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.