lesson 6 factors affecting discharge (hydrographs)
TRANSCRIPT
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What is the relationship is between precipitation and
runoff?
Flood Hydrographs.
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Learning goals
KnowledgeTo know what a flood hydrograph isUnderstandingTo understand the factors affecting
hydrograph shapeSkillsUse key words to describe a flood
hydrographHydrograph interpretation
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Back to Back
• The aim of the task is to draw a diagram as accurately as possible only from you partners description.
• In pairs you will need to decide who will describe the diagram and who will draw it.
• You will have 5 minutes to complete the diagram.
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Lets think…?
What happens to rain water once it has left
the cloud?
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river
evapotranspiration
precipitation
throughflow
surface runoff
groundwater flowwater table
interception
percolation
infiltration
The drainage basin Remember…
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Rainwater will:
• Be lost through the system through evapotranspiration
• Be held in storage in lakes, the soil or underground
• Flow into a river to return, eventually to the sea as run-off.
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In other words…
• The amount of rainwater that reaches the river will be:
Precipitation – (evapotranspiration + storage)
So, river runoff will be less than precipitation
If we have excess precipitation then, we get?
Flooding
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What is a hydrograph?
• A hydrograph shows variations in a river’s discharge over a short period of time, usually during a rainstorm.
• River discharge is the amount of water passing a given point in the river at a particular time.
• The shape of a hydrograph can be influenced by many things
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What does a hydrograph look like?
Label your hydrograph with the following sentences.
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Hydrographs
Hydrographs are graphs which show discharge (the amount of water passing a particular point in a river at a particular time).
What affects the shape of a hydrograph?
Land useType and amount of precipitation
Geology and soil Gradient of the valley sides
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• The line graph shows the discharge.• The bar graph shows the rainfall.• When a storm begins, discharge does not
increase immediately as only a small amount of rain will fall directly into the channel.
• The first water to reach the river will come from surface run-off.
• Water arriving in the river later comes from through-flow.
• The increase in discharge is shown by the rising limb.
• The decrease in discharge is shown by the falling limb.
• The gap between the time of peak (maximum) rainfall and peak discharge (highest river level) is called lag time.
• A river with a short lag time and a high discharge is more likely to flood than a river with a lengthy lag time and a low discharge.
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Let’s re-cap
– Hydrograph– Surface runoff– Lag time– Antecedent rainfall
Choose TWO of the following key terms and in pairs tell your partner all you know about;
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1) What type of flow (choose from surface runoff, throughflow and groundwater flow) causes the rising limb?
3) What is the lag time?
2) Which is the last type of flow to reach the river?
Rising limb
Falling limb
Hydrographs:
Surface runoff
Groundwater flow
4 hours
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What other factors affect
flooding?
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What other factors affect
flooding?
sample
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The way a hydrograph looks will vary according to land use/type:
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How do trees reduce flooding?
trees intercept the rain
evapotranspiration reduces the amount of water that reaches the river
water is taken through the roots and so less enters the river
So the removal of
trees increases the risk of flooding.
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Urbanisation
• Water cannot infiltrate (soak) into pavement
• Where does it go then?
• Will this reach the river quicker or more slowly?
As urban areas grow, risk of flooding increases.
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What do hydrographs look like?
Flashy Subdued
What do you think the differences are between the hydrographs?
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Task
• Look at ‘The shape of the hydrograph’ worksheet
• Your task is to complete the table using the information provided to describe the differences between the 2 hydrographs
• You may work in pairs
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Have achieved what we set out to do?
• Can you recognise a flood hydrograph?
• Do you know how a flood hydrograph works?
• Can you use key words to describe a flood hydrograph?
• Do you know what types of human behaviour can affect a flood hydrograph?
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Homework
Complete the flood hydrographs question sheet for . . . . . . . . .
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Flood hydrographs
What can you remember?
• What does a flood hydrograph look like?
• Can you label the rising and falling limb?
• Can you label lag time?
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Task
• Complete the graph using the discharge data.
• On the completed graph:– Label the peak discharge– Indicate the lag time– Label rising and falling limb
• Calculate the lag time.
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Task
On file paper/in your books explain how the different factors have
affected the shape of the hydrographs.
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Task – flood hydrograph: living graph
What is a living graph?
1.Use the rainfall data to complete a rainfall bar chart
2.Use the discharge data to complete a discharge line graph
3.Label your hydrograph with the 8 labels4.Cut out and stick on the information about
Mr & Mrs Jones (this is the living bit)
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On your hydrograph label the following parts:
• Peak rainfall• Peak discharge• Lag time• Rising limb• Falling limb
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Rising limb
Falling limb
Peak discharge
Peak rainfall
Lag time = Peak discharge (hrs) - Peak rainfall (hrs)
storm flow
normal (base) flow
Hydrographs
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Compare these two hydrographs.
Which one has the largest lag time? Explain your answer.
Hydrographs
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a b
The hydrographs ‘a’ and ‘b’ have been produced from the same storm event but from different drainage basins. Which of the following river basin descriptions are more likely to have produced hydrograph ‘a’ and which are more likely to produced hydrograph ‘b’?
Urban area
Rural area
Impermeable bedrock Clay soil
Gentle valley gradientsDeforested river basin
Chalk bedrock
Hydrographs
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1) What type of flow (choose from surface runoff, throughflow and groundwater flow) causes the rising limb?
3) What is the lag time?
2) Which is the last type of flow to reach the river?
Rising limb
Falling limb
Hydrographs