Table Of ContentW
ASTEWATER
C S
OLLECTION YSTEM
M D
ODELING AND ESIGN
F i r s t E d i t i o n
Authors
Bentley Systems
Thomas M. Walski
Thomas E. Barnard
Eric Harold
LaVere B. Merritt
Noah Walker
Brian E. Whitman
Managing Editor
Thomas E. Barnard
Project Editors
Kristen Dietrich, Adam Strafaci, and Colleen Totz
Contributing Authors
Christine Hill, Gordon McKay,
Stan Plante, Barbara A. Schmitz
Exton, Pennsylvania USA
WASTEWATER COLLECTION SYSTEM MODELING AND DESIGN
First Edition
Copyright © 2007 by Bentley Institute Press
Bentley Systems, Incorporated.
685 Stockton Drive
Exton, Pennsylvania 19341
www.bentley.com
Copyright © 2004 by Haestad Press
Haestad Methods, Incorporated
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Watertown, Connecticut 06795
www.haestad.com
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Indexer: Beaver Wood Associates and Ann Drinan
Proofreading: Beaver Wood Associates
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Page 3 – (1989) Bernard Schoenbaum Page 255 – (2002) Frank Cotham
Page 27 – (2000) Mike Twohy Page 283 – (1971) Chon Day
Page 53 – (1991) Robert Mankoff Page 295 – (2002) Mick Stevens
Page 82 – (1994) Leo Cullum Page 339 – (1999) George Booth
Page 94 – (1988) James Stevenson Page 351 – (1987) Donald Reilly
Page 101 – (1970) James Stevenson Page 393 – (1993) Donald Reilly
Page 108 – (1997) Danny Shanahan Page 400 – (1999) Mort Gerberg
Page 115 – (1994) Bruce Eric Kaplan Page 444 – (1997) Leo Cullum
Page 129 – (1994) Edward Koren Page 458 – (1988) Mischa Richter
Page 142 – (1999) Donald Reilly Page 481 – (2002) Barbara Smaller
Page 159 – (1991) George Booth Page 534 – (1994) Mischa Richter
Page 185 – (1992) Mick Stevens Page 541 – (2001) Donald Reilly
Page 193 – (1995) Al Ross Page 548 – (1997) Leo Cullum
Page 235 – (1989) Mick Stevens
ClientCare, FlowMaster, HAMMER, SewerCAD, StormCAD, WaterCAD, and WaterGEMS are trademarks, service marks,
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Library of Congress Control Number: 2007925242
ISBN: 978-1-934493-03-8
Bentley Systems, Inc. Phone (US): 800-225-2613
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Preface
Wastewater conveyance facilities are usually an overlooked part of urban infrastruc-
ture-that is, until they do not perform well. Then they receive the wrong kind of atten-
tion. Good designers and operators of such systems keep their systems quietly,
efficiently working out of the publicʹs eye. This book is intended to help those individ-
uals who design and operate wastewater collection systems do so effectively. It con-
siders the range of wastewater collection system types, from strictly sanitary sewer
systems to combined systems.
There are many aspects of wastewater collection systems including regulatory, inven-
tory, personnel, and property ownership. This book focuses on the hydraulic aspects
of collection systems. The hydraulic calculations needed for good design and analysis
of systems are very cumbersome and nearly impossible to perform manually. As a
result, use of computer models has become the standard for collection system work.
While this book concentrates on hydraulic models of wastewater collection systems, it
is not solely a book on modeling, but one that takes the reader from basic hydraulics
to model building, load estimation, data collection, calibration, design, and rehabilita-
tion of systems. Even though the emphasis is on gravity systems, the book also covers
force mains and pressure sewers, use of Geographical Information Systems (GIS), and
regulatory aspects of collection systems.
The process of modeling of a collection system varies from one situation to another.
Tools and assumptions that work when designing a new system for a land develop-
ment project are different from those used to analyze overflows from an aging urban
system. The book seeks to capture that breadth. Collection system modeling is such a
diverse subject; it took several authors and several years to do justice to the topic.
Writing this book was especially challenging in that we had so many authors and
reviewers, and there are so many ways to approach collections system modeling.
