Hydrology & Water Resources Engineering Subject Code:150602

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.

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

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

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

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. 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 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.

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.

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.

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: 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: 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.

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.

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.

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.

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?

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

Graph….:

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

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.

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.

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

Alternative Method: Time hr 1st 6-hr 2nd 6-hr Ordi. 12 hr 2 cm UH 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

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

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

Prepared by, Dr Dhruvesh Patel www.drdhruveshpatel.com Image Source: www.google.com