flood hydrographs, tgaw, lesson 3

Big Picture:Big Picture: What is a flood hydrograph?

• What: I will understand why rivers flood.

• How: By analysing the key changes as a river floods

• Why: So I can interpret a flood hydrograph.Key Terms for Today’s lesson

Flood Hydrograph Peak rainfall Precipitation

Peak discharge Lag time

Today is pair work so find a Today is pair work so find a partner!partner!

Connect:Connect: In a nutshell, why do you think rivers flood?

http://www.youtube.com/watch?v=aCHg5r6rFoI

Connect Task:Connect Task: In your pair can you summarise this?

Worcester Floods, 2007

• A river floods when the level of the water exceeds bankfull and flows onto the floodplain.

The flood hydrograph explains this in more detail…

Connect Answer:Connect Answer: Why do rivers flood?

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

3

2

1

Disc

harg

e (m

3 /s)

Surface flow

Through flow

Overland flowRi

sing

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Flood

Hyd

rogr

aph

Activate:Activate: What do the key terms on the flood hydrograph mean?

Drainage Basin Any form of moisture that falls from the atmosphere to land.

Precipitation The period of time between peak rainfall and peak river discharge.

Surface flow The volume of water in a river at any given time.Through flow The area of land drained by a main river and its

tributaries.Discharge The movement of water sidewaysLag time A graph that shows the discharge of a river at a

given point over a period of time.

Hydrograph Occurs when water cannot infiltrate (soak into) the soil any more. The land is saturated therefore water flows on the surface.(Overflow)

Task: Can you match up the key term to the correct definition?

How many did you get right? A. Swap sheets with your partner. How many did they get right out of 7? B. Identify the key termskey terms they still could learn.Extension: can you write the definitions of these key terms for your partner?Drainage Basin Any form of moisture that falls from the atmosphere to

land.Precipitation The period of time between peak rainfall and peak river

discharge.Surface flow The volume of water in a river at any given time.Through flow The area of land drained by a main river and its

tributaries.Discharge The movement of water sidewaysLag time A graph that shows the discharge of a river at a given

point over a period of time.Hydrograph Occurs when water cannot infiltrate (soak into) the soil

any more. The land is saturated therefore water flows on the surface.(Overflow)

Demonstrate:Demonstrate: How do flood hydrographs work?

A Flood Hydrograph

0

20

40

60

80

100

120

140

1

Hours from start of storm

Disc

harg

e (c

umec

s)

Demonstrate:Demonstrate: What’s going on here?!

http://www.youtube.com/watch?v=hJftAYYXpVw

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

3

2

1

Disc

harg

e (m

3 /s)

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

3

2

1

Disc

harg

e (m

3 /s)

mm4

3

2

Rainfall shown in

mm, as a bar graph

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

3

2

1

Disc

harg

e (m

3 /s)

mm4

3

2

Discharge in m3/s, as a line graph

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

3

2

1

Disc

harg

e (m

3 /s)

Risi

ng

limb

mm4

3

2

Rising limbThe rising

flood water in the river

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

3

2

1

Disc

harg

e (m

3 /s)

Risin

g lim

bmm4

3

2

Peak flow

Peak flowMaximum

discharge in the river

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

3

2

1

Disc

harg

e (m

3 /s)

Risin

g lim

b

Recession

limb

mm4

3

2

Peak flow

Recession limbFalling flood water in the

river

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

3

2

1

Disc

harg

e (m

3 /s)

Risin

g lim

b

Recession

limb

Lag time

mm4

3

2

Peak flow

Lag timeTime

difference between

the peak of the rain

storm and the peak

flow of the river

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

3

2

1

Disc

harg

e (m

3 /s)

Surface flow

Risin

g lim

b

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Surface flowNormal

discharge of the river

Demonstrate:Demonstrate: Can you plot a living graph?

A Flood Hydrograph

0

20

40

60

80

100

120

140

1

Hours from start of storm

Disc

harg

e (c

umec

s)

Task:Task: Read the scenarios to mark each phase of the flood onto the living graph.

Extension:Extension: Can you write 3 of your own scenarios. Mark them onto your flood hydrograph.

Homework:Homework: How could the following factors influence

the Flood Hydrograph?• Area• Shape• Slope• Rock Type• Soil

• Land Use• Drainage Density• Precipitation / Temp• Tidal Conditions

©Microsoft Word clipart

Reflect:Reflect: Return to key terms

• Go back to the key terms worksheet you completed at the start of the lesson.

• In your pair test each other to see how many you can recall the definitions to.