Calculating the hydraulics of wastewater collection systems is very difficult. It requires solving equations for partly full and full pipes with pumps and various control structures, and conditions can change in the middle of running a simulation. As a result of these difficulties, numerous methods have been proposed to solve the equations describing collection system hydraulics. Bentley Systems wants to provide as many of these methods as possible so that the engineer analyzing collection system hydraulics can have the appropriate tools available for the problem at hand. This paper describes the methods available and how the Bentley storm and sanitary sewer modeling products address them.

1. www.bentley.com
What Is the Right Way
to Analyze Collection
System Hydraulics?
A Technical Briefing Note
By Dr. Thomas Walski
November 2012

2. What Is the Right Way to Analyze Collection System Hydraulics? 2
Bentley Systems wants to
provide as many of these
methods as possible so that the
engineer analyzing collection
system hydraulics can have the
appropriate tools available for
the problem at hand.
SUMMARY
With the V8i SELECTseries 3 release of Bentley’s storm and sanitary sewer
products, users have unparalleled flexibility in matching the hydraulic solver
with the problem of interest. What makes this all possible is the new file
format that all of the products now share.
Multiple Solvers
Background
Calculating the hydraulics of wastewater collection systems is very difficult. It
requires solving equations for partly full and full pipes with pumps and various
control structures, and conditions can change in the middle of running a simulation.
As a result of these difficulties, numerous methods have been proposed to solve the
equations describing collection system hydraulics. Bentley Systems wants to provide
as many of these methods as possible so that the engineer analyzing collection
system hydraulics can have the appropriate tools available for the problem at hand.
This paper describes the methods available and how the Bentley storm and sanitary
sewer modeling products address them.
Unified File Format
Prior to the V8i SELECTseries 3 release, each of Bentley’s storm and sanitary sewer
products had its own file schema and extension. These were:
• SewerGEMS - .swg
• CivilStorm - .csd
• SewerCAD - .swc
• StormCAD - .stc
These four file types have now been combined into a single file type
.stsw (storm and sanitary sewer). It is no longer necessary to import one type
of sewer file into another. All four products can open and save a .stsw file.
Relationship Between Solvers and Products
Bentley continues to support all four of its storm and sanitary products so that users
can buy the type of product that suits their respective needs (more information follows
to explain each solver). SewerGEMS is the most feature-rich product in that it supports
all four solvers. (In the past it was necessary to use SewerGEMS Sanitary to run the
gradually varied flow (GVF)-convex solver for SewerGEMS users. SewerGEMS Sanitary
is no longer needed since SewerGEMS now supports all solvers.)

3. What Is the Right Way to Analyze Collection System Hydraulics? 3
The other products retain the solvers that were developed for them with the addition
that CivilStorm now also supports the GVF-rational solver. The relationship between
solvers and products is summarized in the table below.
Any .stsw file can be opened from any product and edited. However, only the models
with the supported solvers can actually be calculated.
Overall Approaches
There are two overall approaches used to solve collection system hydraulic problem:
1. St. Venant equations.
The first approach solves the most theoretically correct St. Venant equations for
one dimensional flow with a free surface. They are sometimes called the Dynamic
Wave equations. They consist of a set of non-linear partial differential equations
(continuity and momentum) shown below:
Where A = flow area, t = time, Q = flow, x = distance, g = acceleration due to gravity,
0- = angle between pipe slope and horizontal, h = depth of flow, So = slope of channel,
Sx = slope of hydraulic grade line.
These equations must be solved simultaneously. They cannot be solved analytically,
and because of the nonlinear nature, they are difficult to solve numerically, especially
around transitions to pressure flow, pumps and control structures.
2. Hydrologic routing.
In most cases it is not necessary to solve the full St. Venant equations. Instead
flow through a collection system is divided into two types of calculations: flow
routing, which determines the flow in each pipe link, and hydraulic solutions,
which take the flow and determine depth, velocity, and other hydraulic properties.
There are numerous methods for hydrologic routing; these include convex, kinematic
wave, Muskingum, Puls, etc. Once the flow is known, the hydraulic properties are
usually calculated using either normal depth or GVF equations.
Figure 1: Summary of Relationship Between Solvers and Products.
Product/Solver Implicit GVF-convexExplicit GVF-rational
SewerGEMS
CivilStorm
SewerCAD
StormCAD
x x x x
x
x
x x
x

