HYDRAULIC ANALYSIS OF THE UPPER NORTHWEST INTERCEPTOR SECTIONS 5
AND 6
Deana Lynn Donohue, P.E.
B.S., California State University, San Jose, 1996
PROJECT
Submitted in partial satisfaction of
the requirements for the degree of
MASTER OF SCIENCE
in
CIVIL ENGINEERING
(Water Resources)
at
CALIFORNIA STATE UNIVERSITY, SACRAMENTO
FALL
2009
© 2009
Deana Lynn Donohue, P.E.
ALL RIGHTS RESERVED
ii
HYDRAULIC ANALYSIS OF THE UPPERNORTHWEST INTERCEPTOR SECTIONS 5
AND 6
A Project
by
Deana Lynn Donohue, P.E.
Approved by:
__________________________________, Committee Chair
Ralph Hwang, Ph.D., P.E.,F ASCE
__________________________________, Second Reader
Dave Ocenosak, P.E. Principal Civil Engineer
______________________
Date
iii
Student: Deana Lynn Donohue, P.E.
I certify that this student has met the requirements for format contained in the University format
manual, and that this project is suitable for shelving in the Library and credit is to be awarded for
the project.
__________________________, Graduate Coordinator
Cyrus Aryani, Ph.D, P.E., G.E.
Department of Civil Engineering
iv
___________________
Date
Abstract
of
HYDRAULIC ANALYSIS OF THE UPPERNORTHWEST INTERCEPTOR SECTIONS 5
AND 6
by
Deana Lynn Donohue, P.E.
Project Overview
The Upper Northwest Interceptor Sections 5 and 6 (UNWI 5/6) are part of a regional
gravity sewer system designed and constructed to relieve the capacity deficiencies in the
north eastern portion of Sacramento County. Sacramento Regional County Sanitation
District (SRCSD) defines an interceptor as any pipe that conveys greater than 10 million
gallons per day (mgd) of sewage. The UNWI system conveys flows to the regional
wastewater treatment plant in Elk Grove, located in the southwestern portion of
Sacramento County.
UNWI 5/6 is a 5.5-mile gravity interceptor located along Elkhorn Boulevard starting at
Don Julio Boulevard and terminating at Cherry Lane to the west.
The interceptor
diameter is 48 and 66-inch to convey an ultimate flow of 80.6 mgd. Currently, the
interceptor conveys much less flow than 80.6 mgd and interim flows were considered
during design. This project also includes the design of two hydraulic control structures to
v
channelize and direct flow during low flow periods in the near-term and redirect flow
during periods of maintenance or high flow.
This paper will describe the hydraulic design criteria and process for design of UNWI 5/6
project components including the interceptor and hydraulic control structures.
This
interceptor project has been constructed and is currently in operation.
Information Source
The information included in this report is the actual design criteria and calculations.
Brown and Caldwell was the engineering consultant and contracted to design this project
for SRCSD. I was the project engineer for the design team and participated during both
design and construction.
Final Design Summary
Construction of the UNWI 5/6 project was completed in 2006 and is currently
operational. The hydraulic analysis for this interceptor project was comprehensive and
included considerations for several flow scenarios, hydraulic jumps, sedimentation, and
odor control. The hydraulic design was completed to address near and long-term flow
requirements to operate under a varied range of flows. The final design addresses the
upstream capacity deficiencies and allows for relief of the northern Sacramento area
while the remaining downstream sections under construction. Flow monitoring data is
not available, however, details on the current operation of the project was related during
vi
discussions with the SRCSD Principal Engineer.
Section 4.
______________________, Committee Chair
Ralph Hwang, Ph.D., P.E., F ASCE
______________________
Date
vii
These results are summarized in
ACKNOWLEDGMENTS
I would like to thank my sons, Conrad and Clinton, for their patience and understanding
during the long hours of studying, reading, and lecture time. I hope that they are inspired
by my dedication to education.
I would also like to acknowledge and thank the Civil Engineering Department faculty and
staff at the California State University, Sacramento. I appreciate the dedication and
patience of the staff including Neysa Bush, Professor Ramzi Mahmood, and Ed Dammel.
Finally, I want to express my sincere appreciation and gratitude to Professor Ralph
Hwang for his knowledge, his dedication to me as a student, and his guidance as I
completed the graduate program.
viii
TABLE OF CONTENTS
Page
Abstract ...................................................................................................................................... v
Acknowledgments...................................................................................................................... viii
List of Tables ............................................................................................................................. xi
List of Figures ............................................................................................................................ xii
Chapter
1. INTRODUCTION ............................................................................................................... 1
1.1 Project Background ................................................................................................. 4
1.2 Upper Northwest Project Description ..................................................................... 5
1.2.1 Dry Creek Interceptor .................................................................................... 6
1.2.2 Hydraulic Transitions..................................................................................... 8
1.2.3 Interim Dry Creek Interceptor Connection Alternative Comparison ............. 10
2. INTERCEPTOR OPERATIONAL DESIGN CRITERIA................................................... 14
2.1 Hydraulic Flow Path ............................................................................................... 14
2.2 Pipe and Structure Lining ....................................................................................... 16
2.3 Pipe Slope ............................................................................................................... 16
2.4 Maximum Operational Flexibility .......................................................................... 18
2.5 Safe Maintenance Access ....................................................................................... 18
3. INTERCEPTOR FLOW EVALUATION ........................................................................... 20
3.1 Basis of Wastewater Flow ...................................................................................... 20
3.2 Wastewater Flow .................................................................................................... 22
4. INTERCEPTOR DESIGN EVALUATION ........................................................................ 24
ix
TABLE OF CONTENTS
Page
4.1 Hydraulic Criteria ................................................................................................... 24
4.2 Hydraulic Analysis ................................................................................................. 25
4.2.1 Shearing Stress ............................................................................................... 25
4.2.2 Slope Evaluation ............................................................................................ 26
4.3 Hydraulic Control Structures .................................................................................. 34
4.3.1 UNWI 4/5 Junction Structure ........................................................................ 37
4.3.2 28th Street Junction Structure ......................................................................... 52
5. PROJECT CONCLUSIONS ................................................................................................ 54
Appendices................................................................................................................................. 56
Appendix A. UNWI 4/5 Junction Structure Drawings ............................................................ 57
Appendix B. 28th Street Junction Structure Drawings ............................................................. 65
Appendix C. County Design Criteria ....................................................................................... 67
References .................................................................................................................................. 69
x
LIST OF TABLES
Page
1. Table 1 Comparison of Alternatives for Crossing Dry Creek............................................. 13
2. Table 2 UNWI 5/6 Projected Wastewater Flows ................................................................ 22
3. Table 3 Slope Analysis- 28th Street to Don Julio ................................................................ 31
4. Table 4 Slope Analysis- Cherry Lane to 28th Street............................................................ 32
5. Table 5 Summary of Design Slopes by Reach .................................................................... 33
6. Table 6 Hydraulic Characteristics of 27 and 30-inch Pipes ................................................ 44
7. Table 7 Weir Calculation Results ....................................................................................... 47
8. Table 8 HGL and Water Depths in the UNWI 4/5 Structure and Pump Wet Well ............ 50
xi
LIST OF FIGURES
Page
1. Figure 1 Project Location Map – Northeastern Sacramento County .................................. 2
2. Figure 2 Upper Northwest Interceptor Sections 5 and 6 ..................................................... 3
3. Figure 3 Local Interceptor Layout ...................................................................................... 7
4. Figure 4 Interceptor and Trunk Sewer Flow Insertion Points ............................................. 23
5. Figure 5 Open Channel Flow in Pipe Flowing Less than Full ............................................ 28
6. Figure 6 Visual Basic Program for Calculating Flow Depth .............................................. 29
7. Figure 7 Sample Calculation Spreadsheet .......................................................................... 30
8. Figure 8 28th Street Junction Structure to Existing Elkhorn Structure ................................ 36
9. Figure 9 Schematic of Interim Design Flow HGL along Segment A ................................. 40
10. Figure 10 Weir Elevation at the UNWI 4/5 Junction Structure .......................................... 46
11. Figure 11 Schematic of Interim Design Flow HGL along Segment A ............................... 52
12. Figure A-1 UNWI 4/5 Junction Structure Plan................................................................... 58
13. Figure A-2 UNWI 4/5 Junction Structure Top Plan ........................................................... 59
14. Figure A-3 UNWI 4/5 Junction Structure Section 1 ........................................................... 60
15. Figure A-4 UNWI 4/5 Junction Structure Section 2 ........................................................... 61
16. Figure A-5 UNWI 4/5 Junction Structure Section 3 ........................................................... 62
17. Figure A-6 UNWI 4/5 Junction Structure Section 4 ........................................................... 63
18. Figure A-7 Existing Elkhorn Blvd Junction Structure Modifications Sta. 1+00 ................ 64
19. Figure B-1 Dry Creek Relief Interceptor Junction Structure Plans and Section ................. 66
20. Figure C-1 Design Flow Criteria ........................................................................................ 68
xii
1
Chapter 1
INTRODUCTION
The hydraulic analysis for Sacramento Regional County Sanitation District (SRCSD)
Upper Northwest Interceptor Sections 5 and 6 (UNWI 5/6) design project is
systematically presented in this report. The purpose for this paper is to discuss the
hydraulic design and evaluate the project hydraulic design to the operation of the
completed project.
This UNWI system serves to convey wastewater from northern portion of Sacramento
County to the regional wastewater treatment plant in the southwestern portion of
Sacramento County.
The UNWI system is one of the largest interceptor projects
undertaken by the County of Sacramento and cost over $300 million to design, build, and
construct. I was the project engineer for UNWI Sections 5 and 6; I assisted with the
design and provided technical assistance during the construction process.
The UNWI 5/6 Project is located in the northeastern portion of Sacramento County. A
general area map is depicted on Figure 1.
2
UNWI 5/6 Location
Figure 1 Project Location Map- Northeastern Sacramento County
The interceptor alignment is highlighted in the vicinity and location map shown on
Figure 2.
UNWI Section 6 is located along Elkhorn Boulevard from Don Julio
Boulevard to the 28th Street Structure and UNWI 5 continues along Elkhorn Boulevard
from the 28th Street Structure to the UNWI 4/5 Junction Structure. The interceptor is 48inch diameter along UNWI Section 6 and changes diameter to 66-inch at the 28th Street
Junction Structure where a large amount of flow (12 mgd) enters the system.
3
UNWI 4/5 Junction
Structure
28th Street Junction
Structure
UNWI 7 Transition
Structure
Figure 2. Upper Northwest Interceptor Section 5 and 6
UNWI Sections 5/6 involved many aspects of hydraulic aspects during design including:
wastewater flow projection, slope analysis, hydraulic jump analysis, gravity flow, pump
station design, and weir design.
This report emphasizes the assumptions made and design criteria established for UNWI
5/6 and presents the hydraulic analysis process completed for the design of this project.
The following information is provided in this report:

