San Luis Obispo City Council
August 19, 2025

ATTACHMENT B

City of San Luis Obispo, Council Memorandum
City of San Luis Obispo
Council Agenda Correspondence
DATE: August 19, 2025
TO: Mayor and Council
FROM: Aaron Floyd, Utilities Director
Prepared By: Chris Lehman, Deputy Director - Wastewater VIA: Whitney McDonald, City Manager
SUBJECT: Item 8a. – Sewer Infrastructure Renewal Strategy and Private Sewer Lateral Program Updates
Staff received the following questions regarding updates to the City’s Private Sewer Lateral Programs. The questions are below with staff’s response shown in italics:
1) On page 220 of the packet, the staff report describes that in 2019, when the sewer lateral inspection and offset programs were created, there was no requirement put in place to replace or repair laterals upon sale after a damaged lateral was located…the only requirement was the inspection. Instead, the impression is that the rebate program was put in place to incentivize replacement of damaged laterals.

Is there any kind of fine or citation if there is a Sanitary Sewer Overflow (SSO) as a result of someone’s damaged lateral?
Yes. Notice of Violations (NOVs) and Administrative Citations are issued to a property owner following a Sanitary Sewer Overflow (SSO) caused by an issue with a private sewer lateral. The initial requirement of the NOV is that the private sewer lateral be inspected and that the resulting video inspection be sent to staff to review. After staff evaluate and assign a condition rating to the private sewer lateral, for the majority of enforcement cases related to SSOs, the property owner is required to replace the sewer lateral, which is communicated through an Administrative Citation. City Administrative Citations include a $100 fine (for the first offense).
2) About how many sewer lateral rebates have we been providing each year, and how much money has been spent?
Please see Table 1 on the following page. Staff note that all metrics are displayed utilizing the calendar (vs. fiscal) year, and that the total amount dispersed for 2025 includes $20,000 of retroactive rebate payments made following Council adoption of Resolution No. 11517. Also of note, in the fall of 2024, rebates were increased from $2,000 (outside of capacity-constrained areas) and $3,000 (inside capacityconstrained areas) to $4,000 for all single-family property private sewer lateral replacements.

Sewer Infrastructure Renewal Strategy and Private Sewer Lateral Program Updates Page 2 Table 1: Private Sewer Lateral Replacement Rebates
Year No. of Applicants Amount Dispersed
2019 76 $ 122,000.00
2020 97 $ 229,875.00
2021 71 $ 190,900.00
2022 76 $ 190,000.00
2023 39 $ 99,000.00
2024 29 $ 85,000.00
2025 38 $ 172,000.00
3) Is it more expensive to replace a private sewer lateral for a multi-family housing development than for a single-family residence? Is there any reason we might need to consider a different rebate amount for that type of building? There is a wide range of potential project costs for the replacement of a private sewer lateral. Generally, but not always, replacing the private sewer laterals of a multi-unit dwelling would be more expensive than replacing those tied to a singlefamily home. There are some exceptions where a single-family residence replacement would cost more than a multi-family replacement. While staff believes that expansion of the existing rebate eligibility criteria to include all property types in capacity-constrained areas (as discussed in the Staff Report, Study Session Discussion Item #4) can be reasonably accommodated with existing funding, one risk of this could be deferred rebate awards if funding is depleted due to this new activity. Increased rebate amounts for multi-unit dwellings would have the same potential impact. As the program is demand-driven, it is difficult for staff to forecast the number of future rebate applications, but rebate applications have trended lower since the program’s inception. Staff plan for increased outreach in the coming twelve months, particularly in the capacity-constrained areas of the system. If directed by Council, staff could further evaluate establishing a new rebate amount for multi-unit dwellings. Determining an appropriate amount that would cover all scenarios would be difficult due to the wide variety of property configurations. Any expansion of rebate eligibility or increased rebate amounts will require legal analysis by the City Attorney’s Office.

Sewer Infrastructure Renewal Strategy and Private Sewer Lateral Program Updates Page 3 4) One of the Study Session Questions that staff have asked council to consider is whether or not Council would like to do a general plan amendment to change the maps of constrained areas to take into account updates in data over the last several years. Is it necessary to do a general plan amendment to change these areas, or can it be done by ordinance or resolution?
The City Council incorporated the wastewater collection system capacityconstrained area map into the Water and Wastewater Element (WWE) in May 2018 (WWE, Figure 3). The General Plan Amendment is identified to update this map based on the data and analysis presented in the 2025 Wastewater Collection System Infrastructure Renewal Strategy (WWCIRS). Additional minor text updates will also be proposed to update the Wastewater Service section of the WWE to reflect the recent analysis and report. Therefore, modification of the capacityconstrained area map will require a General Plan Amendment.

