Lesson 6 factors affecting discharge (hydrographs)

What is the relationship is between precipitation and

runoff?

Flood Hydrographs.

Learning goals

KnowledgeTo know what a flood hydrograph isUnderstandingTo understand the factors affecting

hydrograph shapeSkillsUse key words to describe a flood

hydrographHydrograph interpretation

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.

Lets think…?

What happens to rain water once it has left

the cloud?

river

evapotranspiration

precipitation

throughflow

surface runoff

groundwater flowwater table

interception

percolation

infiltration

The drainage basin Remember…

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.

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

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

What does a hydrograph look like?

Label your hydrograph with the following sentences.

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

• 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.

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;

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

What other factors affect

flooding?

What other factors affect

flooding?

sample

The way a hydrograph looks will vary according to land use/type:

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.

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.

What do hydrographs look like?

Flashy Subdued

What do you think the differences are between the hydrographs?

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

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?

Homework

Complete the flood hydrographs question sheet for . . . . . . . . .

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?

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.

Task

On file paper/in your books explain how the different factors have

affected the shape of the hydrographs.

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)

On your hydrograph label the following parts:

• Peak rainfall• Peak discharge• Lag time• Rising limb• Falling limb

Its quiz time!

Link to online flashcards

Rising limb

Falling limb

Peak discharge

Peak rainfall

Lag time = Peak discharge (hrs) - Peak rainfall (hrs)

storm flow

normal (base) flow

Hydrographs

Compare these two hydrographs.

Which one has the largest lag time? Explain your answer.

Hydrographs

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

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