Interceptor Design Criteria,

Hydraulic analysis of the alignment and projected flows, and

Hydraulic analysis of the junction structures.
4
1.1 Project Background
The northern and northeastern portions of the Sacramento County have experienced
extensive growth and the regional interceptor system requires increased capacity to
accommodate the increased sewer flows. UNWI 5/6 is part of the larger Upper Northwest
Interceptor regional sewer system that was identified in the 1993 Sacramento Sewerage
Expansion Study [1]1 to provide relief capacity to existing collection systems that are
served by the SRCSD interceptor system. SRCSD is the agency that provides the major
infrastructure that conveys all the sewage in Sacramento County to be treated at the
regional wastewater treatment plant located in Elk Grove.
The Upper Northwest
Interceptor alignment and flow requirements were evaluated in the Upper Northwest
Interceptor Design Report [2]. The UNWI system was divided into 9 sections for final
design to allow multiple design efforts to occur simultaneously.
1
Denotes reference stated in the References, all references will be called out using this format.
5
1.2 Upper Northwest Project Description
The Upper Northwest Interceptor Sections 5/6 are part of a regional gravity sewer system
designed and constructed to relieve the capacity deficiencies in the northern portion of
Sacramento County.
The UNWI system conveys flows to the regional wastewater
treatment plant in Elk Grove, located in the southwestern portion of Sacramento County.
UNWI 5/6 is a 5.5-mile gravity interceptor is the most upstream gravity section of this
regional system and begins at a transition structure located at the intersection of Elkhorn
Boulevard and Don Julio Boulevard. The interceptor continues along Elkhorn Boulevard
to the intersection of Cherry Lane and Elkhorn Boulevard. The interceptor diameter is 48
and 66-inch to convey an ultimate flow of 80.6 mgd. The UNWI 5/6 accepts flows from
several trunk sewers and a smaller interceptor, Dry Creek Interceptor.
6
1.2.1
Dry Creek Interceptor
The Dry Creek Interceptor extends along the south fork of Dry Creek turning
south across Elkhorn Blvd near Cherry Lane. The Dry Creek interceptor is
currently at capacity and the UNWI 5/6 is designed and constructed, in part, to
relief flows in this interceptor. Since Upper Northwest Interceptor Sections 1
through 4, which are downstream from UNWI Sections 5/6, will not be completed
until approximately the year 2010, interim flows will be directed through the
existing southern portions of the Dry Creek Interceptor by the existing Elkhorn
junction structure.
The Dry Creek Interceptor extends along the south fork of Dry Creek turning
south across Elkhorn Blvd near Cherry Lane. The Dry Creek interceptor is
currently at capacity and the UNWI 5/6 is designed and constructed, in part, to
relief flows in this interceptor. Since Upper Northwest Interceptor Sections 1
through 4, which are downstream from UNWI Sections 5/6, will not be completed
until approximately the year 2010, interim flows will be directed through the
existing southern portions of the Dry Creek Interceptor by the existing Elkhorn
7
junction structure. A schematic of the interceptors in this area are presented in
Figure 32.
Figure 3 Local Interceptor Layout
The existing Elkhorn Junction Structure is located on the north side of Elkhorn
Blvd at the north fork of Dry Creek and combines flow from the Rio Linda
Collector with Dry Creek Interceptor. This structure was constructed with a 30inch RCP pipe stub on the east side to collect additional future flows. The 30-
2
Map provided by SRCSD
8
inch pipe stub was used for the interim connection between UNWI 5 and Dry
Creek Interceptor. Once UNWI 1 through 4 are operational, UNWI 5 will be
connected to UNWI 4. The Upper Dry Creek Interceptor (upstream of Elkhorn
Blvd.) will then flow into UNWI 4 and continues to the Natomas Pumping
Station.
The existing Elkhorn Junction Structure is located on the north side of Elkhorn
Blvd at the north fork of Dry Creek and combines flow from the Rio Linda
Collector with Dry Creek Interceptor. This structure was constructed with a 30inch RCP pipe stub on the east side to collect additional future flows. The 30inch pipe stub was used for the interim connection between UNWI 5 and Dry
Creek Interceptor. Once UNWI 1 through 4 are operational, UNWI 5 will be
connected to UNWI 4. The Upper Dry Creek Interceptor (upstream of Elkhorn
Blvd.) will then flow into UNWI 4 and continues to the Natomas Pumping
Station.
1.2.2
Hydraulic Transitions
There are 2 hydraulic control structures required for UNWI 5/6 including:

