HomeMy WebLinkAboutSDP 2022-0003; FPC RESIDENTIAL - SB 330; SEWER STUDY; 2022-11-29
DEXTER WILSON ENGINEERING, INC.
WATER ● WASTEWATER ● RECYCLED WATER
CONSULTING ENGINEERS
2234 FARADAY AVENUE ● CARLSBAD, CA ● (760) 438-4422
SEWER STUDY
FOR THE
FPC RESIDENTIAL PROJECT
IN THE CITY OF CARLSBAD
November 29, 2022
Ryan Martin
November 29, 2022
FPC Residential Sewer Study
DEXTER WILSON ENGINEERING, INC. PAGE 3
Sewer System Design Criteria
The design criteria used for the evaluation of the sewerage system impacts by the FPC
Residential project are consistent with Volume 1 of the 2022 City of Carlsbad Engineering
Standards: General Design Standards (2022 Volume 1 Engineering Standards). The City’s
Sewer Master Plan and hydraulic model were also used for verification and confirmation of
existing wastewater facilities surrounding the project.
Appendix A includes design criteria excerpts from the 2022 Volume 1 Engineering Standards
document.
Sewer Generation Rates
Chapter 6 in the 2022 Volume 1 Engineering Standards identifies the average daily sewage
flow to be 200 gallons per day (gpd) per Equivalent Dwelling Unit (EDU). The sewer
generation rate for the FPC Residential project is presented in Table 1.
TABLE 1
FPC RESIDENTIAL
SEWER GENERATION RATES 1
Land Use Generation Rate
Residential (Low to Med-High Density) 200 gpd/EDU
1. Based on Volume 1 of the 2022 City of Carlsbad Engineering Standards: General Design
Standards, Chapter 6 – Design Criteria for Sewer Facilities.
Peaking Factors
The peaking factor for sewage flow is identified in the 2022 Volume 1 Engineering Standards.
For average daily flows of less than 100,000 gpd, a peaking factor of 2.5 shall be used to
determine peak daily flows.
Ryan Martin
November 29, 2022
FPC Residential Sewer Study
DEXTER WILSON ENGINEERING, INC. PAGE 4
Manning’s “n”
The gravity sewer analyses are made using a computer program which uses the Manning
Equation for all of its calculations. The Manning’s “n” used by the computer program is held
as a constant for all depths in a circular conduit. The value of Manning’s “n” used for this
sewer study is 0.011 which corresponds with the recommended value in the 2022 Volume 1
Engineering Standards.
Depth and Velocity of Flow in Gravity Sewers
Gravity sewer lines are designed to convey peak dry weather flow. Pipes that are 12-inches
in diameter and smaller are designed to convey this flow with a maximum depth-to-diameter
(d/D) ratio of 0.50. Pipes that are larger than 12-inches in diameter are designed for a
maximum d/D ratio of 0.75. Gravity sewer lines are designed to maintain a minimum velocity
of 2.0 feet per second at peak dry weather flow to prevent the deposition of solids.
Projected Sewer Generation
Based on the sewage generation factor presented in Table 2, the estimated average sewer
generation for the project is calculated in Table 2.
TABLE 2
FPC RESIDENTIAL
AVERAGE DAILY SEWER FLOW
Land Use Quantity Sewage Generation Factor Average Daily Flow
Residential 86 DUs 200 gpd/DU 17,200 gpd
TOTAL 17,200 gpd
Per Table 2, average daily sewer flow for the FPC Residential project is 17,200 gpd or 11.9
gpm. The peak daily flow is 43,000 gpd or 29.9 gpm (Average Daily Flow x 2.5).
Ryan Martin
November 29, 2022
FPC Residential Sewer Study
DEXTER WILSON ENGINEERING, INC. PAGE 5
Existing Sewer System
The existing sewer system in the vicinity of the FPC Residential project includes a 15-inch
gravity sewer line east of the existing NCTD railroad that conveys sewer north to a 27-inch
diameter trunk sewer. North of the project site there are exiting 8-inch gravity sewer lines
in Leeward Street and Windvane Lane. South of the project, existing sewer facilities include
the Knots Lane Lift Station, a sewer pipeline stub from the Knots Lane Lift Station into the
east side of the North County Transit District (NCTD) railroad right-of-way, and conveyance
and treatment facilities downstream of the Knots Lane Lift Station. Figure 2 presents the
existing sewer system in the vicinity of the project.
