This document discusses hyetographs, hydrographs, runoff, and unit hydrographs. It contains the following key points: 1. A hyetograph is a graphical representation of rainfall intensity over time, showing the relationship between rainfall amount and time. A hydrograph shows stream discharge over time. 2. Runoff is the portion of rainfall that flows into streams and rivers. It is affected by rainfall characteristics and basin properties like soil, vegetation and topography. 3. A unit hydrograph represents the runoff from 1 cm of effective rainfall uniformly distributed over a basin and duration. It can be used to estimate flood hydrographs from storm rainfall amounts and distributions.

1. MODULE-2
HYETOGRAPH AND HYDROGRAPH
ANALYSIS
PREPARED BY:-
Prof. ANKIT N PATEL

2. HYETOGRAPH
 A hyetograph is a graphical representation of the
relationship between the rainfall intensity and time.
 It is the plot of the rainfall intensity drawn on the
ordinate axis against time on the abscissa axis.
 The hyetograph is a bar diagram.
 The area under the hyetograph gives the total rainfall
occurred in that period.
 This chart is very useful in representing the
characteristics of storm, and is particularly important in
developing the design storm to predict extreme floods.
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3. Mass curve of rainfall
0
10
20
30
40
50
60
0 20 40 60 80 100 120
Time, hour
accumulatedprecipitation,mm
Mass Curve of Rainfall:
The total accumulated precipitation is plotted against time
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4. Hyetograph of a storm
0
0.1
0.2
0.3
0.4
0.5
0 – 8 8 – 16 16 – 24 24 – 32 32 – 40 40 – 48
Time, hours
Intensity,cm/hr
Hyetograph
-A hyetograph is a graphical representation of the relationship
between the rainfall intensity and time
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5. Bits Edu CampusProf. Ankit Patel 5

6. RUNOFF
 Runoff is that portion of rainfall that is not
evaporated.
 The runoff is defined as the portion of the rainfall
that makes its way towards river or ocean as
surface or subsurface flow.
 The discharge flowing in a river is the runoff from
the basin drained by that river.
 Runoff is usually expressed as volume per unit time,
the common unit being m3/s or cumec.
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7. Classification of Runoff
 Surface runoff:
Water flows over the land and is first to reach the
streams and rivers which ultimately discharge the
water to the sea.
 Inter flow or subsurface flow:
A portion of rainfall infiltrates into surface soil and
depending upon the geology of basins, runs as
subsurface runoff and reaches the streams and rivers.
 Ground water flow or base flow:
It is that portion of rainfall which after infiltration,
percolates down and joins the ground water reservoir
which is ultimately connected to the ocean.
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8. PROCESS OF RUNOFF
 When rainfall a part of it is intercepted by vegetation some of it is
stored as depressions on the ground surface and is known as
depression storage which later infiltrate or evaporates some of the
rain is absorbed by soil.
 If the rain continues further, the water starts infiltrating into the
surface soil and if rate of rainfall exceeds the infiltration rate then
this excess water start collecting on the surface as surface
detention and this water flows overland and joins the stream, river
oceans.
 The water which percolates without joining the water table and
flows below the ground surface and then joins the stream or river as
a sub surface flow and is considered as a part of surface runoff.
 The water that percolates to the ground water table and later after
long time joins the river or stream is known as ground water flow
or base flowBits Edu Campus Prof. Ankit Patel 8

9. PROCESS OF RUNOFF
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10. FACTORS AFFECTING
RUNOFF
1. Climatic Factors
 Type of precipitation
 Rain intensity
 Duration of rainfall
 Distribution of rainfall
 Direction of storm movement
 Soil moisture deficiency
 Other climatic conditions
2. The physiographic Factors
 Size of basin
 Shape of the basin
 Topography of the basin
 Soil characteristics of basin
 Cultivation and vegetative cover in basin area
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11. FACTORS AFFECTING
RUNOFF
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12. FACTORS AFFECTING
RUNOFF
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13. Bits Edu Campus Prof. Ankit Patel 13

14. Bits Edu Campus Prof. Ankit Patel 14

15. Characteristics of drainage basin
1. Divide Lines: The boundary line, along a topographic ridge
separating two adjacent drainage basins is called drainage
divide.
2. Concentration Point: The single point or location at which
all surface drainage form a basins comes together or
concentrates as outflow from the basin in the stream channel is
called concentration point
3. Time of concentration: the time of a concentration of a
drainage basin is the time required by the water to reach the
outlet from the most remote points of the drainage area.
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16. HYDROGRAPH
 A hydrograph is a graphical plot of discharge of a
natural stream or river versus time.
 It shows variations of discharge with time, at a
particular point of a stream.
 It also shows the time distribution of total runoff at
the point of measurement.
 Discharge is plotted on Y-axis and the corresponding
time is plotted on X-axis.
 Flood analysis and derivation of unit hydrograph a
single peaked hydrograph is required.
 Hydrograph analysis is the most widely used method
of analyzing surface runoff.
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17. Hydrograph
Record of River Discharge over a period of time
River Discharge
= cross sectional area
rivers mean
(average) velocity
X
(at a particular point in its course)
Storm Hydrographs
Show the change in discharge caused by a
period of rainfall

