HomeMy WebLinkAbout8a. Sewer Infrastructure Renewal Stratey and Private Sewer Lateral Program Updates Item 8a
Department: Utilities
Cost Center: 6002
For Agenda of: 8/19/2025
Placement: Study Session
Estimated Time: 60 minutes
FROM: Aaron Floyd, Utilities Director
Prepared By: Chris Lehman, Deputy Director - Wastewater
SUBJECT: SEWER INFRASTRUCTURE RENEWAL STRATEGY AND PRIVATE
SEWER LATERAL PROGRAM UPDATES
RECOMMENDATION
1. Receive a presentation and conduct a Study Session on the 2025 Wastewater
Collection System Infrastructure Renewal Strategy, including proposed updated
capacity-constrained areas infiltration and intrusion reduction approaches; and
2. Receive and file the 2025 Wastewater Collection System Infrastructure Renewal
Strategy report, and provide strategic direction regarding the following:
a. Continuance of the Wastewater Flow (Sewer Lateral) Offset Program; and
b. Updated General Plan Water and Wastewater Element Capacity Constrained
Boundaries Map; and
c. Expansion of Private Sewer Lateral Replacement Rebate eligibility to include all
sewer lateral replacements within the newly defined capacity-constrained areas;
and
d. Evaluation of a Private Sewer Lateral Inspection Rebate program.
REPORT-IN-BRIEF
This report provides a summary of the public health and regulatory reasons for existing
Municipal Code and program requirements related to operation and maintenance of a
safe and sanitary sewer system and private sewer lateral infrastructure; Council direction
and action to date regarding these codes and programs ; the status and progress made
through the City’s improvements to public infrastructure and implementation of programs
for private sewer lateral inspections, repairs, and replacements; and a summary and
status of the legal action and settlement agreement that initiated the current codes and
programs. This report also summarizes the results of wastewater flow modeling and the
2025 Wastewater Collection System Infrastructure Renewal Strategy (WWCIRS,
Attachment A), including an updated proposed Capacity-Constrained Area map
(Attachment B). Public outreach was conducted prior to this Study Session, and the
discussion items identified by staff were in part developed based on feedback from
interested members of the public, developers, and realtor groups. The report concludes
with staff’s presentation of discussion items for Council to consider, staff’s
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recommendations regarding these discussion items, with requests for direction regarding
existing programs and subsequent approvals for a General Plan Amendment (specific to
the capacity-constrained area map) and modifications to rebate programs. Figures
presented in the body of the report are available in full size as Attachment C to this
report.
Inflow and Infiltration (I&I) flows in the City of San Luis Obispo are significant and impact
all users of the wastewater system. Inflow is stormwater that enters private sewer lat erals
through direct connections such as a downspout, roof drain, yard drain, or foundation
drain. These direct connections are illegal under City Municipal Code Sections 13.08.030
and 13.08.040. Infiltration is water that flows through the ground and can seep, trickle, or
flow into the wastewater collection system through cracks in sewer mains and/or private
sewer laterals. Effective I&I reduction is necessary to eliminate sanitary sewer overflows
(SSOs) and pipe surcharging, reduce the amount of I&I flow to the Water Resource
Recovery Facility (WRRF), and maintain available pipe capacity to serve planned infill
development. Sanitary sewer overflows are a public health risk and can result in National
Pollutant Discharge Elimination System (NPDES) Permit violations, fines from the
Regional Water Quality Control Board (RWQCB), beach closures by the San Luis Obispo
County Public Health Department, Clean Water Act third -party lawsuits, or a long-term
enforcement agreement referred to as a consent decree. While significant progress has
been made in addressing and managing the risks associated with sewer conveyance
capacity limitations, achieving success will continue to require both public and private
investment.
Development of the 2025 WWCIRS (Attachment A) included the collection of wastewater
flow data, the completion of a system-wide capacity modeling study in 2024, and the
completion of an updated wastewater flow model. Based on the results of the analysis,
recommendations for capital improvement sewer projects are included in the 2025
WWCIRS, which will be utilized by staff as a guidance document to investigate and
evaluate future capital projects for Council’s consideration during the development and
adoption of future financial plans.
As discussed further in this report, staff requests strategic direction from the City Council
regarding the following:
1) Continuance of the Wastewater Flow (Sewer Lateral) Offset Program. Alternatives
to the continuance of this program are presented and discussed in Study Session
Question #1. Continuance of the program without modification would not require
further council action. Should Council decide to direct staff to proceed with one of
the alternatives to continuing the program, staff would need to complete an update
to the City's Municipal Code, which would not be feasibly adopted until sometime
in 2026.
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2) Updated sewer capacity-constrained boundaries map (Attachment B). If Council
directs staff to proceed with an updated boundaries map and retain the existing
Private Sewer Lateral Offset Program, staff plan to present the updated map as an
amendment to the Water and Wastewater Eleme nt of the General Plan to the
Planning Commission on October 22, 2025. Staff would then return to Council with
the Planning Commission’s recommendation on December 2, 2025.
3) Expansion of existing private sewer lateral replacement rebate eligibility to include
all properties within capacity-constrained areas (including multi-family and
commercial). Currently, only single-family residential properties qualify for this
rebate. If Council directs staff to proceed with the rebate program amendment,
staff will provide a draft Resolution to execute expanded rebate eligibility for
Council’s consideration on December 2, 2025.
4) Evaluation of a new short-term rebate to incentivize private sewer lateral
inspections within capacity-constrained areas. If Council directs staff to proceed
with the rebate program, staff anticipates that the new rebate would be
implemented through the 26-27 Supplemental Budget or the 27-29 Financial Plan.
Following an initial pilot program, staff would review the effectiveness of the new
rebate and confirm funding availability before making a recommendation for a
longer-term rebate program.
POLICY CONTEXT
The primary goal of the Wastewater Flow Offset (Private Sewer Lateral) and Rebate
Programs continues to be the reduction of inflow and infiltration from private sewer
laterals and reducing the potential for SSOs, while accommodating housing development
and furthering the City’s Major Goal of Housing and Neighborhood Livability‐Healthy,
Safe, and Affordable.
California River Watch Settlement Agreement
On July 7, 2016, the City entered into a settlement agreement with California River Watch
regarding alleged Clean Water Act violations. The settlement agreement include d a
“sunset” date of July 7, 2023, which mean t, in part, that California River Watch agreed
not to sue the City for the period between July 7, 2016, and July 7, 2023 (refer to further
discussion in the Background section of this Council Agenda Report). The settlement
agreement required the City to consider two Supplemental Environmental Programs
intended to secure significant benefits to the local environment. The following programs
are excerpted from that agreement:
Lateral Inspection and Repair Program: Within one (1) year from the Effective Date of this
Agreement, the City staff shall recommend to the City Council an ordinance establishing
program for the inspection, repair, and/or repla cement of private sewer laterals. The
program proposed will use the following events as a basis or trigger” for inspection, repair,
and/or replacement:
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1. Transfer of ownership of the property if no inspection or replacement of the
sewer lateral occurred within twenty (20) years prior to the transfer;
2. If two (2) or more sanitary sewer overflows are caused by the same private
sewer lateral within the immediate past two (2) years; or
3. Where the application for a building permit indicated the expected addition o f
flow through the private lateral (e.g. addition of bathrooms, bedrooms, addition
living space).
Voluntary Private Lateral Replacement Rebate Program: The City staff shall recommend
a Voluntary Private Lateral Replacement Rebate Program. For three (3) years from
establishment, the Program will provide, on a first come, first served basis, one thousand
($ 1,000) dollars toward the total replacement costs of a residential private lateral, not to
exceed a total of fifty thousand ($ 50,000) dollars annually.
The settlement agreement with California River Watch did not mandate that the City adopt
either of these programs, but it did mandate that the City Council consider an ordinance
to establish such programs. Council provided direction to staff at study sessions on
October 4, 2016, and April 4, 2017. On May 16, 2017, Council considered and did not
approve a proposed private sewer lateral ordinance that would have required mandatory
repair or replacement of defective laterals prior to a change of ownership in real property.
Based on feedback received during recent public outreach, this option has been brought
back to Council for consideration during this Study Session. This option is presented
within Study Session Question #1 regarding continuance of the existing Wastewater Flow
Offset program.
Following the 2017 study sessions, and after deliberation and further study, the City
adopted its Private Sewer Lateral Offset and Private Sewer Lateral Inspection Upon Sale
requirements (August 20, 2019), and Private Sewer Lateral Rebate program (August 20,
2019), which are currently in place today. The current inspection upon sale requirements
do not require the repair or replacement of defective laterals prior to a change in
ownership of real property.
2006 General Waste Discharge Requirements and 2015 WWCIRS
The City completed its prior WWCIRS in April 2015, which will be superseded by the 2025
WWCIRS, presented as Attachment A in this report. The City Council approved an update
to the Water and Wastewater Management Element of the General Plan in May of 2018,
which included the identification of capacity-constrained (sewer conveyance) areas.
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The State Water Resources Control Board (SWRCB) adopted statewide General Waste
Discharge Requirements (GWDR) for publicly owned sanitary sewer systems in 2006.
The GWDR required the development and implementation of a system -specific local
sewer system management plan that documents a comprehensive program for sewer
system operation, maintenance, repair, and reporting of all sanitary sewer overflows into
the California Integrated Water Quality System (CIWQS) reporting database. The City is
required to comply with these statewide requirements. The City Council approved the
2019 Sewer System Management Plan on May 7, 2019, which has since been
superseded (see below).
2023 General Waste Discharge Requirements
The SWRCB adopted an updated statewide GWDR for publicly owned sanitary sewer
systems on June 5, 2023. The updated GWDR requires the development and
implementation of a system-specific local sewer system management plan that
documents a comprehensive program for sewer system operation, maintenance, repair,
and reporting of all sanitary sewer overflows in the California Integrated Water Quality
System (CIWQS) reporting database. The City is required to comply with these statewide
requirements. The City Council approved the (current) 2025 Sewer System Management
Plan on March 4, 2025.
DISCUSSION
Background
Private sewer laterals are the portion of the sewer lines
that connect from a residence or commercial structure
up to and including the point of connection with the
publicly owned sewer main (Figure 1).
The City estimates that there are 13,421 private sewer
laterals citywide, including 3,052 within the proposed
updated (draft) capacity-constrained areas. Notably, this
is a significant reduction from the 7,000 private sewer
laterals located in the current capacity-constrained
areas. These changes are discussed and presented in
the Proposed Updated Capacity Constrained Areas
section of this report.
Pursuant to City Municipal Code Section 13.08.395,
inspection and maintenance of private laterals are the
responsibility of the property owner. Inspections are
required when any of the following events occur: 1) after a sanitary sewer overflow (spill)
occurs, or, 2) upon submittal of a building permit for the addition of a bedroom, bathroom,
or kitchen in a residential structure, or the addition of a non -residential space or an
additional plumbing fixture unit in non-residential structures, or, 3) change in the use of
Figure 1: Private sewer lateral
connection to City sewer main
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the structure (e.g. residential to non-residential resulting in a higher flow or where the
structure has been vacant or unoccupied for more than three years), or, 4) increased
water domestic meter size or addition of a domestic new water meter, or, 5) subdivision
of a property, or, 6) upon findings of I&I, or, 7) upon change in ownership of real property.
Inspections are required regardless of whether the property is in a capacity-constrained
area.
Exceptions to inspection requirements include 1) new construction or if a lateral was
installed within the last 20 years, or 2) a lateral inspection has already been completed
within the last five years, or 3) if the lateral is located within a common interest
development.
Impact of Inflow and Infiltration on the City’s Wastewater Collection System
The City has separate public stormwater and public sewer systems; however, during wet
weather, the City’s wastewater collection system experiences a significant increase in
volume due to storm-related flows. Though not intended to be conveyed or treated by the
wastewater system, stormwater enters wastewater pipes directly through improperly
plumbed drains (inflow) and/or as groundwater that seeps through cracked wastewater
pipes (infiltration). An illustration of common sources of inflow and infiltration (I&I) is
presented below in Figure 2. Significant I&I in the collection system can result in SSOs,
which can have public and environmental health impacts. Under these conditions, peak
flows to the City's WRRF have exceeded 20 million gallons a day (MGD) in a 24-hour
period, where expected flows are under four MGD in a typical 24-hour period. Treatment
of this excess wastewater flow increases energy used for pumping, treatment, and
chemical usage. I&I also consumes capacity in the collection system, resulting in
increased capital requirements to increase pipeline size, energy, and other costly
treatment expenses. The comprehensive flow study completed by the City in 2024
identified multiple locations in the collection system that experienced elevated flows due
to I&I, which is discussed in detail in the 2025 WWCIRS (Attachment A). These elevated
flows result in the identification of capacity-constrained areas of the wastewater collection
system that are unable to accommodate the impacts of I&I safely.
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Figure 2: Sources of Inflow and Infiltration
City’s Progress in Reducing Sanitary Sewer Overflows
The City has made significant progress over the years in repairing and replacing its aging
wastewater infrastructure and in reducing SSOs. The City has 148 miles of wastewater
pipelines and roughly 165 miles of private sewer laterals, making over 50 percent of the
system privately maintained. The City has collected condition data on the public
wastewater collection system through closed -circuit television (CCTV) inspections;
approximately 95 percent of the public system has been inspected since 2013, which
excludes assessment of force mains that staff are unable to inspect due to technology
limitations and without major service disruptions. CCTV inspections, which are ongoing,
have been completed in accordance with the terms of the River Watch settlement.
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Based on condition assessments, pipe deficiencies are prioritized for repair or
replacement through the Sewer Fund's capital improvement program. The City completed
substantial rehabilitation of publicly-owned sewer mains, maintenance holes, and lift
stations, as shown in Figure 3, below. The City invests approximately $1.8 million
annually in collection system maintenance and minor construction repairs. This include s
lift station maintenance, hydro cleaning, and closed -circuit television inspection of the
City's sewer mains.
Figure 3: City CIPs completed, all areas, and approved future Capital Improvement
Projects, from 2015 to present
Future proposed collection system improvements recommended in the 2025 WWCIRS
(Figure 4, see following page), and by the City (Figure 5, see the following page), will
require study, design, and budget adoption.
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Figure 4: Proposed (future) collection system improvements recommended in the 2025
WWCIRS (will require further study and budget adoption)
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Figure 5: Proposed (future) collection system improvements identified by the City (will
require further study and budget adoption)
Since the completion of the 2015 WWCIRS, the City has also:
1) Completed updated modeling of the collection system and prioritization of capital
projects needed to address pipeline age, condition, and capacity issues; these
recommended capital projects are presented in the 2025 WW CIRS. The 2025
WWCIRS resulted in the City purchasing new modeling software, which allows for
some in-house capacity evaluations (i.e., staff conduct the modeling rather than
relying on consultant services) and supports developer-funded contracted
modeling for private projects. It also enables staff to model modified flows to
address capacity issues. The model has been integrated into the City’s GIS
platform, which allows for more frequent minor updates as capital projects and
private development projects are implemented;
2) Updated City Design Standards (Uniform Design Criteria) to require CCTV
inspection and repair or replacement for re-use of an existing private sewer laterals
as specified in Municipal Code Section 13.08.395.C (see Background introductory
paragraph for conditions that trigger these requirements);
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3) Reduced SSOs, or spills from public infrastructure (Figure 6) from a peak of 35 in
2021 to one in 2025, and reduced system failures through a dynamic cleaning and
repair program that utilizes Artificial Intelligence (AI) to address the highest risks
first;
Figure 6: Public (SSO) and Private (PSLD) Sewer Spill Trend1
4) Doubled treatment capacity at the WRRF from 4.5 to 9 million gallons to
accommodate I&I through the WRRF Upgrade Project (2019 -2025); and
5) Installed a network of 22 remote monitoring devices that allow staff to actively
monitor and respond to blockage events to prevent spills (SSOs). First installed in
2019, the city currently has five “Smart Covers” in service, which are depicted as
red circle icons in Figure 7 (see following page). In early 2025, the City purchased
and deployed 17 additional (blue icon) advanced meters across the sys tem,
placing them at key locations identified in the recent flow study. These meters are
intended to support long-term data collection to improve monitoring and will help
identify and refine data that helps identify I&I or capacity issues within the collec tion
system. The five Smart Covers have been installed in high -priority locations, such
as creek crossings and siphons, that have a history of blockages. The 17 additional
monitoring devices may be relocated as needed to respond to emerging concerns
or shifting priorities within the collection system.
1 Water Board requirements for tracking PSLDs changed in 2022. Data for 2025 will be
completed at year end.
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Figure 7: Remote Sewer Level Monitoring Devices
California River Watch Lawsuit Settlement, Sunset, and Remaining Risks
After further evaluation in consultation with the City Attorney’s Office, staff have
determined that modification or elimination of one or more of the City’s private sewer
lateral programs (Wastewater Flow Offset, Inspection Upon Sale, or Rebate) would not
violate the terms of the River Watch settlement agreement, which expired on July 7, 2023.
However, due to the ongoing possibility of private sewer lateral discharges (spills), staff
recommend maintaining the existing private sewer lateral programs to address failing
private infrastructure. Maintaining, and/or refining existing private sewer lateral programs
will help manage public and environmental health risks and demonstrate to Non -
Government Organizations (NGOs) and the City’s regulators that there are safeguards in
place and ongoing progress is occurring to ensure that the City is proactively protecting
the public and environmental health of the community, which will also reduce the risk of
future fines and litigation associated with sewer spills.
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Private Sewer Lateral Programs
Inspection of and Replacement of Existing Private Sewer Laterals
After an ordinance revision was made on August 20, 2019, with some exceptions2, after
January 1, 2020, all private sewer laterals connected to the city’s sewer system are
required to be inspected at the property owner’s sole expense when certain events occur,
such as sewer spills, transfer of property ownership, and certain building permit
conditions3. Following an inspection required by transfer of property ownership, the
inspection results must be disclosed to the purchaser of the property, but
repair/replacement of poor or failed laterals is not triggered by sale alone.
These inspections have produced a dataset critical to the success of the overarching
private sewer lateral programs, which aim to correct private infrastructure deficiencies
with a data-driven approach that is focused on the highest areas of risk.
This program also requires repair or replacement of sewer laterals that do not comply
with the condition requirements identified in the Municipal Code4. However, the current
ordinance does not allow staff to require repair or replacement of private sewer laterals
following an inspection completed as a requirement of a property sale (Inspection Upon
Sale), which would require an update to the City’s Municipal Code and Council’s approval
or an updated ordinance. For all other instances, if the private sewer lateral is not in
compliance with these requirements, then it shall be repaired or replaced to conform to
such standards within 180 days. If an inspection of a noncompliant lateral was required
due to a sanitary sewer overflow or findings of infiltration and intrusion, or a lateral with
defects that have not met the required pipe material requirements as defined in the current
City Standard Specifications and Engineering Standards, then it must be completely
replaced rather than repaired.