Upper Northwest 4/5 Junction Structure

28th Street Junction Structure
9
These hydraulic control structures will divert and channelize flow for a smooth
transition of flow. These junction structures are summarized below.
UNWI 4/5 Junction Structure
An interim junction structure is required to divert UNWI flow into the Lower Dry
Creek Interceptor at Dry Creek and Cherry Lane. This structure is needed in the
interim until UNWI 1 through 4 are constructed and flow can continue directly to
the Natomas Pumping Station. The structure must collect flow from Upper Dry
Creek Interceptor as well as UNWI 5. In the interim it must be able to divert both
interceptor flow streams to the Lower Dry Creek Interceptor and ultimately be
able to divert flow down the future UNWI 4. The final design configuration of
this structure is shown on Figures A-1 through A-6 and provided in Appendix A.
The plan views are shown on Figures A-1 and A-2 and the Sections view of the
structure are shown on Figures A-3 through Figure A-3.
This design
configuration incorporated the existing 30-inch stub on the existing Elkhorn
Junction Structure by extending that pipe to the new interim structure and also
utilizes the existing 27-inch pipe on the Upper Dry Creek Interceptor.
The
existing Elkhorn Junction Structure plan view is shown in Figure A-7. Under
ultimate PWWF the 27-inch pipe and the 30-inch pipe will flow under a
surcharged condition.
10
28th Street Junction Structure
This structure will combine flow from the Dry Creek Relief Interceptor into
UNWI 5 at the intersection of 28th Street and Elkhorn Boulevard. The future
relief interceptor will be a 24-inch gravity pipeline and will enter the junction
structure with matched hydraulic grade lines with UNWI 5 under PWWF. Under
initial conditions with lower flows, there will be a fall into the structure. This
structure layout is shown on Figure B-1 provided in Appendix B and was
designed as part of this project, however, due to delays during construction of the
UNWI Sections 5 and 6 this structure will be constructed as part of the Upper Dry
Creek Relief Project.
1.2.3
Interim Dry Creek Interceptor Connection Alternative Comparison
Three alternatives for the design of the interim connection of UNWI 5 with the
Dry Creek Interceptor were evaluated and are described below. Each option
includes using an interim hydraulic junction structure on the Upper Dry Creek
Interceptor.
Alternative 1 -- Match Hydraulic Grade Line
Alternative 1 will consist of the 66-inch UNWI 5 constructed such that its HGL
at the ultimate PWWF is the same as that of the existing 27-inch Dry Creek
Interceptor. The 30-inch stub in the existing Elkhorn Junction Structure would be
11
extended along Dry Creek Interceptor and, together with the existing 27-inch
pipe, would send all UNWI 5 flow into the Lower Dry Creek Interceptor. By
matching inverts, the 66-inch UNWI 5 would cross beneath the south fork of Dry
Creek approximately 1000 feet upstream of the interim junction structure with
approximately 8 feet of cover.
This alternative provides the greatest amount of ground cover of all of the
identified alternatives; however, the invert of UNWI 5 would be lower than Dry
Creek Interceptor by 3.5 feet and ponding of sewage would occur for approximately
2,700 feet upstream of the structure requiring maintenance efforts to prevent odors.
Alternative 2 -- Match Pipe Inverts
The configuration for Alternative 2 will consist of the same interim junction
structure as Alternative 1 except the invert of UNWI 5 would match that of Dry
Creek Interceptor. UNWI 5 would cross beneath the south fork of Dry Creek
with approximately 6.5 feet of cover which is less than required 10 feet by the
Department of Fish and Game for environmental protection. This option was not
chosen since an adequate amount of depth below Dry Creek was not achieved.
Alternative 3 -- UNWI over Dry Creek Interceptor
12
Under Alternative 3, the 66-inch UNWI 5 would be constructed over the existing
Dry Creek Interceptor. The interim junction structure would house the crossing
and the interim UNWI sewer flows would be sent to the Lower Dry Creek
Interceptor via the existing 27-inch pipe and an extension of the 30-inch stub.
Alternative 3 was not chosen since this option did not allow flexibility of
diverting flows through the existing Lower Dry Creek Interceptor since the
structure would not be connected to the existing infrastructure. Additionally,
where the UNWI 5 pipe crossed Dry Creek, the pipe was in the scour zone of the
creek since the ground cover was not sufficient.
The advantages and disadvantages for each alternative are summarized in Table
1.
Alternative 1 was selected for final design to maximize the hydraulic design
options for diverting flows to the existing infrastructure during interim flow
conditions.
13
Table 1 Comparison of Alternatives for Crossing Dry Creek
Alternative
1
Advantages


2

Greatest amount of
cover over 66 inch
pipe
Flexibility of diverting
flows to existing
infrastructure

