1. Hydrology & Water Resources Engineering
Subject Code:150602

2. Hytograph and Hydrograph Analysis
A hytograph can be defined as a plot of intensity of rainfall against the time interval, represented as a bar chart.
2) Hydrograph:
A Hydrograph is a graphical plot of discharge of a natural stream or
River versus time.

3. 1) Runoff:
Runoff is that portion of rainfall that is not evaporated or infiltrated
In ground
Unit
Discharge m3/s, cusecs, cumecs.

4. Classification of Runoff:
Surface runoff
Inter flow or subsurface flow
Ground water flow or base flow

5. Runoff
Runoff is that portion of rainfall that is not evaporated or intercepted.
Factor Affecting Runoff
Intensity of Rainfall
Soil Characteristics of Catchments
Topography of the Catchments
Shape and Size of Catchments
Cultivation and Vegetative Cover in Catchments
Geological Condition of Catchments
Weather Condition

6. i) Fan-shaped Watersheds ii) Fern-Leaf shaped Watersheds

7. Drainage Basin Characteristics

8. Hydrograph:
A hydrograph is a graphical plot of discharge of a natural stream or
River versus time.
Effective Duration:
It is the time between which the rainfall rate is more than the
Infiltration rate.

9. 2) Basin Lag (tp):
It is the time between centroid of the net rainfall (Pnet) and the
Peak (D) of the hydrograph.
3) Recession Time (tr):
It is the duration of direct runoff after the end of effective duration
of rainfall.
4) Time of concentration (tc):
It is the time in hours taken by rain water that falls at the farthest
Point to reach the outlet of a basin.
This time is equal to the time between point of inflection and of
Effective duration of rainfall.

10. Factors affecting the shape of hydrograph
Intensity and duration of rainfall
Size and shape of basin
Nature and slope of basin
Land use and Land cover
Drainage density
Type of soil and its infiltration rate
Lakes, depressions
Stream characteristics.

11. Separation of Base flow:
Direct run-off:
Direct run-off is that water which reaches the river shortly after it
Falls as rain.
Direct run-off is a overland flow (surface run-off)
2) Base Flow:
Base flow is the initial flow of the river before the rainfall comes.

12. Unit Hydrograph:
A unit hydrograph is hydrograph representing 1 cm of runoff
from a rainfall of some unit duration and specific areal
distribution.
Unit Hydrograph (UH) is defined as the hydrograph of surface
runoff of a catchment area resulting from unit depth (usually 1cm)

13. Propositions of the Unit Hydrograph:
Time invariance:
It means that the direct runoff hydrograph for a given effective rainfall in a catchment is always the same irrespective of when
it occurs.
The direct runoff hydrograph does not depend upon the time
when the strom occurs.
2) Linearity of response:
It means that the relation between the direct runoff discharge and the effective rainfall is linear.
This is the most important proposition of the unit-hydrograph.
3) Fixed base period:
The period during which the direct runoff occurs is called the
base period.

14. Assumptions of Unit Hydrograph Theory:
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 the drainage basin.
3) The base or time duration of the hydrograph of direct run-off due
to an effective rainfall of unit duration is constant.
4) 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. This is known as principle of linearity.
5) 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.

15. Limitation of Unit Hydrograph Theory:
This theory is not applicable to large areas because uniformly distributed effective rainfall can’t be expected in large area. Hence UH theory is suited to catchment area under about 500 km2.
The unit hydrograph method can’t be applied when an appreciable portion of the storm precipitation falls as snow.
In mountainous regions, subject to orographic rainfall, aerial distribution is very uneven, but the patten tends to remain the same from strom to strom, and unit hydrograph theory may not be successfully applied.
The principle of linearity is not partically valid for smaller and larger stroms.
The catchment having large storage like reservoir, lake, low area etc. affect the linear relationship and hence theory cannot be applied.

16. 6) The principle of time invariance is valid only for specified time and condition of drainage basin.
Practically no two stroms have the same nature in space and time period. So, it is not possible to construct unit hydrograph
for each pattern.