Wastewater Flow Development Offset Program
While spills (SSOs) originating from the City’s infrastructure have stabilized and were
reduced to one in 2025, private sewer lateral discharges (PSLDs) continue to be elevated
due to aging systems that commonly have little to no preventative maintenance. Of the
estimated 13,421 private sewer laterals in the City, 7,712 (over 50 percent) of these are
in poor, failed, or unknown condition (Figure 8 on the following page). Where private
sewer laterals in poor, failed, or even fair condition with minor defects connect to the City
sewer, groundwater infiltrates into the sewer, and these areas of the collection system
experience “surcharging,” a condition where the sewer pipe is so full that the wastewater
flow starts backing up in the pipe and sometimes up into the ma intenance holes. This
surcharging is especially prevalent in capacity-constrained areas.
2 Municipal Code Subsection 13.08.395(C)(2) Exceptions for inspection requirements of existing
private sewer laterals.
3 Municipal Code Subsection 13.08.395(C)(1) Inspection of Private Sewer Laterals.
4 Municipal Code Subsection 13.08.395(B) Ownership, Maintenance, and Repair
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Figure 8: Private Sewer Lateral Conditions, Citywide
In 2019, Council directed staff to adopt the current private sewer lateral offset program,
which requires existing private sewer laterals in poor or failed condition to be replaced to
“offset” the impacts of new (private) development in capacity-constrained areas.
Replacement of this existing infrastructure reduces I&I impacts and provides additional
capacity for new development in these areas Offset requirements are calculated based
on the conditions described in City Municipal Code Section 13.08.396. While costs may
vary depending on the length and complexity of the replacement, the typical cost to
replace a private sewer lateral ranges from $10,000 to $15,000, exclusive of City fees
which are approximately $2,000. This cost may be offset for projects that qualify for the
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private sewer lateral rebate, which is currently $4,000. Applicants looking for qualified
offsets may utilize the map of private sewer lateral conditions maintained on the City’s
website at www.slocity.org/laterals. Feedback from the community indicates that this
process has been cumbersome, requiring outreach from applicants to property owners
who have poor or failed laterals. This feedback has resulted in staff developing a new
“opt-in” program that is described later on this page.
Private sewer lateral programs like the City’s are not uncommon in California
communities. The following cities have similar programs: Santa Barbara (since 2006),
Berkeley (since 2006), Richmond (since 2006), Piedmont City (since 2011), Stege
Sanitation District (since 2011), Emeryville (since 2011), Oakland (Since 2012), Ventura
(since 2012), Emeryville (since 2011), Alameda (since 2015), Albany (since (2015), Ojai
Valey Sanitation District (since 2015), Santa Cruz (since 2018), Castro Valley Sanitation
District (since 2019), Monterey (since 2019), and San Mateo (since 2020). Many cities in
the Bay Area were either mandated by the U.S. Environmental Protection Agency or the
Regional Water Quality Control Board to adopt such a program. Other programs were
developed as part of negotiated settlement agreements with San Francisco Baykeeper ,
which is a non-governmental organization (NGO).
Private Sewer Lateral Condition Portal
The City currently has a public portal available at www.slocity.org/laterals that shows the
condition of inspected private sewer laterals (Figure 9, see following page). This map only
displays the data that has been provided to the City through inspection requirements of
private sewer laterals, and a significant number of lateral conditions are “unknown.” Some
laterals conditioned as “poor” or “failed” from inspection u pon sale requirements have not
been replaced. Based on prior Council direction to staff, repair or replacement of these
laterals is not mandated prior to the close of escrow or following. The current City
Municipal Code does not allow staff to require repa ir or replacement of private sewer
laterals following an inspection completed as a requirement of a property sale (Inspection
Upon Sale), which would require an updated ordinance and Council’s approval to amend
the Municipal Code.
Private Sewer Lateral Opt-In Program
In 2025, staff developed a new “opt-in” program to facilitate finding private laterals that
qualify as a wastewater offset. Upon Council adoption of the updated capacity-
constrained boundaries, staff will conduct targeted outreach to any pro perties in these
areas with “poor”, “failed,” or “unknown” conditions. Property owners with private sewer
laterals that are in poor or failed condition will have the opportunity to “opt in” to the offset
program, which will result in a new map symbol or note on the lateral that indicates
willingness to participate in the offset program. Property owners with unknown lateral
conditions will be encouraged to complete a CCTV inspection of their lateral. The typical
cost of a sewer lateral CCTV inspection is around $350. To encourage these inspections,
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staff are recommending for Council’s consideration a new proposed rebate, which is
further described in Study Session Discussion Item #3. A subsequent condition rating of
“poor” or “failed” will allow them an opportunity to participate in the offset program. If a
property owner is required to replace a sewer lateral through the Wastewater Flow Offset
Program, that property owner could select one or more of these laterals on the “opt -in”
list to satisfy the requirement. This would help connect property owners and developers
who are required to complete the Wastewater Flow Offset with property owners seeking
lateral replacement. Participation in the opt-in program would only result in the property
being listed as a participant on the public portal. Personal information, such as the
property owner's name and contact information, would not be provided on the public
portal, and applicants seeking to connect with a participant would be expected to conduct
direct outreach to the opt-in property owner. The City would not be involved beyond
providing this list of opt-in candidates. The offset “matchmaking” program outreach will
begin in January 2026, after adoption of the updated capacity-constrained boundaries,
and staff anticipate population of the updated data set on the public portal by the Spring
of 2026.
Figure 9: City-maintained public portal of all known private sewer lateral conditions
(www.slocity.org/laterals)
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Current Wastewater Collections Capacity Constrained Areas
On the following page, Figure 10 provides a “snapshot” of the private sewer lateral
conditions known to the City as of 2015, when the previous WWCIRS project was
completed. This “baseline” data set assumes that any laterals installed prior to 2015
utilizing plastic materials were in “good” condition. Before the adoption of the 2019 private
sewer lateral inspection requirements, staff did not maintain a comprehensive database
of private lateral conditions as they do today. In addition, since 2019, sewer lateral
inspection methods have been standardized, and staff now utilize a Pipeline Assessment
and Certification Program (PACP) condition grading system. PACP conditioning requires
that staff become certified in the program, which is an unbiased method designed to
achieve consistent and reliable data. It is important to note that most plumbers performing
CCTV inspection analysis are not PACP certified, highlighting the importance of City staff
involvement in the conditioning of private sewer laterals using a standardized
methodology. Property owners are required to submit the CCTV inspection to staff. The
fee for staff to review the inspection and assess the condition of the lateral (poor/failed,
fair, or good) is $86.52 per the 2025-26 Comprehensive Fee Schedule.
Figure 10: Current (Adopted) Capacity-Constrained Areas with (2015) Private Sewer
Lateral Condition Data
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2025 Wastewater Collection System Infrastructure Renewal Strategy
The City completed an updated WWCIRS in July 2025 (Attachment A). This study, similar
to the 2015 project, provided an updated sewer hydraulic model based on real data
collected during the peak wet-weather flows of December 2023 through March 2024.
Additionally, an upgraded modeling software was utilized during the 2025 WWCIRS
project, which is considered by the industry to provide a higher degree of accuracy. This
software leveraged significantly more data (including, but not limited to, additional
elevation data in lieu of assumptions and private sewer lateral inspection data) than what
was available during the last study. In addition to an upgraded sewer modeling software
that was provided with more data, the City has improved its inspection software and
condition rating process (PACP) and leveraged AI tools to manage its data. The resulting
recommendations were developed to aid staff in determining current and future (General
Plan build-out conditions) capacity limitations and recommended capital improvem ent
projects to address City infrastructure issues. If directed to do so, staff plan to return to
City Council on December 2, 2025, with an update to the Water and Wastewater
Management Element of the General Plan, which will include updated capacity-
constrained (sewer conveyance) area boundaries. This process will include further public
education and outreach, and consideration by the Planning Commission for
recommendation to City Council.
Proposed Updated Capacity Constrained Areas
One of the primary deliverables of the 2025 WWCIRS is an updated capacity-constrained
area boundary map. The following Figures (11 through 13) present the prior (2015) and
current (2025) conditions of private sewer laterals overlaid over the current (adopted)
capacity-constrained areas, based on data received through the sewer lateral inspection
program. It is worth highlighting the vast improvement in the quality and quantity of this
dataset, which is the result of six years of private sewer lateral inspections. A basic
summary of condition improvements (Table 1) is as follows:
CC Area
2015 2025 2015 to 2025
Improvement “Good” vs.
“Unknown/Poor”
“Good” vs.
“Unknown/Poor”
Laguna 27% / 73% 31% / 66% 6%
Downtown 13% / 87% 27% / 71% 16%
Foothill 18% / 82% 36% / 62% 22%
Table 1: Analysis of Private Sewer Lateral Condition Progress Towards 100% "Good"
Condition Rating
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Figure 11:Current Capacity Constrained Area, “Laguna”, with (2025) Private Sewer
Lateral Conditions
On the following page, Figure 14 presents the updated boundaries, which are
superimposed over the existing boundaries. Properties within Council-adopted capacity-
constrained areas would be subject to the wastewater f low offset program. As discussed
further below, staff recommend including Proposed Areas 1 and 3 on the updated
capacity-constrained areas map, and excluding the consultant’s recommendation for
Proposed Area 2 (see Figure 14 later in the report), which is small enough to be
addressed through City capital improvement projects which are currently funded in the
adopted 25-27 Financial Plan Staff believe this is the most efficient way to address current
and forecasted future capacity constraints in this area, which would not place additional
burden on property owners and developers in this area.
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Figure 12: Current Capacity Constrained Area, "Downtown", with (2025) Private Sewer
Lateral Conditions
Figure 13: Current Capacity Constrained Area, "Foothill", w ith (2025) Private Sewer
Lateral Conditions
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Figure 14: Old vs New Capacity Constrained Areas
Proposed Area 1 (Figure 15, presenting in Study Session Discussion Item #2; formerly
known as the “Foothill” capacity-constrained area) retains much of the original boundaries
and includes some minor revisions to reflect the current operating conditions of the
system, as demonstrated in the 2023-24 sewer flow study, and discussed in the 2025
WWCIRS report. Updated conditions include consideration of the buildout and future flow
conditions of the California Polytechnic State University, including development of the
University’s Water Reclamation Facility (i.e. wastewater treatment plant), which is
anticipated to be commissioned sometime in 2026 and will initially reduce flows to the
City’s sewer system (but later increase with campus buildout).
Proposed Area 3 (Figure 16, presenting in Study Session Discussion Item #2; formerly
known as the “Downtown” capacity-constrained area) has shifted significantly north,
which reflects the current operating conditions of the system, as demonstrated in the
2023-24 sewer flow study. Proposed Area 3 is presented in Figure 16. Potential reasons
for the removal of the downtown capacity-constrained area and the development of a new
capacity-constrained area further north include 1) aging private and public infrastructure
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in the northern area, as it has been ten years since the last assessment, 2) ground
settlement and impacts from drought, such as increased root intrusion into legacy
materials such as vitrified clay pipe, and 3) significant differences in the severity of
weather monitored between the two flow studies (2012 vs 2023 -24). California entered a
multi-year drought beginning in 2012, which became one of the most severe in state
history (lasting through 2016). The National Centers for Environmental Information
classified large portions of the Central Coast (San Luis Obispo, Santa Barbara, and
Monterey counties) as being in “severe to extreme drought” by late 2012.
Proposed Updates and Amendments for Council Discussion
Study Session Discussion Item #1: Should the Wastewater Flow (Private Sewer
Lateral) Offset Program Continue?
Continuing the Wastewater Flow Offset Program provides several important benefits. It
reduces I&I by requiring the replacement of deteriorated private sewer laterals, which
helps prevent sanitary sewer overflows (SSOs) and improves overall system capacity.
This proactive approach supports new development in capacity -constrained areas
without requiring costly, large-scale public infrastructure projects and demonstrates
compliance with SWRCB mandates, and reduces the City’s exposure to regulatory fines,
litigation, or consent decrees. It also protects public and environmental health by
minimizing sewer spills, distributing responsibility for infrastructure improvements
between property owners and developers, and aligning San Luis Obispo with other
California cities that have implemented similar programs.
However, the program also presents challenges. It places a financial burden on
developers, potentially impacting project feasibility in capacity-constrained areas. The
cost to replace a typical private sewer lateral ranges between $10,000 and $15,000
(average City fees are an additional $2,000), but can exceed $25,000 for more
complicated projects. The program also adds significant administrative complexity as
staff must manage case-by-case offset transactions and outreach. Some stakeholders
have raised equity concerns, noting that the burden falls more heavily on new
development rather than existing property owners with failing laterals. Alternatives such
as shifting replacement requirements to the inspection -upon-sale process or addressing
deficiencies through City-funded capital projects of City infrastructure alone could reduce
these burdens on developers and staff. However, that approach may delay repairs and
lessen the program’s proactive capacity-building impact.
Alternatives to continuing the existing Private Sewer Lateral Offset Program may include:
1. Dissolving the Wastewater Flow (Private Sewer Lateral) Offset Program and
retaining existing Inspection Requirements and the Replacement Rebate
Program. Under this alternative, private Sewer Lateral replacements would still
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be conditionally required, such as after sanitary sewer overflows (spills). Risks
associated with this alternative include increased potential for sewer spills resulting
from intensified5 and new development associated with existing and future sewer
capacity limitations (General Plan buildout conditions). As previously presented in
Figure 5, sewer spills trends have greatly reduced, in part due to private sewer
lateral replacements through the Wastewater Flow Offset Program. To manage
these risks and to accommodate housing development, the City would be required
to make significant additions to its Capital Improvement Plan to upsize and/or
reroute sewer mainlines. This additional financial commitment from the City would
be a burden on all rate payers, which would need to be evaluated in a future rate
study. Further, dissolving the existing Private Sewer Lateral Offset program would
shift the City into a reactive, rather than the current proactive approach, and
increase the potential for future fines, litigation, and negative impacts to the public
and environmental health of the community. Dissolving the existing offset
requirements would require an update to the City’s Municipal Code, which staff
anticipate being able to complete in 2026 if this alternative is chosen.
2. Council may direct staff to dissolve the Wastewater Flow (Private Sewer
Lateral) Offset Program and replace it with requiring replacement of all
Private Sewer Laterals located in capacity-constrained areas, identified as
“Poor/Failed”, during the Inspection Upon Sale process. While this alternative
relieves the burden of resolving sewer capacity issues from the development
community, it reallocates the burden to property owners, which may complicate
property transactions. Complete elimination of the Offset Program would also
increase potential risks associated with intensified development in capacity -
constrained areas, which may not be resolved with the replacement of existing
private infrastructure. Furthermore, to manage these risks and to accommodate
housing development, the City would be required to make significant additions to
its Capital Improvement Plan to upsize and/or reroute sewer mainlines. This
additional financial commitment from the City would be a burden on all rate payers,
which would need to be evaluated in a future rate study. This alternative would
require additional work by staff to complete an update to the City’s Municipal Code,
which staff anticipate being able to complete in 2026 if this alternative is chosen.
3. Council may direct staff to retain the existing Wastewater Flow (Private
Sewer Lateral) Offset Program and add additional requirements for
replacements of all Private Sewer Laterals conditioned as “Poor/Failed”
during the Inspection Upon Sale process. This alternative acknowledges the
shared impacts of new development and existing properties on sewer capacity
constraints. This alternative would require additional work by staff to complete an
5 City Municipal Code 13.08.020(LL) Definitions (“Intensified” Development)
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update to the City’s Municipal Code, which staff anticipate being able to complete
in 2026 if this alternative is chosen.
Study Session Discussion Item #2: Should a General Plan Amendment Adopting
the Staff-Recommended Capacity-Constrained Areas Map be Brought Forward for
Adoption?
If Council direct staff to proceed with adopting a General Plan amendment updating the
capacity-constrained boundaries as shown in Figures 15 and 16, staff plan to return to
City Council on December 2, 2025, with an update to the Water and Wastewater
Management Element of the General Plan, which will include the updated capacity-
constrained (sewer conveyance) area boundaries. This process will consist of further
public education and outreach, and consideration by the Planning Commission for a
recommendation to City Council. Following the adoption of the General Plan Amendment,
staff will conduct additional outreach to property owners and developers in the community
through direct mailing, public workshops, and through the City’s website and social media
channels in early 2026.
Figure 15: New Capacity Constrained Area (1), formerly "Foothill" with some
modifications
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Figure 16: New Capacity Constrained Area (3), formerly "Downtown," with a significant
relocation northward6
Study Session Discussion Item #3: Should Staff Further Evaluate a New Private
Sewer Lateral Inspection Rebate to be Brought Forward to Council in December for
Adoption via Resolution?
Proposed Private Sewer Lateral Inspection Rebate
As one of the remaining limitations of further progress in reducing sewer capacity issues
is condition assessments of private sewer laterals, staff recommend establishing a new
rebate associated with CCTV inspection. This recommendation results from numerous
requests from public stakeholders to establish a funding source to increase the quality
and quantity of the City’s existing dataset of private sewer lateral conditions.
Private sewer lateral conditions that are “unknown” account for 63% of the proposed
Capacity-Constrained Area 1 and 61% of the proposed Capacity-Constrained Area 3.
Incentivizing private property owners to complete condition assessments of these sewer
laterals will result in increased awareness of private infrastructure conditions, which is
anticipated to lead to increased replacements as well as improving the City’s database of
private sewer lateral conditions, which is utilized for offset ”match-making”. The proposed
Private Sewer Lateral CCTV Inspection Rebate amount of $350 is significant enough to
pay for all or most of the cost of inspection for most laterals in the City. Demand for the
6 Private Sewer Lateral Conditions in the Proposed Capacity-Constrained Area 3 have been
obtained through the Sewer Lateral Inspection Program, which includes Inspection Upon Sale
requirements.
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rebate is unknown, and staff are proposing to fund the program thr ough existing funding
appropriated for I&I reduction and the Private Sewer Lateral Replacement Rebate
Program, for Fiscal Years 25-26 ($420,000) and 26-27 ($420,000).
During the development of the 27-29 Financial Plan, or before then if prudent, staff would,
if so directed, evaluate the effectiveness of the new rebate program and analyze
competing demands associated with private sewer lateral replacement rebates before
determining whether to continue the CCTV Inspection Rebate and/or to adjust available
funding for both rebates. The success of the program will be highly dependent on
successful stakeholder outreach and education and may take some time to be successful.
Given this, at the end of each fiscal year, staff recommend carrying over any surplus
funds from the rebate programs to the following fiscal year.
The Private Sewer Lateral Inspection Rebate is recommended to be available to any
property located within the newly established Capacity-Constrained Areas for which the
City’s database indicates an “unknown” condition. This would include all residential and
commercial properties. Due to funding limitations, staff are not recommending retroactive
rebates, and only one rebate per parcel (which will likely exclude “air-space” parcels). The
rebate amount would not exceed the cost of the inspection. If the cost of an inspection is
less than $350, the maximum rebate amount awarded would be 100% of the inspection
cost.