As stated in the introduction to the WWE:
“The City decided to adopt an element addressing water resources and wastewater services because of the vital role of these resources and the far-reaching impacts of water policies on community growth and character.

This element translates the Land Use Element's capacity for development into potential demand for water supply and wastewater services. This element outlines how the City plans to provide adequate water and wastewater services for its citizens, consistent with the goals and policies of other General Plan elements.”
The WWE is the appropriate location for the capacity-constrained areas map because the General Plan provides the framework and long-range plan for the City based on the capacity of urban services, including wastewater collection.

Therefore, identification of the City’s capacity to serve existing and build-out development, including capacity in the City’s sewer system, is important to include in the General Plan. General Plan Elements, including the WWE, also establish the policies and programs that are needed to facilitate the LUE’s identified capacity for development in the City, and the Municipal Code provides the regulations and enforcement authority for implementation of the General Plan.

Sewer Infrastructure Renewal Strategy and Private Sewer Lateral Program Updates Page 4 5) If the City removes some of the previously constrained areas from the map, do we run the risk of having those areas become more constrained again? Is there any reason to leave the previous areas as eligible for the rebates, while adding the additional new areas?
Staff’s recommendation to update the capacity-constrained areas map has been determined and informed through the 2025 Wastewater Collection System Infrastructure Renewal Strategy (WWCIRS). The 2025 WWCIRS builds upon the 2015 WWCRIS and is the most current and complete forecast of the city’s ability to accommodate housing growth and the conditions identified in the 2025 General Plan. While there are still ongoing issues in the previously identified capacityconstrained areas, staff believe that enough improvements and risk management systems have been implemented to accommodate the 2035 General Plan’s forecasted growth in these previously identified areas while controlling sewer spills.

Study Session Discussion Item #4 proposes consideration of expanding rebate eligibility in the newly identified capacity-constrained areas, while retaining existing eligibility for all single-family residences City-wide. 6) Are there any other points in time that could be considered to trigger a requirement for replacing a poor/failing sewer lateral other than sale of a property or failure of the lateral resulting in an SSO?
Yes. Municipal Code Section 13.08.395.(C) defines when the inspection of existing private sewer laterals is required. Triggers include following a Sanitary Sewer Overflow (SSO), upon submittal of a building permit that includes a bedroom/bathroom/kitchen/plumbing fixture addition or increase in the water meter size, and change in ownership of real property (inspection upon sale). After the inspection video has been reviewed, Municipal Code section 13.08.395.(G) provides the circumstances when private sewer laterals must be replaced, which include SSOs, findings of Inflow and Infiltration (I&I), or a pipe (lateral) that has defects that do not meet the required pipe material requirements. This section of the Municipal Code does not explicitly state that the Inspection Upon Sale process or change in ownership of review property can trigger a lateral replacement.

Should Council give direction to staff to further evaluate this option, the Municipal Code could be modified to explicitly state this trigger. 7) Do we have data showing how many laterals are replaced each year as a result of the inspection program or the offset program?
Approximately 350 laterals have been replaced through the Wastewater Flow (Private Sewer Lateral) Offset Program. Staff are currently analyzing records to determine the exact number, which is likely higher. While the Inspection Upon Sale program does not currently require replacement, approximately 426 private sewer lateral rebates have been issued since the current program was adopted in September 2019 (see Question 2).

Sewer Infrastructure Renewal Strategy and Private Sewer Lateral Program Updates Page 5 8) Could a retrofit-upon-sale requirement be required only for laterals meeting the "poor" definition?
Yes. Of the 1,016 private sewer lateral inspections completed through the Inspection Upon Sale Program, 316 met the definition of “Poor” (severe defects, missing portions, large breaks in the pipe, root intrusion). Three met the definition of being “Failed” (the camera could not get through the pipe/complete collapse).

