Lecture 17: Theory and Science of Floods

Question 1

Largest Rainfall Floods Occur in Worlds Largest River basins but….

Answer 1

Tropical Basins generate more water/km2 due to Monsoon climate regime.

Question 2

What is the probable maximum flood?

Answer 2

The largest flood that could physically occur at the location of interest.
It is an extremely rare event and defines the extent of flood-prone land in a floodplain. Likelihood of occurrence has been assessed as between 1 in 10 000 and 1 in 10 000 000 years

Question 3

Penning-Rowsell & Fordham (1994) suggested a typology of flooding events

Answer 3

Winter rainfall floods
Summer convectional storm induced floods 
Snowmelt floods 
Sewer flooding/ urban drainage flooding
Urban basement flooding

Question 4

Floods: key indicators

Answer 4

time to peak 
peak 
volume 
duration 
recession rate

Question 5

What controls peak magnitude?

Answer 5

Volume of rain
Proportion of rain that generates quickflow
- rainfall intensity
- amount of saturation
- catchment geology
- time
Time taken for water to
reach outlet

Question 6

What controls time to peak?

Catchment physical properties

Answer 6

Size
Shape
Slope
Drainage density

Question 7

What controls the rate of recession?

Answer 7

Depends on catchment geology

Question 8

Time to peak and recession - function of

Answer 8

…catchment physical and storage properties.

Constant from event to event.

Question 9

Peak flow and total volume - function of

Answer 9

amount of rainfall/snowmelt, state of catchment, and catchment geology.
Varies from event to event.
Different types of catchment are sensitive to different types of rainfall event

Question 10

Exceedance probability is

Answer 10

the probability that an event greater than or equal to X occurs in any one sample interval (e.g. a year)

Question 11

Return period is

Answer 11

the average time between sample intervals containing an event greater than or equal to X

Question 12

Why do Flood frequency curves vary?

For a given catchment type:

Answer 12

variation in floods depends on variation in flood-producing rainfall

Question 13

Why do Flood frequency curves vary?

For a given climate:

Answer 13

Variation in floods depends on catchment physical properties and geology.

Question 14

Flood Hazard Characteristics

Answer 14

• The magnitude of the flood;
• Rate of floodwater rise;
• Duration of flooding;
• Depth and velocity of flowing water;
• Evacuation problems;
• Effective flood access;
• Size of population at risk;
• Land use;
• Flood awareness/readiness;
• Effective flood warning time.