Study Session Discussion Item #4: Should Staff Further Evaluate Expanding
Eligibility for Private Sewer Lateral Replacement Rebates in Capacity-Constrained
Areas to be Brought Forward to Council for Adoption in December via Resolution?
Proposed Private Sewer Lateral Replacement Rebate Expanded Eligibility
To accelerate improvement in the newly proposed capacity-constrained areas, staff
recommend expanding eligible property types beyond single-family homes (which
includes single-family home replacements associated with offset requirements) to include
multi-family developments and commercial properties. This recommendation is the result
of feedback received in response to a Council Agenda Report presented during the
September 3, 2024, Council Meeting, to evaluate this option. Beyond the expansion of
eligible property types (in capacity-constrained areas only), the same terms and
conditions would apply, and staff proposes that each successful applicant receive a
maximum of $4,000. Similar to the proposed Private Sewer Lateral Inspection Rebate,
the financial impacts of this expanded eligibility are unknown, and additional applications
will be dependent on successful stakeholder outreach and education. Staff are proposing
to fund the expanded program eligibility through existing funding allocated for the Private
Sewer Lateral Rebate Program for Fiscal Years 2025 -2026 ($420,000) and 2026-2027
($420,000). During the development of the 27-29 Financial Plan, or before then if prudent,
staff would evaluate the effectiveness of the expanded rebate program to determine
whether to adjust available funding.
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Previous Council or Advisory Body Action
At a Study Session on April 4, 2017, the City Council provided direction to staff on a
private sewer lateral program, a wastewater flow offset program focused on capacity-
constrained areas, a rebate program, and a reduction in permit fees associated with a
private sewer lateral program (specifically waiving the sewer “wye” connection fee). The
City Council considered a private sewer lateral ordinance, which required mandatory
repair or replacement of defective laterals upon transfer of ownership at its May 16, 2017,
meeting, where the ordinance was not approved due to insufficient inspectors and
contractors that could complete repairs within the escrow closing time frame.
On August 20, 2019, the City Council adopted a resolution to establish a Private Sewer
Lateral Replacement Rebate program, and a resolution adopting a new ordinance
amending Chapter 13.08 of the San Luis Obispo Municipal Code , creating a sewer lateral
inspection and offset program. On September 3, 2024, the City Council adopted a
resolution amending the Private Sewer Lateral Replacement Rebate program, increasing
rebate eligibility and the rebate amount.
Public Engagement
Prior to this Study Session, staff engaged with the development and realtor communities
to discuss the challenges of the offset program and potential improvements to the larger
private sewer lateral programs. Specifically, staff facilitated a focused Developer’s
Roundtable meeting on July 1, 2025, which had good attendance (excluding City st aff, 17
were in attendance). Staff also met with the San Luis Obispo Coastal Association of
Realtors on August 13, 2025, regarding sewer lateral inspection requirements, the rebate
program, and to discuss the potential alternatives mentioned in this report that would
potentially require sewer lateral repair or replacement through the inspection upon sale
process.7 On August 14, 2025, staff met with the San Luis Obispo Chamber of Commerce
Legislative Committee to present the recommendations in this report, r espond to
questions, and solicit feedback. Discussions with local plumbers, realtors, and the
development community about the City’s private lateral programs are ongoing, and staff
have encouraged these communities to attend the August 19, 2025 , City Council meeting
to provide further feedback.
CONCURRENCE
The proposed Study Session topics and proposed updates to the capacity-constrained
areas map and rebate programs have been discussed with the City’s Community
Development Department and City Attorney’s Office.
7 Presently, private sewer lateral inspections are required upon “change in ownership,” and the
inspection results must be disclosed to the purchaser of the property (SLOMC
13.08.395(C)(1)(g)), but repair/replacement of poor or failed laterals is not triggered by sale alone
(see SLOMC 13.08.395(C)(1)(h).)
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Item 8a
ENVIRONMENTAL REVIEW
The California Environmental Quality Act (CEQA) does not apply to the recommended
action to receive and file a presentation on the 2025 WWCIRS and provide strategic
direction on next steps, because this action does not constitute a “Project” under CEQA
Guidelines Sec. 15378. The City Council’s action does not include adoption or approval
of, or commitment to, a policy, program, or General Plan Amendment at this time, and
does not include approval of any action that would have any physical effect on the
environment. Should strategic direction be provided that would lead to the initiation of a
Project, that Project would be evaluated for CEQA compliance.
It is expected that any future action, which would require Council c onsideration and
approval, including but not limited to modifications to the Wastewater Flow (Private Sewer
Lateral) Offset Program, and any new or amended rebate programs would be exempt
from the provisions of CEQA, pursuant to CEQA Guidelines Sections 15307 (Actions by
Regulatory Agencies for Protection of Natural Resources) and 15308 (Actions by
Regulatory Agencies for the Protection of the Environment) because the intent of the
Programs is to reduce I&I and the potential for SSOs and to incentivize sewer lateral
replacements. Implementation of these Programs result in the replacement of existing
private sewer laterals serving existing development, which are exempt from CEQA
pursuant to CEQA Guidelines Section 15303 (New Construction or Conversion of Small
Structures) because the replaced sewer laterals would serve existing development in
existing urbanized areas, and the replacement of poorly functioning sewer laterals would
not have a significant adverse impact on the environment. Continued implementatio n of
these Programs would have a beneficial impact on the environment by reducing the
potential for I&I and overflows due to reduced flows from groundwater and stormwater.
If Council directs staff to dissolve and discontinue the Wastewater Flow Offset Pro gram,
private sewer lateral replacements would still be conditionally required, such as after
sanitary sewer overflows (spills) or other Municipal Code violations. Dissolution of the
Offset Program at this time may result in an increased potential for sewer spills and
associated water quality and environmental effects in identified capacity-constrained
areas. The need for environmental review would be further evaluated if Council directed
staff to discontinue the program.
FISCAL IMPACT
N/A
Budgeted: Yes/No Budget Years: 2025-27
Funding Identified: Yes/No
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Item 8a
Fiscal Analysis:
The 2025-27 Financial Plan and Capital Improvement Program identifies $420,000
annually from the Sewer Fund for Inflow and Infiltration Reduction to support the existing
rebate program. If Council directs staff to proceed with the recommended rebate
programs, subject to subsequent Council approval and adoption, these funds would be
used to fund the proposed inspection and replacement rebate programs.
While the current fiscal year includes sufficient funding for these activities, actual demand,
especially from commercial/multi-family sectors and new rebate applicants, remains
uncertain. As such, staff would monitor program participation and fund utilization during
FY 2025-26 and FY 2026-27 and evaluate program effectiveness and remaining funding.
Staff costs associated with Program implementation are absorbed within existing staffing
levels across the Utilities Department and supported by coordination with the Community
Development and Finance Departments. Broader wastewater system management,
including capital planning, regulatory compliance, and maintenance activities, is funded
through the City’s Sewer Enterprise Fund, as outlined in the adopted Financial Plan.
Funding
Sources
Total Budget
Available
Current
Funding
Request
Remaining
Balance
Annual
Ongoing
Cost
General Fund $0 $ $ $
State
Federal
Fees
Other:
Total $0 $ $ $
ALTERNATIVES
The four study session questions identified by staff are presented below, in addition to
staff-identified alternatives for Council’s consideration and discussion.
1. Study Session Discussion Item #1: Should the Wastewater Flow (Private
Sewer Lateral) Offset Program Continue?
The following are presented as alternatives to Study Session Discussion Item #1,
where staff have requested Council to consider whether to continue the existing
Private Sewer Lateral Offset Program:
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Item 8a
a. Council may direct staff to dissolve the Private Sewer Lateral Offset
Program and retain existing Inspection Requirements and the
Replacement Rebate Program. Private Sewer Lateral replacements would
still be conditionally required, such as after sanitary sewer overflows (spills).
Risks associated with this alternative include increased potential for sewer
spills resulting from intensified and new development associated with existing
and future sewer capacity limitations (General Plan buildout conditions).
Further, dissolving the existing Private Sewer Lateral Offset program would
shift the City into a reactive, rather than the current proactive approach, and
increase the potential for future fines, litigation, and negative impacts to the
public and environmental health of the community. Dissolving the existing offset
requirements would require additional time associated with an update to the
City’s Municipal Code, which would not be feasible until sometime in 2026.
b. Council may direct staff to dissolve the Private Sewer Lateral Offset
Program and replace it with requiring replacement of all Private Sewer
Laterals located in capacity-constrained areas, conditioned as
“Poor/Failed”, during the Inspection Upon Sale process. While this
alternative relieves the burden of resolving sewer capacity issues from the
development community, it reallocates the burden to property owners, which
would increase property transaction burdens and timelines. Comp lete
elimination of the Offset Program would also increase potential risks associated
with intensified development in capacity-constrained areas, which may not be
resolved with the replacement of existing private infrastructure. This alternative
would require additional work by staff to complete an update to the City’s
Municipal Code, which would not be feasible until sometime in 2026.
c. Council may direct staff to retain the existing Private Sewer Lateral Offset
Program and add additional requirements for replacements of all Private
Sewer Laterals conditioned as “Poor/Failed” during the Inspection Upon
Sale process. This alternative acknowledges the shared impacts of new
development and existing properties on sewer capacity constraints. This
alternative would require additional work by staff to complete an update to the
City’s Municipal Code, which would not be feasible until sometime in 2026.
2. Study Session Discussion Item #2: Should a General Plan Amendment
Adopting the Staff-Recommended Capacity-Constrained Areas Map be
Brought Forward for Adoption?
The following is presented as an alternative to Study Session Discussion Item #2,
where staff have requested Council to consider adoption of a General Plan
amendment updating the capacity-constrained boundaries map:
a. Council may direct staff not to amend the General Plan capacity-
constrained boundaries map. Impacts of this alternative include requiring
private sewer lateral replacements in areas that have not been identified as
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capacity-constrained by the 2025 WWCIRS. By not updating the capacity-
constrained boundaries map, staff would be carrying out a program that would
be informed by data from ten years prior, which does not take into account the
current conditions informed by the 2025 WWCIRS.
3. Study Session Discussion Item #3: Should Staff Further Evaluate a New
Private Sewer Lateral Inspection Rebate to be Brought Forward to Council
in December for Adoption via Resolution?
The following is presented as an alternative to Study Session Discussion Item #3,
where staff have requested Council to provide direction to staff to evaluate
establishing a new Private Sewer Lateral Inspection Rebate .
a. Council may direct staff not to proceed with the evaluation of a new
Private Sewer Lateral Inspection Rebate. Impacts of this alternative may be
reduced data provided to the City that would help developers and property
owners locate private sewer laterals to be replaced through offset
requirements.
4. Study Session Discussion Item #4: Should Staff Further Evaluate Expanding
Eligibility for Private Sewer Lateral Replacement Rebates in Capacity-
Constrained Areas to be Brought Forward to Council for Adoption in
December via Resolution?
The following is presented as an alternative to Study Session Discussion Item #4,
where staff have requested Council to provide direction to staff to evaluate
expanding eligibility criteria for Private Sewer Lateral Replacement Rebates :
a. Council may direct staff not to proceed with evaluating expanding
eligibility criteria for Private Sewer Lateral Replacement Rebates. Impacts
of this alternative may include a reduction of private sewer lateral
replacements. Further, by limiting eligibility to only single-family home projects,
excluding multi-family or commercial properties, as the current program
requires, staff believe there would be ongoing equity concerns and limitations
to address private infrastructure repairs in the most critical locations (capacity-
constrained areas). Limiting current eligibility to only single-family homes would
slow progress and improvement in these areas.
ATTACHMENTS
A - 2025 Wastewater Collection System Infrastructure Renewal Strategy (Reading File)
B - Updated proposed sewer capacity-constrained boundaries map
C - Report Figures from this Council Agenda Report. High Resolution Figures are
available through the following link:
https://opengov.slocity.org/WebLink/Browse.aspx?id=212718
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PREPARED BY
Wastewater Collection System
Infrastructure Renewal Strategy
PREPARED FOR
City of San Luis Obispo
DRAFT REPORT | AUGUST 2025
Page 249 of 350
DRAFT REPORT | AUGUST 2025
Wastewater Collection System
Infrastructure Renewal Strategy
Prepared for
City of San Luis Obispo
Project No. 1027-60-23-02
Prepared by: Chris Malone, PE, RCE #51009 Date
QA/QC Review: Jeff Pelz, PE, RCE #46088 Date
Page 250 of 350
W-C-1027-60-23-02-WP
i City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Table of Contents
CHAPTER 1 Introduction .................................................................................................................. 1-1
1.1 Background and Relevant Documents .......................................................................................... 1-1
1.2 Project Objectives .......................................................................................................................... 1-2
1.3 Report Organization ...................................................................................................................... 1-2
CHAPTER 2 System Overview and Land Use Information .................................................................. 2-1
2.1 Collection System Features ........................................................................................................... 2-1
2.2 Land Use and Zoning ..................................................................................................................... 2-4
2.3 Parcel Information ......................................................................................................................... 2-7
CHAPTER 3 Flow Analysis ................................................................................................................ 3-1
3.1 Winter Water Use .......................................................................................................................... 3-1
3.2 Collection System Flow Monitoring .............................................................................................. 3-1
3.2.1 2023/2024 Flow Meter Program ......................................................................................... 3-1
3.2.2 WRRF Flow Information ....................................................................................................... 3-6
3.2.3 City Flow Metering Data ...................................................................................................... 3-8
3.3 Groundwater Infiltration ............................................................................................................... 3-9
3.4 Flow Generation Factor Development ........................................................................................ 3-12
CHAPTER 4 Hydraulic Model Development and System Evaluation Criteria ....................................... 4-1
4.1 Model Development ...................................................................................................................... 4-1
4.1.1 Modeling Software............................................................................................................... 4-1
4.1.2 Model Geometry .................................................................................................................. 4-1
4.1.3 Assignment of Flows to Model Nodes ................................................................................. 4-2
4.1.3.1 Sanitary Flows............................................................................................................. 4-2
4.1.3.2 Diurnal Flow Patterns ................................................................................................. 4-2
4.1.3.3 Groundwater Infiltration ............................................................................................ 4-3
4.1.3.4 Rainfall-Dependent Infiltration and Inflow ................................................................ 4-3
4.2 Model Calibration .......................................................................................................................... 4-3
4.2.1 Criteria for Model Calibration .............................................................................................. 4-3
4.2.2 RTK Method ......................................................................................................................... 4-5
4.2.3 Model Calibration Results .................................................................................................... 4-5
4.3 Design Storm Development......................................................................................................... 4-10
4.4 System Evaluation Criteria .......................................................................................................... 4-10
CHAPTER 5 Model Results and System Capacity Evaluation .............................................................. 5-1
5.1 Previously Identified Deficiencies.................................................................................................. 5-1
5.2 Modeled Existing Conditions Evaluation ....................................................................................... 5-2
5.2.1 Gravity Sewer Results – Existing Conditions ........................................................................ 5-2
5.2.2 Lift Station Results – Existing Conditions ............................................................................. 5-4
5.3 Modeled Buildout Conditions Evaluation ...................................................................................... 5-5
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W-C-1027-60-23-02-WP
ii City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Table of Contents
5.3.1 Gravity Sewer Results –Buildout Conditions ....................................................................... 5-5
5.3.2 Lift Station Results –Buildout Conditions............................................................................. 5-5
5.4 Wastewater Flow Offset Program Evaluation ............................................................................... 5-9
CHAPTER 6 Capital Improvement Program Development ................................................................. 6-1
6.1 Cost Estimating Assumptions ........................................................................................................ 6-1
6.2 Gravity Sewer Condition Assessment ............................................................................................ 6-2
6.3 Capacity-Related Capital Improvements ....................................................................................... 6-5
6.3.1 Gravity Sewer Capacity Improvements................................................................................ 6-5
6.3.2 Inverted Siphon Improvements/Actions .............................................................................. 6-6
6.3.3 Lift Station Capacity Improvements ..................................................................................... 6-6
6.3.4 Additional City-Identified Gravity Sewer Improvements ..................................................... 6-6
6.3.5 WRRF Improvements ........................................................................................................... 6-6
6.4 Proposed Capital Improvement Plan ............................................................................................. 6-8
6.5 Capacity-Constrained Areas .......................................................................................................... 6-8
6.6 Conclusions and Recommendations ........................................................................................... 6-13
LIST OF TABLES
Table 2-1. Summary of Gravity Mains by Pipe Diameter .................................................................... 2-1
Table 2-2. Summary of Gravity Mains by Pipe Material ...................................................................... 2-3
Table 2-3. Summary of Lift Stations and Force Main Information ...................................................... 2-3
Table 2-4. Summary of City Zoning Areas ........................................................................................... 2-4
Table 3-1. Winter Water Use Summary Information .......................................................................... 3-2
Table 3-2. Maximum Storm Return Periods, December 19, 2023 through March 26, 2024 .............. 3-5
Table 3-3. Flow Monitoring Program Summary Statistics ................................................................... 3-5
Table 3-4. WRRF Summary Statistics by Calendar Year ...................................................................... 3-6
Table 3-4. Top Ten WRRF Peak Wet Weather Flows, 2018–2023 ....................................................... 3-8
Table 3-5. City Flow Metering Results, October 2022 through March 2023 ...................................... 3-9
Table 3-5. GWI, February 14–16, 2023 Dry Weather Period, Cumulative Flows .............................. 3-11
Table 3-6. GWI, February 14–16, 2023 Dry Weather Period, Incremental Flows ............................. 3-12
Table 3-5. Proposed Flow Generation Factors .................................................................................. 3-13
Table 4-1. Modeled versus Metered Peak Flows ................................................................................ 4-6
Table 5-1. Modeled Lift Station and Force Main Results – Existing Peak Flow Conditions ................. 5-5
Table 5-2. Modeled Lift Station and Force Main Results –Buildout Peak Flow Conditions ................ 5-9
Table 6-1. Summary of Overall Pipe Score Results .............................................................................. 6-2
Table 6-2. Pipe Score Results by Pipe Diameter – Segment Counts ................................................... 6-4
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W-C-1027-60-23-02-WP
iii City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Table of Contents
Table 6-3. Pipe Score Results by Pipe Diameter – Overall Length ...................................................... 6-4
Table 6-4. Proposed Collection System Improvements .................................................................... 6-10
LIST OF FIGURES
Figure 2-1. Wastewater Collection System ......................................................................................... 2-2
Figure 2-2. General Plan Land Uses ..................................................................................................... 2-5
Figure 2-3. City Zoning Areas ............................................................................................................... 2-6
Figure 3-1. Flow Metering Areas ......................................................................................................... 3-3
Figure 3-2. Four-Gauge Average Rainfall, San Luis Obispo, December 2023 – March 2024 ............... 3-4
Figure 3-3. Monthly WWRF Flows and Rainfall, 2018-2024 ................................................................ 3-7
Figure 3-4. Wastewater Flow Components, January through April 2024 ......................................... 3-10
Figure 4-1. Aggregate Normalized Diurnal Flow Curve for Downstream (Unmetered) Areas ............ 4-4
Figure 4-2. Site SM1A, Model vs Flow Meter, Dry Weather Conditions ............................................. 4-7
Figure 4-3. Site SM1A, Model vs Flow Meter, Wet Weather Conditions ............................................ 4-8
Figure 4-4. Tank Farm: Model vs Flow Meter, January 7–13, 2023 .................................................... 4-9
Figure 4-5. 10-Year, 24-Hour Storm Pattern, SCS Type I Pattern ...................................................... 4-11
Figure 5-1. Gravity Sewer Capacity Exceedances, 2015 WCSIRS......................................................... 5-2
Figure 5-2. Gravity Sewer Results – Existing Conditions ..................................................................... 5-3
Figure 5-3. Flow Depths and Peak Wet Weather Flows, Modeled Buildout 10-Year Storm
Conditions, Existin and Previous Flow Metering Sites ...................................................... 5-6
Figure 5-4. Gravity Sewer Results –Buildout Conditions ..................................................................... 5-7
Figure 5-5. Flow Depths and Peak Wet Weather Flows, Modeled Buildout 10-Year Storm
Conditions, Buildout and Previous Flow Metering Sites .................................................. 5-8
Figure 5-6. Private Sewer Lateral Connections to City Sewer Main .................................................. 5-10
Figure 6-1. Gravity Sewer Pipe Defect Scores ..................................................................................... 6-3
Figure 6-2 City-Identified Gravity Sewer Improvements ..................................................................... 6-7
Figure 6-3. Proposed Collection System Improvements ................................................................... 6-11
Figure 6-4. Previously Identified Capacity-Constrained Areas .......................................................... 6-12
Figure 6-5. Updated Capacity-Constrained Areas Map ..................................................................... 6-13
Figure 6-6. Previous and Planned Collection System Repairs/Improvements .................................. 6-14
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iv City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Table of Contents
LIST OF APPENDICES
Appendix A. Modeled versus Metered Flow Results
LIST OF ACRONYMS AND ABBREVIATIONS
2012 I&I Study Sanitary Sewer Flow Monitoring and Inflow/ Infiltration Study, March 2012 1-1
2015 WCSIRS Wastewater Collection System Infrastructure Renewal Strategy, December 2015 1-1
AACE Association for the Advancement of Cost Engineering 6-1
AG Agriculture 4-3
C/OS Conservation/Open Space 2-6
Cal Poly California Polytechnic State University, San Luis Obispo 1-1
CCTV Closed-Circuit Television 6-2
CIMIS California Irrigation Management Information System 3-6
CIP Capital Improvement Program 1-1
City City of San Luis Obispo 1-1
CIWEM Chartered Institution of Water and Environmental Management 4-5
d/D Depth/Diameter 4-10
DWF Dry Weather Flow 4-1
ENR CCI Engineering News Record Construction Cost Index 6-1
fps Feet Per Second 4-10
gpd Gallons Per Day 3-1
gpm Gallons Per Minute 2-3
GWI Groundwater infiltration 3-8
I&I Infiltration and Inflow 3-1
LS Lift Station 3-8
mgd Million Gallons Per Day 3-1
MH Maintenance Hole 2-1
NOAA National Oceanic and Atmospheric Administration 3-1
Project Wastewater Collection Infrastructure Renewal Strategy 1-1
PVC Polyvinyl Chloride 2-1
RG Rain Gauge 3-1
SCS Soil Conservation Service 4-10
SOI Sphere of Influence 2-3
SSMP Sewer System Management Plan 1-1
SSO Sanitary Sewer Overflow 4-12
VCP Vitrified Clay Pipe 2-1
WRRF Water Resource Recovery Facility 3-6
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August 2025
CHAPTER 1
Introduction
The purpose of this report is to present the methods, results, and recommendations associated with the
City of San Luis Obispo (City) Wastewater Collection Infrastructure Renewal Strategy (Project). The topics
addressed in this Introduction include the following:
• Background and Relevant Documents
• Project Objectives
• Report Organization
1.1 BACKGROUND AND RELEVANT DOCUMENTS
The City of San Luis Obispo is the largest city and the County Seat in San Luis Obispo County. It is located
approximately 8 miles inland along the Central Coast of California. As of January 2024, the California
Department of Finance reported the City as having a population of 48,684 residents. Immediately north
of the City limits lies the California Polytechnic State University, San Luis Obispo (Cal Poly) campus, which,
as of fall 2023, is reported to have an enrollment of 22,279 students and over 2,000 faculty and staff.