4. What Is the Right Way to Analyze Collection System Hydraulics? 4
Pumps and pressure pipes can be solved using a true pressure pipe solve based on
WaterGEMS. Unlike the St. Venant equations, the hydrologic routing methods and
pressure pipe solutions can be applied to steady-state as well as dynamic situations.
The simpler hydrologic routing methods route the flow downstream based on the
assumption that routing accounts for the attenuation of dynamic effects. In cases
where there are substantial backups in the collection systems, hydrologic routing
methods cannot accurately account for the extra flow attenuation, and the
St. Venant solvers should be used.
An additional compromise that a user must accept with hydrologic routing is that
flow splits must be modeled using a rating curve, as opposed to the St. Venant solvers,
which determine flow splits dynamically.
For a system with minimal backups and accurate rating curves for flow split (if they
occur), hydrologic routing and St. Venant solutions produce very similar results and
the hydrologic methods are faster and unconditionally stable.
Solvers Used in Bentley Models
In addition to two overall ways of posing collection system flow equations,
there are numerous ways of solving those equations. These are referred to
as “solvers” in the Bentley models.
The St. Venant equations are solved using finite difference methods that divide
time and distance into discrete approximations of the derivatives in the equations.
Even here there are different ways of setting up and solving these finite difference
equations. The two used by Bentley are:
Implicit solver:
based on implicit numerical methods developed for the FLDWAV model as modified
to account for conditions in collection systems such as drop manholes and transition
between gravity and pressure flow.
Explicit solver:
based on explicit numerical methods developed for the SWMM model as
adopted by Bentley.
The implicit solver is theoretically more stable and can use longer time steps but
there are situations in which the explicit solver can produce better solutions. The
nonlinear nature of the equations can lead to instability in either solver, especially
in situations where there are sudden changes in flow, such as pump starts and stops,
and weirs just beginning to overflow. The explicit solver has the ability to model
complex control logic and perform water quality analyses.
Both the implicit and explicit methods have been available in SewerGEMS
and CivilStorm since these models were developed.

5. What Is the Right Way to Analyze Collection System Hydraulics? 5
In the case of hydrologic methods, Bentley developed two different approaches
for sanitary sewers and storm sewers:
GVF-convex solver:
uses convex routing to determine flow and gradually varied flow (backwater analysis)
to determine hydraulic properties once the flow is known. The collection system
is first divided into gravity and pressure subnetworks. Convex routing is used to
determine flow in the gravity subnetworks, and the WaterGEMS pressure solver
is used to determine flow in the pressure subnetworks. Finally, GVF equations are
used to determine hydraulic grades and velocities. The GVF-convex solver is the
only Bentley solver that can perform both steady and unsteady analyses.
GVF-rational solver:
routes peak storm flows developed using the rational method and then calculates
the hydraulic properties based on those flows. The solver only solves for peak flows,
although there is a way to use rational method C values with dynamic flows
employing a “modified rational method.”
There are two additional solvers available when the explicit solver is selected.
These are the SWMM kinematic wave solution, which is a hydrologic routing
method available through the SWMM model, and the Uniform Flow solution,
which assumes all pipes are at normal depth and does no real flow routing. These
are simply calculation options in the Routing method property when the Explicit
solver is selected.
Which is the Right Solver to Use?
Each of the solvers mentioned above has its own particular strengths and is
more appropriate for a specific type of problem. The situations in which each
is preferred are described below.
Implicit and explicit solvers are best in studies of sewer system overflow, where
handling of flow splits dynamically or storage of water in pond is important.
They work best in systems that are primarily gravity flow, with pumping limited
to simple force mains without complex pressure hydraulics. The explicit solver
has the ability to handle control logic for gravity structures.
The GVF-convex solver is best for new collection system design, especially for
cases in which there is a good deal of pumping or extensive use of pressure sewers.
In general, these systems are designed to not overflow, so the calculation of
overflows and backups should not be important.
The GVF-rational solver is used for stormwater runoff from small areas in which
the assumptions underlying the rational method are valid. These would be typical
of subdivisions, industrial facilities, and commercial areas upstream of any ponds.
Once ponds are involved, one of the dynamic implicit or explicit solvers should be
used or PondPack for the pond analysis.
In addition to two overall ways
of posing collection system flow
equations, there are numerous
ways of solving those equations.
These are referred to as “solvers”
in the Bentley models.

