Environmental Engineering FOURTH EDITION

Water Pollution 67

0.99

0.95

0.90

0.80

0.70

0.50

0.30

0.20

0.10

0.05

0.01

0.5 1 .o 2.0 5.0

Flow (rn3/s)

Figure 4-9. Plot of 7-day, 10-year low flows, for Example 4.2.

EXAMPLE 4.2. Calculate the 7-day, 10-year low flow given the data below.

Lowest flow Lowest flow in

7 consecutive Ranking order of severity

Year days (m3/s) (4 m/(n + 1) (m3/s)

1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977

1.2

1.3

0.8

1.4

0.6

0.4

0.8

1.4

1.2

1 .o

0.6

0.8

0.9

1 2 3 4 5 6 7 8 9

10

11

12

13

1/14 = 0.071

2/14 = 0.143

3/14 = 0.214

4/14 = 0.285

5/14 = 0.357

6/14 = 0.428

7/14 = 0.500

8/14 = 0.571

9/14 = 0.642

10/14 = 0.714

11/14 = 0.786

12/14 = 0.857

13/14 = 0.928

0.4

0.6

0.6

0.8

0.8

0.8

0.9

1 .o

1.2

1.2

1.3

1.4

1.4