Development within the City is undertaken through a planning process that is delineated by its General
Plan, the most recent version of which was published in December 2014, with various element
amendments since that time. The General Plan addresses future development regarding land use,
housing, traffic, water and wastewater, services, and other considerations.
In parallel with its General Plan, the City has undertaken significant work over the past several years to
better understand the condition and hydraulic capacity of its wastewater collection system. Efforts have
included flow monitoring, hydraulic modeling, strategic and long-range planning, and Capital
Improvement Program (CIP) development. Three documents that provide the basis for the City’s
collection system planning efforts include:
• Sanitary Sewer Flow Monitoring and Inflow/ Infiltration Study, March 2012 (2012 I&I Study)
• Wastewater Collection System Infrastructure Renewal Strategy, December 2015
(2015 WCSIRS)
• 2019 Sewer System Management Plan Update, April 2019 (SSMP)
The results of these efforts have been used to develop and to inform the City’s Wastewater Flow Offset
Program, which applies to new and intensified development in capacity-constrained areas of the system,
to bring the City into compliance with regulatory requirements to protect public health and the
environment. Since the adoption of the Wastewater Flow Offset Program, approximately 300 private
sewer laterals have been replaced through conditional offset requirements under Municipal Code
Section 13.08.396. In addition, the 2015 WCSIRS identified improvements to support the buildout
population of 57,200 in accordance with the General Plan. Additional information regarding the
Wastewater Flow Offset Program is presented in Section 5.4 of this report (Wastewater Flow Offset
Program Evaluation).
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August 2025
1.2 PROJECT OBJECTIVES
The purpose of this Project is to refine past modeling results using updated information to reevaluate
both existing and future (buildout) collection system flow conditions and to confirm or revise the list of
previously identified collection system improvements in a manner consistent with the City’s General Plan.
Key objectives of this Project include:
Identifying existing and future system deficiencies and developing a prioritized replacement
program that addresses system capacity and condition deficiencies.
Engaging City staff in the process of developing an updated CIP to provide a long-range
forecast of future collection system improvements.
Evaluating the effectiveness of the ongoing Wastewater Flow Offset and Rebate Programs
and making recommendations to improve accuracy, monitoring, and effectiveness.
1.3 REPORT ORGANIZATION
In addition to this Introduction, this report is organized into the following chapters:
Chapter 2: System Overview and Land Use Information – Includes information on collection system
features and quantities, existing and future land use/development assumptions, and existing
parcel information.
Chapter 3: Flow Analysis – Provides detailed information on dry and wet weather flow monitoring
results, winter water use (as a basis for estimating sanitary flows), and development of land use-based
flow factors.
Chapter 4: Hydraulic Model Development and System Evaluation Criteria – Describes the processes
and procedures for setting up, calibrating, and using the hydraulic modeling to simulate both existing
and future flow conditions; presents the criteria used for evaluating the model results.
Chapter 5: Model Results and System Capacity Evaluation – Presents the model results for existing
and future flow conditions; identifies existing or potential system deficiencies.
Chapter 6: Capital Improvement Program Development – Presents the results of a risk analysis
performed by the City to prioritize needed improvements to address non-capacity-related system
deficiencies. Describes the schedule, costs, and prioritization of capacity and condition-related
collection system improvements over a 20-year timeframe.
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2-1 City of San Luis Obispo
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August 2025
CHAPTER 2
System Overview and Land Use Information
The purpose of this chapter is to present relevant information on the City’s wastewater collection system,
land use/zoning, and parcel data. Key topics addressed in this chapter include:
• Collection System Features
• Land Use and Zoning
• Parcel Information
2.1 COLLECTION SYSTEM FEATURES
The City actively maintains a database of its wastewater collection system. The collection system database
includes information for gravity mains, force mains, lift stations, and service laterals. A schematic of the
City’s collection system is shown on Figure 2-1.
A summary of gravity main pipe quantities by pipe diameter, as taken from the City’s wastewater
collection system database, is presented in Table 2-1. As indicated, 6-inch and 8-inch diameter pipes
account for approximately 83 percent of the total pipe count. The total length of all City owned gravity
sewers is approximately 772,000 feet or roughly 146 miles. There are also 3,082 maintenance holes (MHs)
associated with the gravity mains.
Table 2-1. Summary of Gravity Mains by Pipe Diameter
Pipe Diameter, inches Total Length, feet Pipe Count Percentage of Total Count
6 332,281 1,476 45.6
8 289,927 1,207 37.3
10 53,523 217 6.7
12 26,477 101 3.1
15 20,648 74 2.3
18 19,830 70 2.2
21 3,282 10 0.3
24 8,054 31 1.0
27 2,098 6 0.2
30 5,687 16 0.5
36 5,183 11 0.3
48 1,601 5 0.2
Other (a) 3,312 12 0.4
(a) Either not specified or listed as a non-standard diameter.
A summary of gravity main pipes by pipe material, as taken from the collection system database, is
presented in Table 2-2. As indicated, vitrified clay pipe (VCP) is the most common material, accounting for
more than half of all pipe segments. Not shown in the table is that approximately 88 percent of the VCP
mains are 6-inch and 8-inch diameter. In addition, all pipes larger than 24-inch diameter are listed as
polyvinyl chloride (PVC).
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August 2025
Table 2-2. Summary of Gravity Mains by Pipe Material
Pipe Material Total Length, feet Pipe Count Percentage of Total Count
VCP 429,681 1,803 55.5
PVC (a) 198,045 834 25.7
HDPE (b) 95,416 426 13.1
CIPP (c) 22,774 82 2.5
RPP (d) 12,766 54 1.7
FRCP (e) 5,339 17 0.5
Other (f) 5,283 30 0.9
(a) Polyvinylchloride (including 16,578 feet of HOBAS pipe trunk lines).
(b) High density polyethylene (often a liner within a pipe of different material but may also be the original material).
(c) Cured-in-place pipe (a fiber-reinforced plastic resin-based pipe, typically a liner within a pipe of a different material; CIPP has only been
used in special high-risk scenarios, such as creek crossings, as a short-term risk mitigation method designed to defer permanent
replacement due to budget constraints).
(d) Reinforced plastic pipe (trussed).
(e) Fiberglass reinforced cement pipe.
(f) “Other” pipe types Include asbestos concrete, cast iron, corrugated metal, and ductile iron.
The City operates and maintains a total of nine lift stations. Table 2-3 presents summary information for
these stations, including nominal pumping capacity in gallons per minute (gpm), year constructed, force
main diameter, and force main length.
Table 2-3. Summary of Lift Stations and Force Main Information (a)
Lift
Station
Nominal Flow
Capacity, gpm (b)
Year
Constructed
Force Main
Diameter, in Length, ft
Airport 225 1980 6 839
Buckley 739 2021 6 5,378
Calle Joaquin 766 2024 10 3,656
Foothill 300 1986 4 440
Laguna 3,000 2014 16 2,232
Margarita 400 2018 6 231
Prefumo 35 2004 4 595
Silver City 450 1967 6 765
Tank Farm 3,250 2009 14 3,772
(a) Information provided by City staff, unless otherwise indicated.
(b) Equals the sum of the nominal capacity of each pump with the largest pump out of service, and normal sewer service conditions except
for Airport LS (based on draw and fill analysis performed by West Yost) and Tank Farm (based on pump curve analysis performed by
West Yost).
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2-4 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
2.2 LAND USE AND ZONING
The City’s General Plan contains a land use map indicating land use designations for land within the City
limits and also within the City’s Sphere of Influence (SOI). The SOI is defined by Government Code 56425
as “a plan for the probable physical boundary and service area of a local agency or municipality .” The
City’s General Plan land use designations are shown on Figure 2-2.
In addition, the City maintains a zoning map that defines land use zoning designations for a less extensive
area that is confined to within the City limits. The City’s zoning areas are shown on Figure 2-3, and zoning
information is summarized in Table 2-4. At the direction of City staff, the City’s zoning map is used
preferentially over the General Plan land use map throughout this analysis. As of 2025, no additional
annexation applications were available to be considered in this analysis.
As indicated in Table 2-4, a total of approximately 7,400 acres are zoned. The largest zoning category in
terms of acreage is Conservation/Open Space (C/OS), which accounts for approximately 2,500 acres, the
majority of which is located around the perimeter of the City. The largest flow-producing category is Low
Density Residential (R-1), which accounts for approximately 1,700 acres, or 35 percent of all
flow-producing zoned areas.
Table 2-4. Summary of City Zoning Areas
Abbreviation Zoning Designation Category Gross Area, acres
R-1 Low Density Residential Residential 1,710.0
R-2 Medium Density Residential Residential 633.7
R-3 Medium-High Density Residential Residential 213.8
R-4 High Density Residential Residential 220.5
C-C Commercial-Community Commercial 95.2
C-D Commercial-Downtown Commercial 49.5
C-N Commercial-Neighborhood Commercial 39.8
C-R Commercial-Retail Commercial 200.0
C-S Commercial-Service Commercial 532.0
C-T Commercial-Tourist Commercial 119.1
BP Business Park Other 146.4
M Manufacturing Other 255.7
O Office Other 193.5
PF Public Facility Other 467.0
AG Agriculture Zero Flow (a) 51.6
C/OS Conservation/Open Space Zero Flow (a) 2,499.4
Total 7,427.1
Total, Non-Zero Flow 4,876.1
(a) The AG and C/OS zones allow for one residential unit, but the resulting flows are minimal, such that these categories are
considered zero flow.
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Agriculture
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Medium High Density Residential
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Office
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Service and Manufacturing
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Suburban Residential
Tourist
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2.3 PARCEL INFORMATION
The City maintains parcel data within its GIS database covering areas within and around the City limits.
(The San Luis Obispo Planning Area, as defined in the City’s Land Use Element.) The database includes
17,103 parcels totaling approximately 19,000 acres, which is approximately 2.5 times the total area
represented by the zoned areas within City limits shown on Figure 2-3. Most of the parcels that fall outside
the City limits are designated as Open Space in the City’s General Plan, with a few areas designated as
Rural Residential.
The status of development (occupancy) of any given parcel is not specified in the parcel data. Accordingly,
a visual inspection of the existing parcels using recent satellite imagery was performed to identify whether
a parcel is vacant, partly vacant, or occupied. This effort was generally limited to parcels that are 1 acre in
size or larger. In addition, any low-density residential (R-1) parcels that did not have an active water meter
during the 2022/2023 wet season were classified as vacant. (It is understood that this approach may
overstate the number of vacant R-1 parcels, but this approach is nevertheless conservative in evaluating
the ability of the system to accommodate future flows.)
The resulting vacant and partly vacant parcels are used as the basis for determining areas of future
development within the City and are summarized as follows:
• Vacant (excluding R-1): 337 parcels; 618 acres
• Partly vacant: 44 parcels; 218 acres
• Vacant R-1: 609 parcels; 218 acres
The zoning area boundaries largely but not entirely coincide with the parcel boundaries. Notable
exceptions include a small number of larger undeveloped or partially developed parcels that have multiple
associated zoning designations. Eight such parcels totaling 339 acres were identified, mainly in the
southern part of the City.
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CHAPTER 3
Flow Analysis
The purpose of this chapter is to present an analysis of flows in the City’s wastewater collection system
and to use that information to develop flow generation factors for use in the collection system model.
The major topics addressed in this chapter include:
• Winter Water Use
• Collection System Flow Monitoring
• Groundwater Infiltration
• Flow Generation Factor Development
3.1 WINTER WATER USE
Winter water use provides a means of estimating baseline sanitary flows because it coincides with the
period of minimal landscape irrigation, such that most winter water consumption ends up in the
wastewater collection system. Groundwater infiltration (GWI) can then be calculated as the difference
between dry weather sewer flows and winter water use.
For this analysis, City staff provided winter water use data for 2021/22 and 2022/23. Key statistics for both
winter periods are shown in Table 3-1 and include meter counts for the various categories, meter counts
for meters with non-zero usage, City-wide usage (in units of million gallons per day (mgd)), and per meter
usage in units of gallons per day (gpd). As indicated in the table, total residential and commercial usage
decreased from 2021/22 to 2022/23 by 0.53 mgd (3.97 mgd minus 3.45 mgd). This decrease was likely
due to the unusually wet conditions during the 2022/2023 wet season, which would have reduced
landscape irrigation activities in comparison to the unusually dry 2021/22 wet season. As a result, the
2022/23 winter water use data is likely to be a much better representation of baseline sanitary flows.
3.2 COLLECTION SYSTEM FLOW MONITORING
The following topics related to collection system flow monitoring are discussed in this section:
• 2023/2024 Flow Metering Program
• WRRF Flow Information
• City Flow Metering Data
The flow data presented in this section provide the basis for the calibration of the collection system model,
as described in Chapter 4 of this report.
3.2.1 2023/2024 Flow Metering Program
A collection system flow metering program was conducted during the 2023/2024 wet season and covered
the period of December 19, 2023 through March 26, 2024. The flow metering program consisted of a
reduced list of flow metering sites than those previously monitored in 2010 and 2011 (and documented
in the 2012 I&I Study), with minor adjustments to the exact metering locations. The 2023/2024 metering
sites focused on areas known or suspected to be capacity-constrained and/or to have high levels of
infiltration and inflow (I&I) during wet weather events. The flow metering locations and their tributary
areas are shown on Figure 3-1.
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August 2025
Table 3-1. Winter Water Use Summary Information
Statistic Winter 2022/23 Winter 2021/22
Meter Count
Total 16,728 16,379
Residential 14,384 14,026
Commercial, Industrial, Institutional 1,730 1,736
Landscape 536 546
Recycled 79 72
Meter Count: Usage>0
Total 15,637 15,654
Residential 13,862 13,654
Commercial, Industrial, Institutional 1,502 1,521
Landscape 225 416
Recycled 48 63
Residential/Commercial Usage, mgd
Total 3.45 3.97
Residential 2.20 2.55
Commercial, Industrial, Institutional 1.25 1.42
Flow per Meter, gpd/meter
Residential 158.7 186.8
Commercial, Industrial, Institutional 833.4 930.8
A total of 16 sites were flow metered during the monitoring period. The field activities were conducted
by V&A Consulting Engineering, and Hach FL900 flow metering equipment was used. This technology uses
submerged sensors with pressure transducers to collect depth readings and ultrasonic Doppler sensors to
determine flow velocities.
Daily rainfall totals during the flow monitoring period are shown on Figure 3-2. The results shown in the
figure represent the average of four rain gauges (RG) locations associated with publicly available private
weather stations. The highest single-day rainfall of 1.94 inches occurred on December 20, 2023, and the
highest two consecutive days of rain occurred on December 19–20, 2023 (2.85 inches).