6. What Is the Right Way to Analyze Collection System Hydraulics? 6
Switching Between Solvers
With the SELECTseries 3 version of SewerGEMS, it has become much easier to switch
between solvers in that solver selection is simply a matter of picking the desired solver
in the Calculation Option “Active Numerical Solver.” In general, switching between
solvers is easy as long as the user avoids some model features that are only handled
in a single solver, or are handled much differently between solvers. Complicated pump
controls and flow splits are two of the areas that can be troublesome when switching
solvers, especially between St. Venant and hydrologic routing methods.
A new dialog called the “Compute Center” has been added to enable the user
to easily keep track of the solver and important solver options, and to smoothly
switch between solvers.
Relationship With Hydrology Calculations
In models that involve rainfall-runoff hydrology calculations on catchments, such
as the previous versions of SewerGEMS or CivilStorm, there were limitations as to
which hydrology methods could be used with each hydraulic solver. Now, there are
few limitations in that regard.
It is useful to understand that hydrology calculations are carried out in two different portions
of the models. When the EPA-SWMM Runoff Method is selected or the node-based RTK
method is used (“Apply SWMM RTK Unit Hydrograph?” set to True), the calculations are
performed using methods from the SWMM model, while all other rainfall-runoff hydrology
calculations are performed with methods developed by Bentley.
The Compute Center can be accessed by picking Analysis > Compute Center.

7. What Is the Right Way to Analyze Collection System Hydraulics? 7
If a long-term simulation is to be conducted, any non-steady hydraulic solver can be
used, but it is recommended that one of the EPA SWMM methods be used for the
rainfall-runoff calculations.
Summary Product Table Comparison
To choose the product that best fits your modeling needs, please refer to the summary
comparison table below for sewer and stormwater network products, or contact your
sales representative for more information.
About Bentley Systems
Bentley is the global leader dedicated to providing architects, engineers,
geospatial professionals, constructors, and owner-operators with comprehensive
software solutions for sustaining infrastructure.
Additional information about Bentley’s water solution is available at
www.bentley.com/WTR and in Bentley’s annual report.
© 2012 Bentley Systems, Incorporated. Bentley, the “B” logo, SELECT, WaterGems, SewerCAD, SewerGems, StormCAD, and CivilStorm are either registered
or unregistered trademarks or service marks of Bentley Systems, Incorporated, or one of its direct or indirect wholly owned subsidiaries. Other brands
and product names are trademarks of their respective owners. 9136 10/12.
Summary Product Comparison
SewerGEMS SewerCAD CivilStorm StormCAD PondPack
Automatic sanitary sewer design x x
Sanitary sewer analysis x x x
Automatic storm sewer design x x x
Storm sewer analysis x x x Non surcharged
Inlet analysis and design x x x
Interconnected pond analysis and design x x x
Pond analysis and design x x x x
Pond outlet structure calculations x x x x x
(weir, orifice, riser, vortex valves,
user-defined rating table)
Automated pond and outlet design wizard x
Conduit control structure calculations x x x
(weir, orifice, depth vs. flow, exponential function)
Culvert analysis and design x x x x x
Open channel analysis and design x x x x x
Pump analysis and design x x x Simplified
Low impact development control x x x
analysis and design
Water quality analysis x x Simplified