Title: Hydrographs[Date] Today I will: - Be able to construct and understand flood hydrographs
1Hydrographs DateToday I will- Be able to
construct and understand flood hydrographs
2What 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.
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6Basin lag time
Peak flow
3
Rising limb
Overland flow
Recession limb
2
Discharge (m3/s)
mm
Through flow
4
1
3
2
Base flow
0 12 24 36 48 30 72
Hours from start of rain storm
7The discharge of the river is measured in cumecs
- this stands for cubic metres per second
8Rainfall expressed in mm
3
2
mm
Discharge (m3/s)
4
1
3
2
0 12 24 36 48 30 72
Hours from start of rain storm
9Discharge in m3/s, as a line graph
3
2
mm
Discharge (m3/s)
4
1
3
2
0 12 24 36 48 30
72
Hours from start of rain storm
10 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.
3
Rising limb
2
mm
Discharge (m3/s)
4
1
3
2
0 12 24 36 48 30 72
Hours from start of rain storm
11Peak flow
3
Peak flow is when the Maximum discharge in the
river, the time when the river reaches its
highest flow
Rising limb
2
mm
Discharge (m3/s)
4
1
3
2
0 12 24 36 48 30 72
Hours from start of rain storm
12Peak flow
The recession limb shows that water is still
reaching the river but in decreasing amounts
3
Rising limb
Recession limb
2
mm
Discharge (m3/s)
4
1
3
2
0 12 24 36 48 30 72
Hours from start of rain storm
13The basin lag time is the time it takes for the
water to find its way to the river
Basin lag time
Peak flow
3
Rising limb
Recession limb
2
mm
Discharge (m3/s)
4
1
3
2
0 12 24 36 48 30 72
Hours from start of rain storm
14Basin lag time
Base Flow is the normal discharge of the river
Peak flow
3
Rising limb
Recession limb
2
mm
Discharge (m3/s)
4
1
3
2
Base flow
0 12 24 36 48 30 72
Hours from start of rain storm
15Overland flow
Basin lag time
Peak flow
Through flow
3
Rising limb
Overland flow
Recession limb
Storm Flow
2
mm
Discharge (m3/s)
Through flow
4
1
3
2
Base flow
0 12 24 36 48 30 72
Hours from start of rain storm
16Read 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
17Can you label 1 to 8 accurately?
7.
1.
6.
8.
5.
River discharge (cumecs) Rainfall (mm)
Peak rainfall Falling limb
Rising limb Lag time
Peak discharge Time
4.
2.
3.