Storm return period information for the four rain gauges is summarized in Table 3-2. These results are
developed from the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 precipitation
frequency estimates and are shown for 6-hour, 12-hour, 24-hour, or 48-hour durations. As indicated, the
return periods for the largest storm ranged from 0.6 years to 2.4 years, depending on the rain gauge and
the event duration, with an average return period value of 0.8 years for the 24-hour storm. The model
calibration process (described in Chapter 4) produces system parameters that can be extrapolated to
larger storms, including the 10-year, 24-hour event that provides the basis for this analysis.
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
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Daily Rainfall, inFigure 3-2: Four-Gauge Average Rainfall, San Luis Obispo, December 2023–March 2024
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Summary statistics for the flow metering sites are presented in Table 3-3. Key statistics for each flow
metering location include MH ID, flow metering area designation (from the 2012 I&I Study), pipe diameter
(D), average dry period flow, peak hour flow, time of peak hour flow, peaking factor (defined as the peak
hour flow divided by the average dry flow), and the maximum depth of flow. All flows are presented in
units of mgd. The average dry period flow was calculated from dry days (typically in early to mid-January)
when Cal Poly was in session, when flows had stabilized following previous storm events, and before
groundwater levels had built up to their seasonal highs in late winter.
Table 3-2. Maximum Storm Return Periods, December 19, 2023 through March 26, 2024
Rain Gauge (a) 6-Hour Duration 12-Hour Duration 24-Hour Duration 48-Hour Duration
RG Northwest 1.0 0.8 0.6 0.8
RG North 2.4 2.1 1.2 1.3
RG East 1.7 1.5 1.0 1.4
RG Southwest 1.0 0.9 0.7 1.1
4-Gauge Average 1.3 1.2 0.8 1.1
(a) Rain gauge locations were as follows: RG Northwest, Montrose Drive near Skyline Drive; RG North, Serrano Drive near Serrano Circle;
RG East, Johnson Avenue near Pismo Street; RG Southwest, Oceanaire Drive near Mariner Cove.
Table 3-3. Flow Monitoring Program Summary Statistics
Metering
Area
MH
ID
Pipe
D, in
Avg Dry
Flow, mgd
Peak Hour
Flow, mgd
Time of
Peak Flow
Peaking
Factor
Max
Depth, in (a)
A.1 I06-39 10 0.322 1.192 2/19/2024 3:00 3.7 4.1
A.2 H06-5 6 0.010 0.130 2/19/2024 1:00 13.5 3.2
A.3 H05-38 6 0.003 0.041 2/19/2024 3:30 14.5 2.9
A.4 H04-37 10 0.024 0.264 2/4/2024 15:30 11.1 2.7
B.1 J08-43 15 0.309 2.008 2/4/2024 17:00 6.5 7.1
B.2 I06-48 10 0.038 0.426 2/19/2024 2:00 11.3 28.7
B.3 I06-41 8 0.011 0.217 2/19/2024 1:45 19.2 4.1
H.1 J10-52 12 0.058 0.819 2/4/2024 15:45 14.1 4.4
H.2 J11-45 12 0.077 0.647 2/19/2024 4:00 8.4 3.4
J.1 J12-10 18 0.272 1.638 2/4/2024 15:15 6.0 7.9
M.1A H15-16 21 0.378 1.246 2/19/2024 11:15 3.3 10.3
M.2 G13-15 12 0.092 0.269 3/2/2024 8:45 2.9 5.8
P.1 K09-56 10 0.074 0.244 2/4/2024 14:45 3.3 3.9
P.2 L09-20 8 0.047 0.236 2/4/2024 15:15 5.0 4.9
Q.1 K10-22 8 0.030 0.241 2/19/2024 3:45 7.9 4.3
Q.2 L10-17 6 0.017 0.278 2/5/2024 13:00 16.4 11.7
(a) Yellow highlighting indicates surcharging at peak flow conditions.
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It is notable that peak flows did not coincide with the largest storm event at any of the flow meters.
Instead, peak flows occurred on February 4–5, 2024 (maximum 24-hour rainfall of 1.38 inches),
February 19, 2024 (maximum 24-hour rainfall of 1.44 inches), and March 2, 2024 (maximum 24-hour
rainfall of 1.56 inches). These results indicate that saturated soils and high groundwater levels are likely
at least as significant as rainfall magnitude in influencing peak wet weather flow conditions. As indicated
in the table, two flow metering locations (SB2 and SQ2) underwent surcharging above the pipe crown
during peak flow conditions.
3.2.2 WRRF Flow Information
Influent flow information from the City’s Water Resource Recovery Facility (WRRF) is used in this analysis
for determining total collection system flow for dry and wet weather conditions, both specific to the flow
monitoring period described above and more generally for recent historical periods. In support of this
effort, City staff provided average daily and peak instantaneous flows for each calendar day for the
six-year period of January 2018 through July 2024. Based on this information, the average and peak flows
for each month are shown on Figure 3-3. In addition, Figure 3-3 also shows total monthly rainfall and
maximum 2-day rainfall for each month, as taken from the California Irrigation Management Information
System (CIMIS) rain gauge #52 located adjacent to the practice field near the intersection of Via Clara and
Highland Drive on the Cal Poly campus.
Table 3-4 shows summary statistics by calendar year for the WRRF and the CIMIS rain gauge. Key statistics
include annual average flow, average dry weather flow (Qa), peak wet weather flow (Qp), the date on
which the Qp occurred, the two-day rainfall associated with the Qp, and the total annual rainfall. In general,
the Qa value is the average for the month of October, which corresponds to when Cal Poly is in session
and therefore provides a reasonable baseline condition for maximum occupancy conditions. The average
of the Qa values is 3.10 mgd, which is 93.2 percent of the winter water use for the 2022/2023 period. This
result is consistent with return-to-sewer ratios, which commonly range from 90 to 95 percent in other
areas during wet winters.
Table 3-4. WRRF Summary Statistics by Calendar Year
Year
Annual Average
Flow, mgd
Qa,
mgd (a)
Qp,
mgd
Date
of Qp
2-Day Rainfall
at Qp, in (a)
Total Annual
Rainfall,in (a)
2018 3.26 3.21 19.91 3/22/2018 4.07 17.93
2019 3.62 3.19 20.72 12/3/2019 0.99 28.58
2020 2.90 2.93 14.69 3/16/2020 2.50 11.04
2021 2.96 3.05 17.17 12/23/2021 3.25 21.43
2022 3.02 3.14 19.09 12/10/2022 2.88 14.87
2023 4.38 (b) 25.94 1/9/2023 6.05 34.85
2024 4.13 (c) 14.91 4/14/2024 2.28 (b)
(a) CIMIS gauge #52, San Luis Obispo, Lat/Long: 35.305442, -120.66178
(b) Data not available due to WRRF construction activities and instrumentation commissioning.
(c) Incomplete year; not all data are available.
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0
5
10
15
20
25
30
35
400
5
10
15
20
25
30
35
40
Jan-18Mar-18May-18Jul-18Sep-18Nov-18Jan-19Mar-19May-19Jul-19Sep-19Nov-19Jan-20Mar-20May-20Jul-20Sep-20Nov-20Jan-21Mar-21May-21Jul-21Sep-21Nov-21Jan-22Mar-22May-22Jul-22Sep-22Nov-22Jan-23Mar-23May-23Jul-23Sep-23Nov-23Rainfall, inInfluent Flow, mgdFigure 3-3. Monthly WWRF Flows and Rainfall, 2018–2024
Peak Flow Total Rainfall Average Flow Max 2-Day Rainfall
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Table 3-5 shows flow and rainfall statistics for the 25 largest peak wet weather flow events during the
period of 2018 through 2023. As indicated in the table, the top four peak wet weather flows all occurred
in January and March 2023. The largest such flow (25.94 mgd) occurred on January 9, 2023, during which
6.05 inches of rain fell on consecutive days, and 5.38 inches of rain fell in 24 hours. This latter result
corresponds to a 9.4-year return period for a 24-hour event (based on NOAA Atlas 14 precipitation
frequency estimates). Based on a review or historical data, City staff determined that peak flows during
this event provide an appropriately conservative estimate of the peak flow associated with a 10 -year,
24-hour storm event.
Table 3-5. Top Ten WRRF Peak Wet Weather Flows, 2018–2023
Rank Peak Flow, mgd Date of Peak Flow Total Daily Flow, mgd 2-Day Rainfall, in (a)
1 25.94 1/9/2023 17.00 6.05
2 25.65 3/14/2023 7.75 3.92
3 24.59 3/10/2023 16.54 3.09
4 21.47 1/14/2023 10.07 2.25
5 20.72 12/3/2019 3.07 0.99
6 19.91 3/22/2018 4.07 4.07
7 19.09 12/10/2022 5.06 2.88
8 18.15 12/31/2022 6.92 2.28
9 17.17 12/23/2021 6.63 3.25
10 16.83 1/17/2019 4.43 1.78
(a) CIMIS gauge #52, San Luis Obispo, Lat/Long: 35.305442, –120.66178
3.2.3 City Flow Metering Data
The City provided flow and/or pump operations data for the following lift station (LS) facilities and gravity
flow metering locations:
• Airport LS
• Laguna LS
• Margarita LS
• Tank Farm LS
• Cal Poly (gravity)
• Madonna Inn (gravity)
For all of the above except Airport LS and Tank Farm LS, the City provided flow data on 5-minute intervals.
For Airport LS and Tank Farm LS, flows were calculated based on an analysis of pump curve information
and pump operational data. The focus of the data obtained for the above sites was the period of
October 2022 through March 2023, which covers the high-flow periods at the WRRF discussed above.
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Table 3-6 summarizes average and peak flows at the above sites for the period of October 2022 through
March 2023. As indicated in the table, peak flow conditions occurred on January 9, 2023 at all sites except
Madonna Inn. The low peaking factor of 2.3 at Madonna Inn, in combination with the fact that peak flows
occurred on a day with no rainfall, indicates that flows from this site are not significantly influenced by
wet weather conditions.
Table 3-6. City Flow Metering Results, October 2022 through March 2023
Metering
Location
Avg Dry
Flow, mgd
Peak Hour
Flow, mgd
Time of
Peak Flow
Peaking
Factor
Airport LS 0.037 0.392 1/9/23 6:35 10.7
Laguna LS 0.782 3.629 1/9/23 16:00 4.6
Margarita LS 0.031 0.211 1/9/23 15:40 6.9
Tank Farm LS 0.481 2.380 1/9/23 14:50 4.9
Cal Poly 0.244 1.699 1/9/23 15:45 7.0
Madonna Inn 0.027 0.064 3/2/24 18:15 2.3
3.3 GROUNDWATER INFILTRATION
GWI at a given location in the collection system can be estimated by calculating the difference between
the average flows during dry weather periods and the winter water use coming from all upstream areas.
GWI varies over time and is generally lowest in the late dry season and highest in the winter or early spring
following prolonged rainy periods. Figure 3-4 shows the average rate of GWI throughout the period of
January through April 2024. As indicated in the figure, GWI increased each month through the wet season,
indicating increasing groundwater levels. Given an estimated Qa value of 3.10 mgd (as discussed above),
GWI expressed as percentage of Qa increases from 16 percent in January 2024 to 57 percent in April 2024.
GWI calculations were also done for each of the tributary areas that were flow metered during the
2023/2024 flow monitoring period. Table 3-7 shows cumulative flows at each flow meter for the dry
weather period of February 14–16, 2023, while Table 3-8 shows incremental flows (defined as the
calculated results with all upstream flow meter results subtracted out) for the same period. Flow
meter B.1 is excluded from these tables due to multiple upstream flow splits that prevent the
determination of the tributary area.
Key findings from the analysis of GWI include:
The A.1 area (shown on Figure 3-1 above) in the largest contributor of GWI of any area in
terms of both incremental volume (~0.44 mgd) and GWI per unit area (~1,800 gpd/acre).
Other areas that contributed approximately 0.10 mgd or more include H.2 (~0.10 mgd), SJ1
(~0.23 mgd), M.1A (~0.14 mgd), and all areas downstream of the metering sites (calculated
by difference as 0.29 mgd).
Other areas that contributed GWI at a rate of 1,000 gpd/acre or more include B.3
(~1,000 gpd), H.2 (~1,100 gpd/acre), and Q.2 (~1,000 gpd/acre).
Based on the flow metering results, the net GWI contributions from areas B.2, H.1, P.1, and
Q.1 are estimated to be zero.
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GWI=0.46 mgd
GWI=1.39 mgd
GWI=1.58 mgd GWI=1.73 mgd
0
2
4
6
8
10
12
14
16
180
1
2
3
4
5
6
7
8
9
Jan-24 Feb-24 Mar-24 Apr-24 Rainfall, inFlow, mgdFigure 3-4. Wastewater Flow Components, January through April 2024
Qa GWI Total Rainfall
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Table 3-7. GWI, February 14–16, 2023 Dry Weather Period, Cumulative Flows (a)
Metering
Location
Tributary
Area, acres
Cumulative Winter
Water Use, mgd
February 14–16
Flow, mgd
February 14–16,
GWI, mgd
Cumulative GWI,
Rate gpd/acre
A.1 359.4 0.212 0.670 0.458 1,274
A.2 45.7 0.015 0.019 0.005 103
A.3 24.8 0.0038 0.0064 0.0026 103
A.4 67.5 0.023 0.035 0.012 170
B.2 49.8 0.049 0.076 0.027 537
B.3 33.0 0.020 0.054 0.034 1,038
H.1 91.9 0.078 0.138 0.060 653
H.2 90.7 0.079 0.178 0.098 1,082
J.1 563.5 0.384 0.618 0.234 415
M.1A 609.3 0.416 0.577 0.161 265
M.2 165.2 0.085 0.102 0.018 107
P.1 108.5 0.095 0.108 0.013 121
P.2 79.6 0.062 0.084 0.022 271
Q.1 48.0 0.038 0.060 0.022 456
Q.2 26.0 0.015 0.041 0.025 980
Madonna 30.4 0.014 0.022 0.008 248
Cal Poly 516.9 0.288 0.395 0.107 206
Airport 136.6 0.051 0.047 0 0
Laguna 1,036.0 0.695 1.056 0.361 349
Margarita 40.1 0.030 0.028 0 0
Tank Farm 736.0 0.351 0.582 0.231 314
WRRF 4,696 3.313 4.657 1.328 283
(a) Cumulative flows refer to results calculated irrespective of any upstream flow meters.
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Table 3-8. GWI, February 14–16, 2023 Dry Weather Period, Incremental Flows (a)
Metering
Location
Net Tributary
Area, acres
Incremental Winter
Water Use, mgd
February 14–16
Incremental Flow, mgd
February 14–16,
Incremental GWI, mgd
Incremental GWI,
Rate gpd/acre
A.1 246.1 0.174 0.609 0.435 1,768
A.2 21.0 0.011 0.013 0.002 102
A.3 24.8 0.004 0.006 0.003 103
A.4 67.5 0.023 0.035 0.012 170
B.2 16.9 0.029 0.022 0 0
B.3 33.0 0.020 0.054 0.034 1,038
H.1 43.8 0.040 0.037 0 0
H.2 90.7 0.079 0.178 0.098 1,082
J.1 563.5 0.384 0.618 0.234 415
M.1A 413.7 0.317 0.453 0.136 329
M.2 165.2 0.085 0.102 0.018 107
P.1 28.9 0.033 0.024 0 0
P.2 79.6 0.062 0.084 0.022 271
Q.1 22.0 0.023 0.020 0 0
Q.2 26.0 0.015 0.041 0.025 980
Madonna 30.4 0.014 0.022 0.008 248
Cal Poly 516.9 0.288 0.395 0.107 206
Airport 136.6 0.051 0.047 0 0
Laguna 426.7 0.280 0.479 0.200 468
Margarita 40.1 0.030 0.028 0 0
Tank Farm 572.9 0.297 0.582 0.285 498
WRRF 1,102.9 1.051 1.938 0.888 805
(a) Incremental flows refer to results calculated with all upstream flow meter results subtracted out.
3.4 FLOW GENERATION FACTOR DEVELOPMENT
For this analysis, proposed flow generation factors are derived on a per acre basis for each category
included in the City’s zoning map (shown on Figure 2-3 in Chapter 2 of this report). The intent of these
factors is that they be used to evaluate future collection system flow conditions, and that they be applied
either to existing vacant parcels or to parcels that are slated for redevelopment. For existing occupied
parcels that are not slated for redevelopment, it is assumed that the existing winter water use
characteristics of the given parcel would remain applicable in the future, such that future flows are
assumed to equal current flows.
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It can be expected that a high degree of parcel-to-parcel flow variability exists within each land use
category. Therefore, for planning of future collection system conditions, it is appropriate to develop flow
generation factors that have a high degree of conservatism. The method used in this analysis to derive
such factors is referred to here as the “Upper Average Method” and involves the following steps:
For existing occupied parcels, establish the land use category of each parcel by overlaying
the City’s GIS Land Use layer upon the existing parcel layer.
For each parcel, determine the daily average flow generation (qi) based on the winter water
use associated with that parcel.
For each parcel, determine the per acre flow generation rate (ri) by dividing the parcel flow
generation by the parcel acreage (ai), such that ri = qi/ai.
For each land use category (LU), calculate the average per acre flow generation rate (RLU) by
dividing the total winter water use for that category (QLU) by the total occupied acreage for
that category (ALU), such that RLU = QLU/ALU.
For each land use category, calculate the average per acre flow generation rate for only
those parcels that are above RLU, such that:
Flow Generation Factor = Average(ri) where ri > RLU
The results obtained using this method are shown in Table 3-9. The proposed flow generation factors are
rounded to the nearest 100 gpd/acre.
Table 3-9. Proposed Flow Generation Factors
Code Description
Flow Generation, gpd/acre
Average Upper Average Proposed Factor
AG Agriculture 0 0 0
BP Business Park 524 3,757 3,800
C/OS Conservation/Open Space 0 0 0
C-C Community Commercial 1,108 2,337 2,300
C-D Downtown Commercial 2,106 7,183 7,200
C-N Neighborhood Commercial 1,340 3,241 3,200
C-R Retail Commercial 670 2,506 2,500
C-S Service Commercial 296 1,116 1,100
C-T Tourist Commercial 1,415 3,400 3,400
M Manufacturing 359 1,369 1,400
O Office 826 1,647 1,600
PF Public Facility 148 500 500
R-1 Low Density Residential 567 1,126 1,100
R-2 Medium Density Residential 956 2,020 2,000
R-3 Medium-High Density Residential 1,365 3,190 3,200
R-4 High Density Residential 1,785 4,722 4,700
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CHAPTER 4
Hydraulic Model Development and System Evaluation Criteria
The purpose of this chapter is to present the steps involved in developing and calibrating the collection
system model and establishing the criteria used for evaluating system capacity deficiencies. The major
elements of this chapter include:
• Model Development
• Model Calibration
• Design Storm Development
• System Evaluation Criteria
4.1 MODEL DEVELOPMENT
The key elements of collection system model development addressed in this section include the following:
• Modeling Software
• Model Geometry
• Assignment of Flows to Model Nodes
4.1.1 Modeling Software
The hydraulic model of the City’s collection system uses AquaTwin Sewer software developed by
Aquanuity. AquaTwin Sewer is a fully dynamic modeling program that operates within an ArcGIS Pro
environment and is powered by the EPA SWMM5 engine (version 5.2.3). The flow input fields include:
• Inflow Dry Weather Flow (DWF)
• Inflow Direct
• Inflow Rainfall-Dependent Infiltration and Inflow (RDII)
For this model, the Inflow DWF field is used to represent sanitary flow, the Inflow Direct field is used to
represent GWI, and the Inflow RDII is used to represent RDII. Sanitary inputs are assigned to each model
node (MH or lift station), and a diurnal flow pattern is assigned to each such input. GWI is treated as a
constant value throughout each simulation. RDII is dependent on the magnitude of the design storm and
on the wet weather calibration parameters.
