c. Principles of Design.
(1) Lateral sewers, branch sewers, and usually, main sewers are laid to
permit gravity flow of their contents (see Figure 1-4). Usually the slope is such that the
wastewater flow rate is 2 feet per second or more when the line is half full or full. This is
known as the "scouring velocity," below which solid material would tend to settle out in
the line. Sewer pipes are of sufficient size to carry a peak flow of about 3.7 times the
average flow expected from the area served. For gravity flow lines, sewer pipe of
vitrified tile; concrete; cement-asbestos; or bituminous-impregnated fiber may be used.
For force mains and stream crossings, cast-iron or cement-asbestos pipe is used.
Caulking of suitable material is used in the joints.
Figure 1-4. Good and bad grades for sewer lines.