Different from the Study Session Discussion in 2017, where staff and Council evaluated requiring replacement of these laterals before closing of escrow, staff are currently recommending that these laterals be replaced within 180 days, which is consistent with replacement requirements for all other triggers and currently incorporated into the existing Municipal Code (for those other triggers – see correspondence question number 6). Some minor revisions to the Municipal Code would be required to explicitly state that the results of the Inspection Upon Sale program may trigger a replacement requirement of certain laterals (as defined in Municipal Code Section 13.08.395(G). The 180-day timeline may be extended through the Director's discretion, given that good faith efforts have been made. 9) How does the presence of accessory dwelling units (ADUs) in various neighborhoods impact the sewer lateral analysis?
The modeling effort included water usage data from actual water meter reads during the flow monitoring periods, which included all properties with a water meter. The build-out projections were based on assumptions of potential development including ADUs based on underlying zoning and past and potential future trends of development for the life of the 2035 General Plan. For example, a factor of five percent was applied to single-family residential (R-1) zoned parcels, which results in an assumed estimate of approximately 400 ADUs in this zone alone. One of the many benefits of the City’s updated sewer model is that it will be easier to adjust the model based on actual development over time.

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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 laterals 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 Element 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 included a “sunset” date of July 7, 2023, which meant, 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 replacement 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 of 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 includes 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 WWCIRS. 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 system, 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 collection 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 NonGovernment 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 maintenance 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 upon 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 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 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 capacityconstrained boundaries, staff will conduct targeted outreach to any properties 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 improvement 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 capacityconstrained (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
“Good” vs. Improvement
“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 capacityconstrained areas would be subject to the wastewater flow 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", with (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 capacityconstrained 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 capacityconstrained (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 through 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 capacityconstrained 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 staff, 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, respond 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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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 consideration 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 implementation 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 Program, 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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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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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. 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. 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 capacityconstrained 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 capacityconstrained 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 CapacityConstrained 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 (capacityconstrained 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

PREPARED BY
Wastewater Collection System
Infrastructure Renewal Strategy

PREPARED FOR
City of San Luis Obispo
DRAFT REPORT | AUGUST 2025

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

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

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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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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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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1-1 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy 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).

Chapter 1
Introduction
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1-2 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy 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
Wastewater Collection System Infrastructure Renewal Strategy 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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Chapter 2
Overview and Land Use Information
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2-3 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy 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).

Chapter 2
Overview and Land Use Information
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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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Chapter 2
Overview and Land Use Information
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2-7 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy August 2025

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 limitsshown 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) parcelsthat 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.

W-C-1027-60-23-02-WP
3-1 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy August 2025
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.

Chapter 3
Flow Analysis
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3-2 City of San Luis Obispo
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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

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Figure 3-2: Four-Gauge Average Rainfall, San Luis Obispo, December 2023–March 2024

Chapter 3
Flow Analysis
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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
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Nov-22
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Rainfall, in
Influent Flow, mgd
Figure 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 0 18
1 2
3 4
5 6
7 8
9 Jan-24 Feb-24 Mar-24 Apr-24
Flow, mgd
Rainfall, in
Figure 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 0:001:002:003:004:005:006:007:008:009:00
10:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:00 0:00
Normalized Flow
Time 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 timeto-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

W-C-1027-60-23-02-WP
City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy August 2025
0 0.1

0.2 0.3

0.4 0.5

0.6 0.7

0.8 0.9
1 Flow, mgd
Figure 4-2. Site SM1A, Model vs Flow Meter, Dry Weather Conditions Flow Meter
Model Output

W-C-1027-60-23-02-WP
City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy August 2025
0 0.1

0.2 0.3

0.4 0.5

0.6 0.7

0.8 0.9
0 1

0.2 0.4

0.6 0.8
1 1.2

1.4 1.6

1.8 2
Rainfall, in/hr
Flow, mgd
Figure 4-3. Site SM1A, Model vs Flow Meter, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

W-C-1027-60-23-02-WP
City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy August 2025
0 0.25

0.5 0.75
1 1.25

1.5 1.75
0 2

0.5 1

1.5 2

2.5 3

3.5 4
Rainfall, in/hr
Flow, mgd
Figure 4-4. Tank Farm: Model vs Flow Meter, January 7–13, 2023 Rainfall
Flow Meter
Model Output

Chapter 4
Hydraulic Model Development and System Evaluation Criteria W-C-1027-60-23-02-WP
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)

W-C-1027-60-23-02-WP
City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy August 2025
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 23 Rainfall, in/hr
Hour
Figure 4-5. 10-Year, 24-Hour Storm, SCS Type I Pattern

W-C-1027-60-23-02-WP
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

Chapter 5
Model Results and System Capacity Evaluation
W-C-1027-60-23-02-WP
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.

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S Higuera St
Orcutt
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California
Polytechnic
State University

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Water Resource Recovery
Facility
Gravity Sewers
No Restriction, Diameter ≥12"
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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
0 2,500 5,000 Existing Conditions
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City of San Luis Obispo
Wastewater Collection System
Infrastructure Renewal Strategy
Prepared by: Prepared for:

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 5-2

Chapter 5
Model Results and System Capacity Evaluation
W-C-1027-60-23-02-WP
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.