4.1.2 Model Geometry
The collection system model includes all gravity mains, MHs, lift stations, and force mains in the City’s
sewer GIS database. The City’s GIS database specifies pipe, MH, and lift station locations, pipe diameters,
and pipe connectivity. The GIS database also contains partial information on MH rim and pipe invert
elevations. For locations where such elevations were lacking, this analysis made use of some combination
of the following:
• Information from field measurements or record drawings provided by the City
• Information from the previous model
• Interpolation of inverts between known points
• Minimum standard pipe slope assumptions
• LIDAR data (MH rims only)
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In determining system connectivity, a relatively small number of errors needed to be corrected, including:
• MH ID numbers not matching the upstream or downstream MH ID of the adjacent pipe ID
• Pipe information showing the upstream and downstream MHs reversed
• Pipe information with either the upstream or downstream MHs not specified
Model geometry was also checked and adjusted by performing a flow simulation and observing unrealistic
levels of surcharging or flow fluctuation. Corrections were made on a case-by-case basis.
4.1.3 Assignment of Flows to Model Nodes
The assignment of flows to model nodes involves assigning the following flow elements:
• Sanitary Flows
• Diurnal Flow Patterns
• Groundwater Infiltration
• Rainfall-Dependent Infiltration and Inflow
4.1.3.1 Sanitary Flows
As described in Chapter 3, winter water use from 2022/2023 provides the basis for sanitary flow inputs to
the collection system model. Assigning water use values to individual model nodes is a multi-step process
involving the following steps:
Water meters are assigned to parcels based on matching of street addresses and/or physical
proximity. The street address fields in the water meter database and the parcel database
often do not match exactly, and in some cases could not be matched at all, in which case the
meter-parcel match was established based on physical proximity.
Parcels were matched to collection system MH ID number based on physical proximity. In a
small number of cases, parcels had to be reassigned by visual inspection because the
proximity-based match was determined to be incorrect.
For any given MH (model node), winter water use from all contributing parcels is summed
up to provide the daily average flow input associated with that node.
In some cases, a given parcel may have multiple assigned water meters, while in other cases, a parcel may
have no assigned meters, even though the parcel is occupied. The latter situation is common for medium
and high-density residential parcels, and for many commercial parcels, where an apartment/
condominium complex or business operation occupies multiple parcels but is served by only one City
water meter. By contrast, low-density residential areas typically have a one-to-one correlation between
water meters and occupied parcels.
4.1.3.2 Diurnal Flow Patterns
Diurnal flow patterns were derived for each of the flow meters that were deployed during the 2023/2024
wet season. These patterns are based on data from dry weather days early in the wet season when GWI
was comparatively low. The patterns are calculated on hourly intervals and are expressed in terms of
normalized flow (defined as the hourly flow value divided by the average flow throughout the diurnal
period), with the result being that the average of the values on the curve equals 1.0 regardless of location.
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For this analysis, the assumed diurnal flow pattern assigned to the flows entering a given node is assumed
to be the same diurnal flow pattern observed for the given flow metering area within which the node is
located. For example, if a node is located upstream of flow meter A.1 but downstream of flow meters A.2,
A.3, and A.4, that flow input is assigned the diurnal pattern for flow meter A.1.
Because flow data for the WRRF are only available as daily flow totals (and daily peak flows), a diurnal
curve could not be developed from WRRF data. Instead, areas downstream of all the flow meters are
assigned an aggregate diurnal flow curve that was derived by averaging of all the other diurnal flow curves.
This aggregate curve is shown on Figure 4-1.
4.1.3.3 Groundwater Infiltration
GWI is assigned to the model by multiplying the area tributary to each node by the assumed GWI rate, as
shown in Table 3-6 in Chapter 3. As with the diurnal flow patterns, the assignment of GWI rates is based
on the flow metering area within which the node is located. For example, if a given flow metering area is
determined to have a GWI rate of 1,000 gpd/acre, and if a given node is connected to 10 acres of parcels,
the total GWI entering the model at that node is assumed to be 10,000 gpd. The tributary area to a given
node is assumed to exclude parcels with an assigned land use of either Agriculture (AG) or
Conservation/Open Space (C/OS). In addition, for existing conditions only, any currently vacant parcels
are also excluded from the calculation of the assigned parcel area.
4.1.3.4 Rainfall-Dependent Infiltration and Inflow
As with the assignment of diurnal flow patterns and GWI rates, model nodes are assigned RDII-related
parameters based on the flow metering area within which they are located. The derivation (calibration)
of RDII-related parameters is discussed in the next section.
4.2 MODEL CALIBRATION
Elements of model calibration discussed in this section include:
• Criteria for Model Calibration
• RTK Method
• Model Calibration Results
4.2.1 Criteria for Model Calibration
The criteria used for assessing the accuracy of the model calibration are based on standards developed
by the Chartered Institution of Water and Environmental Management (CIWEM). The criteria for assessing
the accuracy of the calibration of dry weather flow conditions are summarized as follows:
• The volume of modeled versus metered flow should be in the range of ±10-percent
• The modeled versus metered peak flow rate should be in the range of ±10-percent
• The modeled versus metered peak flow depth should be in the range of ±0.5 feet
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0:001:002:003:004:005:006:007:008:009:0010:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:000:00Normalized FlowTime of Day
Figure 4-1. Aggregate Normalized Diurnal Flow Curve for Downstream (Unmetered) Areas
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The criteria used for assessing the accuracy of the calibration of wet weather flow conditions are
summarized as follows:
• The modeled versus metered peak flow rate should be in the range of minus 15 percent to
plus 25 percent
• The volume of modeled versus metered flow during and following the calibration storm
should be in the range of minus 10 percent to plus 20 percent
• The modeled versus metered non-surcharged flow depth should be in the range of ±0.5 feet
• The modeled versus metered depth of surcharge should be in the range of minus 0.5 feet to
plus 1.6 feet
In general, the above criteria are viewed as objectives rather than as inviolable standards. If one or more
of these criteria are not met, any such deviation is viewed in terms of whether the model maintains
acceptable accuracy while remaining adequately conservative in its predictive capabilities, based on
professional judgment. Of the above criteria, agreement between modeled and metered peak flows is
considered the most important because peak flows define the sizing needs of existing and future
collection system facilities.
4.2.2 RTK Method
This analysis uses the RTK method for wet weather calibration of the collection system model. The RTK
parameters are defined as follows:
• R: The portion of total rainfall within the tributary area that enters the collection system in
the form of RDII.
• T: The time from the onset of rainfall to the peak of the RDII response.
• K: The ratio of the time-to-recession to the time-to-peak of the hydrograph, where the time-
to-recession is defined as the time from the peak to the point in time where there is no
longer any appreciable RDII response.
The RTK parameters are specific to each flow-metered area for which flow data exist, including areas
downstream of the meters used in the flow monitoring program but upstream of the WRRF. The resultant
RTK parameters established through calibration are then applied to the collection system model using the
10-year, 24-hour design storm.
For future development areas, City staff have provided instructions that assumed RDII rates should be
reduced by approximately 50 percent in comparison to existing development areas. The justification for
this assumption is that newly constructed collection system facilities will be significantly less porous than
older facilities.
4.2.3 Model Calibration Results
For the flow metering locations from the 2023/2024 wet season flow monitoring program, the model
results were calibrated to the period of February 16–22, 2024. The first two days represent dry weather
baseline flow condition immediately preceding the February 18–19 wet weather calibration storm. During
the February 16–17 period, flows had stabilized after a period of declining GWI after a series rainfall events
in late January and early February.
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For the various flow metered lift station and City SCADA-connected locations described in Chapter 3, the
model results were calibrated to the period of January 7–13, 2023. This period includes the major storm
event that occurred on January 9, 2023, discussed in Chapter 3.
A typical result for the February 16–17, 2024 dry weather period is shown on Figure 4-2, which shows the
calibration results for flow metering site M.1A serving areas of southwest San Luis Obispo west of the
US 101 highway. The results for site M.1A for the entire February 16–22, 2024 period are shown on
Figure 4-3. A typical result for the January 7–13, 2023 period is shown on Figure 4-4, which shows the
calibration results for Tank Farm LS.
Time series plots comparing modeled and metered flow results for the flow metering locations discussed
in Chapter 3 are shown in Appendix A. A comparison of modeled and metered peak flow results is
presented in Table 4-1.
In addition, the model was calibrated to match modeled versus measured peak flows at the WRRF during
the January 9, 2023 storm event. City staff confirmed that WRRF flows during this event were a valid
representation of 10-year, 24-hour storm conditions.
Table 4-1. Modeled versus Metered Peak Flows
Metering
Site
Calibration
Period
Modeled Peak
Flow, mgd
Metered Peak
Flow , mgd
Modeled Peak/
Metered Peak, %
A.1 Feb 16–22, 2024 1.158 1.192 102.93
A.2 Feb 16–22, 2024 0.126 0.130 103.47
A.3 Feb 16–22, 2024 0.0415 0.0409 98.66
A.4 Feb 16–22, 2024 0.187 0.180 96.26
B.2 Feb 16–22, 2024 0.409 0.426 104.28
B.3 Feb 16–22, 2024 0.226 0.217 96.04
H.1 Feb 16–22, 2024 0.698 0.708 101.47
H.2 Feb 16–22, 2024 0.636 0.647 101.65
J.1 Feb 16–22, 2024 1.586 1.610 101.54
M.1A Feb 16–22, 2024 1.269 1.246 98.16
M.2 Feb 16–22, 2024 0.213 0.237 111.36
P.1 Feb 16–22, 2024 0.188 0.165 87.95
P.2 Feb 16–22, 2024 0.167 0.180 108.10
Q.1 Feb 16–22, 2024 0.234 0.241 102.96
Q.2 Feb 16–22, 2024 0.151 0.153 101.35
Cal Poly Jan 7–13, 2023 1.694 1.699 100.28
Airport LS Jan 7–13, 2023 0.397 0.392 98.78
Laguna LS Jan 7–13, 2023 3.374 3.629 107.54
Margarita LS Jan 7–13, 2023 0.209 0.211 101.31
Tank Farm LS Jan 7–13, 2023 2.385 2.369 99.37
WRRF Jan 7–13, 2023 25.926 25.940 100.05
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0
0.1
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Flow, mgdFigure 4-2. Site SM1A, Model vs Flow Meter, Dry Weather Conditions
Flow Meter
Model Output
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Rainfall
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Rainfall, in/hrFlow, mgdFigure 4-4. Tank Farm: Model vs Flow Meter, January 7–13, 2023
Rainfall
Flow Meter
Model Output
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Hydraulic Model Development and System Evaluation Criteria
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4-10 City of San Luis Obispo
Infrastructure Renewal Strategy
July 2025
4.3 DESIGN STORM DEVELOPMENT
For modeling purposes, 10-year, 24-hour design storm flow conditions provide the basis for assessing
collection system capacity deficiencies. The 10-year, 24-hour design storm pattern is based on a US Soil
Conservation Service (SCS) Type I rainfall pattern, which is applicable to coastal and valley areas of central
and southern California. The SCS Type I storm pattern is shown on Figure 4-5.
The NOAA estimates for the 10-year, 24-hour storm for the CIMIS rain gauge and the four other rain
gauges referred to in Chapter 3 are as follows:
• CIMIS: 5.46 inches
• RG Northwest: 5.09 inches
• RG North: 4.89 inches
• RG East: 4.88 inches
• RG Southwest: 4.38 inches
In general, rainfall magnitude appears to decrease toward the west and south, such that the northeast
portion of the City has the highest rainfall magnitudes, and the southwest portion of the City has the
lowest. Given this spatial variation, using a value from near the downtown area appears to be
appropriately conservative. Specifically, at the intersection of Highway 101 and Highway 1, the NOAA 10-
year, 24-hour storm event magnitude is estimated to be 4.78 inches. Accordingly, this value is used in this
analysis for the modeling of existing and future peak flow conditions.
4.4 SYSTEM EVALUATION CRITERIA
The following set of system evaluation criteria is used in this analysis. These criteria are application at
peak flow conditions:
Gravity mains:
• Sanitary sewer overflow (SSO): Surcharging to within 1 foot of the ground surface
• Excessive surcharging: Surcharging 1 to 4 feet from the ground surface
• Moderate surcharging: Surcharging 4 to 8 feet from the ground surface
• Minor surcharging: Surcharging more than 8 feet from the ground surface
• d/D exceedance: No surcharging but the d/D criteria from the 2015 WCIRS is exceeded
Force mains:
• Over capacity: Any force main flowing in excess of 8 fps
Pump stations:
• Over capacity: Any pump station that exceeds its firm pumping capacity (same as the
2015 WCIRS)
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23Rainfall, in/hrHour
Figure 4-5. 10-Year, 24-Hour Storm, SCS Type I Pattern
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5-1 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
CHAPTER 5
Model Results and System Capacity Evaluation
The purpose of this chapter is to present the wastewater collection system modeling results for both
existing and future flow (buildout) conditions, and to evaluate and revise (if appropriate) the City’s existing
Wastewater Flow Offset Program. The key sections of this chapter include:
• Previously Identified Deficiencies
• Modeled Existing Conditions Evaluation
• Modeled Buildout Conditions Evaluation
• Wastewater Flow Offset Program Evaluation
5.1 PREVIOUSLY IDENTIFIED DEFICIENCIES
A total of 14 gravity sewer improvement projects were identified in the 2015 WCSIRS. The projects all
were modeled as being capacity deficient, and several of them also exhibited significant condition-related
problems. The project locations are shown graphically on Figure 5-1 (equivalent to Figure 6-2 in the
2015 WCSIRS). The exceedances represent future flow conditions; however, the 2015 WCSIRS showed
very little difference in the number of capacity exceedances (using the d/D criteria discussed in Chapter 4)
between existing and future. Specifically, the 2015 WCSIRS model showed 245 gravity sewer capacity
exceedances for existing conditions and 251 gravity sewer capacity exceedances for future conditions.
Figure 5-1. Gravity Sewer Capacity Exceedances, 2015 WCSIRS
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5-2 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
The 2015 WCSIRS analysis indicated that only one lift station, Calle Joaquin LS, significantly exceeded its
firm capacity rating for existing and future conditions and was scheduled to be upgraded accordingly. In
addition, the 2015 WCSIRS stated that the Airport LS, Foothill LS, Margarita LS, and Silver City LS were
scheduled to be replaced due to poor condition. Since 2015, the Margarita LS and Calle Joaquin LS have
been upgraded, and the Buckley LS has been added to the system. Replacement of the Airport LS,
Foothill LS, and Silver City LS has been postponed, pending a review of improvement alternatives.
5.2 MODELED EXISTING CONDITIONS EVALUATION
Model results for existing development conditions for both gravity sewers and lift stations are described
in this section.
5.2.1 Gravity Sewer Results – Existing Conditions
Modeled gravity sewer results for each gravity pipe segment under existing 10-year storm peak flow
conditions are shown graphically on Figure 5-2. The various pipe segments are categorized as follows:
• C-0 – Capacity-driven SSO (freeboard <1 foot)
• C-1 – Capacity-driven excessive surcharging (freeboard<4 feet)
• C-2 – Capacity-driven moderate surcharging (freeboard>4 feet and <8 feet)
• C-3 – Capacity-driven minor surcharging (freeboard>8 feet)
• C-4 – No surcharging; d/D exceeds criteria used in the 2015 WCSIRS
• B-0 – Backwater-driven SSO (freeboard <1-foot)
• B-1 – Backwater-driven excessive surcharging (freeboard<4 feet)
• B-2 – Backwater-driven moderate surcharging (freeboard>4 feet and <8 feet)
• B-3 – Backwater-driven minor surcharging (freeboard>8 feet)
Backwater-driven surcharging refers to pipe segments that are below their full-pipe capacity under peak
flow conditions but are nevertheless experiencing surcharging due to downstream capacity restrictions.
This distinction is important because for a pipe that flows below its capacity under peak flow conditions
but experiences surcharging, upsizing of that segment may not be justified. Backwater-driven surcharging
can be addressed by increasing the capacity of downstream facilities or redirecting flow into other parts
of the collection system.
Page 289 of 350
TFRRWU
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Laguna Lake
California
Polytechnic
State University
TFRRWU Water Resource Recovery
Facility
Gravity Sewers
No Restriction, Diameter ≥12"
No Restriction, Diameter <12"
C-0 − Capacity-Driven SSO
C-1 − Capacity-Driven Excessive
Surcharging
C-2 − Capacity-Driven Moderate
Surcharging
C-3 − Capacity-Driven Minor
Surcharging
C-4 − No Surcharging; d/D
Criteria Exceeded
B-0 − Backwater-Driven SSO
B-1 − Backwater-Driven
Excessive Surcharging
B-2 − Backwater-Driven
Moderate Surcharging
B-3 − Backwater-Driven Minor
Surcharging
Gravity Sewer Results
Existing Conditions02,500 5,000
FeetN
City of San Luis Obispo
Wastewater Collection System
Infrastructure Renewal Strategy
Prepared for:Prepared by:
Fig 5-3: W:\Clients\1027 City of San Luis Obispo\60-23-02 WW Inf Strategy\GIS\MXD\Figures\Workshop1\SLO.aprx - mjimenez - 7/10/2025
Figure 5-2Page 290 of 350
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5-4 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
The collection system model indicates the following gravity sewer restrictions for existing conditions:
Approximately 5,300 feet of 10-inch and 12-inch diameter pipe along Foothill Boulevard,
Ramona Drive, Broad Street, Murray Drive, and Chorro Street.
Approximately 3,800 feet of 6-inch and 8-inch diameter pipe from Monterey Street to
San Luis Drive.
Approximately 700 feet of 6-inch diameter sewer from Fredericks Street to Grand Avenue.
Approximately 1,300 feet of 6-inch and 8-inch diameter pipe along Broad Street from
Mountain View Street to Peach Street.
Approximately 1,300 feet of 6-inch diameter pipe from a point between Murray and
West Streets to Montalban Street.
Approximately 700 feet of 8-inch diameter sewer from California Boulevard to Casa Street.
