1. Storm Hydrographs
Yr12IB Drainage Basins

2. The Storm (flood) Hydrograph
A = Discharge
B = Lag time
C = Peak discharge
D = Storm flow
E = Base flow
F = Falling limb
G = Rising limb

4. Match the label to the correct letter
Watershed c
Confluence
Tributary b
Source d a

5. What affects the shape of a hydrograph?
Hydrographs
Hydrographs are graphs which show discharge (the amount of water passing a particular point in a river at a particular time).
Geology and soil
Gradient of the valley sides
What affects the shape of a hydrograph?
Type and amount of precipitation
Land use

6. 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

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

8. Hydrographs Rising limb Falling limb
1) What type of flow (choose from surface runoff, throughflow and groundwater flow) causes the rising limb?
2) Which is the last type of flow to reach the river?
3) What is the lag time?

9. Hydrographs b a
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’?
Chalk bedrock
Impermeable bedrock
Urban area
Clay soil
Deforested river basin
Rural area
Gentle valley gradients

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

11. Hydrographs
Imagine that these two areas have had the same amount of rain over a 24-hour period. Draw a possible hydrograph for each of these areas. A B

12. Flood hydrographs

13. In some drainage basins the discharge of a river can rise slowly or quickly in response to a period of rainfall. This can be shown on the Flood Hydrograph. When a storm begins, discharge does not rise immediately as only a little of the rain will fall directly into the channel. The shape of the hydrograph will very much depend on a number of factors.
Key: Definitions:
Discharge-Amount of water passing a given point at a given time.
Lag Time- The difference between peak rainfall and Peak discharge
Rising Limb – Discharge rising in the river (i.e. water level rises)
Falling Limb – Water level falls in the river.

15. Which conditions give a Flashy Hydrograph
Which conditions give a Flashy Hydrograph? Which will give a lower, later peak discharge?

16. Heavy storms Well distributed rainfall
Permeable Rock Impermeable Rock
Dense Vegetation Little Vegetation
Long & thin Basin Short & round Basin
Steep slopes Gentle slopes
Flood control scheme No river management
Rural area Urban area