Proposed Private Sewer System
Figure 3 presents the proposed private sewer system for the FPC Residential project. As
shown in Figure 3, the private sewer system for the project will consist of private gravity
sewer lines onsite, a private sewer lift station, a private force main, and replacing an existing
public manhole with a new public manhole.
A private sewer lift station will be required for the project due to the invert elevations of
existing sewer facilities in the vicinity of the project. Pad elevations within the site range
from 40 feet to 46 feet. To avoid a private force main in City of Carlsbad right-of-way, the
City of Carlsbad Utilities Department directed the project to pump sewage east across the
NCTD railroad to an existing 15-inch sewer line.
Onsite private gravity sewer lines will convey sewage to the southeast corner of the project
to a private sewer lift station where sewage will be pumped east across the NCTD railroad.
East of the NCTD railroad, the force main will discharge into a new public manhole. The
new public manhole will replace an existing manhole located along the existing 15-inch sewer
line east of the NCTD railroad. Because the private force main is proposed to cross the NCTD
railroad, trenchless construction will be required for installation of the force main.
Ryan Martin
November 29, 2022
FPC Residential Sewer Study
DEXTER WILSON ENGINEERING, INC. PAGE 8
Private Sewer Lift Station. The minimum recommended force main size to carry raw
sewage for this project is 2.5-inch diameter and the maximum recommended is 4-inch
diameter. At 2.5-inch diameter grinder pumps would be required, but larger submersible
pump options will be evaluated during final design. A 4-inch force main is presented in
Figure 3, but the force main size is subject to change during final design. A dual force main
configuration to provide force main redundancy will be considered during final design.
For sewage pumping, the minimum recommended sewage flow velocity is three feet per
second (fps). In order to achieve three feet per second sewage flow velocity in a 2.5-inch
diameter force main, the private lift station pumping capacity must be at least 46 gpm. This
pumping capacity is greater than the calculated peak sewage flow of 32.8 gpm from the
project. In order to achieve three feet per second sewage flow velocity in a 4-inch diameter
force main, the private lift station pumping capacity must be at least 120 gpm.
The detailed design of the private lift station is beyond the scope of this study. However, it
is appropriate to provide some basic design parameters for this lift station. The private lift
station will consist of a wet well with two submersible pumps. Each pump will be capable of
pumping the design peak flow, thereby providing 100 percent pumping capacity redundancy.
A pump control panel will provide motor starters for the pumps and control the pump
operation based on liquid level in the wet well.
The lift station will include a valve vault in which an emergency force main connection will
be located. This connection will enable the submersible pumps to be connected to temporary
hoses so that sewage can be pumped into a truck in the event that the force main needs
repair. In addition, if both the submersible pumps fail, a temporary pump can be brought to
the site and connected to the force main piping so that sewage pumping can continue while
the main sewage pumps are repaired.
Emergency sewage storage will be provided in an underground vault which will store up to
ten (10) hours of average sewage flow. This will provide time for a response to any problem
at the lift station. The control panel will include a manual transfer switch and receptacle for
a portable emergency power generator. If a prolonged power outage occurs, a generator can
be brought to the site to maintain power to the sewage pumps.
Ryan Martin
November 29, 2022
FPC Residential Sewer Study
DEXTER WILSON ENGINEERING, INC. PAGE 9
Finally, an alarm system will be part of the pump control panel. Alarms will be
communicated via telephone to those designated to respond to any problems at the lift
station.
The design of the private lift station and private gravity sewer system will be undertaken
with the preparation of final engineering plans for the FPC Residential project.
Offsite Public Sewer System Analysis
Buildout of the Ponto beachfront area was evaluated in the City of Carlsbad 2019 Sewer
Master Plan Update (Master Plan). In the Master Plan it is assumed that flows from the
Ponto beachfront area will be conveyed to the existing Knots Lane Lift Station for pumping.
