1 March 1997
(2) Velocity. Sewers will be designed to provide a minimum velocity of 0.60 meters per
second (2.0 feet per second) at the average daily flow, or average hourly flowrate, and a
minimum velocity of 0.75 to 1.05 m/s (2.5 to 3.5 fps) at the peak diurnal flowrate, as determined
in paragraph 3-1. When velocities drop below 0.30 m/s (1.0 fps) during periods of low flow,
organic solids suspended in the wastewater can be expected to settle out in the sewer.
Sufficient velocity (0.75 to 1.05 m/s (2.5 to 3.5 fps)) must be developed regularly, once or twice
daily as a minimum, to resuspend and flush out solids which may have been deposited during
low flows. A velocity of 0.75 m/s (2.5 fps) minimum is required to keep grit and sand suspended.
However, new sewers which are properly designed and constructed should contain only minor
quantities of grit or sand. Maximum velocity is set at 3.00 m/s (10.0 fps) in the event that grit
becomes a problem.
(3) Slope. Assuming uniform flow, the value of S in the Manning formula is equivalent to
the sewer invert slope. Pipe slopes must be sufficient to provide the required minimum velocities
and depths of cover on the pipe. Although it is desirable to install large trunk and interceptor
sewers on flat slopes to reduce excavation and construction costs, the resulting low velocities
may deposit objectionable solids in the pipe creating a buildup of hydrogen sulfide, and thus will
(4) Cover. Adequate cover must be provided for frost protection. Generally, a minimum
0.6 meters (2 feet) of earth will be required to protect the sewer against freezing. Where frost
penetrates to a considerably greater depth and lasts for an appreciable length of time, the wastes
may not contain sufficient heat to prevent the gradual cooling of surrounding earth and buildup of
an ice film inside the pipe. Under these conditions, greater cover will be required. Sufficient
cover must also be provided to protect the pipe against structural damage due to superimposed
surface loadings. Concentrated and uniformly distributed loads are discussed in chapter 5.
b. Design procedure. After a preliminary layout has been made, a tabulation will be prepared
in convenient form setting forth the following information for each sewer section:
- Designation of manholes by numerals or letters.
- Contributing populations - resident and nonresident.
- Design flows - average, daily peak, and extreme peak.
- Length of sewer.
- Invert elevations.
- Invert slope or gradient.
- Pipe diameter and roughness coefficient.
- Flow depths at design flows.
- Velocities at design flows.
- Depths of cover on the pipe - maximum and minimum.
c. Hydraulic profile. In most situations where small to medium sized gravity sewers are
installed in long runs, it will be safe to assume uniform flow throughout the entire length of
conduit. However, in cases where larger sewers, 600-millimeter (24-inch) diameter and above,
are constructed in runs of less than 30 meters (100 feet), and with a number of control sections