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524 Chapter 14 Design of Sewer Systems

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Column 18 column 15 (245 to 285 gpcd) 1>(7.48 24 60 60) ft /s
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column 15 (245 to 285 gpcd) (1.54 10 ). For example, in Section a, 1080 265
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(1.54 10 ) 0.440 ft /s or 12.46 L/s.
Column 19 Sum of column 5 times rate of infiltration. For example, in Section a, 49
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0.09>100 0.044 ft /s or 1.246 L/s.
Column 20 Sum of columns 16 to 19.
Columns 21 to 29 record the size of sewer for required capacity and available or required grade
together with depth and velocity of flow. For example, in Section a, an 8-in. sewer laid on a
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grade of 6.8>850 0.008 or 0.8% will discharge Q 0.82 ft /s (23.22 L/s) at a velocity of
2.35 ft/s (0.716 m/s) when it flows full. Hence, for q>Q 0.795>0.82 0.971, d>D
0.796, v>V 1.16, or d 8 0.796 6.37 in. (160 mm) and v 2.35 1.16 2.72 ft/s
or 0.83 m/s.
Columns 28 through 31 are taken from profiles of streets and sewers.
Column 28, Section b, shows a drop in the manhole of (113.20 113.03) 0. 17 ft (0.052 m)
compared with Col. 28, Section a. This allows for a full drop of 0.17 12 2 in. (51 mm),
thus offsetting the increase in the diameter of the sewer from 8 to 10 in. (203 to 254 mm).
Column 32 arithmetic mean of columns 30 and 31.

14.10 CAPACITY DESIGN IN STORM DRAINAGE
As shown in a previous chapter, the rational method of estimating runoff from rainfall pro-
vides a common hydrologic basis for the capacity design of storm-drainage systems.
Rainfall rates are normally expressed in terms of in./h or mm/h and are available from the
nation’s weather bureau. All models of rainfall are empirical approximations based on pre-
vious records statistically interrelated, and are represented by the following general hyper-
bolic equation:

i X>(t Y) (U.S. Customary or SI Units) (14.2)
where i rate of rainfall, in in./h or mm/h; t total duration of the rain storm, in min; X a
constant determined from previous records; and Y another constant determined from previ-
ous records. Both X and Y vary with location, topography, and relative frequency of storms.
The axiom of surface runoff analysis and design can be represented by the following
empirical equation:
Q cia (U.S. Customary Units) (14.3)
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where Q runoff, ft /s; c a runoff coefficient; i intensity of rainfall, in./h; and A
drainage area from which runoff takes place, acres. A similar empirical equation using
the metric system is shown in below:

Q cia (SI Units) (14.4)
where Q runoff, in L/h; c a runoff coefficient; i intensity of rainfall, in mm/h; and
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A drainage area from which runoff takes place, in m . Another set of SI units can be: Q,
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2
m /s; i, m/h; and a, m .
The designer must arrive at the best possible estimates of c, the runoff-rainfall ratio,
and i, the rainfall intensity, with the area a being determined by measuring tributary sur-
faces. Because both c and i are variable in time, storm flows reaching a given point in the
drainage system are compounded of waters falling within the time of concentration.