Engineers designing new systems have a much different perspective than those ana-
lyzing old systems with overflow problems. It was not uncommon for an author to
write about topic x, and another author to comment, ʺWhy are we discussing topic x?
No one does it that way.ʺ So weʹd delete it and a reviewer would say, ʺWhy donʹt you
discuss topic x? Thatʹs how I solve these problems.ʺ There is really no single ʺrightʺ
way to do things, so we tried to cover as much ground as possible.
Chapter Overview
Chapter 1 provides general background information on wastewater collection sys-
tems and lays the historical groundwork for the remainder of the book.
xii Preface
Chapter 2 presents steady gravity flow hydraulic theory from the most basic princi-
ples to somewhat more advanced topics such as tractive forces and gradually varied
flow.
Chapter 3 presents the basics of unsteady flow, from the most theoretically correct
approach using the full Saint-Venant equations to various simplifications of those
equations. It includes issues posed by appurtenances such as manholes and problems
caused by surcharging in pipes that normally flow partly full.
Not all wastewater collection system pipes can flow by gravity. The principles of
pumping and pressurized flow are presented in Chapter 4.
With the hydraulic principles covered in the earlier chapters, Chapter 5 continues
with an overview of the process of creating hydraulic models for a variety of situa-
tions. It then discusses the practical aspects of obtaining and using the data needed for
sound hydraulic analyses.
Design and analysis of collection systems are driven by the loads placed on the sys-
tem. Dry weather loads, presented in Chapter 6, are especially important in new sani-
tary systems, and a variety of methods for estimating them are discussed. Wet
weather flows (Chapter 7) tend to be most important in combined sewer systems and
older sanitary systems with a good deal of infiltration and inflow. These contributions
to loading can be as simple as unit loads or as complicated as hydrologic runoff mod-
els.
Models need data for calibration and verification, and the methods to collect this type
of flow and precipitation data are presented in Chapter 8. When working with exist-
ing systems, Chapter 9 describes how these measurements are used to calibrate and
validate the model so that model behavior closely mimics the behavior of the real col-
lection system over a range of conditions.
Chapter 10 presents how new collection systems are designed, with an emphasis on
the application of hydraulic models.
With most existing systems, the concern is not so much with installing new sewers but
in assessing the capacity and performance of the systems and using modeling to
answer questions about the available capacity and the need for rehabilitation. Chapter
11 describes the requirements for this type of study, which are considerably different
from those encountered when installing new sewers.
There are two ways in which pumping and pressure pipes are used in collection sys-
tems. The first consists of force mains and pumping stations as described in Chapter
12, while the second is a pressure sewer system with individual pumps at each cus-
tomer as described in Chapter 13.
Utilities are increasingly relying on GIS data for model building. Chapter 14 discusses
use of GIS data in wastewater modeling.
Collection systems must be designed and operated in compliance with increasingly
strict regulations. An overview of regulatory practices is presented in Chapter 15.
Convention
While this book is entitled Wastewater Collection System Modeling and Design and the
word ʺwastewaterʺ is used frequently in the book, some prefer the term ʺsewage.ʺ
xiii
These words are used interchangeably throughout the text, as are the terms ʺwastewa-
ter collection systemsʺ and ʺsewage systems.ʺ The word ʺsewerʺ is used to describe
the piping in the systems.
For units, English units are used first, with equivalent metric values given alongside.
There is an extensive set of unit conversion tables in Appendix B.
Continuing Education and Problem Sets
Also included in this text are approximately 100 hydraulics and modeling problems
to give students and professionals the opportunity to apply the material covered in
each chapter. Some of these problems have short answers, and others require more
thought and may have more than one solution. The accompanying CD in the back of
the book contains an academic version of Bentley Systems SewerCAD soft-ware (see
ʺAbout the Softwareʺ on page xxi), which can be used to solve many of the problems,
as well as data files with much of the given information in the problems pre-entered.
However, we have endeavored to make this book a valuable resource to all
modelers, including those who may be using other software packages, so these data
files are merely a convenience, not a necessity.
