Flood Hydrographs

Starter ActivityWhat do you think the graph below shows? Discuss with your partner.

LO: How are flood hydrographs used to analyse flooding?

LO: How are flood hydrographs used to analyse flooding?

Flood Hydrographs

Flood hydrographs, also known as storm hydrographs, show how a river’s discharge changes after a storm.

River discharge is the volume of water flowing through a river channel. This is the total volume of water flowing through a channel at any given point and is measured in cubic metres per second (cumecs).

= cross sectional areariver’s mean or average velocityx

(at a particular point in its course)

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (

m3/s

)

Base flow

Through flow

Overland flow

Basin lag time

mm

4

3

2

Peak flow

LO: How are flood hydrographs used to analyse flooding?

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (

m3/s

)

LO: How are flood hydrographs used to analyse flooding?

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (

m3/s

)

mm

4

3

2

Rainfall shown in mm, as a bar graph

LO: How are flood hydrographs used to analyse flooding?

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (

m3/s

)

mm

4

3

2

Discharge in m3/s, as a line graph

LO: How are flood hydrographs used to analyse flooding?

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (

m3/s

)

mm

4

3

2

The rising flood water in

the river

LO: How are flood hydrographs used to analyse flooding?

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (

m3/s

)

mm

4

3

2

Peak flow Peak flow

Maximum discharge in

the river

LO: How are flood hydrographs used to analyse flooding?

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (

m3/s

)

mm

4

3

2

Peak flow

Falling flood water in the

river

LO: How are flood hydrographs used to analyse flooding?

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (

m3/s

)

Basin lag time

mm

4

3

2

Peak flow Basin lag time

Time difference between the

peak of the rain storm and the

peak flow of the river

LO: How are flood hydrographs used to analyse flooding?

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (

m3/s

)

Base flow

Basin lag time

mm

4

3

2

Peak flow

Base flow

Normal discharge of

the river

LO: How are flood hydrographs used to analyse flooding?

LO: How are flood hydrographs used to analyse flooding?

Storm Hydrographs

Task1. Use your knowledge on flood hydrographs and the data you

complete your own flood hydrograph. 2. Use times and discharge figures from your graph (and table) to

describe: The shape of the rising limb The shape of the falling limb and base flow

ExtensionNot every flood hydrograph looks the same. Suggest and explain two factors you think will affect the appearance of a hydrograph.

LO: How are flood hydrographs used to analyse flooding?

TaskComplete the table to show the impact each factor has on infiltration, overland flow and lag time.

Several factors affect the shape of a flood hydrograph…

Drainage basin factor

Impact on infiltration

Impact on overland flow and throughflow

Impact on lag time

Steep slopes

Gentle slopes

Porous rocks

Impermeable rocks

Urban land uses

Planting more trees

LO: How are flood hydrographs used to analyse flooding?