a smaller size than the inlet pipe due
d. Determine slope.
to a more favorable slope, it maybe
e. Choose pipe size.
necessary to lower the invert to the
pipe to provide for entry head or
f. Check actual velocity.
increased velocity head or both. With
this
exception,
the
practice
of
g. Determine invert elevations.
lowering the invert of the outlet pipe
to theoretically compensate for entry
In order to simplify this process a
head and similar losses in the manhole
worksheet such as the one in figure 12
is
of
questionable
value
from
a
should be used.
A profile of the
practical point of view.
Where the
sewer system should be drawn as the
invert of the inlet pipe would be more
design proceeds.
These steps will be
than 18 inches above the manhole floor
discussed in turn.
a drop connection should be provided.
11. SEWER LAYOUT DETERMINATION
e. Drop
manholes.
The
only
difference between the standard and
The development of final sewer plans
drop manhole is the difference of
must await the final site plan, the
elevation above the bottom of the
completion of field surveys, and to
manhole of the inverts of the entrance
some extent, the establishment of
and exit pipes.
The drop manhole is
floor
grades.
However,
the
utilized to reduce velocities, clear
development of economical site plans
and
reduce
construction
often requires concurrent preliminary
effort for ditching along laterals and
planning of the sewer system.
common sewers.
a. Location.
The
location
of
f. Lampholes.
If the only reason
lateral and branch sewers will depend
for placing a manhole is that is 400
not only upon topography but upon the
or 600 feet from the last one, and if
type and layout of the housing to be
the velocity is at least 3 feet per
served.
Normally, the most practical
second,
then
lampholes
may
be
location would be along one side of the
substituted at every other manhole
street.
In other cases they may be
location.
Lampholes are only large
located behind the buildings midway
enough to permit lowering a lamp to
between streets.
In closely built-up
give light for sewer inspection.
areas and particularly where the street
is very wide or already paved, it may
g. Pump stations.
Pumping should
be
advantageous
and
economical
to
be
avoided
in
the
design
of
a
construct laterals on each side of the
collection system unless analysis of
street.
Main, trunk, and outfall
the
initial
and
operating
costs
sewers should follow the most feasible
conclusively indicate it would be more
route to the point of disposal.
All
practical than gravity flow, with due
sewers should be located outside of
consideration given to the situation
roadways as much as practicable so that
which might result from pump failure.
the number of roadway crossings is
reduced to a minimum. A sewer from one
10. SEWER DESIGN
building should not be constructed
under another building or remain in
The steps in designing a sewer system
service
where
a
building
is
are as follows:
subsequently constructed over it if any
other practical location for the sewer
a. Determine sewer layout.
is available. Where no other location
is suitable, necessary measures should
b. Locate manholes.
be taken to assure accessibility for
future
excavation
and
complete
c. Determine flow rates.
4-17
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