Bentley also publishes a solutions guide that is available to instructors and
professionals.
We hope that you find this culmination of our efforts and experience to be a core
resource in your engineering library, and wish you the best with your modeling
endeavors.
Thomas M. Walski, PhD, PE, DEE
Vice President of Engineering and Product Development
Haestad Methods, Inc.
Foreword
Libraries are wondrous institutions. Rows and rows of books create a feeling of awe
as they rise high above the floors of the great libraries of the world, such as the Biblio-
thèque Nationale in Paris, The John P. Robarts Research Library at the University of
Toronto, and the Library of Congress in Washington, D.C. But, the true magnificence
of libraries lies in their ability to set in motion imagination and excitement in learning.
The books and other materials they house provide the opportunity to understand and
benefit from what others have experienced, and expand our ability to make informed
decisions. Indeed, the mission statement of the Library of Congress states the
Library’s aim to “sustain and preserve a universal collection of knowledge and cre-
ativity for future generations.”
Study of the decisions, actions, and results of others’ past activities under conditions
analogous to one’s own situation helps in predicting the results of current decisions.
Social science scholars study human behavior and develop hypotheses or explana-
tions of what to expect from humans in certain group interactions. Military scientists
explore the principles that control the conduct of war, and then apply those principles
to battle conditions. Scientists study the research and experiments of other scientists
to better understand the causes and effects of material, force, and environment varia-
tion. Each of these examples represents the premise of simulation and model software
development, which is that the past helps predict the future when combined with sci-
entific or mathematical principles.
Like libraries, models can activate the imagination and generate excitement because
they provide information and knowledge that, in turn, enables more informed deci-
sions or solutions. Decision makers use modeling tools to simulate actual conditions
and generate performance and functional assessments significantly more quickly than
with real-time observations. Generally, the only technical restriction on what can be
simulated is the complexity and power of the model.
The value and benefit of using models is increasingly being recognized. In the waste-
water industry, utilities use hydraulic models in all phases of the collection and treat-
ment system’s life cycle: planning, design, construction, and operation. Hydraulic
models allow costly and complicated collection systems to be comprehensively evalu-
ated prior to expending a communityʹs limited resources to solve service or regula-
tory issues. Proposed infrastructure components can be assessed against the risk of
not achieving a utility’s mission and goals, or of incurring harmful health and envi-
ronment consequences due to poor or unexpected performance.
Once wastewater collection and treatment systems have been built and put into ser-
vice, their performance can be measured. Utility decision makers can then use the
measurement information to gauge earlier decisions and expectations. Knowledge
gained from past experience will aid the utility in achieving future goals, and can
assist others in similar situations if documented and exchanged in technical publica-
tions, manuals, or reference books.
This book has done an excellent job of capturing and communicating the knowledge
of several experienced hydraulic modelers. It presents a good, balanced perspective of
the alternatives and informational needs of a modeler starting a modeling project,
including how to build and assemble the hydraulic components and apply the soft-
ware tool efficiently.
Undoubtedly, this book too will appear in libraries and be on the shelves of hydraulic
modelers who want to gain from the knowledge and experiences of their peers and
predecessors.