Approximately 1,800 feet of 15-inch diameter sewer from Monterrey Street to Marsh Street.
In addition, the collection system model indicates the following restrictions associated with inverted
siphons for existing conditions:
Approximately 140 feet of dual inverted siphon beneath San Luis Obispo Creek on
Nipomo Street near Monterey Street.
Approximately 380 feet of dual inverted siphon beneath Brizzolara- Creek on Santa Rosa Street
(Highway 1).
Approximately 80 feet of dual inverted siphon beneath Laguna Lake drainage channel near
Oceanaire Drive. (This siphon is already scheduled for replacement.)
Approximately 110 feet of inverted siphon beneath San Luis Obispo Creek on Marsh Street.
Figure 5-3 shows flow depths and peak wet weather flows for existing conditions at the various 2023/2024
gravity sewer flow metering sites monitored during the 2023/2024 wet season, as described in Chapter 3.
As indicated, flow metering sites A.1 and B.2 (both located on Ramona Drive) are modeled as flowing full,
which is consistent with Item 1 in the above list of system restrictions.
5.2.2 Lift Station Results – Existing Conditions
Lift station pump capacity and force main results for modeled existing peak flow conditions are
summarized in Table 5-1. The model indicates that the firm pumping capacity is exceeded at Airport LS,
although the Silver City LS is very close to its firm capacity at peak flow conditions. Force main velocities
are well below the 8 fps threshold for all lift stations.
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5-5 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Table 5-1. Modeled Lift Station and Force Main Results – Existing Peak Flow Conditions
Lift
Station
Nominal Firm
Capacity, mgd
Peak Flow,
mgd (a)
Force Main
Diameter, in
Force Main
Peak Velocity, ft/s (a)
Airport 0.32 0.44 6 3.5
Buckley 1.06 0.044 6 0.3
Calle Joaquin 1.10 0.60 10 2.5
Foothill 0.43 0.083 4 1.5
Laguna 4.32 2.82 16 3.1
Margarita 0.58 0.24 6 1.9
Prefumo 0.050 0.025 4 0.4
Silver City 0.65 0.64 6 5.1
Tank Farm 4.68 2.52 14 3.7
(a) Capacity exceedances shown in yellow.
5.3 MODELED BUILDOUT CONDITIONS EVALUATION
Model results for buildout development conditions for both gravity sewers and lift stations are described
in this section.
5.3.1 Gravity Sewer Results –Buildout Conditions
Modeled gravity sewer results for each gravity pipe segment under buildout 10 -year storm peak flow
conditions are shown graphically on Figure 5 -4. While gravity sewer exceedances are marginally more
severe than for existing conditions, no new system defici encies have been identified under buildout
flow conditions.
Figure 5-5 shows flow depths and peak wet weather flows for buildout conditions at the various
2023/2024 flow metering sites described in Chapter 3. These results are generally similar to but slightly
more severe than those shown on Figure 5-3 below.
5.3.2 Lift Station Results –Buildout Conditions
Lift station pump capacity and force main results for modeled buildout peak flow conditions are
summarized in Table 5-2. The model indicates that firm pumping capacity is exceeded at Airport LS,
Calle Joaquin LS, and Silver City LS, although the exceedances at Calle Joaquin LS and Silver City LS are
slight. Force main velocities are below the 8 fps threshold for all stations.
Page 292 of 350
City of San Luis Obispo
Wastewater Collection Infrastructure Renewal Strategy
August 2025W-C-1027-60-23-02
Source: City of San Luis Obispo
Figure 5-3. Flow Depths and Peak Wet Weather Flow, Modeled Buildout 10-Year Storm Conditions,
Existing and Previous Flow Metering Sites
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Gravity Sewers
No Restriction, Diameter ≥12"
No Restriction, Diameter <12"
C-0 − Capacity-Driven SSO
C-1 − Capacity-Driven Excessive
Surcharging
C-2 − Capacity-Driven Moderate
Surcharging
C-3 − Capacity-Driven Minor
Surcharging
C-4 − No Surcharging; d/D
Criteria Exceeded
B-0 − Backwater-Driven SSO
B-1 − Backwater-Driven
Excessive Surcharging
B-2 − Backwater-Driven
Moderate Surcharging
B-3 − Backwater-Driven Minor
Surcharging
Gravity Sewer Results
Buildout Conditions02,500 5,000
FeetN
City of San Luis Obispo
Wastewater Collection System
Infrastructure Renewal Strategy
Prepared for:Prepared by:
Fig 5-4: W:\Clients\1027 City of San Luis Obispo\60-23-02 WW Inf Strategy\GIS\MXD\Figures\Workshop1\SLO.aprx - mjimenez - 7/9/2025
Figure 5-4Page 294 of 350
City of San Luis Obispo
Wastewater Collection Infrastructure Renewal Strategy
August 2025W-C-1027-60-23-02
Source: City of San Luis Obispo
Figure 5-5. Flow Depths and Peak Wet Weather Flow, Modeled Buildout 10-Year Storm Conditions,
Buildout and Previous Flow Metering Sites
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5-9 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Table 5-2. Modeled Lift Station and Force Main Results –Buildout Peak Flow Conditions
Lift
Station
Nominal Firm
Capacity, mgd
Peak Flow,
mgd (a)
Force Main
Diameter, in
Force Main
Peak Velocity, ft/s (a)
Airport 0.32 0.73 6 5.8
Buckley 1.06 0.44 6 3.4
Calle Joaquin 1.10 1.13 10 4.7
Foothill 0.43 0.086 4 1.5
Laguna (b) 4.32 3.69 16 4.1
Margarita 0.58 0.24 6 1.9
Prefumo 0.050 0.026 4 0.5
Silver City 0.65 0.68 6 5.4
Tank Farm 4.68 4.50 14 6.5
(a) Capacity exceedances shown in yellow.
5.4 WASTEWATER FLOW OFFSET PROGRAM EVALUATION
The following topics relevant to the City’s Wastewater Flow Offset Program are addressed in this section:
• Program Background
• Program Effectiveness and Remaining Risks
5.4.1 Program Background
Private sewer laterals are the sewer lines that connect from a residence or commercial structure up to
and including the point of connection with the publicly owned sewer main, as shown on Figure 5-6.
Pursuant to City Municipal Code Section 13.08.395, inspection and maintenance of private laterals are the
responsibility of the property owner.
As of 2025, the City estimates that there are 13,421 private sewer laterals citywide, including 3,052
such laterals within the proposed updated (draft) capacity -constrained areas. Notably, this total is a
significant reduction from the 7,000 private sewer laterals located in the capacity-constrained areas
developed in 2015.
Significant I&I in the collection system can result in SSOs, which can have public and environmental
health impacts. As discussed in Chapter 3, peak flows into the WRRF have exceeded 25 mgd during
major storm events, whereas average dry weather flows are t ypically below 4 mgd. This excess
wastewater flow increases the costs of pumping, treatment, and chemical usage at the WRRF, and it
consumes capacity in the collection system, resulting in capital requirements to increase pipeline size
and pump station capacity.
The primary goal of the Wastewater Flow Offset (Private Sewer Lateral) and Rebate Programs continues
to be the reduction of I&I from private sewer laterals in an effort to reduce the potential for SSOs, while
accommodating development and furthering the City’s major goal of “Housing and Neighborhood Livability
— Healthy, Safe, and Affordable.”
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Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Source: City of San Luis Obispo
Figure 5-6. Private Sewer Lateral Connection to City Sewer Main
In the fall of 2019, the City Council directed staff to initiate the current private sewer lateral offset
program, which requires existing infrastructure in poor condition to be replaced to offset the impacts of
new development in capacity-constrained areas. Replacement of this existing infrastructure reduces I&I
impacts on the wastewater collection system, providing additional capacity for new development in these
areas. The City adopted its Private Sewer Lateral Offset and Private Sewer Lateral Inspection Upon Sale
requirements (September 3, 2019), and Private Sewer Lateral Rebate program (August 20, 2019), which are
currently in place today.
Private sewer lateral programs similar to the City’s program are common in California communities. City
staff have identified the following cities with similar programs: Santa Barbara (since 2006), Berkeley (since
2006), Richmond (since 2006), Piedmont City (since 2011), Stege Sanitation District (since 2011), Emeryville
(since 2011), Oakland (Since 2012), Ventura (since 2012), Emeryville (since 2011), Alameda (since 2015),
Albany (since (2015), Ojai Valey Sanitation District (since 2015), Santa Cruz (since 2018), Castro Valley
Sanitation District (since 2019) Monterey (since 2019), and San Mateo (since 2020). Many cities in the Bay
Area were either mandated by the U.S. Environmental Protection Agency or the Regional Water Quality
Control Board to adopt such programs. Other programs were developed as part of negotiated settlement
agreements with non-governmental organizations, such as San Francisco Baykeeper.
5.4.2 Program Effectiveness and Remaining Risks
SSOs from the City’s public wastewater collection system have declined from a peak of 35 in 2021 to one
in the first half of 2025. This reduction is attributable to:
1. A dynamic cleaning and repair program that utilizes artificial intelligence (e.g., Argon (formerly
SCREAM) by Jacobs) to address the highest risks first,
2. Significant capital improvement projects, and
3. An extensive SmartCover™ monitoring system that alerts City staff to wastewater flow levels at
key collection system locations.
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Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Nevertheless, private sewer lateral discharges (PSLDs), or spills originating from private sewer laterals
continue to be elevated (as shown by the blue line on Figure 5-7) due to aging systems that commonly
undergo little to no preventative maintenance. (State requirements for tracking PSLDs began in 2022.
Data for 2025 will be completed at year end.) Private sewer laterals in poor condition connect to the City
sewer, such that stormwater infiltration from cracks in the lateral exacerbate problems in
capacity-constrained areas of the collection system. The cumulative effect of such infiltration can result
in downstream surcharging where sewer pipe capacity is exceeded, and flow starts backing up in the pipes
and manholes.
Source: City of San Luis Obispo
Figure 5-7: Public (SSO) and Private (PSLD) Sewer Spill Trends
The Wastewater Flow Offset Program provides several important benefits. It reduces I&I by requiring the
replacement of deteriorated private sewer laterals, which helps prevent SSOs and PSLDs, and improves
overall system capacity. This proactive approach: 1) supports new or intensified development in
capacity-constrained areas while minimizing the need for costly, large-scale public infrastructure projects;
2) demonstrates compliance with state and federal mandates; and 3) reduces the City’s exposure to
regulatory fines, litigation, and consent decrees. It also protects public and environmental health by
minimizing sewer spills while distributing responsibility for infrastructure improvements between property
owners and developers, and aligning San Luis Obispo with other California cities that have implemented
similar programs.
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6-1 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
CHAPTER 6
Capital Improvement Program Development
The purpose of this chapter is to present an updated CIP for the City to use as a basis for its collection
system infrastructure improvement planning. The major topics addressed in this chapter include:
• Cost Estimating Assumptions
• Gravity Sewer Condition Assessment
• Capacity-Related Capital Improvements
• Proposed Capital Improvement Plan
• Capacity-Constrained Areas
• Conclusions and Recommendations
6.1 COST ESTIMATING ASSUMPTIONS
Cost estimates prepared for this report are developed in accordance with the guidelines of the Association
for the Advancement of Cost Engineering (AACE) International for a Class 5 Estimate. AACE International
defines a Class 5 Estimate in the following manner:
Class 5 Estimate: This estimate is prepared based on limited information, where little more than
proposed plant type, its location, and the capacity are known. Strategic planning purposes include,
but are not limited to, market studies, assessment of viability, evaluation of alternate schemes,
project screening, location and evaluation of resource needs and budgeting, and long-range capital
planning. Examples of estimating methods used would include cost/capacity curves and factors,
scale-up factors, and parametric and modeling techniques. The expected accuracy ranges for this
class estimate are -20 to -50 percent on the low side and +30 to +100 percent on the high side.
Construction and capital cost estimates are presented in January 2025 dollars corresponding to an
Engineering News Record Construction Cost Index (ENR CCI) of 13766 (20-city average). Construction costs
were developed based on a combination of data supplied from manufacturers, bids on other wastewater
facilities design projects built by other public agencies, construction costs previously estimated by
West Yost, and standard cost estimating guides. Subsequent preliminary and detailed design efforts will
serve to refine and confirm the estimates presented herein. For ongoing budgeting and planning, costs
should be adjusted to account for inflation and other construction market conditions that may occur
between now and the time a project is constructed.
The development of cost estimates presented in this chapter involves the following cost estimation
methods and assumptions to obtain project base costs:
1. For open-cut gravity sewer construction projects, a unit cost of $35 per inch-diameter
lineal-foot plus $15,000 per manhole is assumed. These costs are assumed to include
excavation/backfill, sheeting and shoring, pipe construction, dewatering, lateral/sewer main
connection, repaving, and mobilization/demobilization.
2. For pump station force main piping, an assumed unit cost of $25 per inch-diameter
lineal-foot is used. These costs are assumed to include excavation/backfill, sheeting and
shoring, pipe construction, dewatering, repaving, and mobilization/demobilization.
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Per City staff, capital costs for the projects presented in this chapter are obtained by marking up the base
costs using to the same mark-ups defined in the 2015 WCSIRS. These mark-ups equate to 58 percent of
base construction costs, and are listed below:
• Construction Contingency: 25 percent
• Project Cost Allowances
— Project Development: 15 percent
— Project Construction Support: 10 percent
It is assumed that the recommended facilities will be developed in public rights-of-way or on public
property; therefore, land acquisition costs have not been included. Construction cost estimates do not
include costs for annual O&M nor any allowance for project financing costs.
6.2 GRAVITY SEWER CONDITION ASSESSMENT
An assessment of gravity sewer physical condition was performed for the City by Jacobs, utilizing a
proprietary tool, Argon (formerly SCREAM), that utilized sewer closed-circuit television (CCTV) and system
data. For each sewer segment, a score of 0 to 100 (where numbers increase with the severity of observed
defects) was assigned based on such considerations as structural condition, maintenance concerns, sags,
root intrusion, corrosion, and infiltration and inflow potential. Based on the results, an overall condition
score of 0 to 5 was assigned, where the scoring reflects the following range of conditions:
• 0 – No observed defects
• 1 – Minimal defects
• 2 – Good condition
• 3 – Fair condition
• 4 – Poor condition
• 5 – Severely defective
The spatial distribution of pipe defect scores is shown on Figure 6-1. A summary of overall pipe score
results by pipe count and total length is presented in Table 6-1. The pipe segment count results are
presented by pipe diameter in Table 6-2. The total pipe lengths by pipe diameter are shown in Table 6-3.
Table 6-1. Summary of Overall Pipe Score Results
Pipe Score Description Pipe Segment Count Total Length, feet
5 Severe 339 81,881
4 Poor 320 80,266
3 Fair 369 87,231
2 Good 329 74,614
1 Minimal Defects 210 48,557
0 No Defects 1,366 282,852
<null> Not Inspected (a) 327 not specified
(a) The following lines were not inspected: small diameter/CIPP pipes, force mains, and siphons. Additionally, pipes not assessed with
Pipeline Assessment Certification Program ratings are excluded.
Page 300 of 350
101
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Figure 6-1
Gravity Sewer
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City of San Luis Obispo
Wastewater Collection System
Infrastructure Renewal Strategy
Prepared for:Prepared by:
Fig 6-1: W:\Clients\1027 City of San Luis Obispo\60-23-02 WW Inf Strategy\GIS\MXD\Figures\Workshop1\SLO.aprx - mjimenez - 7/9/2025
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6-4 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Table 6-2. Pipe Score Results by Pipe Diameter – Segment Counts
Pipe
Diameter, in
Segment Count by Pipe Score Total
4&5
Percent of
Total (4&5) 5 4 3 2 1 0
6 289 245 246 180 85 327 534 81.0
8 38 50 75 85 80 783 88 13.4
10 5 9 21 38 20 119 14 2.1
12 3 7 14 6 9 36 10 1.5
15 2 4 6 4 4 35 6 0.9
18 1 2 4 9 4 35 3 0.5
21 0 1 0 0 2 6 1 0.2
24 1 2 1 3 6 18 3 0.5
27 0 0 0 2 0 0 0 0
30 0 0 0 0 0 0 0 0
36 0 0 0 1 0 0 0 0
48 0 0 1 1 0 0 0 0
Total 339 320 368 329 210 1,359 659 100
Table 6-3. Pipe Score Results by Pipe Diameter – Overall Length
Pipe
Diameter, in
Overall Length (feet) by Pipe Score Total
4&5
Percent of
Total (4&5) 5 4 3 2 1 0
6 70,194 61,282 55,415 34,533 17,985 59,472 131,476 81.1
8 8,387 11,781 18,753 21,808 18,411 158,412 20,168 12.4
10 1,198 2,430 5,227 8,622 5,008 26,920 3,628 2.2
12 749 2,365 3,717 2,279 2,812 10,442 3,114 1.9
15 737 879 1,865 1,323 1,171 9,877 1,616 1.0
18 438 577 1,143 3,241 1,271 9,886 1,015 0.6
21 0 507 0 0 827 1,828 507 0.3
24 178 445 131 1,380 1,072 3,544 623 0.4
27 0 0 0 897 0 0 0 0
30 0 0 0 0 0 0 0 0
36 0 0 0 386 0 0 0 0
48 0 0 500 145 0 0 0 0
Total 81,881 80,266 86,751 74,614 48,557 280,381 162,147 100
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6-5 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Tables 6-2 and 6-3 also show the total pipe amounts (by segment count and overall length, respectively)
that have a defect score of either 4 or 5. As indicated, approximately 94 percent of all such pipes are either
6-inch or 8-inch diameter. The scoring results indicate that 37 such pipe segments (approximately
10,500 feet) have diameters of 10 inches or greater, and only 13 such segments (approximately
3,800 feet) have diameters of 15 inches or greater.
The improvement projects identified in this report are based on capacity needs, rather than pipe
condition. Measures to correct condition deficiencies may be a combination of maintenance activities and
repairs performed by the City or by outside contractors. The specific types and extent of repair efforts will
depend on defect location, pipe diameter, defect severity, applicability of various repair methods, and
other factors.
6.3 CAPACITY-RELATED CAPITAL IMPROVEMENTS
Capital improvements identified in this chapter fall into the following categories:
• Gravity Sewer Capacity Improvements
• Gravity Sewer Inverted Siphon Improvements/Actions
• Lift Station Capacity Improvements
• Additional City-Identified Gravity Sewer Improvements
• WRRF Improvements
6.3.1 Gravity Sewer Capacity Improvements
The following gravity sewer improvements (G) have been identified to accommodate modeled existing
and buildout peak flow conditions:
Project G-1: Foothill-to-Chorro sewer – Upsize to 15-inch diameter approximately 5,300 feet
of 10-inch and 12-inch diameter pipe along Foothill Boulevard, Ramona Drive, Broad Street,
Murray Drive, and Chorro Street. It should be noted that it may be more efficient to
eliminate the parallel flow path that exists along Murray and Chorro Streets and replace it
with a single line; however, this decision should be made at the predesign stage.