Smooth hydraulic flow
line




3

Constructed over
existing interceptor




Disadvantages
Ponding of
wastewater
requiring odor
control
Creek Crossing
Less cover over
66 inch pipe
Less flexibility for
diverting flows to
existing
infrastructure
Creek Crossing
Depth of pipe
cover was not
sufficient
More expensive
structure
Flatter slope
No flexibility for
diverting flows to
existing
infrastructure
Comment
Selected Option
to ensure
greatest
hydraulic
flexibility for
design of
structure and
piping
14
Chapter 2
INTERCEPTOR OPERATIONAL DESIGN CRITERIA
A summary of the design criteria for the Upper Northwest Interceptor Sections 5/6 and
the evaluations conducted that provided the basis of design is presented in this section.
The design criteria adopted for this project are a compilation of the Sacramento Regional
County Sanitation District (SRCSD) design standards, along with the project team’s
experience in designing and constructing major sewer interceptors. The operational
design criteria are established to ensure that the SRCSD operational requirements are met
during design for all flow scenarios considered.
The design for UNWI 5/6 is based on several criteria that focus on the ability to construct
the project and to provide a long-term facility requiring infrequent maintenance. Criteria
that were considered include hydraulic flow path, operational flexibility, and safe
maintenance access.
2.1 Hydraulic Flow Path
A smooth hydraulic flow path along the interceptor will minimize turbulence and offgassing that reduces flow capacity and overall odors in the interceptor. Additionally, the
hydraulic flow path is impacted by many design factors including lining, slopes, curves,
and existing utilities.
15
Sedimentation
Sedimentation of gravel or other objects along the interceptor would reduce flow capacity
over time and require maintenance for removal. An analysis of the shearing stress
required to prevent sedimentation was completed during design of the interceptor. The
interceptor slopes were designed to establish a minimum velocity of 2 fps during periods
of flow and to provide the minimum shearing stress to prevent sedimentation as discussed
in Section 3.4.1.
Odor Control
Air movement in large pipelines is sometimes a poorly handled phenomenon and is often
overlooked. Improper air handling can create concentrated odors and adverse hydraulic
effects when the entrapped air is eventually vented. For this reason, proper air passage
was assured throughout the length of the pipeline. Specific areas of concern are air
connections across drop structures and siphons and areas of hydraulic jumps where the
tail water may inundate the pipe impeding the flow of air.
The design of this interceptor
provided for adequate air flow along the pipeline and provided continuity of air flow at
structures.
UNWI Section 6 connects to UNWI 7 at a transition structure located approximately 300
feet west of the intersection of Elkhorn and Don Julio Boulevards. UNWI 7 is a force
16
main that passes through the site of the SRCSD Corporation Yard for the northern
collection/conveyance system at the northeast corner of Don Julio Blvd and Elkhorn
Blvd. Upstream of UNWI 7 is the Van Maren Pump Station. At that location, an odor
control dosing station was constructed which suppresses sulfide development in UNWI
5/6 to reduce odors. That odor control system was designed as part of the Van Maren
Pump Station so there is no odor control system included in the UNWI 5/6 design.
2.2 Pipe and Structure Lining
The installed RCP interceptor and hydraulic control structures were lined with PVC for
protection against hydrogen sulfide corrosion.
The PVC liner covers the full inner
circumference inside the interceptor. The liner was not be welded along the flow line to
allow for dissipation of groundwater from behind the PVC liner.
All structures were lined with PVC and portions of the structures that could not be
effectively lined by T-lock PVC liners manufactured by Ameron were lined with a
differently anchored PVC lining product such as those manufactured by Arrow-Lock.
2.3 Pipe Slope
The pipe slope for each reach between manholes is designed to maintain a minimum
velocity of 2 feet per second (fps) during low flow periods to maximize shear stress for
prevention of debris sedimentation along the interceptor.
17
Horizontal and Vertical Curves
Horizontal and vertical curves impact velocities and flow transitions along the interceptor
to avoid turbulence and off-gassing. The allowable radius of curvature was specified to
maintain a constant velocity and smooth transition of flow to minimize sedimentation and
odor production. Horizontal and vertical curves were limited to a maximum of 45
degrees.
Existing Utilities
Minimum horizontal clearance of 10 feet is recommended for parallel utilities to
minimize the potential of damage along the alignment. In select locations, the horizontal
clearance is less than 10 ft. Adjustments to the alignment were made during design based
on the information provided on the utility maps and field confirmation by potholing of
facilities near the interceptor alignment.
Minimum vertical clearance was 12 inches to provide increased constructability of the
interceptor and to provide the required clearance beneath potable waterlines.
In the event that the pipe slope was impacted adversely and the design slope could not be
maintained, the existing utility was realigned.
18
2.4 Maximum Operational Flexibility
A wide range of flows are accommodated with the final interceptor design. Current
wastewater flows in the interceptor are very low, under certain operational scenarios zero,
since the upstream portions of this project are not connected. The projected minimum
design flow of 3-mgd for the initial operation scenario provides a minimum flow velocity
of approximately 2.5-fps. The projected maximum design flow of 80.6 mgd will provide
a minimum flow velocity of approximately 8 fps. The interceptor design slope was
analyzed for average dry weather (minimum flow), peak dry weather flow, and peak wet
weather flow (maximum flow) to ensure operational requirements were realized under all
conditions.
2.5 Safe Maintenance Access
A critical aspect of locating the interceptor and the maintenance holes along the selected
alignment is to provide a safe working area for maintenance crews.
Maintenance Holes
Access to the gravity portion of the interceptor will be through maintenance holes.
SRCSD standard interceptor maintenance holes will be used throughout the project. The
maintenance hole structures were 60-inch diameter chimney style with flat top, frame and
cover mounted to a cast-in-place encasement surrounding the pipeline. The structures
19
have an interior, sealed cover that allows direct access to the interceptor pipe. The
encasement encapsulated the entire pipe segment between joints. At maintenance holes,
“short” segments of pipe were used to allow the entire pipe joint to be encapsulated in
concrete.
Venting of specific maintenance holes was considered for odor control;
however, this was not necessary. SRCSD bolts and gaskets all manholes to force air to
travel to a downstream control structure for treatment. For this project, the air travels to
the Arden or Natomas Pump Station for odor control. Covers for the maintenance holes
were designed to be 36-inch cast iron covers in accordance with SRCSD standards.
During final design, placement of the interceptor manholes was optimized to allow for
maintenance access safety. Access to the maintenance holes may be accomplished by
temporarily closing the outside traveled lane and the bike lane. Lane closures typically
provide 20-feet of working area for the maintenance crew to operate with traffic on only
one side of the work area.
.
20
Chapter 3
INTERCEPTOR FLOW EVALUATION
This section provides a summary of the Upper Northwest Interceptor Sections 5 and 6
service area flow evaluation. The overall project flow and pipe capacity were evaluated
during the preliminary design on a regional basis. The flow and pipe capacity were also
evaluated for project specific flows and capacity.
3.1 Basis of Wastewater Flow
The design flows for the UNWI Sections 5 and 6 were determined using information
found in the County Sanitation District No. 1 (CSD-1) Trunk Sewer Master Plan. The
flows were obtained from the District in a spreadsheet format that presented both
Average Dry Weather (ADWF) and Peak Wet weather
(PWWF) flows for future
projection years including 2010, 2020, and Build out.
The flow data included flow projections from the proposed “trunk sheds” for a 48-hour
time period in 15-minute increments. Trunk sheds are small tributary sewer areas that
were identified for conveyance of sewer flows to a particular location (manhole). The
trunk shed flow insertion points were based on location of the proposed future trunks and
existing trunks identified during the master planning efforts. The minimum, average, and
maximum flows were identified for each of the flow insertion points from available data
21
provided by DSRSD. The design flows for the UNWI 5/6 were determined by totaling
the insertion point flows. The 19 flow insertion points identified in the CSD-1 Master
Plan were grouped into several aggregate flow points that represented estimated manhole
locations. These insertion points and the associated flows are shown in Figure 4. It was
important to identify these manhole locations and the approximate location for flow
entering the system to understand the required depth of the interceptor since the trunk
sewers typically require greater depth due to slope requirements and distance to the
interceptor. The grouped insertion points were reviewed and approved by SRCSD and
CSD-1.
The design flows calculated in the CSD-1 Trunk Sewer Master Plan were determined by
an Equivalent Single family dwelling unit (ESD) accounting system. An ESD is defined
as the average base wastewater flow contribution from a single-family dwelling. The
tributary area for the UNWI Sections 5 and 6 was divided into “trunk sheds” as part of
that analysis. The Average Dry Weather flow (ADWF) was generated by using the area
of the “trunk sheds” tributary to the UNWI Sections 5/6 and the land use data. The base
wastewater and inflow/infiltration (I/I) flow rates were calculated by assigning ESD’s per
acre for each of the “trunk sheds” based on the land use data. The ESD’s were converted
to an ADWF by using the land use and County design flow criteria is shown on Figure
22
C-1 provided in Appendix C for gallons per day per acre per ESD (gpd/acre/ESD). The
design flow or Peak Wet Weather flow (PWWF) was generated using the ADWF and
applying a peaking factor determined by County standards. The UNWI Sections 5/6
Tributary Area design flow was determined to be 84.8 mgd.
3.2 Wastewater Flow
Wastewater flows for the UNWI Sections 5/6 alignments are summarized in Table 2.
The pipe was sized for open channel flow using the ultimate wastewater flows projected
for the overall area served by the interceptor.
Table 2 UNWI 5/6 Projected Wastewater Flows
Flow Condition
Average Dry
Weather Flow,
(mgd)
Peak Dry
Weather Flow,
(mgd)
Peak Wet
Weather Flow,
(mgd)
2010
2020
Build Out
6.1
6.2
17.4
9.1
9.2
26.2
32.6
42.9
84.8
During the wastewater flow analysis, it was determined that the flows anticipated to enter
the system near the termination point (UNWI 4/5 Structure) would likely enter the system
downstream of this project.
(3.99+0.14+0.017) to 80.6 mgd.
Therefore, the design flow was reduced by 4.2 mgd
23
Figure 4 Interceptor and Trunk Sewer Flow Insertion Points
24
Chapter 4
INTERCEPTOR DESIGN EVALUATION
This section presents the interceptor design evaluation including: hydraulic design criteria
and the hydraulic analysis for all project components.
4.1 Hydraulic Criteria
The design of this project was based on accepted design criteria for open channel, gravity
pipe flow under all flow scenarios and establishing a smooth flow path along the
interceptor. The design criteria used for the hydraulic analysis are summarized below.
Design criteria associated with the hydraulic analysis included the following:

Gravity pipe shall flow in an open channel condition for all flow conditions.

