Hydrographs show variations in a river's discharge over time, usually during a rainstorm. The shape of a hydrograph is influenced by factors like land use, precipitation amounts, geology, and soil. A hydrograph has a rising limb as discharge increases and a falling limb as it decreases. Lag time is the delay between peak rainfall and peak discharge as water from precipitation moves through the landscape into the river. Urbanization can increase flooding risk by preventing water infiltration into paved surfaces.

1. What is the relationship is
between precipitation and
runoff?
Flood Hydrographs.

2. Learning goals
Knowledge
To know what a flood hydrograph is
Understanding
To understand the factors affecting
hydrograph shape
Skills
Use key words to describe a flood hydrograph
Hydrograph interpretation

3. 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.

4. Lets think…?
What happens to rain
water once it has left
the cloud?

5. river
evapotranspiration
precipitation
water table
interception
percolation
infiltration
The drainage basin Remember…

6. 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.

7. 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

8. 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

9. What does a hydrograph look like?
Label your
hydrograph
with the
following
sentences.

11. 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

12. • 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.

13. 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;

14. 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

15. What other
factors
affect
flooding?

16. What other
factors
affect
flooding?
sample

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

18. 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.

19. 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.

20. What do hydrographs look like?
Flashy Subdued
What do you think the differences are between
the hydrographs?

21. 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

22. 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?

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

24. 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?

25. 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.

26. Task
On file paper/in your books explain how
the different factors have affected
the shape of the hydrographs.

27. 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 labels
4.Cut out and stick on the information about Mr
& Mrs Jones (this is the living bit)

28. On your hydrograph label the
following parts:
• Peak rainfall
• Peak discharge
• Lag time
• Rising limb
• Falling limb

29. Its quiz time!
Link to online flashcards

30. Rising
limb
Falling
limb
Peak discharge
Peak rainfall
Lag time =
Peak discharge (hrs) - Peak rainfall (hrs)
storm flow
normal (base) flow
Hydrographs

31. Compare these two hydrographs.
Which one has the largest lag time? Explain your answer.
Hydrographs

32. 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

33. 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