Higher GeographyHydrosphere

Hydrographs [Date]

Today I will:- Be able to construct and understand flood hydrographs

Higher GeographyHydrosphere

What is a hydrograph?

A hydrograph is a graph showing the rate of flow

(discharge) versus time past a specific point in a river. The

rate of flow is typically expressed in cubic meters

(m3).

They are used to show how rivers respond to a flood. Hydrographs are used to help predict flooding

events therefore influence implementation of flood prevention measures.

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Higher GeographyHydrosphere

0 12 24 36 48 30 72Hours from start of rain storm

3

2

1

Dis

char

ge (m

3 /s)

Base flow

Through flow

Overland flowRi

sing

limb

Recession limb

Basin lag time

mm4

3

2

Peak flow

0 12 24 36 48 30 72Hours from start of rain storm

3

2

1Dis

char

ge (m

3 /s)

The discharge of the river is measured in cumecs - this

stands for cubic metres per second

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (m

3 /s)

mm4

3

2

Rainfall expressed in mm

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (m

3 /s)

mm4

3

2

Discharge in m3/s, as a line graph

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (m

3 /s)

Risi

ng li

mb

mm4

3

2

The rising limb is when the normal (base) flow of

the river starts to rise when run-off, ground and

soil water reaches the river.

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (m

3 /s)

Risin

g lim

bmm

4

3

2

Peak flow

Peak flow is when the Maximum discharge in the

river, the time when the river reaches its highest

flow

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (m

3 /s)

Risin

g lim

b

Recession limb

mm4

3

2

Peak flow

The recession limb shows that water is still reaching the river but in

decreasing amounts

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (m

3 /s)

Risin

g lim

b

Recession limb

Basin lag time

mm4

3

2

Peak flowThe basin lag time is

the time it takes for the water to find its way to

the river

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (m

3 /s)

Base flow

Risin

g lim

b

Recession limb

Basin lag time

mm4

3

2

Peak flowBase Flow is the

normal discharge of the river

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

char

ge (m

3 /s)

Base flow

Through flow

Overland flowRi

sing

limb

Recession limb

Basin lag time

mm4

3

2

Peak flow

Overland flow

Through flow

+

=Storm Flow

Higher GeographyHydrosphere

Read the information on page 230 and 231 of the NWW Textbook. Copy the table below into your jotter

and fill in the information gathered.

Reasons for Differences in Flood Hydrographs

LONG LAG TIME SHORT LAG TIME

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1.

2.

5.

4.

6.

7.

8.

3.

Riverdischarge (cumecs)

Rainfall (mm)

Peak rainfall Falling limb

Rising limb Lag time

Peak discharge Time

Can you label 1 to 8 accurately?