1CC606 HYDROLOGY

CHAPTER 4HYDROGRAPH ANALYSIS

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Hydrograph

A hydrograph is a graph showing discharge/ velocity or other properties of water flow with respect to time

(Hidrograf ialah graf yang menunjukkan pelepasan / halaju atau harta lain aliran air terhadap masa)

There are several types of hydrograph which include Natural hydrographUnit hydrograph

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Natural hydrograph

A natural hydrograph is one recorded at a stream gauging site and is a finger print of the upstream drainage area’s response to rainfall

A B

C

D

E

Where,AB - baseflow recessionBC - rising limbCD - falling limbDE - baseflow recession

Figure : Component of streamflow hydrograph

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The component of natural hydrograph

STREAMFLOW HYDROGRAPH

Baseflow separation

Q(t) = Q0e-(t-

t0)/k

S(t) = kQ(t)

Baseflow separation techniques

Straight line method

Fixed base length method

Variable slope method

-Separate baseflow & direct runoff-Normal depletion curve or master baseflow recession curve

Straight line method

Draw a horizontal line segment (A‐B) from beginning of runoff to intersection with recession curve

Fixed base length method

Draw line segment (A – C) extending baseflow recession to a point directly below the

hydrograph peak

Draw line segment (C – D) connecting a point N time periods after the peak

N = number of days from the time to peak of hydrograph measured to the end of direct runoff (time between C-DA = watershed area

Variable slope method

Draw line segment (A‐C) extending baseflow recession to a point directly below the hydrograph

peak

Draw line segment B‐E extending baseflow recession backward to a point directly below the inflection point

Draw line segment (C‐E)

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Unit hydrograph

The hydrograph of direct runoff that results from 1-inch (or 1 unit) of excess precipitation spread uniformly in space and time over a watershed for a given duration.

Hydrograph unit adalah satu dyhrograph (hubungan kadaalir, Q dengan masa ) air larian yang dihasilkan dari satu unit hujan berkesan dalam satu temph tertentu yang diagihkan secara seragam dalam satu kawasan tadahan

The key points : 1-inch of EXCESS precipitation Spread uniformly over space - evenly over the watershed Uniformly in time - the excess rate is constant over the

time interval There is a given duration pertaining to the storm – NOT

the duration of flow!

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Usage uf UH

To get in relationship betweenprecipitation with runoff.

Provide continuous dischargerecords

Knowing the peak flow, total flowvolume and duration of flow for aproject.

Obtain information critical periodfor the permitted level flow.

As a tool to get something original

hidrograph rainfall.

Bagi mendapatkan hubungan hujan dangan air larian permukaan.

Memberikan rokod kadar alir secara berterusan

Mengetahui aliran puncak , jumlah aliran sispadu dan tempoh aliran bagi sesuatu projek.

Mendapatkan maklumat tempoh bagi aliran tahap kritikal yang dibenarkan.

Sebagai alat mendapatkan hidrograph asli sesuatu kejadian hujan.

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a) Total storm hydrographb) Hydrograph minus baseflow, rainfall minus lossesc) Hydrograph adjusted to be a 2-hr unit hydrograph

(a)

(b)

(c)

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Changing the Duration of UH

Very often, it will be necessary to change the duration of the unit hydrograph. Storms occur in all shapes (rainfall amount) and sizes (durations)

The most common method of altering the duration of a unit hydrograph is by the S-curve method.

The S-curve method involves continually lagging a unit hydrograph by its duration and adding the ordinates.

For the present example, the 6-hour unit hydrograph is continually lagged by 6 hours and the ordinates are added.

