Hydrograph
Record of River Discharge over a period of time
Storm Hydrographs
Show the change in discharge caused by a
period of rainfall
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
Construction
©Microsoft Word clipart
Of
Storm (flood)
Hydrographs
Basin lag time
Peak flow
Discharge (m3/s)
3
2
mm
Through flow
4
1
Overland
flow
3
2
0
Base flow
12
24
36
48
30
72
Hours from start of rain storm
Discharge (m3/s)
3
2
1
0
12
24
36
48
30
72
Hours from start of rain storm
Rainfall shown
in mm, as a
bar graph
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
30
72
Hours from start of rain storm
The rising
flood water in
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
Peak flow
Maximum
discharge in
the river
36
30
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24
48
72
Hours from start of rain storm
Peak flow
Discharge (m3/s)
3
Falling flood
water in the
river
2
mm
4
1
3
2
0
12
24
36
48
30
72
Hours from start of rain storm
Basin lag time
Basin lag time
Peak flow
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24
36
48
Time
difference
between the
peak of the
rain storm
and the peak
flow of the
river
30
72
Hours from start of rain storm
Basin lag time
Base flow
Peak flow
Discharge (m3/s)
3
Normal
discharge of
the river
2
mm
4
1
3
2
0
Base flow
12
24
36
48
30
72
Hours from start of rain storm
Basin lag time
Overland flow
Through flow
Discharge (m3/s)
3
2
mm
=
Overland
flow
Storm Flow
Through flow
4
1
+
Peak flow
3
2
0
Base flow
12
24
36
48
30
72
Hours from start of rain storm
Draw your flood hydrograph
Analysis: what factors are going to lead to a flood hydrograph like A
and a flood hydrograph like B? Complete next slide with a list
A
A: solid line
B: dotted line
©Microsoft Word clipart
A: grassy area,
B:
Factors influencing
Storm Hydrographs
• Area
• Land Use
• Shape
• Drainage Density
• Slope
• Precipitation
• Rock Type
• Soil
©Microsoft Word clipart
Boscastle floods (watch the clip – think
about why the flood was so bad)
Boscastle flood
Complete the sentences of the following
slides to explain why some areas flood
badly (line B) and others have less
severe or no floods ( line A)
Area
Large basins…
Larger size means longer lag time as….
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Shape
Elongated basin will …..whereas a circular one
of the same size will….
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Slope
steep slopes will mean….
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Rock Type
Permeable rocks mean rapid infiltration and
so…
Whereas impermeable rocks will mean…
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Soil
thick soil will mean…
sand will allow …
Whereas clay is more impermeable…
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Land Use
Urbanisation will mean…
Afforestation will mean…
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Drainage Density
A higher density of rivers will mean a
hydrograph for downstream will have …
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Precipitation & Temperature
Short intense rainstorms….
If there have been extreme temperatures,
the ground can be frozen or solid….
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
With reference to areas you have studied
and studies you have completed, explain the
factors that are likely to lead to flooding.
(10)