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Underground Flow Equalization System

Current Underground Flow Equalization System Progress

Below are a live view from our construction site camera and a 3D visualization of the current phase of the project. In order to start the live camera view, click on the three lines symbol and click the LIVE button. The video will load in a few seconds. This page is best viewed with the Chrome browser and may not be fully functional with Internet Explorer.

Please click here for the recording of the UFES Virtual Community Meeting from Dec. 21, 2020. The project and construction team provide details of the project and answer questions from the community.

Construction Camera

 Visualization of current project phase

What Is the Underground Flow Equalization System and Why Is It Important?

The City of San Mateo is currently under a Cease and Desist Order issued by the Regional Water Quality Control Board to eliminate sanitary sewer overflows (SSOs). SSOs occur primarily during heavy rains, when excess water enters the sewer collection system and overwhelms its capacity. During these events, sewage flows out of the system onto the City’s streets. The Underground Flow Equalization System (UFES) consists of an underground 5.3-million-gallon structure that will help eliminate SSOs in San Mateo and comply with the Cease and Desist Order.

A Proven Technology

These facilities are becoming more widely used across the entire country to provide additional capacity in sewer systems during heavy rains to help prevent sewer overflows. Seattle, Sacramento, Pacifica, and Daly City use them.

Project Schedule

Due to the scale and complexity, the UFES project will be constructed in 6 phases, with pipeline work occurring concurrently, over approximately three years. More information about the UFES project can be found here.

Construction Start: July 2020
Construction Finish: Fall 2020

Construction Start: November 2020
Construction Finish: Winter 2021

Construction Start: Early 2021
Construction Finish: Fall 2021

Construction Start: Fall 2021
Construction Finish: Winter 2022

PHASE 5: STRUCTURE (Concrete, Mechanical, Electrical)
Construction Start: Winter 2022
Construction Finish: Spring 2023

Construction Start: Winter 2022
Construction Finish: Summer 2023

Pipeline Construction in Saratoga Drive:
Construction Start: Winter 2020
Construction Finish: Fall 2022

Pipeline Construction in S. Delaware Street:
Construction Start: Fall 2021
Construction Finish: Fall 2022

project status iconProject Status

The UFES project is making great progress. Phase 5 construction is well underway, and includes installation of the mechanical and electrical equipment, concrete base, walls, columns, beams, and top slab for the UFES structure.

As of May 1, 2022, over 3,000 cubic yards of concrete have been poured in the at the Event Center Site, and the floor of the underground holding structure is complete.

Pipeline work continues along Saratoga Drive between South Delaware Street and the main entrance to the San Mateo County Event Center. Pipeline rehabilitation work along Saratoga Drive will occur between the hours of 7 AM and 5 PM from May 9, 2022 through May 31, 2022. Some activities will extend through the night and possibly the weekend. The majority of the construction activity and night work will occur at Pit 2. Pits 1 and 3 are the end points of the pipelining procedure where the ends of the hardened resin material will be cut open to allow flow. 

Map of pipeline pit locations on Saratoga.

Saratoga Drive is anticipated to open temporarily in early June for activities at the Event Center, and will close again in July for final pipeline work.

Traffic signs have been placed in specific areas to ensure the safety of drivers. Please pay attention to these traffic signs, and note that traffic patterns are subject to change.

Updated on May 10, 2022

upcoming work iconUpcoming Work

The following construction work is planned for the UFES project from May 16, 2022 – May 30, 2022 (Please note, schedules can change. We will post this information biweekly throughout 2022):

  • Inside the Event Center Site: additional concrete pours requiring after-hours approval
  • S. Delaware Street: equipment installation and removal related to pipeline work will continue
  • Saratoga Drive: The City’s contractor is rehabilitating an existing pipeline along Saratoga Drive, between South Delaware Street and the main entrance to the San Mateo County Event Center. The City has approved work from 5 p.m. until 8 p.m. Monday, May 16, 2022 through Thursday, May 19, 2022. The curing process for the pipeline liner requires 48 hours of continuous use of a generator and boiler; therefore, the City is allowing their contractor to work continuously from 7 a.m. Tuesday, May 17, 2022 through 8 p.m. Thursday, May 19, 2022.

Alerts will be sent via newsletter and text message ahead of work requiring traffic updates, work hour changes, or expected to generate excess noise at the Event Center site.

We thank you for your continued patience and cooperation as we make steady progress toward the completion of our sewer system improvements.

Updated on May 16, 2022

Construction Monitoring

As part of our ongoing commitment to the community living and working near the UFES project, the City is providing live camera feeds to view the on-site activities, and is monitoring dust, noise, vibration, and groundwater levels for the duration of construction. Dust, noise, and vibration monitors are placed at the two project fence lines along Saratoga Ave and Bay Meadows Park. Groundwater monitoring wells are located on-site and in the surrounding neighborhoods. After the City completes a quality control review of the monitoring data, the City will post summarized monitoring updates below. The City will be utilizing best management practices to prevent dust, noise, vibration, and groundwater levels from exceeding the threshold levels; however, if threshold exceedances occur during the reporting timeframe, the City will report the causes and corrective actions taken.

Project not to exceed 90 dBA at the property line at Saratoga Ave and Bay Meadows Park

Week: May 2 – May 6, 2022
There were no observed exceedances this week.

Project not to exceed vibrations of 1.0 inches per second for more than 30 seconds

Week: May 9 – May 15, 2022
There were no construction-related exceedances observed this week.

Project not to exceed dust particulate levels of 82 pounds per day

Week: May 2 – May 6, 2022
There was one exceedance this week due to wind. Corrective actions were taken and dust levels returned to normal.  

