from an average of 30 to 300 ppm to less than 5 ppm; at the third site PCBs were reduced
from 40 ppm to less than 3 ppm.STORM-WATER RUNOFF RATE
AND RAINFALL INTENSITY
What is the storm-water runoff rate from a 40-acre (1.6-km^2 ) industrial site having an im-
perviousness of 50 percent if the time of concentration is 15 min? What would be the ef-
fect of planting a lawn over 75 percent of the site?
Calculation Procedure:- Compute the hourly rate of rainfall
Two common relations, called the Talbot formulas, used to compute the hourly rate of
rainfall R in/h are R = 360/(/ + 30) for the heaviest storms and R = W5/(t + 15) for ordi-
nary storms, where t = time of concentration, min. Using the equation for the heaviest
storms because this relation gives a larger flow rate and produces a more conservative de-
sign, we see R = 3607(15 + 30) = 8 in/h (0.05 mm/s). - Compute the storm-water runoff rate
Apply the rational method to compute the runoff rate. This method uses the relation Q =
AIR, where Q = storm-water runoff rate, fWs; A = area served by sewer, acres; /= coeffi-
cient of runoff or percentage of imperviousness of the area; other symbols as before. So Q
= (40)(0.50)(8) = 160 fWs (4.5 m^3 /s). - Compute the effect of changed imperviousness
Planting a lawn on a large part of the site will increase the imperviousness of the soil.
This means that less rainwater will reach the sewer because the coefficient of impervious-
ness of a lawn is lower. Table 12 lists typical coefficients of imperviousness for various
surfaces. This tabulation shows that the coefficient for lawns varies from 0.05 to 0.25. Us-
ing a value of I= 0.10 for the 40(0.75) = 30 acres of lawn, we have Q = (3O)(0.10)(S) = 24
fWs (0.68 m^3 /s).
The runoff for the remaining 10 acres (40,460 m^2 ) is, as in step 2, Q = (10)(0.5)(8) =
TABLE 12. Coefficient of Runoff for Various Surfaces
Surface Coefficient
Parks, gardens, lawns, meadows 0.05-0.25
Gravel roads and walks 0.15-0.30
Macadamized roadways 0.25-0.60
Inferior block pavements with uncemented 0.40—0.50
joints
Stone, brick, and wood-block pavements with 0.75-0.85
tightly cemented joints
Same with uncemented joints 0.50-0.70
Asphaltic pavements in good condition 0.8 5-0.90
Watertight roof surfaces 0.70-0.95