Minimum Depth to Diameter (d/D) ratio of 0.75

Minimum dry weather flow velocity of 2 fps

Minimum peak dry weather flow velocity of 3 fps.

Minimum shearing stress of 0.07 pounds per square foot (discussed in Section
3.4)

Manning’s “n” factor of 0.013.

Maximum design velocity of 10 fps or less, unless unusual topography prevents
lower velocities in the pipe.
25
These criteria were based on the consideration of re-suspending solids and limiting the
generation and release of odors.
4.2 Hydraulic Analysis
The hydraulic analysis for this project consisted of determining the shearing stress to
maintain solids suspension and identifying slope based on established flow scenarios.
The diameter size was established during the pre-design completed by others and was
verified during the slope analysis for this project.
4.2.1 Shearing Stress
Determining the required flow to re-suspend solids in a pipeline is based on the
shearing stresses available along the pipe wall that interface with the flow. The
1993 Sacramento Sewerage Expansion Study [1] provided criteria for evaluating
shearing forces at the pipe/water interface for gravity pipes. This criterion is in
accordance with the ASCE Manuals and Reports on Engineering Practice No. 60,
“Sulfide in Wastewater Collection and Treatment Systems.” Shearing stress is
calculated using the following relation:
T0 = wRS
Where:
T0 = shearing stress in pounds per square foot (lbs/sq ft)
26
w= specific weight (lbs/cubic foot)
R= hydraulic radius (ft)
S= pipe invert slope
The result is an average shearing stress and is considered adequate near the
bottom of the pipe when the flow in the pipe is one-third full. For most practical
purposes, the value obtained from this relation is considered valid for situations
where flow depths are less than one-third of the full pipe flow and the velocity is a
minimum of 2 feet per second (fps). The recommended range for shearing stress
is 0.03 to 0.08 lbs/sq ft. The lower end of the range is recommended for grit
particles (approximately 1 mm). Historically, SRCSD has used a minimum
shearing stress equal to 0.07 lbs/sq ft for larger interceptor pipelines such as
UNWI Sections 5 /6.
4.2.2 Slope Evaluation
The hydraulic analysis for the UNWI 5/6 project was completed using two
spreadsheet models. Both hydraulic spreadsheet models were developed using
Microsoft Excel and completed to determine slopes, velocities, and depth of flow
for each of the projected flows listed in Table 2.
27
A slope analysis was performed to determine the minimum slope that could be
used for the pipe design to maintain minimum shear stress of 0.07 and the
maximum slope to remain at or below critical flow. Maintaining pipe flow at or
below critical flow is important to minimizing turbulence and odor generation.
Additionally, the flow depth was limited to a minimum d/D ratio to 0.75 to allow
air flow along the pipe and prevent an anaerobic condition. Therefore, evaluating
the range of acceptable pipe slopes for both minimum and maximum flow was
performed.
Manning’s Equation was used for the slope analysis. Since the pipe is flowing
less than full, the hydraulic radius (R) needed to be determined for a cross section
that is a fraction of a circle. The cross section of the less than full pipe flow is
presented on Figure 5.
28
E
D
A
C

