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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

Peak discharge

Precipitation Lag time


Today is pair work so find a partner!


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

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


Connect Task: In your pair can you summarise this?

Worcester Floods, 2007


Connect Answer: Why do rivers flood? • A river floods when the level of the water exceeds bankfull and flows onto the floodplain. The flood hydrograph explains this in more detail…


h p a Basin lag time r g l i mb Rising

Discharge (m3/s)

Through flow

4

1

b

mm

Overland flow

l im

2

Peak flow

i on ss ce Re

d o o l F

H

o r yd 3

3 2

0

Surface flow

12

24

36

48

30

72

Hours from start of rain storm


Activate: What do the key terms on the flood hydrograph mean? Task: Can you match up the key term to the correct definition? Drainage Basin

Precipitation Surface flow Through flow Discharge

Any form of moisture that falls from the atmosphere to land. The period of time between peak rainfall and peak river discharge. The volume of water in a river at any given time. The area of land drained by a main river and its tributaries. The movement of water sideways

Lag 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)


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 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 Surface flow

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

Lag 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: How do flood hydrographs work? A Flood Hydrograph 140

Discharge (cumecs)

120 100 80 60 40 20 0 1 Hours from start of storm


Demonstrate: What’s going on here?! http://www.youtube.com/watch?v=hJftAYYXpVw


Discharge (m3/s)

3 2 1

0

12

24

36

48

30

72

Hours from start of rain storm


Rainfall shown in mm, as a bar graph

Discharge (m3/s)

3 2 mm 4

1

3 2

0

12

24

36

48

30

72

Hours from start of rain storm


Discharge in m3/s, as a line graph

Discharge (m3/s)

3 2 mm 4

1

3 2

0

12

24

36

48

30

72

Hours from start of rain storm


Rising limb

l im b Rising

Discharge (m3/s)

3

The rising flood water in the river

2

mm 4

1

3 2

0

12

24

36

48

30

72

Hours from start of rain storm


Peak flow

36

30

l i mb

Maximum discharge in the river

Rising

Discharge (m3/s)

3

Peak flow

2

mm 4

1

3 2

0

12

24

48

72

Hours from start of rain storm


Recession limb Peak flow l i mb

2

lim

Rising

i on ss ce Re

mm

b

Discharge (m3/s)

3

Falling flood water in the river

4

1

3 2

0

12

24

36

48

30

72

Hours from start of rain storm


Lag time

Lag time

l i mb

2

l im

Rising

i on ss ce Re

mm

b

Discharge (m3/s)

3

Peak flow

4

1

3 2

0

12

24

36

48

Time difference between the peak of the rain storm and the peak flow of the river 30

72

Hours from start of rain storm


Basin lag time

l i mb Rising

Normal discharge of the river

l im

2

Peak flow

i on ss ce Re

mm

b

Discharge (m3/s)

3

Surface flow

4

1

3 2

0

Surface flow

12

24

36

48

30

72

Hours from start of rain storm


Demonstrate: Can you plot a living graph?

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

A Flood Hydrograph 140 120 Discharge (cumecs)

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

100 80 60 40 20 0 1 Hours from start of storm


Homework: How could the following factors influence the Flood Hydrograph? • Area

• Land Use

• Shape

• Drainage Density

• Slope

• Precipitation / Temp

• Rock Type

• Tidal Conditions

• Soil ©Microsoft Word clipart


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.


flood hydrographs, tgaw, lesson 3