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What Is the Right Way to Analyze Collection

System Hydraulics?A Technical Briefing Note

By Dr. Thomas Walski

November 2012

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.)

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 xx

x

x xx

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.

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.

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.

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 xanalysis and design

Water quality analysis x x Simplified