

When there is a long lag time, the graph will be…

Answer 4

Subdued

Question 5

When there is a short lag time, the graph will be…

Answer 5

Flashy

Question 6

What is a river regime?

Answer 6

The variability in a rivers discharge throughout the year

Question 7

How can we define ‘distribution’?

Answer 7

1. Magnitude
2. Frequency

Question 8

What is mitigation?

Answer 8

Reducing flood risk through hard & soft engineering schemes

Question 9

What is prediction?

Answer 9

Forecasting when a flood will occur

Question 10

Some advantages of dams & reservoirs are…

Answer 10

• water is held back by the dam and released gradually
• water is usually stored in a reservoir behind the dam
• water can be used for hydroelectric or for recreation purposes

Question 11

Some disadvantages of dams & reservoirs are…

Answer 11

• expensive to build
• sediment is often trapped behind the wall = erosion downstream
• settlements & agricultural land may be lost

Question 12

List some hard engineering techniques for rivers

Answer 12

• dams & reservoirs
• artificial levees & embankments
• channelisation
• containment
• dredging

Question 13

What is channelisation?

Answer 13

A deliberate attempt to alter the natural geometry of a river

Question 14

What are some advantages of channelisation?

Answer 14

• floodplain reclaimed
• speeds up flow & moves water fast downstream
• creates space on floodplains
• act as route ways

Question 15

What are some disadvantages of channelisation?

Answer 15

• natural beauty & ecology lost
• natural filter & storage capacity of floodplain lost
• cannot prevent major flood events (and may worsen them)
• become heavily polluted
• high cost

Question 16

What is containment?

Answer 16

An extreme form of channelisation, when the river is entirely contained underground within channels

Question 17

What are some advantages of containment?

Answer 17

• prevents flooding within the contained area
• allows urban function
• upfront costs are high -> but reduced in the long term

Question 18

What are some disadvantages of containment?

Answer 18

• can be difficult to maintain (access problems)
• loss of ecology & beauty
• high construction cost
• reduces the variety for recreational space within the city

Question 19

What is dredging?

Answer 19

Removal of sediment from the channel in order to increase capacity and channel efficiency

Question 20

What are the ways of dredging?

Answer 20

1. Dredging machine
2. Reservoir/ abandoned meander loops

Question 21

What are some advantages of dredging?

Answer 21

• maintains the natural river channel
• limits upfront costs of big infrastructure projects
• sediments rich in nutrients = economic value

Question 22

What are some disadvantages of dredging?

Answer 22

• ecologically disruptive
• high ongoing costs (labor intensive)
• can be an eyesore, noisy

Question 23

What is afforestation?

Answer 23

Process of establishing a forest

Question 24

Why is afforestation good?

Answer 24

• Increased interception and surface runoff
• protection of vegetation (wildfires, overgrazing and deforestation)

Question 25

What is the main aim of wetland/ floodplain restoration?

Answer 25

Bring back floodplains as a natural source of flood waters

Question 26

What is the case study for wetland restoration?

Answer 26

Kissimmee river, Florida

Question 27

What are some advantages of wetland/ floodplain restoration?

Answer 27

• low cost
• significant prevention of floods
• cost savings
• good ecological benefits

Question 28

What are some disadvantages of wetland/ floodplain restoration?

Answer 28

• wetlands need to be large
• local flooding impacts
• loss of agriculturally productive land may

Question 29

What is floodplain zoning?

Answer 29

A systems for setting the level of flooding risk for different areas of a country

Question 30

What are some advantages of riverbank conservation?

Answer 30

• low cost
• maintains relative normality of river
• reduces frequency of floods

Question 31

What are some disadvantages of river bank conservation?

Answer 31

• reduces natural ecology of river bank
• labor intensive

Question 32

What is the case study for flood mitigation?

Answer 32

1. Jakarta
2. Bangkok
3. Mississippi