UH lagging/superposition method

UH for particular watershed is defined for specific duration, D of rainfall

The linear property of UH can be used to generate UH of larger duration

*Developed 2-hr UH by adding two 1-hr UHs. The second one lagged by 1hr, adding together and divide by 2

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Example1:

Based on data given below, convert 1-hr UH to 3-hr UH

Time (hr) UH (1-hr)

0 0

1 20

2 45

3 95

4 75

5 55

6 35

7 15

8 0

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Solution

Time (hr) UH (1-hr) Lagged 1-hr UH Lagged 1-hr UH

0 0 0 0

1 20 0 0

2 45 20 0

3 95 45 20

4 75 95 45

5 55 75 95

6 35 55 75

7 15 35 55

8 0 15 35

9 0 0 15

10 0 0 0

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Sum of all 1-hr UH 3-hr UH0 0.0

20 6.7 (20/3)65 (40+25) 21.7 (65/3)

160 (95+45+20)

0 (untill it become 0) 00 1 2 3 4 5 6 7 8 9 10

0

10

20

30

40

50

60

70

80

90

100

UH 1hrlagged 1-hr UHlagged 1-hr UH3-hr UH

Time

Q

[(added 3 1-hr UH) / 3]

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Exercise 1: State the steps to convert 1h UH to 3h UH (super posising/legging method)

Answer :

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S-curve method

Allows construction of UH of any durationAssume UH of duration, D is known and we

wish to generate UH for the same watershed with duration D’.

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S-curve method

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Example 1 : S-curve method

Convert the following tabulated 2-hr UH to a 3-hr UH using S

curve method. Time (hr) 2-hr UH0 01 752 2453 2954 2705 1956 957 708 459 20

10 0

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Solution

Time (hr) 2-hr UH 2-hr lagged UH's Sum

0 0 01 75 22 hr 752 245 0 4hr 2453 295 75 3704 270 245 0 5155 195 295 75 5656 95 270 245 0 6107 70 195 295 75 6358 45 95 270 245 0 6559 20 70 195 295 75 655

10 0 45 95 270 245 0 65511 20 70 195 295 75 655

SameValue or repeat

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Time (hr)S-curve

ordinateS-curve

lagged 3 hr Difference3-hr UH ordinate

0 0 0 01 75 75 50.02 245 245 163.33 370 0 370 246.74 515 75 440 293.35 565 245 320 213.36 610 370 240 160.07 635 515 120 80.08 655 565 90 60.09 655 610 45 30.0

10 655 635 20 13.311 655 655 0 0.0

A B C D E

B - C D X (2/3)

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0 1 2 3 4 5 6 7 8 9 10 110

100

200

300

400

500

600

700

0

75

245

370

515

565

610635

655 655 655 655

0

50

163.3

246.7

293.3

213.3

160

8060

3013

00

75

245

370

515

565

610635

655

S-curve

s-curvehydrographshifted s-curve

Q (

cfs

)

C

B

25

1 2 3 4 5 6 7 8 9 10 110

50

100

150

200

250

300

350

0

50

163.3

246.7

293.3

213.3

160

80

60

30

13.30

Hidrograph UH- 2h and UH -3h

UH-2J

UH-3J

Q(m

3/s

)

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Exercise 2 :

Jadual dibawah menunujukkan Ordinat UH – 2 jam bagi sebuah kawasan tadahan. Tukarkan tempoh Uh tersebut kepada UH-5 jam dengan menggunakan kaedah lengkungan S. Lakarkan graf UH dan lengkung S yang menunujukkan perubahan tersebut.

Time (hour) Ordinat UH- 2hr (ft3/s)

0 0

1 75

2 250

3 300

4 275

5 200

6 100

7 75

8 50

9 25

10 0

Anwer : Time (hr)

2-hr UH 2-hr lagged UH's

S-curve ordinate

S- curved lagged 6 hour

differences

6-hr UH

0 01 752

2453 2954 2705 1956 957

708 459 20

100

11 012 013 0

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Quiz 2

Based on data given below, convert 2-hr UH to 6-hr

UH.a) Super posising/legging method with graph

b) S curve method also with graph

Time (hr) 2-hr UH0 0

1 85

2 215

3 350

4 290

5 205

6 105

7 90

8 60

9 40

10 0

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Time (hr) 2-hr UH 2-hr lagged UH's

Sum 6-hr UH

0 01 852 2153 3504 2905 2056 1057 908 609 40

10 011 0

12 0

13 0

14 0

Super posising/legging method

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Time (hr) 2-hr UH 2-hr lagged UH's    

Sum  S-curve lagged 6 hr

differences 6-hr UH

0

0

 

1

85

 

2

215

 

3

350

 

4

290

 

5

205

 

6

105

 

7

90

 

8

60

 

9

40

 

10

0 11 0

12 0

13

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S curve method