Although the Master Plan assumes all sewer flows from the project site will be conveyed
south to the Knots Lane Lift Station, the sewer flows from the Knots Lane Lift Station are
ultimately conveyed north from the station to the same existing 15-inch sewer line that the
FPC Residential project is proposing to connect to. In the Master Plan there are no
deficiencies identified in or downstream of the existing 15-inch sewer line under buildout of
the Carlsbad sewer service area (CSSA) which includes the Ponto beachfront area. Pertinent
excerpts from the City’s Sewer Master Plan are included in Appendix B for reference.
To confirm that there is sufficient capacity in the existing 15-inch sewer line east of the
project, the existing offsite sewer system was analyzed under existing flow conditions and
under existing flows plus proposed flows from the FPC Residential project. Existing flows
were estimated by counting the number of units in the sub-basin served by the existing 15-
inch sewer line in question and multiplying the total by the appropriate sewer generation
factor. Appendix C includes an exhibit that shows the sewer basins served by the existing
15-inch sewer line east of the project; the sewer basin consists of 527 existing residential
units and two clubhouses.
As-built drawings were obtained from the City of Carlsbad to determine the sewer slopes of
the existing 15-inch sewer line east of the project for the following analyses. The as-built
drawings are included in Appendix D for reference.
Ryan Martin
November 29, 2022
FPC Residential Sewer Study
DEXTER WILSON ENGINEERING, INC. PAGE 10
Existing System Under Existing Flows. Appendix E presents the results for the existing
sewer system analysis under existing flows. Exhibit A presents a Manhole Diagram for the
system. The results for this analysis show that under existing flows the existing 15-inch
public sewer line east of the project has a maximum depth-to-diameter ratio of 0.20. Flow
velocities range from 1.60 fps to 1.61 fps under existing flows.
Existing System Under Existing Plus Project Flows. Appendix F presents the results
for the existing and proposed sewer system analysis under existing flows plus project flows.
Exhibit A presents a Manhole Diagram for the system. The results for this analysis show
that under existing plus project flows the existing 15-inch public sewer line east of the project
the maximum depth-to-diameter ratio increases from 0.20 to 0.22. Flow velocities in the
existing system range from 1.60 fps to 1.71 fps under existing plus project flows.
Based on the Master Plan and the results of the public sewer analysis in this report, there is
sufficient capacity in the existing public sewer system to serve the project.
Conclusions and Recommendations
The following conclusions have been made related to providing sewer service to the FPC
Residential project.
1. The FPC Residential project will receive sewer service from the City of Carlsbad.
2. Due to the invert elevations of existing sewer facilities in the vicinity of the project, a
private sewer lift station is required to serve the FPC Residential project.
3. Figure 3 presents the proposed private sewer system for the FPC Residential project.
The proposed private sewer system will consist of private gravity sewer lines onsite,
a private sewer lift station, a private force main, and replacing an existing public
manhole with a new public manhole.
APPENDIX A
CITY OF CARLSBAD
SEWER SYSTEM DESIGN CRITERIA
APPENDIX B
EXCERPTS FROM THE
CITY OF CARLSBAD
2019 SEWER MASTER PLAN UPDATE
APPENDIX C
SEWER SUB-BASINS
EXISTING 15-INCH SEWER LINE
EAST OF PROJECT
APPENDIX D
AS-BUILT DRAWINGS
EXISTING 15-INCH SEWER LINE
EAST OF PROJECT
APPENDIX E
ANALYSIS OF
EXISTING 15-INCH SEWER LINE
EAST OF PROJECT
EXISTING FLOWS
DATE:
FOR:SHT 1 OF 1
509-142 BY:
M.G.D.C.F.S.
