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
increased velocity head or both. With
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
should be used.
A profile of the
practical point of view.
sewer system should be drawn as the
invert of the inlet pipe would be more
These steps will be
than 18 inches above the manhole floor
discussed in turn.
a drop connection should be provided.
11. SEWER LAYOUT DETERMINATION
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
utilized to reduce velocities, clear
development of economical site plans
often requires concurrent preliminary
effort for ditching along laterals and
planning of the sewer system.
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
Normally, the most practical
location would be along one side of the
substituted at every other manhole
In other cases they may be
Lampholes are only large
located behind the buildings midway
enough to permit lowering a lamp to
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.
construct laterals on each side of the
collection system unless analysis of
Main, trunk, and outfall
sewers should follow the most feasible
conclusively indicate it would be more
route to the point of disposal.
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.
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
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
c. Determine flow rates.