17. Application of Unit Hydrograph Theory:
The unit hydrograph method is used for the estimation of the
maximum flood discharge of a stream.
Use of unit hydrograph for deriving a flood hydrograph resulting from rainfall of unit duration.
Use of unit hydrograph for deriving a flood hydrograph resulting from a series of rainfalls each of same unit duration.
Use of unit hydrograph for deriving a unit hydrograph of other duration.

18. Ex.1 The ordinates of 3 hr unit hydrograph are given:
Time hr 3 6 9 12 15 18 21 24 27 30
Ordi
Cum. 10 25 20 16 7 5
Find the ordinate of a 6 hr unit hydrograph for the same basin
Analytically. Also sketch this unit hydrograph. What is the peak
Value of discharge in this unit hydrograph?

19. Time Hr. 1
1st 3 hr 2
2nd 3 hr 3
Ordi. 6 hr 2cm
4 =2+3
Ordi. 6hr unit
5 = 4/2 - 10 5 6 25 35
17.5 9 20 45
22.5 12 16 36 18 15 28 14 09 21
10.5 07 8 24 05 7 27 03 4 30 00
1.5 33

20. Graph….:

21. Ex.2 The ordinates of 8 hr unit hydrograph are given:
Time hr
Ord. 8 hr
Ordi. 8 hr
0.0 44
79.0 4
5.5 48
57.0 8
13.5 52
42.0 12
26.5 56
31.0 16
45.0 60
22.0 20
82.0 64
14.0 24
162.0 68
9.5 28
240.0 72
6.6 32
231.0 76
4.0 36
165.0 80
2.0 40
112.0 84
1.0
Obtain a 24 hr unit hydrograph with graph

22. S- Hydrograph:
S- hydrograph is hydrograph of direct surface discharge that would
Result from successive storms each of which producing effective
Rainfall of 1 cm in unit duration.
S –hydrograph is therefore a continuously rising curve in the form
Of letter S, till a constant value of discharge is reached.

23. Ex.1 A 6-hr UH for a basin has the ordinates:
Time hr 6 12 18 24 30 36 42 48 54 60 66
Ordi. 6hr (m3/s) 20
150
120 90 50 32 22 10
Determine the ordinates of 12-hr UH. Using S-curve method.

24. Time hr
Ordi. 6hr.
Offset
Ordi.
T1 =6hr
S-curve
Lagged
T2 =12 hr
Diff.
12 hr.
UH. 1 2 3 4 5
4-5 =6
6 * t1/t2 - 6 20 10 12 60 80 40 18
150
230
210
105 24
120
350
270
135 30 90
440 36 66
506
156 78 42 50
556
116 58 48 32
588 82 41 54 22
610 27
620 16 05

25. Alternative Method: Time hr 1st 6-hr 2nd 6-hr Ordi. 12 hr 2 cmUH 1 2 3
4 = 2+3
5 = 4/2 - 6 20 10 12 60 80 40 18
150
210
105 24
120
270
135 30 90 36 66
156 78 42 50
116 58 48 32 82 41 54 22 27 16 05 72

26. Ex.2 A 4-hr UH has the following ordinates:
Compute the ordinate of S-Curve .
Also compute 5 hr UH for basin.
Time hr
Ordi. UH 11 41 1 6 12 34 2 36 13 27 3 66 14 23 4 91 15 17 5
106 16 93 9 7 79 18 8 68 19 58 20 10 49 21

27. Time hr
Ordi. UH
Offset
T1 = 4 hr
m3/s
S-Curve
T2 =5 hr
Diff.
Ordinates
5 hr UH
=6 x t1/t2 1 2 3 4 5
4-5=6 7 - 6 36 29 66 53 91 73
106
112 90 93
129
123 98 79
145
109 87 8 68
159 74 9 58
170 63 10 49
178 11 41
186 57 46 12 34

28. Prepared by, Dr Dhruvesh Patel www.drdhruveshpatel.com
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