Reggie Rowe, PE
CH2M Hill
Table of Contents
Preface xi
About the Software xv
Chapter 1 Introduction to Wastewater Collection System Modeling 1
1.1 Wastewater Collection System Overview 1
Terminology..............................................................................................2
Sources of Wastewater............................................................................2
Types of Conveyance...............................................................................3
1.2 Modeling 5
Applications of Collection System Models..........................................5
Types of Collection System Modeling..................................................7
1.3 Historical Perspective on Collection System Analysis 7
Collection Systems...................................................................................7
Hydraulics History................................................................................11
Historical Summary..............................................................................16
1.4 The Modeling Process 16
Chapter 2 Steady Gravity Flow Hydraulics 23
2.1 Fluid Properties 24
Density and Specific Weight................................................................24
Viscosity..................................................................................................24
Fluid Compressibility............................................................................27
Vapor Pressure.......................................................................................27
2.2 Fluid Statics and Dynamics 28
Static Pressure........................................................................................28
Absolute Pressure and Gauge Pressure.............................................29
Velocity and Flow..................................................................................30
Reynolds Number..................................................................................31
Velocity Profiles.....................................................................................31
2.3 Fundamental Laws 32
Conservation of Mass............................................................................32
ii Table of Contents
Conservation of Energy........................................................................33
Conservation of Momentum................................................................36
2.4 Hydraulic Design Variables 38
Flow Rate or Discharge.........................................................................38
Channel/Pipe Slope................................................................................38
Depth of Flow.........................................................................................38
Velocity....................................................................................................39
2.5 Energy and Head Losses 40
Energy Equation.....................................................................................41
2.6 Hydraulic Elements 44
Open-Top Cross Sections......................................................................44
Closed-Top Cross Sections....................................................................46
Noncircular Cross Sections...................................................................49
2.7 Manning’s n Variation 49
Calculating n with the Darcy-Weisbach Equation............................50
Variation of n with Depth.....................................................................52
Recommended Values of Manning’s n................................................53
2.8 Minor Losses in Junction Structures 54
Energy-Loss Method.............................................................................55
Composite Energy-Loss Method.........................................................60
2.9 Tractive Force Self-Cleansing 63
Tractive Tension.....................................................................................63
Sediment Carrying Capacity – Experimental Analysis....................65
Camp Formula........................................................................................69
Yao’s Method...........................................................................................69
Abwassertechnische Vereinigung (ATV) Method.............................71
Additional Considerations...................................................................71
2.10 Specific Energy and Critical Flow 72
Specific Energy.......................................................................................72
Froude Number......................................................................................74
Subcritical and Supercritical Flow.......................................................74
Hydraulic Jumps....................................................................................75
Flow Profiles...........................................................................................76
Backwater Curves..................................................................................77
2.11 Hydraulics of Flow-Control Structures 78
Orifices.....................................................................................................79
Weirs........................................................................................................79
Gates........................................................................................................84
Chapter 3 Unsteady Gravity Flow Hydraulics 91
3.1 Basics of Unsteady Flow Analysis 93
3.2 Types of Routing 94
3.3 Hydrodynamic Equations 95
Saint-Venant Equations.........................................................................95
Approximation to Hydrodynamic Equations...................................96
iii
Diffusion Analogy.................................................................................97
Kinematic Wave.....................................................................................97
Muskingum Routing.............................................................................98
Muskingum-Cunge Routing................................................................98
Convex Routing...................................................................................100
Weighted Translational Routing........................................................100
Level Pool Routing..............................................................................100
Summary of Methods..........................................................................101
3.4 Complications to Routing Methods 101
Manholes and Junction Tables...........................................................103
Surcharging..........................................................................................103
Overflows and Diversions..................................................................104
Parallel Pipes and Loops.....................................................................105
Flow Reversal.......................................................................................106
Dry Pipes...............................................................................................107
Drop Structures....................................................................................107
Chapter 4 Force Main and Pumping Hydraulics 113
4.1 Friction Losses 113
Darcy-Weisbach Equation..................................................................116
Colebrook-White Equation and the Moody Diagram....................117
Hazen-Williams Equation..................................................................118
Swamee-Jain Equation........................................................................120
Manning Equation...............................................................................120
Pipe Roughness Changes...................................................................121
Comparison of Friction Loss Methods.............................................121
4.2 Minor Losses 122
Minor Loss Valve Coefficients...........................................................123
4.3 Energy Addition – Pumps 125
Pump Head-Discharge Relationship................................................125
System Head Curves...........................................................................126
Other Pump Characteristic Curves...................................................128
Fixed-Speed and Variable-Speed Pumps.........................................129
Affinity Laws for Variable-Speed Pumps........................................129
Power and Efficiency...........................................................................130
Chapter 5 Model Construction 137
5.1 Developing the Modeling Plan 137
5.2 The Modeling Process 138
Purpose and Objectives of a Model...................................................138
Develop Alternatives...........................................................................141
Scales of Models...................................................................................141
Software Selection and Training........................................................143
Define Data Requirements.................................................................143
Identify Data Sources..........................................................................144