18. 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

19. Construction
Of
Storm (flood)
Hydrographs

20. 0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Discharge(m3/s)
Base flow
Through flow
Overland
flow
Basin lag time
mm
4
3
2
Peak flow

21. 0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Discharge(m3/s)

22. 0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Discharge(m3/s)
mm
4
3
2
Rainfall shown
in mm, as a
bar graph

23. 0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Discharge(m3/s)
mm
4
3
2
Discharge in
m3/s, as a
line graph

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

25. 0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Discharge(m3/s)
mm
4
3
2
Peak flow
Peak flow
Maximum
discharge in
the river

26. 0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Discharge(m3/s)
mm
4
3
2
Peak flow
Falling flood
water in the
river

27. 0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Discharge(m3/s)
Basin lag time
mm
4
3
2
Peak flow
Basin lag time
Time
difference
between the
peak of the
rain storm
and the peak
flow of the
river

28. 0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Discharge(m3/s)
Base flow
Basin lag time
mm
4
3
2
Peak flow
Base flow
Normal
discharge of
the river

29. 0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Discharge(m3/s)
Base flow
Through flow
Overland
flow
Basin lag time
mm
4
3
2
Peak flow
Overland flow
Through flow
+
=
Storm Flow

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

31. COMPONENTS OF A HYDROGRAPH
• Rising limb: The rising limb of hydro graph, also known
as concentration curve, reflects a prolonged increase in
discharge from a catchment area, typically in response
to a rainfall event
• Recession (or falling) limb: The recession limb extends
from the peak flow rate onward. The end of storm flow
(direct runoff) and the return to groundwater-derived
flow (base flow) is often taken as the point of inflection
of the recession limb. The recession limb represents
the withdrawal of water from the storage built up in
the basin during the earlier phases of the hydrograph.
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32. • Peak discharge: the highest point on the
hydro graph when the rate of discharge is
greatest
• Lag time: the time interval from the center of
mass of rainfall excess to the peak of the
resulting hydrograph
• Time to peak: time interval from the start of
the resulting hydro graph
• Discharge: the rate of flow (volume per unit
time) passing a specific location in a river or
other channel
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33. SEPERATION OF BASE
FLOW
The total runoff comprises direct runoff and base flow
1. Direct runoff: it is that water which reaches the river shortly
after it falls as rain. It is a overland flow
2. Base Flow: It is the initial flow of the river before the rainfall
comes. It is sustained or dry weather flow of the river resulting
from the outflow of perennial or almost permanent ground water
contribution that reaches the river.
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34. Unit Hydrograph
Unit Hydrograph is defined as the
hydrograph of surface runoff of a
catchment area resulting from unit
depth of rainfall excess or net rainfall
occurring uniformly over the basin
and at uniform rate for a specified
duration.
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35. UNIT HYDROGRAPH
 The effective rainfall also called rainfall excess is that part of total
rainfall that enters the stream directly.
 The specified or unit duration is that period within which the
effective rainfall is considered to be uniformly distributed.
 A unit hydrograph is designed according to the unit duration
 Since the area under a hydrograph represents the total runoff, it is
evident from the definition of unit hydrograph that the area under
the unit hydrograph represents a direct runoff of 1 cm
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36. Text in
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ASSUMPTIONS
 The effective rainfall is uniformly distributed within its duration or
specified period of time
 The effective rainfall is uniformly distributed over the entire area of
drainage basin.
 The base or time duration of the hydrograph of direct runoff due to
an effective rainfall of unit duration is constant.
 The ordinates of all the direct runoff hydrographs of a common base
period are directly proportional to the total amount of direct runoff
represented by each hydrograph.
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37. Text in
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Text in
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 For a given drainage basin the hydrograph of direct runoff
corresponding to a given period of rainfall reflects all the physical
characteristics of the basin.
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38. LIMITATIONS OF UH
 This theory is not applicable to large areas because uniformly
distributed effective rainfall cannot be expected in large area.
 The UH method cannot be applied when an appreciable portion of
the storm precipitation falls as snow
 Practically no two storms have the same nature in space and time
period. So it is not possible to construct UH for each pattern.
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39. 39
Derived Unit Hydrograph
0.0000
100.0000
200.0000
300.0000
400.0000
500.0000
600.0000
700.0000
0.0000 0.5000 1.0000 1.5000 2.0000 2.5000 3.0000 3.5000 4.0000
Total
Hydrograph
Surface
Response
Baseflow
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40. APPLICATION OF UH
 Use of UH for deriving a flood hydrograph resulting from rainfall
of unit duration
 Use of UH for deriving a flood hydrograph resulting from a series
of rainfalls each of same unit duration
 Use of UH for deriving a UH of other duration.
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