O
D (ft)
d (ft)
B
Figure 5 Open Channel Flow in Pipe Flowing Less than Full
The line ADC represents the flow line and d is the depth of flow equal to 0.75D.
The pipe diameter is denoted by D (ft). Point O represents the center of the circle.
The equation for R is:
R= A/P [3]
Where:
R= Hydraulic Radius of a Circle
A= Area of Flow = Area of the Circle – (Area of Sector AOCE – Area of
Triangle AOCD)
P = Wetted Perimeter = Arc ABC
29
Angle = cos-1 (0.25D/0.5D)
Area of Sector AOCE = [(2*)Deg.]*(1/4 D2)
Length of Arc ABC = D – [(2*)Deg.]*(D)
Area of Triangle AOCD = 2*1/2*(0.25D)*(0.25D tan )
This equation was entered into Microsoft Excel using a visual basic program so
that the d/D value, slope, and friction coefficient could be entered. The visual
basic program is presented in Figure 6.
FLDEPTH
=RESULT(1)
=ARGUMENT("Q",1)
=ARGUMENT("D",1)
=ARGUMENT("S",1)
=SET.NAME("DEPTH",0)
=SET.NAME("STEP",0.0005)
=0.0135*Q/S^0.5
=SET.NAME("DEPTH",DEPTH+0.1)
=ROUND(2*ACOS(1-2*DEPTH/(D/12)),4)
=(D/12)^2.6667*(C10-SIN(C10))^(5/3)/(20.16*C10^(2/3))
=IF((C11)<C8,GOTO(C9))
=SET.NAME("DEPTH",DEPTH-0.1)
=SET.NAME("DEPTH",DEPTH+STEP)
=ROUND(2*ACOS(1-2*DEPTH/(D/12)),4)
=(D/12)^2.6667*(C15-SIN(C15))^(5/3)/(20.16*C15^(2/3))
=IF(C16<C8,GOTO(C14))
=IF(12*DEPTH>D,GOTO(C21))
=SET.NAME("DEPTH",12*DEPTH)
=RETURN(DEPTH)
=RETURN(D)
Figure 6 Visual Basic Program for Calculating Flow Depth
30
The spreadsheet calculated full pipe flow, flow depth, hydraulic radius, and
Froude number based on these design criteria entered.
Critical slope was
determined by entering slope values until a Froude number of 1 was obtained as a
result. A sample of the spreadsheet is presented in Figure 7.
Figure 7 Sample Calculation Spreadsheet
The results of the analysis for minimum slope are presented in Table 3 and Table
4. Since a large inflow will occur into the UNWI Sections 5 and 6 at 28th Street
from the Upper Dry Creek Relief and the McClellan Air Force Base, the slope
31
criteria are presented from both 28th Street to Don Julio Boulevard and Cherry
Lane to 28th Street, the location of a hydraulic control structure.
Initial
wastewater flows in the interceptor will be very low and, under certain
operational scenarios, zero.
A minimum flow of 3 mgd was used for the
hydraulic analysis.
Table 3 Slope Analysis – 28th Street to Don Julio
Pipe
Diameter
(in)
Slope
48
0.0022
48
0.00373
48
0.0022
48
0.00434
3
4
Flow
(Qmin)
(mgd)
3
3
Critical Slope based on minimum flow
Critical Slope based on minimum flow
Flow
(Qmax)
(mgd)
42.8
42.8
d/D
0.18
0.16
0.80
0.61
Shear
Velocity Stress
(t)
(fps)
3.1
0.06
3.7
0.09
6.1
0.28
8.2
0.30
Comment
Minimum
Initial Flow
Ultimate
PWWF
32
Table 4 Slope Analysis- Cherry Lane to 28th Street
Slope
0.0014
66
66
0.00375
3
0.0012
80.8
0.90
5.6
0.13
66
6
80.8
0.57
9.1
0.34
Pipe
Diameter
66
0.0037
Flow
(Qmax)
(mgd)
Shear
Velocity Stress
(t)
(fps)
Comment
2.5
0.04
Minimum
3.6
0.08 Initial Flow
Flow
(Qmin)
(mgd)
3
d/D
0.13
0.10
Ultimate
PWWF
As shown in Table 3, critical slope for the 48-inch diameter pipe 0.0037 for a
flow of 3 mgd and 0.0043 for a flow of 42.5 mgd. The selected slope for this
section was 0.0037 to achieve the maximum velocity at minimum flow and to
remain close to critical slope and maximum velocity at maximum flow.
The second spreadsheet was established to identify manhole to manhole slope
requirements based on utility conflicts, connection points, and depth of pipe
cover. The slope was adjusted between manholes as necessary and the hydraulic
capacity was compared again to the design criteria. The slope for the 66-inch
diameter pipe section was selected based on the shear stress, velocity, and d/D
values along with additional criteria of depth of the pipe. The pipe depth for this
5
6
Critical Slope based on minimum flow
Critical Slope based on minimum flow
33
section was critical since construction of pipe depths greater than 25 feet become
difficult and cost preventive. In an effort to keep the pipe depth less than 25 feet
and to meet the required inverts at both junction structures, the flattest slope
possible was selected. A summary of slopes based on this evaluation is presented
in Table 5.
Table 5 Summary of Design Slopes by Reach
Velocity at
Maximum Q
Start Station End Station
(fps)
Comment
Section 6- Maximum Flow, 42.8 mgd
264+23
285+00
5.67
260+69
264+23
Supercritical Utility Conflict
Pipe
Diameter
Slope
48
48
0.0037
0.0176
48
0.0037
239+73
260+69
48
0.2580
239+45
239+73
Supercritical Utility Conflict
48
48
0.0037
0.0136
205+86
200+81
239+45
205+86
5.67
Supercritical Grade Change
48
0.0037
66
0.0011
69+00
140+00
5.07
66
0.0030
19+77
69+00
8.38
66
0.0021
2+36
19+77
7.23
5.67
140+00
200+81
5.67
Section 5- Maximum Flow, 80.6 mgd
Lowered for
Upper Dry
Creek Relief
UNWI Section 6 has several locations where it was required to increase the pipe
depth to avoid existing utilities which could not be moved or the grade changed
and the minimum pipe cover could not be met. The minimum pipe cover required
34
to minimize pipe loading was 10 feet.
At all these locations, the flow is
supercritical and a hydraulic jump will occur. Hydraulic jumps will damage the
pipe by scouring. To prevent damage to the pipe from scour, the pipe was lined
for the entire circumference of the pipe. In addition, the potential for air venting
was analyzed at these locations.
The slopes for UNWI Section 5 were maintained generally within the slope range
evaluated. However, the reach between Station 69+00 and Station 140+00 is just
under the minimum slope of 0.0012 due the need to lower the 28th Street
Structure to accommodate the Upper Dry Creek Relief. An economical analysis
was performed to determine whether it would cost more to lower this structure or
pump the Upper Dry Relief Interceptor flows. It was determined that the least
costly solution was to increase the depth of the UNWI Section 5 pipe. However,
due to the distance available to the UNWI 4/5 structure, the slope was reduced
slightly to meet the required invert at that structure.
4.3 Hydraulic Control Structures
Hydraulic control structures are designed to divert and channelize flow for a smooth
transition at pipe diameter changes or flow convergence points. For this project, two
junction structures were required. The UNWI 4/5 Junction Structure was designed to
35
channelize incoming flow and outgoing flow, accommodate a pipe change, and provide
interim flow control. The 28th Street Junction Structure was designed for both a pipe
diameter change and a point for flow convergence from 2 pipes.
Both structures are
located along UNWI Section 5. A schematic of the layout from the 28th Street Junction
Structure to the existing Elkhorn Junction Structure is shown in Figure 8. The schematic
shows UNWI Section 5 divided into two segments for the purpose of discussion.
36
Figure 8 28th Street Junction Structure to Existing Elkhorn Structure
The segment of UNWI Section 5 between the UNWI 4/5 Junction Structure and the
existing Elkhorn Junction Structure has been labeled Segment A and the project portion
between the 28th Street Junction Structure and the UNWI 4/5 Junction Structure has been
labeled as Segment B.
37
4.3.1 UNWI 4/5 Junction Structure
The UNWI 4/5 Junction Structure will collect flow from Upper Dry Creek
Interceptor as well as from UNWI Sections 5 and 6. Sewer flow from the UNWI
Sections 5 and 6 enters the UNWI 4/5 Junction Structure from a 66 inch pipe that
originates at an upstream junction structure, the 28th Street Junction Structure
discussed in Section 4.3.2.
The UNWI 4/5 junction structure was designed to perform as a diversion of
sewer flow from the UNWI 4/5 Junction Structure to the UNWI Sections 1
through 4 and flow will continue directly to the Natomas Pumping Station. In the
interim, flow from the Dry Creek Interceptor, Rio Linda Interceptor, and UNWI
Sections 5/6 will initially be sent down the existing Lower Dry Creek Interceptor.
The hydraulic criteria used for the design of UNWI 4/5 Junction Structure
included:

