Flooding 1

Key idea: The amount of water in a river fluctuates due to a number of reasons

Lesson objectives

• To know the terms:Flooding, hydrograph, rising limb, falling limb, lag time, drainage basin

• To be able to describe the characteristics of a hydrograph

Starter activity….

A DRAINAGE BASIN

DISCHARGE

VELOCITY

WATERSHED The boundary of a drainage basin

The amount of water in a river channel at a given time

The area of land from which precipitation flows into a stream.

The speed of the river

A flood occurs when…

…..the discharge is so great that all the water can no longer be contained within the channel so that the river overflows its banks

Factors affecting discharge

•Amount and type of rainfall

•Temperature

•Previous weather conditions

• Relief

•Drainage Density

•Rock Type

• Soil

• Land Use

• Slope

Rising limb

Falling limb

Peak discharge

Peak rainfall

storm flow

normal (base) flow

Hydrographs

I = Lagtime

Show the change in discharge caused by a period of rainfall

Storm hydrographA graph which shows the relationship between precipitation and the level of a river

APPROACH SEGMENT = base flow• The discharge of the river before it rains.RISING LIMB• Results from a rapid increase in rainwater

reaching the river.LAG TIME • The difference between the time of the

heaviest rainfall and the maximum level and /or discharge of the river.

FALLING LIMB • Some rainwater is still reaching the river,

but in decreasing amounts – throughflow and groundwater flow

Why Construct & Analyse Hydrographs ?

To find out discharge patterns ofa particular drainage basin

Help predict flooding events,therefore influence implementation of flood prevention measures ©Microsoft Word clipart

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

charg

e (

m3/s

)

Base flow

Through flow

Overland flowR

isin

g

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Floo

d Hyd

rogr

aph

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

charg

e (

m3/s

)

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

charg

e (

m3/s

)

mm4

3

2

Rainfall shown in

mm, as a bar graph

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

charg

e (

m3/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

charg

e (

m3/s

)

Ris

ing

lim

b

mm4

3

2

Rising limbThe rising

flood water in the river

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

charg

e (

m3/s

)

Ris

ing

limb

mm4

3

2

Peak flow

Peak flow

Maximum discharge in

the river

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

charg

e (

m3/s

)

Ris

ing

limb

Recession

limb

mm4

3

2

Peak flow

Recession limbFalling flood water in the

river

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

charg

e (

m3/s

)

Ris

ing

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Basin lag time

Time difference between the peak of the rain storm and the peak flow of the river

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

charg

e (

m3/s

)

Base flow

Ris

ing

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Base flow

Normal discharge of

the river

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

charg

e (

m3/s

)

Base flow

Through flow

Overland flowR

isin

g

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Volume of water reaching the river from surface run off

Overland flow = surface run off

Through flow

Volume of water reaching the river through horizontal movement of water through the soil and underlying rock layers

• After a period of heavy rainfall, the discharge increases.

• The lagtime is the difference in time between the peak of the rainstorm and the peak of the discharge. The shorter this is and the steeper the rising limb the greater the risk of flooding.

• Complete task 1

Plenary

Homework

Worksheet - task 2 a - c