Higher Geography
Hydrosphere
Hydrographs
[Date]
Today I will:
- Be able to construct and understand flood hydrographs
Higher Geography
Hydrosphere
What is a hydrograph?
A hydrograph is a graph
showing the rate of flow
(discharge) versus time past
a specific point in a river. The
rate of flow is typically
expressed in cubic meters
(m3).
They are used to show how
rivers respond to a flood.
Hydrographs are used to help predict flooding
events therefore influence implementation of
flood prevention measures.
Higher Geography
Hydrosphere
Higher Geography
Hydrosphere
Higher Geography
Hydrosphere
Basin lag time
Peak flow
Discharge (m3/s)
3
Overland
flow
2
mm
4
1
Through flow
3
2
0
Base flow
12
24 36 48
30 72
Hours from start of rain storm
Discharge (m3/s)
3
The discharge of the river is
measured in cumecs - this
stands for cubic metres per
second
2
1
0
12
24
36
48
30
Hours from start of rain storm
72
Rainfall expressed
in mm
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24 36 48
30 72
Hours from start of rain storm
Discharge in m3/s,
as a line graph
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24
36
48
Hours from start of rain storm
30
72
The rising limb is when
the normal (base) flow of
the river starts to rise
when run-off, ground and
soil water reaches the
river.
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24 36 48
30 72
Hours from start of rain storm
Peak flow
Discharge (m3/s)
3
Peak flow is when the
Maximum discharge in the
river, the time when the
river reaches its highest
flow
2
mm
4
1
3
2
0
12
24 36 48
30 72
Hours from start of rain storm
Peak flow
The recession limb
shows that water is still
reaching the river but in
decreasing amounts
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24 36 48
30 72
Hours from start of rain storm
Basin lag time
Peak flow
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24 36 48
30 72
Hours from start of rain storm
The basin lag time is
the time it takes for the
water to find its way to
the river
Basin lag time
Peak flow
Discharge (m3/s)
3
2
mm
4
1
3
2
0
Base flow
12
24 36 48
30 72
Hours from start of rain storm
Base Flow is the
normal discharge of
the river
Overland flow
Basin lag time
Peak flow
+
Through flow
3
Discharge (m3/s)
=
Overland
flow
2
Storm Flow
mm
4
1
Through flow
3
2
0
Base flow
12
24 36 48
30 72
Hours from start of rain storm
Higher Geography
Hydrosphere
Read the information on page 230 and 231 of the
NWW Textbook. Copy the table below into your jotter
and fill in the information gathered.
Reasons for Differences in Flood Hydrographs
LONG LAG TIME
SHORT LAG TIME
Higher Geography
Hydrosphere
Can you label 1 to 8
accurately?
7.
1.
6.
5.
4.
2.
3.
8.
River
discharge
(cumecs)
Rainfall (mm)
Peak rainfall
Falling limb
Rising limb
Lag time
Peak discharge
Time