Presentation on theme: "Hydrology & Water Resources Engineering"— Presentation transcript:
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 orRiver versus time.
3 1) Runoff:Runoff is that portion of rainfall that is not evaporated or infiltratedIn groundUnitDischarge m3/s, cusecs, cumecs.
4 Classification of Runoff: Surface runoffInter flow or subsurface flowGround water flow or base flow
5 RunoffRunoff is that portion of rainfall that is not evaporated or intercepted.Factor Affecting RunoffIntensity of RainfallSoil Characteristics of CatchmentsTopography of the CatchmentsShape and Size of CatchmentsCultivation and Vegetative Cover in CatchmentsGeological Condition of CatchmentsWeather Condition
8 Hydrograph:A hydrograph is a graphical plot of discharge of a natural stream orRiver versus time.Effective Duration:It is the time between which the rainfall rate is more than theInfiltration rate.
9 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.
10 Factors affecting the shape of hydrograph Intensity and duration of rainfallSize and shape of basinNature and slope of basinLand use and Land coverDrainage densityType of soil and its infiltration rateLakes, depressionsStream characteristics.
11 Separation of Base flow: 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.
12 Unit Hydrograph:A unit hydrograph is hydrograph representing 1 cm of runofffrom a rainfall of some unit duration and specific arealdistribution.Unit Hydrograph (UH) is defined as the hydrograph of surfacerunoff 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 whenit occurs.The direct runoff hydrograph does not depend upon the timewhen 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 thebase period.
14 Assumptions of Unit Hydrograph Theory: The effective rainfall is uniformly distributed within its durationor specified period of time.The effective rainfall is uniformly distributed over the entirearea of the drainage basin.3) The base or time duration of the hydrograph of direct run-off dueto 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 hydrographfor each pattern.
17 Application of Unit Hydrograph Theory: The unit hydrograph method is used for the estimation of themaximum 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 hr36912151821242730OrdiCum.1025201675Find the ordinate of a 6 hr unit hydrograph for the same basinAnalytically. Also sketch this unit hydrograph. What is the peakValue of discharge in this unit hydrograph?
21 Ex.2 The ordinates of 8 hr unit hydrograph are given: Time hrOrd. 8 hrOrdi. 8 hr0.04479.045.54857.0813.55242.01226.55631.01645.06022.02082.06414.024162.0689.528240.0726.632231.0764.036165.0802.040112.0841.0Obtain a 24 hr unit hydrograph with graph
22 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.
23 Ex.1 A 6-hr UH for a basin has the ordinates: Time hr612182430364248546066Ordi. 6hr (m3/s)201501209050322210Determine the ordinates of 12-hr UH. Using S-curve method.