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15.2 SewerCAD 557
Each ordinate of the outflow hydrograph is calculated as follows:

1 1
O = I t-¢t + a1 - bI t (15.3)
t
c c
where
Q t outflow at current time step (L/s, gpm)
c convex routing coefficient
I t t inflow at previous time step (L/s, gpm)
I t inflow at current time step (L/s, gpm).

15.1.3 Hydrologic and Hydraulic Time Steps
SewerCAD uses two distinct time steps when running an extended-period simulation:

1. Hydrologic time step: This time step is used to calculate the routed hydrographs
and represents the time increment of all hydrographs generated during the analysis.
The hydrologic time step is also used as the calculation increment for the pressure
calculations.
2. Hydraulic time step: This time step represents how often the hydraulic calculations
are performed for gravity flow. Flows are interpolated from the previously gener-
ated hydrographs using the hydraulic time step, and are then used to perform the
gradually varied flow analysis for that time step.

The hydrologic time step should be less than or equal to the hydraulic time step, and
the hydraulic time step should be a multiple of the hydrologic time step.

15.2 SEWERCAD
SewerCAD is an easy-to-use program for the design and analysis of wastewater collection
systems. SewerCAD’s intuitive graphical interface and data exchange capabilities make it
easy to develop and load complex models of combined gravity and pressure networks.
Using SewerCAD, you can design and analyze the gravity portion of the system according
to either a gradually varied flow calculation or a standard capacity analysis.
The automatic design capabilities of SewerCAD allow you to design the system based
on user-defined constraints for velocity, cover, and slope. You can also design for partially
full pipes, multiple parallel sections, maximum section size, and invert/crown-matching
criteria and also allow for drop structures. You can disable the design feature on a pipe-by-
pipe basis, to design all or only a portion of your system.
SewerCAD’s loading model provides complete support for all types of sanitary and
wet-weather loads. Sanitary loads are typically estimated based on a number of contribut-
ing units, with a specified average load per unit (such as 260 L/d per apartment resident).
The unit is typically a measure of population such as residents or employees; however,
loads can also be based on other criteria such as contributing area or user-defined counts of
items indicative of loading behavior. See Table 15.1 for a list of standard loading sources
and their associated average base loads.
SewerCAD’s unit sanitary load library is completely user customizable and supports
population, area, discharge, and count-based sanitary loads, as well as hydrographs (flow
versus time data) and load patterns. Wet-weather inflow can also be added to the model as
instantaneous loads, hydrographs, or base loads with associated patterns. Infiltration can
be computed based on pipe length, pipe diameter/length, pipe surface area, or unit count.