Base Flow Separation

 Methods of Base Flow Separation

The surface-flow hydrograph is obtained from the total storm hydrograph by separating the quick-response flow from the slow response runoff. It is usual to consider the interflow as a part of the surface flow in view of its quick response. Thus only the base flow is to be deducted from the total storm hydrograph to obtain the surface flow hydrograph.  There are three methods of base-flow separation that are in common use.

 Method 1

In this method the separation of the base flow is achieved by joining with a straight line the beginning of the surface runoff to a point on the recession limb representing the end of the direct runoff.

 In Fig..1, pointA represents the beginning ofthe direct runoff off and it is usually easy to identify in view of the sharp change in the runoff rate at that point. Point B, marking the end of the direct runoff is rather difficult to locate exactly.

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Fig..1. Method 1 for base flow separation.

An empirical equation for the time interval N (days) from the peak to the point B is

Description: Description: 232.webp (23.1)

WhereA is drainage area in km2and N is in days. Points A and B are joined by a straight line to demarcate to the base flow and surface runoff. This method of base-flow separation is the simplest of all the three methods.

 

Method 2

In this method the base flow curve existing prior to the commencement of the surface runoff is extended till it intersects the ordinate drawn at the peak (point C in Fig. 23.2). This point is joined to point B by a straight line. Segment AC and CB demarcate the base flow and surface runoff. This is probably the most widely used base-flow separation procedure.

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Fig. 2. Method 2 for base flow separation.

 

 Method 3

In this method the base flow recession curve after the depletion of the flood water is extended backwards till it intersects the ordinate at the point of inflection (line EF in Fig. 23.3). Points A and F are joined by an arbitrary smooth curve. This method of base-flow separation is realistic in situations where the groundwater contributions are significant and reach the stream quickly.

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Fig. 3. Method 3 for base flow separation.

The surface runoff hydrograph obtained after the base-flow separation is also known as direct runoff hydrograph (DRH).

 

Example 1

The following are the ordinates of the hydrograph of flow from a catchment area of 770 km2 due to a 6-h rainfall. Derive the ordinates of DRH. Make suitable assumptions regarding the base flow.

Time from beginning of storm

(h)

0

6

12

18

24

30

36

Discharge

(m3/s)

42

65

215

360

400

350

270

Time from beginning of storm

(h)

42

48

54

60

66

72

Discharge

(m3/s)

205

145

100

70

50

42

 

Answer:

Given: catchment area (A) = 770 km2

Using equation 23.1,

Description: Description: 2324.webp

Description: Description: 235.webp

From given data, with our convenience, base flow = 42 m3/s at 72 h

Therefore, DRH = Flood Hydrograph – Base flow

 

Time from

beginning

of storm

Discharge

Base flow

DRH

h

m3/s

m3/s

m3/s

0

40

42

-2

6

65

42

23

12

215

42

173

18

360

42

318

24

400

42

358

30

350

42

308

36

270

42

228

42

205

42

163

48

145

42

103

54

100

42

58

60

70

42

28

66

50

42

8

72

42

42

0

Example 2

The daily stream flow data at a site having a drainage area of 6500 km2 are given in the following table. Separate the base flow using the above three methods.

Time (days)

Discharge (m3/s)

1

1600

2

1550

3

5000

4

11300

5

8600

6

6500

7

5000

8

3800

9

2800

10

2200

11

1850

12

1600

13

1330

14

1300

15

1280

 

Answer

1. Plot the total runoff hydrographMethod 1: join point A, the beginning of direct runoff, to point B, the end of direct runoff. Both points are selected by judgment.

Description: Description: 238.webp

 2. Method 2: Extend the recession curve before the storm up to point C below the peak. Join point C to D, computed using equation

Description: Description: 2325

Description: Description: 239.webp

 3. Method 3: Extend the recession curve backward to point E. Join point E to A

 

 Description: Description: 2310.webp

4. The ordinates DRH by three methods are given in Table

Table Ordinates of DRH by different methods

Time

Total runoff

Base flow

Direct runoff

Method 1

Method 2

Method 3

Method 1

Method 2

Method 3

(days)

(m3/s)

(m3/s)

(m3/s)

(m3/s)

(m3/s)

(m3/s)

(m3/s)