820 816 363.00 363.0 200 72,600 2.5 181,500 0.182 0.281 15 0.174 0.040846 0.25073 0.20 0.1123 1.60 361 Existing Units and 2 Clubhouses
816 812 0.00 363.0 200 72,600 2.5 181,500 0.182 0.281 15 0.174 0.040846 0.25073 0.20 0.1123 1.60
812 808 0.00 363.0 200 72,600 2.5 181,500 0.182 0.281 15 0.174 0.040846 0.25073 0.20 0.1123 1.60
808 804 0.00 363.0 200 72,600 2.5 181,500 0.182 0.281 15 0.176 0.040614 0.25004 0.20 0.1118 1.61
804 800 166.00 529.0 200 105,800 2.5 264,500 0.265 0.409 27 0.160 0.012948 0.25826 0.11 0.0501 1.61 166 Existing Units
Total EDUS Min Slope Max dn/D
529.0 0.16 0.20
JOB NUMBER:
11/29/2022
LENGTH (ft)IN-LINE
EDUs
TOTAL
EDUs
SEWAGE
PER EDU /
DAY (gpd)
LINE FROM TO
REFER TO PLAN SHEET:
Offsite Sewer Analysis - FPC Residential Project - Existing System, Existing Flow
Dexter Wilson Engineering, Inc.
SEWER STUDY SUMMARY
NOTESVELOCITY
(f.p.s.)
LINE SIZE
(inches)
DESIGN
SLOPE
(%)
DEPTH K' (1)dn (feet)dn/D(2)Ca for
Velocity(3)
PEAK FLOW (DESIGN
FLOW)
AVG. DRY
WEATHER
FLOW (gpd)
PEAKING
FACTOR
PEAK
FLOW
(gpd)
1 K' based on n = 0.011
2 dn/D using K' in Brater King Table 7-14
3 From Brater King Table 7-4 based on dn/D \\ARTIC\Eng\509142\Sewer\2022-11-29 Ponto Redevelopment Sewer Analysis_East.xlsx Existing Flow
APPENDIX F
ANALYSIS OF
EXISTING 15-INCH SEWER LINE
EAST OF PROJECT
EXISTING FLOWS PLUS PROJECT FLOWS
DATE:
FOR:SHT 1 OF 1
509-142 BY:
M.G.D.C.F.S.
820 816 363.00 363.0 200 72,600 2.5 181,500 0.182 0.281 15 0.174 0.040846 0.25073 0.20 0.1123 1.60 361 Existing Units and 2 Clubhouses
816 812 86.00 449.0 200 89,800 2.5 224,500 0.225 0.347 15 0.174 0.050523 0.27868 0.22 0.1306 1.70 86 Proposed Units
812 808 0.00 449.0 200 89,800 2.5 224,500 0.225 0.347 15 0.174 0.050523 0.27868 0.22 0.1306 1.70
808 804 0.00 449.0 200 89,800 2.5 224,500 0.225 0.347 15 0.176 0.050235 0.27788 0.22 0.1300 1.71
804 800 166.00 615.0 200 123,000 2.5 307,500 0.308 0.476 27 0.160 0.015053 0.27767 0.12 0.0557 1.69 166 Existing Units
Total EDUS Min Slope Max dn/D
615.0 0.16 0.22
11/29/2022
JOB NUMBER:
LINE SIZE
(inches)LINE FROM TO LENGTH (ft)IN-LINE
EDUs
TOTAL
EDUs NOTES
REFER TO PLAN SHEET:
Offsite Sewer Analysis - FPC Residential Project - Existing System, Existing + Project Flow
Dexter Wilson Engineering, Inc.
SEWER STUDY SUMMARY
DESIGN
SLOPE
(%)
DEPTH K' (1)dn (feet)dn/D(2)Ca for
Velocity(3)
VELOCITY
(f.p.s.)
SEWAGE
PER EDU /
DAY (gpd)
AVG. DRY
WEATHER
FLOW (gpd)
PEAKING
FACTOR
PEAK
FLOW
(gpd)
PEAK FLOW (DESIGN
FLOW)
1 K' based on n = 0.011
2 dn/D using K' in Brater King Table 7-14
3 From Brater King Table 7-4 based on dn/D \\ARTIC\Eng\509142\Sewer\2022-11-29 Ponto Redevelopment Sewer Analysis_East.xlsx Existing + Project Flow
EXHIBIT A
MANHOLE DIAGRAM