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Higher GeographyHydrosphere
Hydrographs [Date]
Today I will:- Be able to construct and understand flood hydrographs
Higher GeographyHydrosphere
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 GeographyHydrosphere
Higher GeographyHydrosphere
Higher GeographyHydrosphere
0 12 24 36 48 30 72Hours from start of rain storm
3
2
1
Dis
char
ge (m
3 /s)
Base flow
Through flow
Overland flowRi
sing
limb
Recession limb
Basin lag time
mm4
3
2
Peak flow
0 12 24 36 48 30 72Hours from start of rain storm
3
2
1Dis
char
ge (m
3 /s)
The discharge of the river is measured in cumecs - this
stands for cubic metres per second
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (m
3 /s)
mm4
3
2
Rainfall expressed in mm
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (m
3 /s)
mm4
3
2
Discharge in m3/s, as a line graph
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (m
3 /s)
Risi
ng li
mb
mm4
3
2
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.
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (m
3 /s)
Risin
g lim
bmm
4
3
2
Peak flow
Peak flow is when the Maximum discharge in the
river, the time when the river reaches its highest
flow
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (m
3 /s)
Risin
g lim
b
Recession limb
mm4
3
2
Peak flow
The recession limb shows that water is still reaching the river but in
decreasing amounts
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (m
3 /s)
Risin
g lim
b
Recession limb
Basin lag time
mm4
3
2
Peak flowThe basin lag time is
the time it takes for the water to find its way to
the river
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (m
3 /s)
Base flow
Risin
g lim
b
Recession limb
Basin lag time
mm4
3
2
Peak flowBase Flow is the
normal discharge of the river
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (m
3 /s)
Base flow
Through flow
Overland flowRi
sing
limb
Recession limb
Basin lag time
mm4
3
2
Peak flow
Overland flow
Through flow
+
=Storm Flow
Higher GeographyHydrosphere
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 GeographyHydrosphere
1.
2.
5.
4.
6.
7.
8.
3.
Riverdischarge (cumecs)
Rainfall (mm)
Peak rainfall Falling limb
Rising limb Lag time
Peak discharge Time
Can you label 1 to 8 accurately?
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