Maintain subcritical flow in the 66-inch diameter pipe,

Interim flow equal to 26 mgd,

Build out flow equal to 80.6 mgd,

Mannings’ “n” equal to 0.013, and
38

Entrance loss coefficient into the structure of 0.1
The UNWI 4/5 Junction Structure was constructed with a weir and a pump station
to convey flow through the existing downstream sections of the Lower Dry Creek
Interceptor. A three mgd pump station was designed as part of this structure to
discharge sewage to a bay located on the south side of the structure. The weir
was designed to allow sewage to spill during periods of maintenance or in the
event that the pump either failed or the flow reached 26 mgd.
The existing Elkhorn Junction Structure was modified to allow sewage to flow to
the Lower Dry Creek Interceptor from the UNWI 4/5 Junction Structure until the
remaining sections of the UNWI system are constructed. The existing junction
structure has a 27-inch diameter opening and a 30-inch diameter opening that has
a plug to prevent infiltration of groundwater into the structure since a connection
had not been constructed. In an effort to use the existing Elkhorn Structure pipe
openings at, the invert elevations for the 27 and 30-inch diameter pipes at the
UNWI 4/5 Junction Structure that connected to the existing Elkhorn Structure
needed to be determined to allow for conveyance of the interim flow of 26 mgd.
Additionally, the weir length and the pump station hydraulics needed to be
calculated based on the hydraulics required to convey the interim flow.
39
The hydraulic design for the 66 inch pipe discharging into the UNWI 4/5
Structure, the Elkhorn Structure connection, the UNWI 4/5 Structure weir and
pump station are presented in the following sections.
Hydraulic Grade Line for the 66-inch Diameter Pipe
The 66-inch diameter pipe into the UWNI 4/5 Junction Structure is the section of
pipe between the 28th Street Structure and this structure. This pipe will carry a
build out flow of 80.6 mgd and the required freeboard between the hydraulic
grade line and the invert of the 27 and 30-inch diameter pipes in this pipe was
designed to be a minimum of 6-inches. This design will allow wastewater to be
stored in the 66-inch diameter pipe in the event of a failure downstream or
maintenance in the structure. Based on the pipeline profile and slope analysis
performed, the invert of the 66-inch diameter pipe was set at 20.42 feet at the
UNWI 4/5 Junction Structure to allow for an adequate slope and the minimum
freeboard required. The crown elevation in the 66 inch pipe is at 25.92 feet. The
flow depth in the pipe at build out flow of 80.6 mgd is 3.37 feet and the HGL is
23.79 feet.
40
Connection to the Existing Elkhorn Junction Structure
The hydraulic calculation for the flow available through the 27 and 30-inch
diameter pipes assumed that the headloss through both pipes were equal, the pipes
were flowing full, and the required total flow through the pipes was 26 mgd. A
schematic of the design including HGL at the interim flow of 26 mgd is shown in
Figure 9.
Ground
Elevation 47 ft
Ground Elevation
47 ft
Figure 9. Schematic of Interim Design flow HGL along Segment A
The invert elevation at the existing Elkhorn Junction Structure was set at 24.49
feet. The invert elevation at the UNWI 4/5 Junction Structure discharge bay was
set based on the slope required to provide 26 mgd of flow, the hydraulic grade
41
line of the existing 27 inch Dry Creek Interceptor, and the hydraulic grade line of
the 66 inch diameter pipe.
The total flow in each pipe was calculated by determining the headloss using the
following equation for each pipe diameter and setting these equations equal to
each other:
hL= n2Q2*L/2.21*R4/3*A2 (ft) [4]
Where:
n = Manning’s friction coefficient
Q= pipe flow (cfs)
L= length of pipe section (ft)
R= hydraulic radius (ft)
A= area of the pipe (ft2)
Since the Manning’s friction coefficient was assumed to be equal for both pipes
this term and the constant (2.21) cancel each other when setting the equations for
these pipes equal to each other. The hydraulic radius was determined for both the
pipe diameters. The hydraulic radius (R) was calculated using the following
equation:
R=A/W (ft) [4]
42
Where:
R= hydraulic radius of the pipe (ft)
A= area of the pipe (ft)
W= the wetted perimeter (ft)
The equation has two unknowns, Q27 and Q30 and was solved for one of the
unknowns in terms of the other unknown. The total flow is equal to the sum of
the flow in each pipe, therefore, the flow for one was solved by adding the two
flows and subtracting from the total flow of 26 mgd. The second flow was
determined from subtracting the calculated flow and subtracting from the total
flow of 26 mgd. The pipe flow units used were mgd since the units cancel for all
other components of the calculation. Based on the calculation, the flow was
determined to be 11.2 mgd and 14.8 mgd in the 27-inch and 30-inch diameter
pipes, respectively.
The velocity in each pipe was determined based on Q= VA and calculated to be
4.4 and 4.6 fps for the 27 and 30-inch pipes, respectively. The velocity was
calculated to determine the minor headloss in each pipe using the equation HL=
kV2/2g [4].
Where:
HL= headloss (ft)
43
k = headloss coefficient
V = velocity (fps)
g = 32.2 ft/s2
The minor headloss due to entrance or pipe bends in each pipe was less than 0.1
and determined to be negligible. The pipe friction losses were determined using
the Hazen Williams headloss equation for submerged pipes. The pipe friction
loss equation used was:
hf = 3.022 * V1.85 * L/ C1.85 * D1.165 [4]
Where:
hf = pipe friction loss (ft)
V = velocity (fps)
L = pipe length (ft)
C= Hazen Williams pipe friction coefficient
D = pipe diameter (ft)
The headloss was originally assumed to be equal and as would be expected the
headloss was calculated to be 0.57 feet for both pipes for lengths of 253 feet for
the 27-inch pipe and 263 feet for the 30-inch diameter pipe. The discharge bay
invert was set 1 foot above the build out flow HGL for the 66 inch pipe at 24.81
feet and slightly below the existing 27 inch Dry Creek Interceptor calculated
44
invert of 24.85 feet. The slope for the 27 and 30 inch pipe was then calculated to
be .0018 based on the inverts at each structure. The pipe flow for these pipes was
not dependant on the slope since the pipes are submerged and under pressure.
The hydraulic characteristics for the 27 and 30-inch pipes are summarized in
Table 6.
Table 6 Hydraulic Characteristics of 27 and 30-inch Pipes
Pipe
Diameter
(inch)
27
30
Slope
0.0018
0.0018
Flow Q
(mgd)
14.8
11.2
Velocity
(fps)
4.4
4.6
Headloss (feet)
0.57
0.57
Weir
The weir was designed to allow 26 mgd to spill into the discharge bay during
interim operations. The height and length of the weir were calculated using a side
weir calculation along with elevations determined by the pump station design.
The pump station is discussed in more detail in the following section.
The weir height was set to allow the discharge bay to fill to approximately 1 foot
over the 30 inch pipe crown plus up to 6 inches of freeboard between the water
level and the top of the weir. To determine the weir height, the hydraulic grade
line in the pump station needed to be determined. The HGL in the discharge bay
45
was determined to be 27.88 feet based on the invert elevation of 24.81 feet and
adding the 30 inch diameter pipe and the head loss of 0.57 feet
(24.81+30/12+0.57 = 27.88 feet). The target freeboard between the top of the
weir and the hydraulic grade line within the discharge bay was 6 inches.
However, the head over the 27 and 30 –inch pipes required to ensure the design
flow of 26 mgd, allowed for a 0.25 feet freeboard above this HGL to the top of
weir elevation of 28.1 feet. The height of the water over the weir was chosen to
be 0.96 feet, to keep the height of the water over the weir below 1 foot. A
schematic view of the UNWI 4/5 Junction Structure weir elevation and HGL’s is
shown in Figure 10.
46
Ground Elevation
47 ft
Figure 10 Weir Elevation at the UNWI 4/5 Junction Structure
The length of the weir was determined by using the following equation:
Q=Cw*L*h(3/2) [3]
Where:
Q= wastewater flow (mgd)
Cw= Weir coefficient
L= length of weir (feet)
h= height of water over the weir
47
The height of water over the weir was assumed to be 1.0 foot, the weir length was
increased by 0.25 feet for each calculation iteration, and the weir coefficient was
assumed to be 3.3 which is a typical value for rectangular discharge weirs. Based
on the design flow of 26 mgd and the head over the weir of 1 foot, the length of
the weir was determined to be 12.25 feet in length. However, the weir length was
set at 13 feet to accommodate other design concerns such as existing pipe and
structure rebar design. The head over the weir would be 0.96 feet at a flow of 26
