1 March 1997
where nonuniform flow may occur, a plot of the hydraulic profile is recommended. For process
and plant piping at wastewater treatment facilities, a hydraulic profile is always required.
Methods used to calculate and plot hydraulic profiles including backwater curves, drawdown
curves and hydraulic jumps, will conform to those presented in standard hydraulics textbooks.
d. Critical flow. Gravity sewers will ordinarily be designed to maintain subcritical flow
conditions in the pipe throughout the normal range of design flows. However, there are
exceptions in which super critical flow may be required, and will be justified. Minimum sized
sewers (150- and 200-millimeter (6- and 8-inch)) designed to discharge very low flows, must
occasionally be placed on slopes steeper than critical in order to provide minimum velocities. In
addition, small to medium sized sewers when required to discharge unusually large flows, may
necessitate super critical slopes. Finally, steep slopes may be unavoidable due to natural
topography and ground conditions. Where super critical flow will occur, care must be taken in
the design to insure that downstream pipe conditions do not induce a hydraulic jump or other flow
disturbance. Depths of flow within 10 to 15 percent of critical are likely to be unstable and will be
avoided where pipes will flow from 50 to 90 percent full. Critical depths for various flows and
e. Computer programs. Numerous commercial computer programs are available for
modeling, data management (such as location, diameter, depth, slope, and capacity of each
sewer component), design, and analysis of sewer systems. As well as being used to design new
systems, such programs can assist in assessing impacts of changes and additions to existing
systems and thus help optimize designs. When the software interfaces with the design district's
or installation's GIS system (generally Intergraph Microstation or Autocad), three-dimensional
models allow designers to detect interference with other utilities.
3-3. REQUIRED PUMPING CAPACITY. Proper selection of the number and capacity of
pumping units is dependent upon the quantity and variation of wastewater flows to be handled.
Except as indicated below for small stations, pumping units will be selected to handle the normal
daily range of wastewater flows generated in the service area. The number and capacity of
pumps provided will be sufficient to discharge the minimum, average, peak daily and extreme
peak flowrates. Pumping capacity will be adequate to discharge the peak flowrates with the
largest pump out of service. Pumps utilized for treatment plant processes, recycling and
bypassing of flows, etc., will be based on criteria developed in TM 5-814-3/AFM 88-11, Vol. 3.
Consideration will be given to future conditions which may occur during the life of the station.
Normally, where future development and population increases are projected for the area, pumps
will be designed for initial conditions only, and the station will be provided adequate room for
expansion of pumping capacity at a later date. Expansion of pumping capacity can be
accomplished with the installation of additional pumping units, larger pumps, impellers, drive
units, adjustable or variable speed drives. However, some situations may warrant provision of
capacity for future increases initially, for economic or other reasons. Each case will be analyzed
a. Small stations. Pumping stations required for small remote areas which generate extreme
peak flowrates of less than 45 L/s (700 gpm), and where the possibility of future expansion is
unlikely, and grinder pump installations serving three or more buildings, will be provided with two
identical pumping units. Each pumping unit will be of the constant speed type, and will be
capable of discharging the extreme peak wastewater flowrate. The station will be designed to
alternate between zero discharge and peak discharge. This arrangement will provide 100
percent standby capacity to allow for necessary maintenance and repairs. Pneumatic ejector
stations will be provided with duplex ejectors each sized for the extreme peak flowrate.