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518 Chapter 14 Design of Sewer Systems
this instance, the sewer becomes deeper and deeper until it is more economical to lift the
sewage by placing a pumping station in the line. Specifically for Case a in Fig. 14.15, the
sewer grade is held at minimum (0.33%), and h h L(g s) 0.9 3(0.033 0.33)
2
1
1.8 ft or 0.55 m, the depth increasing by (1.8 0.9) 0.9 ft, or 0.27 m.
Case b is unusual in that the required sewer grade is the same as the street grade.
Therefore, the depth of the sewer remains unchanged.
Case c introduces a street grade steep enough to provide the required capacity in an 8-in.
(200-mm), rather than a 10-in. (250-mm), parallel conduit. Arriving at minimum depth, there
is no possibility of utilizing the available fall in part or as a whole to recover minimum depth
as in Cases d and e. The reduced pipe size becomes the sole profit from the steep street grade,
provided usually that the upstream sewer is also no greater than 8 in. (200 mm).
Case d aims at maximum reduction of excess depth by replacing a 10-in. (250-mm)
sewer on minimum grade or, in accordance with Eq. 14.1, s g (h h )>L. For h 0,
2
1
2
s 1.00 (0.9 0.0)>3 0.70%, which is more than the required minimum of 0.33%.
Hence the sewer can be brought back to minimum depth, or 7 ft.
Case e is like Case d, but full reduction to minimum depth is not attainable, because s
0.70 (1.9 0.0)>3 0.07%. This is less than the required minimum of 0.33%. Hence the
minimum grade must be used, and h 1.9 3(0.70 0.33) 0.8 ft (0.244 m).
2
Case f illustrates how high velocities can be avoided by introducing drop manholes on
steep slopes. Case f parallels Case d but provides a drop of 1.1 ft (0.335 m) to place the sewer
on minimum grade and give it minimum velocity. Such action is normal only when grades are
extraordinarily steep. Excessive drops and resulting excessive sewer depth can then be avoided
by breaking the drops into two or more steps through insertion of intermediate drop manholes.
Because the sewers flow nearly full, no attention needs to be paid to actual velocities
and depths of flow in these illustrative cases. Consideration of actual depths and velocities
of flow is generally restricted to the upper reaches of sewers flowing less than half full.
There, the designer may have to forgo self-cleaning grades for lateral sewers. Otherwise,
they might reach the main sewer at an elevation below that of the main itself, and the main
would have to be lowered to intercept them. Normally this would be expensive.
Generally speaking, the designer should try for the fullest possible exploitation of the
capacity of minimum-sized sewers before joining them to larger sewers. The implications
are demonstrated in Fig. 14.16. There Scheme a keeps lateral flows from joining the main

1 1 1 1 1
Lateral 2 4 6 8
1
2 1 4 1 6 1 8 10 19
Main
Scheme (a)

1 1 1 1 1
Lateral 1 1 1 1

2
3 3 7 3 11 3 15 3 19
Main
Scheme (b)

Figure 14.16 Relative Utilization of the
Capacity of Lateral Sewers.