Hydrology & Water Resources Engineering

Subject Code:150602

Hytograph and Hydrograph Analysis1) Hytograph: 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 orRiver versus time.

1) Runoff: Runoff is that portion of rainfall that is not evaporated or infiltratedIn groundUnitDischarge m3/s, cusecs, cumecs.

Classification of Runoff: 1) Surface runoff2) Inter flow or subsurface flow3) Ground water flow or base flow

Runoff

Runoff is that portion of rainfall that is not evaporated or intercepted.

Factor Affecting Runoff

1)Intensity of Rainfall2) Soil Characteristics of Catchments3) Topography of the Catchments4) Shape and Size of Catchments5) Cultivation and Vegetative Cover in Catchments6) Geological Condition of Catchments7) Weather Condition

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

Drainage Basin Characteristics

Hydrograph:A hydrograph is a graphical plot of discharge of a natural stream or River versus time.

1) Effective Duration:It is the time between which the rainfall rate is more than the Infiltration rate.

2) Basin Lag (tp):It is the time between centroid of the net rainfall (Pnet) and thePeak (D) of the hydrograph.

3) Recession Time (tr):It is the duration of direct runoff after the end of effective durationof rainfall.

4) Time of concentration (tc):It is the time in hours taken by rain water that falls at the farthestPoint to reach the outlet of a basin.This time is equal to the time between point of inflection and ofEffective duration of rainfall.

Factors affecting the shape of hydrograph1) Intensity and duration of rainfall2) Size and shape of basin3) Nature and slope of basin4) Land use and Land cover5) Drainage density6) Type of soil and its infiltration rate7) Lakes, depressions8) Stream characteristics.

Separation of Base flow:1) Direct run-off:Direct run-off is that water which reaches the river shortly after itFalls 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.

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)

Propositions of the Unit Hydrograph:

1) 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.

Assumptions of Unit Hydrograph Theory:

1) The effective rainfall is uniformly distributed within its duration or specified period of time.2) 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.

Limitation of Unit Hydrograph Theory:

1) 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.

2) The unit hydrograph method can’t be applied when an appreciable portion of the storm precipitation falls as snow.

3) 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.

4) The principle of linearity is not partically valid for smaller and larger stroms.

5) The catchment having large storage like reservoir, lake, low area etc. affect the linear relationship and hence theory cannot be applied.

6) The principle of time invariance is valid only for specified time and condition of drainage basin.

7) 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.

Application of Unit Hydrograph Theory:The unit hydrograph method is used for the estimation of the maximum flood discharge of a stream. 1) Use of unit hydrograph for deriving a flood hydrograph resulting

from rainfall of unit duration.2) Use of unit hydrograph for deriving a flood hydrograph resulting

from a series of rainfalls each of same unit duration.3) Use of unit hydrograph for deriving a unit hydrograph of other

duration.

Ex.1 The ordinates of 3 hr unit hydrograph are given:

Time hr

0 3 6 9 12 15 18 21 24 27 30

OrdiCum.

0 10 25 20 16 12 9 7 5 3 0

Find the ordinate of a 6 hr unit hydrograph for the same basin Analytically. Also sketch this unit hydrograph. What is the peakValue of discharge in this unit hydrograph?

Time Hr. 1

1st 3 hr 2

2nd 3 hr 3

Ordi. 6 hr 2cm4 =2+3

Ordi. 6hr unit5 = 4/2

0 0 - 0 0

3 10 0 10 5

6 25 10 35 17.5

9 20 25 45 22.5

12 16 20 36 18

15 12 16 28 14

18 09 12 21 10.5

21 07 9 16 8

24 05 7 12 6

27 03 5 8 4

30 00 3 3 1.5

33 - 0 0 0

Graph….:

Ex.2 The ordinates of 8 hr unit hydrograph are given:

Time hr Ord. 8 hr Time hr Ordi. 8 hr

0 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

S- Hydrograph:S- hydrograph is hydrograph of direct surface discharge that wouldResult from successive storms each of which producing effectiveRainfall of 1 cm in unit duration.

S –hydrograph is therefore a continuously rising curve in the formOf letter S, till a constant value of discharge is reached.

Ex.1 A 6-hr UH for a basin has the ordinates:

Time hr 0 6 12 18 24 30 36 42 48 54 60 66

Ordi. 6hr (m3/s)

0 20 60 150 120 90 66 50 32 22 10 0

Determine the ordinates of 12-hr UH. Using S-curve method.

Time hr

Ordi. 6hr.

OffsetOrdi.T1 =6hr

S-curveOrdi.

S-curveLaggedT2 =12 hr

Diff. Ordi.12 hr.UH.

1 2 3 4 5 4-5 =6 6 * t1/t2

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

Time hr 1st 6-hr 2nd 6-hr Ordi. 12 hr2 cm

Ordi. 12 hr.UH

1 2 3 4 = 2+3 5 = 4/2

0 0 - 0 0

6 20 0 20 10

12 60 20 80 40

18 150 60 210 105

24 120 150 270 135

30 90 120 210 105

36 66 90 156 78

42 50 66 116 58

48 32 50 82 41

54 22 32 54 27

60 10 22 32 16

66 0 10 10 05

72 - 0 0 0

Alternative Method:

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 Time hr Ordi. UH

0 0 11 41

1 6 12 34

2 36 13 27

3 66 14 23

4 91 15 17

5 106 16 13

6 93 17 9

7 79 18 6

8 68 19 3

9 58 20 1

10 49 21 0

Time hr

Ordi. UH

OffsetT1 = 4 hrm3/s

S-Curve

S-CurveT2 =5 hr

Diff. Ordinates5 hr UH=6 x t1/t2

1 2 3 4 5 4-5=6

7

0 0 - 0 - 0 0

1 6 - 6 - 6 5

2 36 - 36 - 36 29

3 66 - 66 - 66 53

4 91 0 91 - 91 73

5 106 6 112 0 112 90

6 93 36 129 6 123 98

7 79 66 145 36 109 87

8 68 91 159 66 93 74

9 58 112 170 91 79 63

10 49 129 178 112 66 53

11 41 145 186 129 57 46

12 34

Prepared by, Dr Dhruvesh Patel

Prepared by, Dr Dhruvesh Patel

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