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Chapter 15

Sewerage Systems: Modeling

and Computer Applications

The hydraulics in the gravity portion of sanitary and storm sewers are analyzed with the
same techniques used in open-channel flow. When the flow depth is constant, Manning’s
equation is used. When obstructions or changes in pipe slope exist, the gradually varied
flow analysis procedure is applied. The pressure portion of the sanitary sewer is analyzed
in the same manner as a water distribution system, as discussed in Chapters 6 and 7. The
primary difference is that flow is generally withdrawn from a water distribution network,
whereas in a sewer system, flow is injected into force mains.
In this chapter, SewerCAD is used as a demonstration for the application of modeling
and computer techniques in the sewer design process. More comprehensive reviews of
sewer design were introduced in Chapters 13 and 14.

15.1 EXTENDED-PERIOD SIMULATIONS

An extended-period simulation (EPS) model represents how a sewer network will behave
over time. This type of analysis allows the user to model how wet wells fill and drain;
pumps toggle on and off; and pressures, hydraulic grades, and flow rates change in re-
sponse to variable loading conditions and automatic control strategies formulated by the
modeler. EPS is a useful tool for assessing the hydraulic performance of alternative pump
and wet-well sizes.
The SewerCAD algorithm proceeds in a general downstream direction toward the out-
fall, following the procedure described here:

1. The analysis begins in the gravity portion of the network. The hydrographs enter
the gravity system and are successively routed and summed as the flows approach
the downstream wet well or outfall. Ultimately, the total inflow hydrograph to the
wet well is determined.
2. Knowing the inflow into the wet well, the pressure calculations for the force main
system bounded by the wet well are performed. In addition to flow velocities and
pressures, the levels in the wet well over time are determined.
3. The calculation then returns to the gravity portion of the network discussed in step 1.
The hydraulics and HGL (hydraulic grade line) profiles are calculated throughout the
gravity system for each time step using the known level of the wet well as the bound-
ary condition for the backwater analysis.

The process repeats, continuing through the system downstream of the pressure net-
work until an outlet is reached.
As a hydrograph is routed through a conduit, it undergoes changes in shape and tem-
poral distribution caused by translation and storage effects. SewerCAD uses one of two

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