Project G-2: Monterey-to-San Luis sewer – Upsize to 10-inch diameter approximately
3,800 feet of 6-inch and 8-inch diameter pipe from Monterey Street to San Luis Drive.
Project G-3: McCollum-to-Grand sewer – Upsize to 8-inch diameter approximately 700 feet
of 6-inch diameter sewer from Fredericks Street to Grand Avenue.
Project G-4: Mountain View-to-Peach sewer – Upsize to 10-inch diameter approximately
1,300 feet of 6-inch and 8-inch diameter pipe along Broad Street from Mountain View Street
to Peach Street.
Project G-5: West-to-Lincoln sewer – Upsize to 10-inch diameter approximately 1,300 feet of
6-inch diameter pipe from a point between Murray and West Streets to Montalban Street.
Project G-6: California-to-Casa sewer – Upsize to 10-inch diameter approximately 700 feet
of 8-inch diameter sewer from California Boulevard to Casa Street.
Project G-7: Monterey-to-Marsh sewer – Upsize to 21-inch diameter approximately
1,800 feet of 15-inch diameter sewer from Monterrey Street to Marsh Street.
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6-6 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
6.3.2 Inverted Siphon Improvements/Actions
The following gravity sewer inverted siphon (S) capacity improvements/actions have been identified:
Project S-1: Nipomo Street siphon – Monitor water surface elevations upstream of the dual
inverted siphon beneath San Luis Obispo Creek.
Project S-2: Santa Rosa Street siphon – Monitor water surface elevations upstream of the
dual inverted siphon beneath Brizzolara Creek.
Project S-3: Marsh Street siphon – Monitor water surface elevations upstream of the dual
inverted siphon beneath San Luis Obispo Creek.
Project S-4: Oceanaire Drive siphon – Replace approximately 110 feet of inverted siphon
beneath Laguna Lake drainage channel (currently scheduled improvement).
6.3.3 Lift Station Capacity Improvements
The following lift station (L) capacity upsizing requirements have been identified to accommodate
modeled existing and/or buildout conditions:
Project L-1: Airport LS – Upsize firm capacity from the current 225 gpm (0.32 mgd) to
310 gpm (0.44 mgd) for existing conditions and 510 gpm (0.73 mgd) for buildout conditions.
Project L-2: Calle Joaquin LS – Modeled flow conditions indicate a slight exceedance of firm
pumping capacity at buildout. Monitor lift station operations over time to determine if peak
flows approach or surpass firm pumping capacity
Project L-3: Silver City LS – Modeled flow conditions indicate a slight exceedance of firm
pumping capacity at buildout. Monitor lift station operations over time to determine if peak
flows approach or surpass firm pumping capacity.
There are no identified force main improvements for any of the lift station facilities.
6.3.4 Additional City-Identified Gravity Sewer Improvements
The City has identified a series of gravity sewer improvements that are needed to address collection
system maintenance and reliability concerns. The improvements are shown on Figure 6-2. The exact
quantities and costs associated with these improvements are not addressed in this report.
6.3.5 WRRF Improvements
Major improvements at the WRRF are under construction and nearly complete as of 2025. WRRF
improvements of note include upgrades both to treatment processes and to wastewater storage facilities.
Plans exist to double influent storage (equalized storm flow) capacity, which will allow the WRRF to
accommodate peak flows resulting from RDII during wet weather events while reducing the impacts of
those flows on the downstream treatment processes. In parallel, continued RDII reduction measures in
the collection system will help preserve capacity at the WRRF for sanitary flows. The costs associated with
the WRRF improvements are not addressed in this report.
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6-8 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
6.4 PROPOSED CAPITAL IMPROVEMENT PLAN
The above listed gravity sewer capacity, siphon, and lift station capacity projects are summarized in
Table 6-4 and shown schematically on Figure 6-3. For each project, Table 6-4 describes the existing
facilities, proposed improvements, time frame for improvements, estimated capital costs, and any
associated action items. Monitoring recommendations are included to track system performance and
confirm or refine the severity of the predicted capacity restrictions, as appropriate. The aforementioned
City-identified gravity sewer and WRRF improvements are not included on Figure 6-3 or Table 6-4.
6.5 CAPACITY-CONSTRAINED AREAS
Based on the analyses presented in the 2015 WCSIRS, three capacity-constrained areas were identified.
These areas represent the tributary areas upstream of previously identified significant gravity sewer
capacity constraints. The areas in question are shown in green on Figure 6-4 and are described as follows:
• Northwest (Foothill) area: Areas of the City bounded by Highway 101, Highway 1, and the
City limits (including flow metering basins A and B from the 2015 WCSIRS).
• Central (Downtown) area: Areas of the City east of Highway 101 roughly between Marsh
and South Streets (including all or part of flow metering basins H, J, P, Q and R from the
2015 WCSIRS).
• Southwest (Laguna) area: Areas of the City west of Highway 101 and south of Laguna Lake
(including flow metering basin M and a portion of basin L from the 2015 WCSIRS).
The areas in question became the focus of the City’s service lateral offset program, in which the City
sought to reduce infiltration and inflow by replacing private service laterals. In addition, the Southwest
area saw a major trunk sewer replacement in which a deteriorating trunk sewer running under a farm
field was replaced when the area in question (now San Luis Ranch) was developed.
Based on the most recent wet weather flow metering data and current collection system modeling, both
the Southwest area and the Central area are no longer considered to be capacity-constrained, while one
new area has been identified. Accordingly, Figure 6-5 shows the following three groups:
• Red: Current areas that were previously identified (equivalent to the Northwest area
described above)
• Green: Newly identified capacity-constrained areas
• Blue: Previous areas no longer considered to be capacity-constrained
The red area corresponds to the tributary area for gravity sewer capacity improvement project G-1 listed
above. The green areas correspond to the tributary areas for gravity sewer capacity improvement projects
G-2 through G-7 listed above.
The average age of the City-owned sewers in the red area is 62 years, while the average age of the
City-owned sewers in the green areas is 66 years. Many of the service laterals that discharge into these
lines are of similar ages. Sewer pipes and service laterals of more advanced age are likely to deteriorate
at a faster rate than will happen in newer areas. Accordingly, in the ten years since to 2015 WCSIRS was
published, some deterioration of sewer pipes and service laterals has undoubtedly occurred.
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Table 6-4. Proposed Collection System Improvements
Project ID Project Name Existing Facilities Proposed Improvements Time Frame Estimated Capital Cost, $M Action Item
G-1 Foothill-to-Chorro sewer
~5,300 lineal‑feet of 10‑inch and 12‑inch diameter pipe along
Foothill Boulevard, Ramona Drive, Broad Street, Murray Drive, and
Chorro Street
Upsize to 15-inch diameter 2030 $4.83M
Proceed with predesign of improvements; may be
performed in phases
G-2 Monterey-to-San Luis sewer
~3,800 feet of 6-inch and 8-inch diameter pipe from Monterey
Street to San Luis Drive Upsize to 10-inch diameter TBD $2.39M
Perform flow or flow depth monitoring to confirm
severity of restriction
G-3 McCollum-to-Grand sewer
~700 feet of 6-inch diameter sewer from Fredericks Street to Grand
Avenue Upsize to 8-inch diameter TBD $0.41M
Perform flow or flow depth monitoring to confirm
severity of restriction
G-4 Mountain View-to-Peach sewer
~1,300 feet of 6-inch and 8-inch diameter pipe along Broad Street
from Mountain View Street to Peach Street.Upsize to 10-inch diameter TBD $0.87M
Perform flow or flow depth monitoring to confirm
severity of restriction
G-5 West-to-Lincoln sewer
~1,300 feet of 6-inch diameter pipe from a point between Murray
and West Streets to Montalban Street Upsize to 10-inch diameter TBD $0.92M
Perform flow or flow depth monitoring to confirm
severity of restriction
G-6 California-to-Casa sewer
~700 feet of 8-inch diameter sewer from California Boulevard to
Casa Street Upsize to 10-inch diameter TBD $2.74M
Perform flow or flow depth monitoring to confirm
severity of restriction
G-7 Monterey-to-Marsh sewer
~1,800 feet of 15-inch diameter sewer from Monterrey Street to
Marsh Street Upsize to 21-inch diameter TBD $2.16M
Perform flow or flow depth monitoring to confirm
severity of restriction
S-1 Nipomo Street siphon ~140 lineal feet of inverted siphon beneath San Luis Obispo Creek TBD TBD TBD
Confirm siphon configuration; monitor flow depths
imediately upstream of siphon
S-2 Santa Rosa Street siphon ~380 lineal feet of inverted siphon beneath Brizzolara Creek TBD TBD TBD
Confirm siphon configuration; monitor flow depths
imediately upstream of siphon
S-3 Marsh Street siphon ~80 lineal feet of inverted siphon beneath San Luis Obispo Creek TBD TBD TBD
Confirm siphon configuration; monitor flow depths
imediately upstream of siphon
S-4 Oceanaire Drive siphon
~110 lineal feet of inverted siphon beneath Laguna Lake drainage
channel Replace existing dual siphon 2030 TBD Currently scheduled improvement, per City staff
L-1 Airport LS Existing firm capacity of 225 gpm
Upsize to 310 gpm for existing
conditions and 510 gpm for
buildout conditions
TBD TBD
Monitor flows and/or pump operations to confirm
severity of restriction
L-2 Calle Joaquin LS Existing firm capacity of 500 gpm TBD TBD TBD
Monitor flows and/or pump operations to confirm
timeframe for capacity restrictions
L-3 Silver City LS Existing firm capacity of 450 gpm TBD TBD TBD
Monitor flows and/or pump operations to confirm
timeframe for capacity restrictions
W-C-1027-60-23-02-WP
City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
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Chapter 6
Capital Improvement Program Development
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6-11 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Source: 2015 WCSIRS
Figure 6-4. Previously Identified Capacity-Constrained Areas
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6-13 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy
August 2025
Additionally, since 2015, the City has worked to improve the collection system by addressing capacity
issues identified in the prior WCSIRS and performing rehabilitation projects. The City continues to plan
sewer improvement projects aimed at reducing I&I flows. Much of the work that has been accomplished
is located within the previously identified capacity-constrained areas. The implementation of such
improvements is a key factor in why two of these areas are no longer considered to be
capacity-constrained.
The projects identified in this report, along with the City's planned projects, provide a road map for further
reducing I&I flows. The projects that the City has already accomplished and is planning to implement are
displayed on Figure 6-6, along with the capacity improvement projects identified above.
6.6 CONCLUSIONS AND RECOMMENDATIONS
Major conclusions and recommendations from this chapter include:
Capital costs developed in this chapter are consider ed Class 5 estimates, per
AACE International.
The identified CIP projects are not intended to address physical pipe condition deficiencies,
some of which will be addressed through maintenance activities and in-house repairs, with
specific action to be determined based on location, pipe diameter, defect severity, repair
method, and other factors.
Identified capital improvements are categorized as follows:
— Gravity sewer improvements to address existing and buildout capacity deficiencies
— Gravity sewer inverted siphon improvements/actions
— Lift station capacity improvements
Major improvements at the WRRF are under construction and nearly complete as of 2025.
The costs of such improvements are not addressed in this report.
City-identified gravity sewer improvements to address collection system maintenance and
reliability concerns are shown on Figure 6-2, but the exact quantities and costs associated
with these improvements are not addressed in this report.
Proposed CIP improvements are summarized in Table 6-4 and shown schematically on
Figure 6-2. Monitoring recommendations are included to track system performance and
confirm or refine the severity of the predicted capacity restrictions, as appropriate.
Estimated capital costs are provided for standard gravity sewer improvements and force mains.
No costs estimates are included for inverted siphons or lift station capacity improvements,
owing to uncertainties about the nature of and/or need for such improvements.
Two of the three areas previously identified as capacity-constrained areas are no longer
identified as capacity-constrained. Newly identified areas are located in older parts of the
system, where ongoing system deterioration is likely a factor.
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Modeled versus Metered Flow Results
Appendix A
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Sewer Infrastructure Renewal Strategy and
Private Sewer Lateral Program Updates (Item 8a)
August 19, 2025
Presented by:
Chris Lehman – Wastewater Deputy Director
Agenda
1.Background
2.2025 Wastewater Collection System Infrastructure
Renewal Strategy (WWCIRS)
3.City Investments
4.Stakeholder Outreach
5.Study Session Discussion Items
Private sewer laterals are the portion of the sewer lines
that connect from a residence or commercial structure
up to and including the point of connection with the
publicly owned sewer main.
The City estimates that there are 13,421 private sewer
laterals citywide.
Private Sewer Laterals
Private Sewer Laterals – Inflow & Infiltration (I&I)
Reduced spills from public infrastructure from a peak of
35 in 2021 to one in 2025. Spills from private
infrastructure remain elevated.
Private Sewer Laterals – Impacts of I&I & Progress
Reduced Sewer Spills – Why Should I Care?
Sewer spills pose a health risk to private property and
occupants, and the public.
Sewer spills have impacts on the environment –
polluting our creeks and sensitive habitats.
Sewer spills have financial impacts on the City and
ratepayers:
Fines from the City’s regulator may be $10/gallon (a
10,000-gallon spill could result in a fine of $100,000
The City may be (and has been) subject to third-
party lawsuits
Operating costs associated with remediation and
enforcement
Private Sewer Lateral Offset Program
Background
Riverwatch lawsuit settlement (now sunset, but risks
remain)
City was targeted due to spills
Spills result from limited sewer capacity
Prior Council direction:
Offset requirements for new development rather
than condition replacements for inspection upon
sale findings.
Wastewater Collection System
Infrastructure Renewal Strategy
(WWCIRS)
The 2025 Wastewater Collection System
Infrastructure Renewal Strategy (WWCIRS) was
completed in August 2025 (updating the 2015 Study).
Wastewater flow modeling
Updated proposed Capacity-Constrained Area
map
WWCIRS
WWCIRS
WWCIRS
WWCIRS – Capacity Constrained Area Changes
City Investments
Water Resource Recovery Facility Inflow
Equalization Expansion (4.5 to 9 Million Gallons)
City Investments
Water Resource Recovery Facility Capacity Upgrades
$5,730,000*
Wastewater Collection System Capital Improvements
$1.4M (2019-2024 annual avg.)
Wastewater Collection System Maintenance
$300,000 (annually; excludes operating costs of $1.5M)
Private Sewer Lateral Rebate Program
$420,000 ($85,000 - $200,000 awarded annually)
*Does not include increased treatment costs.
Stakeholder Outreach
Stakeholder Outreach
Ongoing outreach to the following communities:
Realtors
Developers
Plumbers
Property owners
Specific outreach events
Developer's Roundtable (July 1, 2025)
SLO Coastal Association of Realtors (August 13, 2025)
Chamber of Commerce Leg. Committee (August 14, 2025)
Study Session Discussion Items
Study Session Discussion Item #1:
Should the Wastewater Flow (Private Sewer Lateral) Offset Program Continue?
1. Dissolving the Wastewater Flow (Private Sewer Lateral) Offset Program and retaining existing Inspection Requirements and the Replacement Rebate Program.
Impacts: Increased potential for spills and fines, reactive vs. proactive
Next steps: Update to City Municipal Code (2026)
2. Dissolve the Wastewater Flow (Private Sewer Lateral) Offset Program and replace it with requiring the replacement of all Private Sewer Laterals located in capacity-constrained areas, identified as “Poor/Failed”, during the Inspection Upon Sale process.
Impacts: Increased burden on property owners, increased potential for spills associated with intensified development in capacity-constrained areas.
Next steps: Update to City Municipal Code (2026)
3. Retain the existing Wastewater Flow (Private Sewer Lateral) Offset Program and add additional requirements for replacements of all Private Sewer Laterals conditioned as “Poor/Failed” during the Inspection Upon Sale process.
Impacts: Shared burden between property owners and developers
Next Steps: Update to City Municipal Code (2026)
Next Steps: None, if the Program continues without modification.
Alternatives to continuing the existing Private Sewer Lateral Offset Program include:
Study Session Discussion Item #2:
Should a General Plan Amendment Adopting the Staff-Recommended Capacity-Constrained Areas Map be Brought Forward for Adoption (If the Offset Program is not dissolved)?
1. Council may direct staff not to amend the General Plan Capacity-Constrained Map.
Impacts: Staff would administer the program through inaccurate/old data.
Next Steps: None
Next steps: If directed to proceed with amending the General Plan, Staff would proceed
to the Planning Commission in October and return to Council on December 2, 2025, with
a draft resolution.
Alternatives to amending the General Plan Capacity-Constrained Map include:
Study Session Discussion Item #3
Should Staff Further Evaluate a New Private Sewer Lateral Inspection Rebate (for capacity-constrained areas) to be Brought Forward to Council in December for Adoption via Resolution?
1. Council may direct staff not to proceed with the evaluation of a new Private Sewer Lateral Inspection Rebate.
Impacts: Fewer inspections performed in high-risk/capacity-constrained areas, and less data available, which assists the Wastewater Flow Offset Program, informs property owners, and helps the City in the overall assessment of private infrastructure.
Next steps: None
Next steps: If directed to further evaluate a new Private Sewer Lateral Inspection
Rebate, Staff would further assess the potential rebate program, including terms,
limitations, and program administration. Staff’s evaluation would be brought back to
Council on December 2, 2026, with a draft resolution for Council’s consideration.
Alternatives to adopting a new Private Sewer Lateral Inspection Rebate include:
Study Session Discussion Item #4
Should Staff Further Evaluate Expanding Eligibility for Private Sewer Lateral Replacement Rebates in Capacity-Constrained Areas to be Brought Forward to Council for Adoption in December via Resolution?
1. Council may direct staff not to proceed with evaluating expanding eligibility
criteria for Private Sewer Lateral Rebates.
Impacts: Reduced private sewer lateral replacements in capacity-constrained areas.
Potential equity impacts to property types that are currently not qualified.
Next Steps: None
Next steps: If directed to further evaluate expanding eligibility of the (existing)
Private Sewer Lateral Replacement Rebate, Staff would further assess the potential
expansion, including terms, limitations, and program administration. Staff’s evaluation
would be brought back to Council on December 2, 2026, with a draft resolution for
Council’s consideration.
Alternatives to expanding eligibility of the existing Private Sewer Lateral Replacement
Rebate include:
Chris Lehman
City of SLO Utilities Dept.
Deputy Director, Wastewater
805.431.4372
clehman@slocity.org
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Extra Slides
Private Sewer Lateral Condition Online (Public) Map
www.slocity.org/laterals
Private Lateral Conditions – Vitrified Clay Pipe
Private Lateral Conditions – Failed Lateral
Void in pipe
Private Lateral Conditions – Failed Lateral
Void in pipe
Offset joint
Private Sewer Laterals - Conditions
Private Sewer Laterals
City Capital Improvement Projects Completed
Recommended City Capital Improvement Projects
Baseline Data Set of Private Sewer Lateral Conditions