Q

sec 360

where Q = peak flow (m3/s); A = drainage area (ha); C = runoff coefficient (weighted); I = rainfall intensity (mm/hr); 360 = Conversion constant.

The second method is to use tables and graphs that have metric units. When this method is used the coefficient of runoff is taken from Table 17.6. The time of concentration is found in Table 17.7. The rainfall intensity is taken from Figure 17.7.

Table 17.6. Recommended coefficient of runoff values for various selected land uses, SI units.

Hydrologie soils group*

Hydrologie soils group*

Table 17.6. Recommended coefficient of runoff values for various selected land uses, SI units.

Land use

Description

A

B

C

D

Cultivated Land

without conservation treatment

0.49

0.67

0.81

0.88

with conservation treatment

0.27

0.43

0.67

0.67

Pasture or Range Land Meadow

poor condition

0.38

0.63

0.78

0.84

good condition

0.25

0.51

0.65

good condition

0.41

0.61

Wood or Forest Land

thin stand, poor cover, no mulch

0.34

0.59

0.70

good cover

0.45

0.59

Open Spaces, Lawns, Parks, Golf

Courses, Cemeteries

Good Condition

grass cover on 75% or more

0.25

0.51

0.65

Fair Condition

grass cover on 50% to 75%

0.45

0.63

0.74

Commercial; and Business Area

85% impervious

0.84

0.90

0.93

0.96

Industrial Districts

72% impervious

0.67

0.81

0.88

0.92

Residential

average % impervious

Average Lot Size (acres)

1/8

65

0.59

0.76

0.86

0.90

1/4

38

0.29

0.55

0.70

0.80

1/3

30

0.49

0.67

0.78

1/2

25

0.45

0.65

0.76

1

20

0.41

0.63

0.74

Paved Areas

parking lots, roofs, driveways, etc

0.99

0.99

0.99

0.99

Streets and Roads

paved with curbs and storm sewers

0.99

0.99

0.99

0.99

gravel

0.57

0.76

0.84

0.88

dirt

0.49

0.69

0.80

0.84

Note: Values are based on NRCS (formerly the SCS) definitions and are average values. Source: Technical Manual for Land Use Regulation Program, Bureau of Inland and Coastal Regulations, Stream Encroachment Permits, New Jersey Department of Environmental Protection * Group A Soils: High infiltration (low runoff). Sand, loamy sand, or sandy loam. Infiltration rate > 0.3 inch/hr when wet.

Group B Soils: Moderate infiltration (moderate runoff). Silt loam or loam. Infiltration rate 0.15 to 0.3 inch/hr when wet.

Group C Soils: Low infiltration (moderate to high runoff). Sandy clay loam. Infiltration rate 0.05 to 0.15 inch/hr when wet.

Group D Soils: Very low infiltration (high runoff). Clay loam, silty clay loam, sandy clay, silty clay, or clay. Infiltration rate 0 to 0.05 inch/hr when wet.

Table 17.7. Time of concentration (minutes) SI units.

Drainage way length (m) Drainage way change _

in elevation (m) 100 150 200 250 500 1,000 1,500 2,000

1

4

6

8

11

23

55

85

120

2

3

4.5

6.5

8

18

40

65

95

5

2

3

4.5

6

13

29

44

65

10

1.5

2.4

3.5

4.5

9

22

36

50

15

1.25

2

3

4

8

20

33

42

20

0.25

1.75

2.5

3.5

7

17

27

1 50 100

50 40

250 200

1 50 100

50 40

s

1 5 20 30 45 60 Duration (minutes)

90 120

1 5 20 30 45 60 Duration (minutes)

90 120

FIGURE 17.7. Rainfall intensity, duration, and frequency for metric units.

Problem: Determine the peak rate of runoff for a 50 hectare cultivated field with conservation practices and soil group B. The drainage way has a change of elevation of 2 m and a length of 1,000 m. The design calls for a 25-year storm.

Solution:

m3 sec

0 0

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