Page 548 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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506 Chapter 14 Design of Sewer Systems
built in place, some by tunneling. Hydraulically and structurally, they share the properties
of grade aqueducts.
Sewers are laid deep enough with adequate ground cover to:
1. Protect them against breakage by traffic shock.
2. Keep them from freezing.
3. Permit them to drain the lowest fixture in the premises served.
Common laying depths are 3 ft (0.90 m) below the basement floor and 11 ft (3.35 m)
below the top of building foundations (12 ft or more for basements in commercial dis-
tricts), together with an allowance of 0.3 in. per ft (2.5%) for the slope of the building
sewer. At this slope a 6-in. (150-mm) sewer flowing full will discharge about 300 gpm or
3
3
40 ft /min (1,135 L/min or 1.13 m /min) at a velocity of 3.5 ft/s (1.0 m/s). In the northern
United States, cellar depths range from 6 to 8 ft and frost depths from 4 to 6 ft (1.22 to 1.83 m).
A 2-ft (0.60-m) earth cover will cushion most shocks. The deep basements of tall buildings
are drained by ejectors or pumps.
As shown in Fig. 14.5, manholes are channeled to improve flow, and the entrance of
high-lying laterals is eased by constructing drop manholes rather than going to the expense
of lowering the last length of run. In their upper reaches, most sewers receive so little flow
that they are not self-cleaning and must be flushed from time to time. This is done by
1. Damming up the flow at a lower manhole and releasing the stored waters after the
sewer has almost filled
2. Suddenly pouring a large amount of water into an upstream manhole
3. Providing at the uppermost end of the line a flushing manhole that can be filled
with water through a fire hose attached to a nearby hydrant before a flap valve,
shear gate, or similar quick-opening device leading to the sewer is opened
4. Installing an automatic flush tank that fills slowly and discharges suddenly. Apart
from the cost and difficulties of maintenance, the danger of backflow from the
sewer into the water supply is a disadvantage of automatic flush tanks.

14.3 COLLECTION OF STORMWATERS

Much of the suspended load of solids entering storm drains is sand and gravel. Because fine
sand is moved along at velocities of 1 ft/s (0.3 m/s) or more and gravel at 2 ft/s (0.60 m/s) or
more, recommended minimum velocities are 2.5 to 3 ft/s (0.75 to 0.90 m/s), or about 0.5 ft/s
(0.15 m/s) more than for sanitary sewers. The following factors determine the capacity of
storm drains:
1. Intensity and duration of local rainstorms
2. Size and runoff characteristics of tributary areas
3. Economy of design, determined largely by the opportunity for quick discharge of
collected stormwaters into natural water courses.
Rate of storm runoff is ordinarily the governing factor in the hydraulic design of storm
drains. To prevent inundation of streets, walks, and yards and flooding of basements and
other low-lying structures, together with attendant inconvenience, traffic disruption, and
damage to property, storm sewers are made large enough to drain away, rapidly and without
becoming surcharged, the runoff from storms shown by experience to be of such intensity
and frequency as to be objectionable. The heavier the storm, the greater but less frequent the
potential inconvenience or damage; the higher the property values, the more sizable is the