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Good for Avery Hill Water Landforms and people
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Flooding
What is meant by a river flood?
A river floods when the level of the water
exceeds bankfull and flows onto the
floodplain.
Floodplain
Rivers regularly flood. When they do so they
deposit some of their load. This material is
known as silt.
The land on either side of a river that floods
is known as its floodplain.
Floodplain
Higher land
Floodplain
Notice how brown the water is. It is this colour because of the silt it is carrying..
Our school is on the floodplain of the River Thames but is protected by flood defences
Edge of floodplain
The Thames
Dagenham Park
Dagenham Heathway
Floodplain worksheet
Complete the floodplain worksheet
Flood hydrographs
These are used to show the relationship
between rainfall and the discharge of a river
Discharge is the volume of water flowing
past a point in a river during a period of time.
It is usually measured in cubic metres per
second i.e. cumecs
TIME
DIS
CH
AR
GE
(cu
mec
s)
FLOOD HYDROGRAPH
TIME days or hours
DIS
CH
AR
GE
(cu
mec
s)
FLOOD HYDROGRAPH
Bankfull level
Lag time Peak discharge
Peak rainfall
Key terms
• Peak rainfall
• Peak discharge
• Lag time
• Bankfull level
Write out simple definitions of these terms.
• A hydrograph shows us the difference between the peak rainfall and peak discharge. This difference is called lag time. The greater the lag time, the less chance there is of a flood; while a short lag time shows that water has reached the river channel quickly.
• A rise in discharge on the graph is called the rising limb; the falling limb shows a decrease in discharge.
• The flood hydrograph below shows:• rainfall peaking at six hours • discharge peaking at 11 hours • lag time - the time between peak rainfall and peak
discharge - is five hours.
There is a lag time because _______________________________________________________________________________
Extension for the clever onesThere are two peaks to my hydrographbecause ________________________________________________________________________________________________________________________________
Deforestation and flooding
Rock
Trees
River
Soil
When it rains water falls to the ground. In a forested
drainage basin the trees ___________________________
_______________________________________________
_______________________________________________
_______________________________________________
_______________________________________________
_______________________________________________
_______________________________________________
Less water reaches the river and the water that does arrive
takes a long time to reach it. This means that the river has
A chance to discharge the rainwater and so flooding is less
likely.
intercept transpiration roots
Rock
Concrete
Drains
Urbanisation and flooding
River
TIME
DIS
CH
AR
GE
(cu
mec
s)
FLOOD HYDROGRAPH
Bankfull level
Lag time Peak discharge
Peak rainfall
Explain why there a time lag between peak
rainfall and peak discharge?
Use your diagram of the drainage basin to
help you.