Chapter 5
Model Results and System Capacity Evaluation
W-C-1027-60-23-02-WP
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 deficiencies 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.

City of San Luis Obispo
Wastewater Collection Infrastructure Renewal Strategy
August 2025 W-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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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
0 2,500 5,000 Buildout Conditions
Feet
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City of San Luis Obispo
Wastewater Collection System
Infrastructure Renewal Strategy
Prepared by: Prepared for:

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 5-4

City of San Luis Obispo
Wastewater Collection Infrastructure Renewal Strategy
August 2025 W-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

Chapter 5
Model Results and System Capacity Evaluation
W-C-1027-60-23-02-WP
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 typically 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.”

Chapter 5
Model Results and System Capacity Evaluation
W-C-1027-60-23-02-WP
5-10 City of San Luis Obispo
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.

Chapter 5
Model Results and System Capacity Evaluation
W-C-1027-60-23-02-WP
5-11 City of San Luis Obispo
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.

W-C-1027-60-23-02-WP
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.

Chapter 6
Capital Improvement Program Development
W-C-1027-60-23-02-WP
6-2 City of San Luis Obispo
Wastewater Collection System Infrastructure Renewal Strategy August 2025
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.

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Force Main
Pipe Defect Score
5: Severe Defects
4: Poor Condition
3: Fair Condition
2: Good Condition
1: Minimal Defects
0: No Defects
Figure 6-1
Gravity Sewer
0 2,500 5,000 Pipe Defect Scores
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City of San Luis Obispo
Wastewater Collection System
Infrastructure Renewal Strategy
Prepared by: Prepared for:

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

Chapter 6
Capital Improvement Program Development
W-C-1027-60-23-02-WP
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
5 4 3 2 1 0 Total (4&5)
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
5 4 3 2 1 0 Total (4&5)
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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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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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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6.4 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.

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

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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Capital Improvement Program Development
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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 projectsidentified 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 considered 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.

Modeled versus Metered Flow Results
Appendix A

0 0.1

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0 1

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Rainfall, in/hr
Flow, mgd
Site A.1: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

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Site A.2: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
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Model Flow, mgd

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0 1.2

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Site A.3: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

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Flow, mgd
Site A.4: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

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Flow, mgd
Site B.2: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

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0.45 0.5
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Flow, mgd
Site B.3: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

0 0.2

0.4 0.6

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0 1.2

0.2 0.4

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Rainfall, in/hr
Flow, mgd
Site H.1: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

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Flow, mgd
Site H.2: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

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2.5 Rainfall, in/hr
Flow, mgd
Site J.1: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

0 0.1

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0 1

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Flow, mgd
Site M.1A: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

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Flow, mgd
Site M.2: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

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Flow, mgd
Site P.1: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

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Flow, mgd
Site P.2: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

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Rainfall, in/hr
Flow, mgd
Site Q.1: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

0 0.1

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0.25 Rainfall, in/hr
Flow, mgd
Site Q.2: Modeled vs Metered Flows, Wet Weather Conditions Rainfall
Metered Flow, mgd
Model Flow, mgd

0 0.5
1 1.5
2 0 2.5

0.5 1

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2.5 Rainfall, in/hr
Flow, mgd
CalPoly: Model vs Flow Meter, January 7–13, 2023
Rainfall
Flow Meter
Model Output

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Flow, mgd
Airport: Model vs Flow Meter, January 7–13, 2023
Rainfall
Flow Meter
Model Output

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Flow, mgd
Laguna: Model vs Flow Meter, January 7–13, 2023
Rainfall
Flow Meter
Model Output

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Flow, mgd
Margarita: Model vs Flow Meter, January 7–13, 2023
Rainfall
Flow Meter
Model Output

0 0.25

0.5 0.75
1 1.25

1.5 1.75
0 2

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1.5 2

2.5 3

3.5 4
Rainfall, in/hr
Flow, mgd
Tank Farm: Model vs Flow Meter, January 7–13, 2023
Rainfall
Flow Meter
Model Output

0 0.2

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1.2 1.4

1.6 1.8
0 2

0.02 0.04

0.06 0.08

0.1 0.12
Rainfall, in/hr
Flow, mgd
Buckley: Model vs Flow Meter, January 7–13, 2023
Rainfall
Flow Meter
Model Output