1

1600

1600

1600

1600

0

0

0

2

1550

1550

1550

1550

0

0

0

3

5000

1520

1480

1500

3480

3520

3500

4

11300

1500

1400

1450

9800

9900

9850

5

8600

1450

1700

1400

7150

6900

7200

6

6500

1450

1950

1400

5050

4550

5100

7

5000

1450

2300

1400

3550

2700

3600

8

3800

1400

2550

1400

2400

1250

2400

9

2800

1380

2800

1380

1420

0

1420

10

2200

1380

2200

1380

820

0

820

11

1850

1380

1850

1380

470

0

470

12

1600

1350

1600

1350

250

0

250

13

1330

1330

1330

1330

0

0

0

14

1300

1300

1300

1300

0

0

0

15

1280

1280

1280

1280

0

0

0

 Description: Description: 2312.webp

Effective Rainfall Hyetograph

Effective rainfall (also known as Excess rainfall) (ER) is that part of the rainfall that becomes direct runoff at the outlet of the watershed. It is thus the total rainfall in a given duration from which abstractions such as infiltration and initial losses are subtracted. For purposes of correlating DRH with the rainfall which produced the flow, the hyetograph of the rainfall is also pruned by deducting the losses. Figure 23.4 shows the hyetograph of a storm. The initial loss and infiltration losses are subtracted from it. The resulting hyetograph is known as effective rainfall hyetograph (ERH). It is also known as excess rainfall hyetograph.

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Fig.4.Effective rainfall hyetograph.

Both DRH and ERH represent the same total quantity but in different units. Since ERH is usually in cm/h plotted agains1 time, the area of ERH multiplied by the catchment area gives the total volume of direct runoff which is the same as the area of DRH. Theinitial loss and infiltration losses are estimated based on the available data of the catchment.

 

Example 3

A 4-hour storm occurs over an 80 km2 watershed. The details of the catchment are as follows:

Sub Area

Φ index

Hourly rain (mm)

km2

mm/h

1st hour

2nd hour

3rd hour

4th hour

15

10

16

48

22

10

25

15

16

42

20

8

35

21

12

40

18

6

5

16

15

42

18

8

 

Calculate the runoff from catchment and the hourly distribution of the effective rainfall whole catchment.

Answer:

Description: Description: 2315.webp

Totalrunoff = 2.46Mm3

Hourly distribution of the effective rainfall for the whole catchment:

 

Effective rainfall (mm)

1st hour

1.4375

2nd hour

25.375

3rd hour

0

4th hour

3.9375

 

Example 4

A storm in a certain catchment had three successive 6-h intervals of rainfall magnitude of 3.0 cm, 5.0 cm and 4.0 cm, respectively. The flood hydrograph at the outlet of the catchment resulting from this storm is as follows:

Time

(h)

0

6

12

18

24

30

36

42

Flood hydrograph ordinates

(m3/s)

30

480

2060

4450

6010

6010

5080

3996

Time

(h)

48

54

60

66

72

78

Flood hydrograph ordinates

(m3/s)

2866

1866

1060

500

170

30

 

If the area of the catchment is 8791.2 km2, estimate the index of the storm. Assume the base flow as 30 m3/s.

 

Answer

Flood hydrograph ordinates = DRH ordinates +Base flow ordinates

Direct runoff(cm) = Description: Description: 2318.webp

Where

is direct runoff ordinates (m3/s),  is time interval between successive ordinates (h), A is catchment area (km2)

Time

Flood hydrograph Ordinates

Base flow

DRO

H

m3/s

m3/s

m3/s

0

30

30

0

6

480

30

450

12

2060

30

2030

18

4450

30

4420

24

6010

30

5980

30

6010

30

5980

36

5080

30

5050

42

3996

30

3966

48

2866

30

2836

54

1866

30

1836

60

1060

30

1030

66

500

30

470

72

170

30

140

78

30

30

0

= 34188

 

Therefore,

Direct runoff (cm) = Description: Description: 2320.webp

Directrunoff (cm)  = 8.4

Therefore

Description: Description: 2321.webp

φ = 0.2cm/h 

 

 

Rainfall (cm)

3

5

4

Time interval (h)

6

6

6

Rainfall intensity (cm/h)

0.5

0.833

0.667

 index (cm/h)

0.2

0.2

0.2

Excess rainfall intensity

0.3

0.633

0.467

 

 Elemental Hydrograph

If a small, impervious area is subjected to a constant rate rainfall, the resulting runoff hydrograph will appear much as above, and is known as elemental hydrograph (Fig. 23.5). In the beginning, there will be surface detention (rainfall-runoff) so as to start the sheet flow over the surface. At point B, known as point of equilibrium, outflow rate equals inflow rate. When rainfall ends (at C), recession starts, i.e., outflow rate and detention volume increases.

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Fig. 5.Elemental hydrograph.