TABLE 13. Coefficient of Runoff for Various Areas
Area Coefficient
Business:
Downtown 0.70-0.95
Neighborhood 0.50-0.70
Residential:
Single-family 0.30-0.50
Multiunits, detached 0.40-0.60
Multiunits, attached 0.60-0.75
Residential (suburban) 0.25-0.40
Apartment dwelling 0.50-0.70
Industrial:
Light industry 0.50-0.80
Heavy industry 0.60-0.90
Playgrounds 0.20-0.35
Railroad yards 0.20-0.40
Unimproved 0.10-0.3040 ft^3 /s (1.1 m^3 /s). Hence, the total runoff is 24 + 40 = 64 fWs (1.8 m^3 /s). This is 160 - 64
= 96 fWs (2.7 m^3 /s) less than when the lawn was not used.
Related Calculations. The time of concentration for any area being drained by
a sewer is the time required for the maximum runoff rate to develop. It is also defined as
the time for a drop of water to drain from the farthest point of the watershed to the sewer.
When rainfall continues for an extended period T min, the coefficient of impervious-
ness changes. For impervious surfaces such as watertight roofs, / = 77(8 + T). For im-
proved pervious surfaces, / = 0.377(20 + T). These relations can be used to compute the
coefficient in areas of heavy rainfall.
Equations for R for various areas of the United States are available in Steel—Water
Supply and Sewerage, McGraw-Hill. The Talbot formulas, however, are widely used and
have proved reliable.
The time of concentration for a given area can be approximated from t = 1(LISi^2 )^113
where L = distance of overland flow of the rainfall from the most remote part of the site,
ft; S = slope of the land, ft/ft; i = rainfall intensity, in/h; other symbols as before. For por-
tions of the flow carried in ditches, the time of flow to the inlet can be computed by using
the Manning formula.
Table 13 lists the coefficient of runoff for specific types of built-up and industrial ar-
eas. Use these coefficients in the same way as shown above. Tables 12 and 13 present
data developed by Kuichling and ASCE.
SIZING SEWER PIPES FOR VARIOUS
FLOWRATES
Determine the size, flow rate, and depth of flow from a 1000-ft (304.8-m) long sewer
which slopes 5 ft (1.5 m) between inlet and outlet and which must carry a flow of 5 mil-
lion gal/day (219.1 L/s). The sewer will flow about half full. Will this sewer provide the
desired flow rate?