hydrograph & flood

7/29/2019 Hydrograph & Flood

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GOVT. ENGINEERING COLLEGE

AURANGABADP.T. CIVIL ENGI IIIYR.

Laxman Dhakane BE10S01P0

Amol Late BE10S01P014 Mukesh Tongire BE10S01P0


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Graphical representation of time (hours) versusdischarge (cfs or cms) at a particular point on astream or channel which drains the watershed

area


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Shape represents basin characteristics

Area

Shape

Stream pattern Land/channel properties/slopes

Infiltration capacity


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Rising Limb

Crest Segment

Falling Limb (Recession Curve)


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Lag Time Time interval from the center of mass of the rainfall-excess to

the peak of the resultant hydrograph

Time to Peak

Time interval from the start of the rainfall-excess to the peak ofthe resultant hydrograph

Rainfall Duration Time interval from the start to the end of rainfall-excess

Time of Concentration Time interval from the end of the rainfall-excess to the point of

inflection on the hydrograph recession curve or

Time required for runoff to travel from the hydraulically mostdistant point on the watershed to the point of interest


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Associated with the time duration andfrequency of the rainfall-excess producing it

If the duration D of the rainfall-excess is 30

minutes and the frequency is 2 years, thehydrograph is referred to as a 30-minute, 2-year frequency hydrograph


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Area under the hydrograph represents avolume of water (total volume of rainfall thatfell on the basin and appeared as runoff)

If the area under the hydrograph represents 1inch (or 1 cm) of rainfall-excess over the entiredrainage basin then the hydrograph is called aUNIT hydrograph


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Identical rainfalls with the same antecedentconditions produce identical hydrographs

The time bases of all hydrograph from rainfalls

of the same duration with the same antecedentconditions are equal

Superposition: If the storm duration is thesame, the hydrograph of a 2 storm is twice theamount of a 1 storm.


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0

200

400

600

800

1000

1200

0 5 10 15 20 25 30

Discharge(cfs)

Time (hrs)

Hydrographs

Unit Hydrograph

2" Rainfall Event


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Development can cause less infiltration and morerunoff. In addition, runoff velocities increase becauseof smoother surfaces.

The net effect is that development can increase peakflow, resulting in inadequately sized drainagestructures

Most storm water management regulations require thatpost-development peak runoff rates be equal to or lessthan the predevelopment peak runoff rates

This can be done by constructing detention basinswhich temporarily store the water

Hydrographs are used to design detention basins (timeis an issue)

Sometimes, only the peak runoff flow is needed.


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The S-Curve also known as S-hydrograph ishydrograph is produced by continuouseffective rain fall at constant rate for an infiniteperiod. It curves obtained by summation ofinfinite D-h unit hydrograph spaced D-h apart.Series of D-h hydrograph arranged with theirstarting points D-h apart. At any given time theordinates of the various curves occurring that

time coordinate summed up to obtain S-CURVE. A smooth curve through theseordinates result in an S-Shaped curve called S-curve


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By definition an S-curve is obtained by addinga string of D-h unit hydrographs each laggedby D-hours from one another . Further if Tb=base period of the unit hydrograph, addition ofonly Tb/D unit hydrograph are sufficient toobtain the S curve.


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0.00

10000.00

20000.00

30000.00

40000.00

50000.00

60000.00

010

20

30

40

50

60

70

80

90

100

110

120

Flow(cfs)

Time (hrs.)


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Prediction

This method provides a hydrograph thatpredicts the behavior of a flood from astorm of any duration


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