mgd.
Based on the results of the weir length analysis, a weir length of 13 feet
was selected since the design flow is satisfied and the head over the weir is less
than 1 foot. The weir calculation results are summarized in Table 7.
Table 7 Weir Calculation Results
Flow Q (mgd)
26.13
26.08
26.79
Head over Weir (ft)
1.00
0.96
0.80
Length of Weir
12.25
13.00
17.50
Pump Station
The pump station was designed to discharge 3 mgd of flow during periods when
the total flow was less than 26 mgd.
The pump hydraulics consisted of
determining the wet well size, force main size, the pump on/off levels, and the net
positive suction head available (NPSHA).
48
The assumptions were made that the pump station would be 10 foot in diameter
and the hydraulic grade line would match the hydraulic grade line (HGL) in the
junction structure. The wet well size was chose to be 10-foot diameter, since this
could easily be constructed using 120-inch diameter pipe sections.
The pump wet well volume was based on minimum operational requirements for
the pump using the industry standard of maximum pump start of 6 times per hour.
The wet well volume equation is Q*t/2, where Q is the flow in gpm and t is the
pump cycle time. This equation is based on the wet well and pumps having
uniform pumping rate and the minimum pump cycle time occurring when the rate
of inflow is equal to one half the discharge rate. Additionally, the effective wet
well volume is based on 2.5 times the discharge rate to ensure that the maximum
pump starts per hour does not exceed 6 times which is recommended for pumps
having a uniform pumping rate. Using 2100 gpm and 10 feet diameter, the
volume of 5250 gallons is calculated.
The pump wet well and the 66 inch pipe HGL would naturally equalize to the
same elevation due to the connection of these two structures. Therefore, an
iterative process was used to determine both the HGL in both structures (UNWI
4/5 and the pump wet well). The volume of the UNWI 4/5 Structure channel was
49
assumed to be rectangular and the volume was expressed as the height the water
times width of the channel. The channel width is 5.5 feet and the depth of the
water is an unknown. The pump wet well is cylindrical and the volume can be
expressed as V = h x A; where V is volume (gallons), h is height of water (feet),
and A is the area of the wet well (square feet). The wet well area is the area a
circle (PI *D2/4); where D is the diameter (feet). The height of the water in the
wet well was calculated to be 8.94 feet. The HGL minus the water height is equal
to the invert in each structure. The invert was already determined to be 20.42 for
the UNWI 4/5 Junction Structure. Therefore, there are two equations:
HGL1 = 20.42 + h1
HGL2 = Wet Well Invert + 8.94
Setting the two equations equal to each other the resulting equation is:
11.48 + h1 = Wet Well Invert
Solving this equation using trial and error, the results are shown in Table 8.
50
Table 8 HGL and Water Depths in the UNWI 4/5 Structure and Pump Wet Well
Description
Invert Elevation (ft)
UNWI 4/5 Structure
20.42
Wet Well
15.94
Height of Water (ft)
4.46
8.94
HGL Elevation (ft)
24.88
24.88
Based on the results, during pumping the HGL in the UNWI 4/5 Junction
Structure will be approximately 1 foot above the HGL at the build out flow. For
this to occur, the pump on/off level will need to be set near the HGL elevation.
The force main was sized using Q=VA, with the flow discharge equaling 3 mgd
and holding the velocity at 5 fps. The force main size was determined to be 10
inches in diameter.
The pump on/off levels were based on the junction structure hydraulics and the
pump hydraulics.
The design of pump systems requires an analysis of the
potential for low pressure at the suction side of the pump to ensure that the pump
fluid does not reach the boiling point. The boiling point of a fluid is reached
when the fluid is below the vapor pressure at fluid temperature. The pressure
analysis is important to prevent cavitation, reduced efficiency and damage. The
51
net positive suction available (NPSHA) is an equation used to compare the total
head at the suction side of the pump close to the impeller and the vapor pressure
of the fluid at the associated temperature. This suction head equation is based on
the Energy Equation and expressed as the sum of the static and velocity head.
The vapor pressure head is expressed as the ration of the vapor pressure to the
vapor head. NPSHA is expressed as the difference between suction head and
pressure heads. The NPSHA was calculated using the following equation:
NPSHA= Hbar+hs-Hvap-Hvol- FS
Where:
Hbar = barometric pressure, 32.7 ft
hs = static head (vertical height between pump impeller center line and
water surface) (ft)
Hvap = vapor pressure, 1.6 feet
Hvol = 2 feet
FS= Factor of Safety, 3 feet
The static head was calculated to be 8.2 feet, based on the discharge piping invert
elevation set at 25.47 feet and subtracting the impeller centerline (15.94 +1.33)
feet. The pipe minor and friction losses were assumed to negligible. The NPSHA
was calculated to be 34.3 feet based on the stated equation (32.7+8.2-1.6-2-
52
3=34.3). The pump was chosen based on these pumping characteristics and
keeping the net positive suction head required for the pump below the calculated
NPSHA to prevent cavitation.
4.3.2
28th Street Junction Structure
The 28th Street Junction Structure was designed to channelize flow from 2
incoming pipes and accommodate pipe diameter changes. The hydraulic design
criteria is the same for the UNWI 4/5 Junction Structure. The hydraulic analysis
of this structure also considered maintaining some flow below the pipe crown to
allow air flow along the interceptor to minimize odor generation. A schematic of
the water surface elevation along Segment B is shown on Figure 11.
Ground Elevation
47 ft
Ground Elevation
79 ft
Figure 11 Schematic of Interim Design flow HGL along Segment A
53
This structure is the junction for convergence of 2 pipes, the upstream UNWI
Section 6 and the Upper Dry Creek Relief pipe. The Upper Dry Creek Relief is a
36-inch diameter pipe that will enter this structure from the north along 28th
Street. The crown elevation of both the 48-inch and 66-inch pipes were matched
at elevation 52.02 feet to prevent submergence of the 66 inch pipe.
The invert
elevation of the Upper Dry Creek Relief pipe was set approximately 1 foot above
the invert of the 48-inch pipe to ensure a freeboard within the pipe to prevent
submergence.
.
54
Chapter 5
PROJECT CONCLUSIONS
Construction of the UNWI 5/6 was completed in 2006 and is now operational. The
hydraulic analysis of this interceptor was comprehensive and included considerations for
flow regimes, hydraulic jumps, and odor control. The UNWI 4/5 Junction Structure was
designed to operate under minimum, interim, and build out flow scenarios.
At this time, SRCSD staff have not been able to use the pump station due to the valve
failure between the UNWI 4/5 Structure and the pump wet well. The valve is stuck
partially open likely due to debris stuck in the valve seating or mechanical failure for
some other reason. Maintenance of the valve would require either draining the structure
channel and removing the debris or digging up the valve and replacing it. The valve
would be difficult to access since it is located between two structures that would require
support and the valve is buried approximately 15 feet deep.
Currently, SRCSD has decided to allow the channel to back up and send the flow over
the weir since odor has not been a concern.
Based on the difficulty maintaining the isolation valve between the pump wet well and
the UNWI 4/5 structure, I would consider the following design changes:
55

Installing a gate valve within the pump wet well for ease of access for
maintenance, and

Designing the inlet piping between the UNWI 4/5 Structure with a slope to
increase velocity and minimize debris sedimentation.
.
56
APPENDICES
57
APPENDIX A
UNWI 4/5 Junction Structure Drawings
65
APPENDIX B
28th Street Junction Structure Drawings
67
APPENDIX C
County Design Criteria
68
Figure C-1 County Design Criteria
69
REFERENCES
[1] Montgomery Watson. Final Report- Sacramento Sewerage Expansion Study- 1994 Update.
Prepared for Sacramento Regional County Sanitation District and Sacramento County Sanitation
District No. 1. August, 1994.
[2] Carollo Engineers, Inc. Upper Northwest Interceptor Design Report. May, 1999.
[3] Ranald V. Giles, Jack B. Evett, and Cheng Liu. Fluid Mechanics and Hydraulics 3rd Edition.
McGraw-Hill, 1994.
[4] Linsley, Ray K., Franzini, Joseph B., Freyberg, David L., Tchobanoglous, George. WaterResources Engineering. 4th ed. New Jersey: McGraw-Hill,1992.
[5] McGhee, Terence J. Water supply and sewerage. 6th ed. New Jersey: McGraw-Hill, 1991.