1. Flooding 1
Key idea:
The amount of water in a river
fluctuates due to a number of
reasons

2. Lesson objectives
• To know the terms:
Flooding, hydrograph, rising limb, falling
limb, lag time, drainage basin
• To be able to describe the
characteristics of a hydrograph

3. Starter activity….
The area of land from which
A DRAINAGE BASIN precipitation flows into a
stream.
DISCHARGE
The amount of water in a river
channel at a given time
VELOCITY The speed of the river
WATERSHED The boundary of a drainage
basin

4. A flood occurs when…
…..the discharge is so great that all the
water can no longer be contained within
the channel so that the river overflows
its banks

5. Factors affecting discharge
•Amount and type of rainfall • Soil
•Temperature • Land Use
• Slope
•Previous weather conditions
• Relief
•Drainage Density
•Rock Type

6. Hydrographs
Peak discharge
Falling
Rising
limb
limb
storm flow
normal (base) flow
I = Lagtime Peak rainfall
Show the change in
discharge caused
by a period of
rainfall

7. Storm hydrograph
A graph which shows the relationship between
precipitation and the level of a river

8. APPROACH SEGMENT = base flow
• The discharge of the river before it rains.
RISING LIMB
• Results from a rapid increase in rainwater
reaching the river.
LAG TIME
• The difference between the time of the
heaviest rainfall and the maximum level and
/or discharge of the river.
FALLING LIMB
• Some rainwater is still reaching the river, but
in decreasing amounts – throughflow and
groundwater flow

9. Why Construct & Analyse
Hydrographs ?
• To find out discharge patterns of
a particular drainage basin
Help predict flooding events,
therefore influence implementation
of flood prevention measures
©Microsoft Word clipart

10. ph
g ra Basin lag time
d ro Peak flow
y 3
H
od
limb
Re
lo
ce
Discharge (m3/s)
F
Rising
ss
Overland
2
io
flow
nl
im
mm
b
4 Through flow
1 3
2
Base flow
0 12 24 36 48 30 72
Hours from start of rain storm

11. Discharge (m3/s) 3
2
1
0 12 24 36 48 30 72
Hours from start of rain storm

12. Rainfall shown
3 in mm, as a
bar graph
Discharge (m3/s)
2
mm
4
1 3
2
0 12 24 36 48 30 72
Hours from start of rain storm

13. Discharge in
3 m3/s, as a
line graph
Discharge (m3/s)
2
mm
4
1 3
2
0 12 24 36 48 30 72
Hours from start of rain storm

14. Rising limb
The rising
3 flood water in
the river
li m b
Discharge (m3/s)
Rising
2
mm
4
1 3
2
0 12 24 36 48 30 72
Hours from start of rain storm

15. Peak flow
Peak flow Maximum
3 discharge in
the river
limb
Discharge (m3/s)
Rising
2
mm
4
1 3
2
0 12 24 36 48 30 72
Hours from start of rain storm

16. Recession limb
Peak flow
Falling flood
3 water in the
limb
river
Re
Discharge (m3/s)
ce
Rising
ss
2
ion
mm
li m
b
4
1 3
2
0 12 24 36 48 30 72
Hours from start of rain storm

17. Basin lag time
Basin lag time
Peak flow Time
3 difference
between the
limb
Re
ce
peak of the
Discharge (m3/s)
Rising
ss
2
io
rain storm and
nl
im
mm
the peak flow
b
4
1 3
of the river
2
0 12 24 36 48 30 72
Hours from start of rain storm

18. Base flow
Basin lag time
Peak flow
Normal
3 discharge of
limb
the river
Re
ce
Discharge (m3/s)
Rising
ss
2
io
nl
im
mm
b
4
1 3
2
Base flow
0 12 24 36 48 30 72
Hours from start of rain storm

19. Basin lag time
Peak flow
3
limb
Re
ce
Discharge (m3/s)
Rising
ss
Overland
2
io
flow
nl
im
mm
b
4 Through flow
1 3
2
Base flow
0 12 24 36 48 30 72
Hours from start of rain storm

20. Overland flow = surface run off
Volume of water reaching the river from
surface run off
Through flow
Volume of water reaching the river through
horizontal movement of water through the
soil and underlying rock layers

21. • After a period of heavy rainfall, the
discharge increases.
• The lagtime is the difference in time
between the peak of the rainstorm and
the peak of the discharge. The shorter
this is and the steeper the rising limb
the greater the risk of flooding.
• Complete task 1

22. Plenary

23. Homework
Worksheet - task 2 a - c