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1. LAXMI INSTITUTE
OF TECHNOLOGY,
SARIGAM.
PRESENTATION ON:-
HYDROGRAPH

2. Presented by:
NAME:
BHANDARI NEENAD
PRAJAPATI RIYA
RATHOD NEHA
SHAIKH MOHD. AMIR N.
ENROLLMENT NO.:
150860106006
150860106054
150860106055
150860106062

3. Content:3
 Hydrograph
 Components
 Factors affecting Hydrograph
 Unit Hydrograph
 Assumptions, Derivation, Limitation of Unit Hydrograph
 Application
 Method of superposition
 S – Hydrograph
 Distribution Hydrograph

4. Introduction:
HYDROGRAPH :
 A Hydrograph is a graph showing variation of discharge (Q) with time (T), at a particular point of stream
or river.
 Discharge is usually expressed in cumecs or hectare meter per day and time is expressed in hours or days.
Discharged is plotted on y-axis and corresponding time is plotted on x-axis.

5. Components of hydrograph:5
1) Rising Limb
2) Peak of Crest Segment
3) Recession Curve

6. Component :6
 Rising Limb : Also known as concentration curve – represent the increase in discharge due to gradual building
up of storage in channel and the catchment surface.
 Peak of crest segment : The peak flow occur when the runoff from various parts of the catchment
simultaneously contribute amounts to achieve the maximum amount of flow at basin outlet.
 Recession Limb : Extent from the point of infection at the end of the crest segment to the commencement of the
natural groundwater flow represent the withdrawal of water from the storage built up in the basin during the
earlier phase of hydrograph.

7. Factors affecting hydrograph :7
• Area
• Shape
• Slope
• Rock Type
• Soil
• Land Use
• Drainage Density
• Precipitation / Temp
• Tidal Conditions

8. Unit hydrograph :8
 A unit hydrograph is a hydrograph representing 1 cm of runoff from a rainfall of some unit duration.
 For example: A 3-hours unit hydrograph indicates the hydrograph which gives 1 cm depth of direct runoff
when a storm of 3-hour duration occurs uniformly over the catchment.

9. Assumptions of Unit hydrograph :9
• The excess rainfall has a constant intensity within the effective duration.
• The excess rainfall is uniformly distributed throughout the whole drainage area.
• The base time of the DRH (the duration of direct runoff) resulting from an excess rainfall of given
duration is constant.
• The ordinates of all DRH’s of a common base time are directly proportional to the total amount of direct
runoff represented by each hydrograph.
• For a given watershed, the hydrograph resulting from a given excess rainfall reflects the unchanging
characteristics of the watershed.

10. Limitations :10
 The unit hydrograph theory cannot be applied to catchment areas greater than 5000 km sq.
 The unit hydrograph theory cannot be applied to very small catchments with area less than 2 km sq.
 This theory cannot be applied when the major portion of the storm is in the form of snow.
 This theory is not very accurate. The accuracy obtained is + 10%.
 A large number of unit hydrographs are required for a catchment because as the unit duration changes,
the unit hydrograph also changes.

11. Derivation:11
 A unit hydrograph method is used for the estimation of the maximum flood discharge of a stream as well
as for developing a flood hydrograph corresponding to any anticipated rainfall.
 In order to derive the unit hydrograph from storm hydrograph of same unit duration the following steps
are followed.
 From the past records, select a hydrograph resulting from an isolated, intense, short duration rainfall over
the entire drainage basin.

12. 12
 Separate the base flow (ground water flow) from the direct runoff.
 Subtracting the ordinates of base flow from the total ordinates, find the ordinates of direct runoff.
 Construction of unit hydrograph:

13. 13
 Compute the depth of direct runoff as under.
Depth of direct runoff = Volume of the direct
runoff
Area of the drainage basin
=(∑O*t*3600) m^3
(A*10^6) m^2
= 0.36∑O*t cm
A
Where,
∑O = Sum of direct runoff ordinates in cumec
T = time interval in hours between the successive ordinates.
A = Area of drainage basin in square kilometers.
 By dividing each of the direct runoff by the depth of direct runoff, the ordinates of unit hydrograph are
obtained.
Ordinates of unit hydrograph = Ordinate of
direct runoff
Direct runoff in cm

14. Application :14
 Once a unit hydrograph for a duration try has been developed for a basin, the storm hydrograph for that
basin for any other storm of different intensity but the same duration can be easily developed.
 To draw maximum flood hydrograph for the design of spillways.
 It can be used for flooding and flood warning based on the observed rainfall in the basin.
 To determine runoff from a storm of unit duration.

15. Methods for derivation of UH15
 Method of superposition
 S-curve method.

16. Method of superposition.16
 If a unit hydrograph is available and it is desired to estimate a unit hydrograph of nD-hr UH where, n is
an integers which can be easily detremine by superposing n-UH with each graph separated from the
previous D-hr UH.

17. Example:
17
 Given the ordinated of the 4-hr UH. Derive the ordinates of a 16-hr UH for the same catchment.
T(hrs) 4-hr UH
(m3/s)
1 2 3 4 DRH
5=(1+2+3
+4)
16-hr
UH(DRH
/4)
0 0 0 - - - 0 0
4 20 20 0 - - 20 5
8 80 80 20 0 - 100 25
12 130 130 80 20 0 230 57.5
16 150 150 130 80 20 380 95
20 130 130 150 130 80 490 122.5
24 90 90 130 150 130 500 125
28 52 52 90 130 150 422 105.5
32 27 27 52 90 130 299 74075
36 15 15 27 52 90 184 46
40 5 5 15 27 52 99 24.75
44 0 0 5 15 27 47 11.75
48 0 0 0 5 15 20 5
52 0 0 0 0 5 5 1.25
56 0 0 0 0 0 0 0

18. S – Hydrograph :18
 S-Hydrograph or S-curve is a hydrograph that is produced by a continuous effective rainfall at a constant
rate for indefinite period.
 It is continuous rising curve, in the form of letter S, till equilibrium is reached.
 At the time of equilibrium, the S-curve will represent a runoff discharge given by:
 Qo = ( A*100*100) * Ro = A . Ro cumecs
100*3600 36
Where ,
A = area of catchment in hectares.
Ro = constant rate of effective rainfall (cm/hour).

19. 19
 If the catchment area A is in km^2, the discharge represent by S-curve at the time of equilibrium is given
by,
 Qo = ( A* 1000*1000 )*Ro = 2.78 ARo cumecs
100*3600
 Qo = 2.78 A cumecs
To
 Ro = 1
To
where, To = unit time duration

20. S – Hydrograph :20

21. Example:
21
 A 6-hr UH for a basin has the following oridnates. Detremine the ordinates of 12-hr UH
using s-curve method.
Time(hr.) Ordinates
of 6-hr UH
Offset
ordinatest1
= 6hr
S-curve
ordinates
S-curve
lagged by
12 hrs
Difference Ordinates
of 12-hr
UH
1 2 3 4 5 6=4-5 7=t1 / t2 *6
0 0 - 0 - 0 0
6 20 0 20 - 20 10
12 60 20 80 0 80 40
18 150 80 230 20 210 105
24 120 230 350 80 270 135
30 90 350 440 230 210 105
36 66 440 506 350 156 78
42 50 506 556 440 116 58
48 32 556 588 506 82 41
54 22 588 610 556 54 27
60 10 610 620 588 32 16
66 0 620 620 610 10 05

22. THANK YOU