Click link below for additional information

Map of project location.Where Will the UFES Be Located?

The UFES will be located on the San Mateo County Event Center property between Saratoga Drive and South Delaware Street. After an extensive site selection process, including more than 20 public meetings and hearings, the east corner of the Event Center parking lot was determined to be the most feasible location. Several presentations on UFES and the site selection process can be found on our Presentations page.

Representation of Site PreparationRepresentation of Site Preparation

Phase 1: Site Preparation

During this phase, the site will be cleared, construction trailers and equipment will be brought to the site, construction fencing will be installed, and dust control measures will be in place.

At this time, equipment to monitor groundwater, vibration, dust, and noise will be installed. The City performed baseline assessments of properties neighboring the Event Center site to assess existing conditions and begin monitoring for settlement.

Phase 2: Shoring Installation

During this phase, shoring construction equipment will be brought to the site. The shoring system will safely hold the area open during excavation. The contractor has selected a low-vibration and low-noise system for the installation of the shoring system that has been approved by the City.

Once the shoring equipment is installed, groundwater will be pumped out of the work area to maintain stability of the excavation walls. This dewatering method should not cause the groundwater levels to change from typical conditions observed in recent groundwater level tests conducted beyond the Event Center property line. The groundwater has been tested to confirm there is no contamination.

During construction, the City will monitor groundwater levels, noise, dust, and vibration. Enforceable thresholds will be set and enforced, where work can be stopped if work exceeds the specified limits. The City will keep you informed of any exceedances and corrective actions taken.

The condition of properties assessed prior to construction will be re-assessed for settlement throughout the construction process.

Representation of Shoring InstallationRepresentation of Shoring Installation

The figures below show the effects of dewatering, and groundwater levels, in the Bay Meadows and Fiesta Gardens neighborhoods.

Representation of Excavation WorkControlling Groundwater in the Bay Meadows Neighborhood

Representation of Excavation WorkControlling Groundwater in the Fiesta Gardens Neighborhood

Representation of Excavation WorkRepresentation of Excavation Work

Phase 3: Excavation

Once the shoring is complete, excavation will begin and the soil inside the shoring will be removed. The soil has been tested to ensure there is no contamination. Dust control measures will be in place.

The City will continue to keep you informed of any exceedances of dust, noise, vibration, and groundwater levels, and corrective actions taken.

Phase 4: Foundation Micro-Pile Installation

A network of foundation piles will be drilled to support the structure and prevent uplift (floating) from the naturally high groundwater levels, and to keep it in place during seismic events. Seven different pile types and installation methods were evaluated. Micro-piles were selected because they meet the technical requirements, while being quieter and vibrating less than other methods. Noise from the installation of the micro-piles is expected be below the City’s noise ordinance – this sets a level of 90 dBA at the property line. The City will monitor noise throughout the construction process and keep you informed of any exceedances and corrective actions taken.

Representation of Micro-Piles under UFES StructureRepresentation of Micro-Piles under UFES Structure

Comparison of Activity Noise to Decibel LevelThe figures below show the expected noise levels in the Bay Meadows and Fiesta Gardens neighborhoods during the installation of the micro-piles.

The City will continue to keep you informed of any exceedances of dust, noise, vibration, and groundwater levels, and corrective actions taken.


Section showing anticipated sound levels from micropiling activities in the Bay Meadows neighborhoodSection Showing Anticipated Sound Levels from Micropiling Activities in the Bay Meadows Neighborhood

Section showing anticipated sound levels from micropiling activities in the Fiesta Gardens neighborhooSSection Showing Anticipated Sound Levels from Micropiling Activities in the Fiesta Gardens Neighborhood

Representation of Phase 5 WorkRepresentation of Phase 5 Work

Phase 5: Structure

During Phase 5, the concrete base, walls, column, beams, and roof elements of the UFES structure will be built, and mechanical and electrical equipment will be installed. The new electrical building will be installed as well. Concrete trucks and pumping equipment will be on site.

The City will continue to keep you informed of any exceedances of dust, noise, vibration, and groundwater levels, and corrective actions taken.

Phase 6: Site Finishing

Once the concrete structure is completed, the Event Center storage area will be repaved, with access hatches to components of the underground concrete structure. A new fence will be installed along Saratoga Drive, along with new plantings. The permanent, active odor control facility will be installed on the far west portion of the site, away from the park. The remainder of the site will be repaved and restored for Event Center usage.

The construction crews will be demobilized, and the new facility will undergo startup and testing. Neighboring properties participating in the ongoing condition assessments will receive their final, post-construction condition assessments.

The City will continue to keep you informed of any exceedances of dust, noise, vibration, and groundwater levels, and corrective actions taken.

Representation of Phase 6 Finished SiteRepresentation of Phase 6 Finished Site

Map of project location.How Will the Diversion Structure, Diversion Sewers, and Force Main Be Built?

To transport wastewater to the UFES, a diversion system, which includes a diversion structure, diversion sewers, and force main, is required along Delaware Street and Saratoga Avenue. To prevent SSOs, this diversion system will divert excess flows from the Delaware sewer trunk line, and convey the excess flows to the 5.3-million-gallon underground structure for temporary storage.

The diversion structure will be installed south of the intersection of Saratoga Avenue and Delaware Street. A new 36-inch diversion sewer will be installed using a combination of trenchless (horizontal directional drilling) and open trench construction methods along portions of Saratoga Avenue and Delaware Street. An 18-inch force main along portions of Saratoga Avenue and Delaware Street will be rehabilitated using a trenchless method to inject a lining into the existing pipe. If new pipe is needed for this force main, horizontal directional drilling methods will be used.

Traffic Management

Traffic control and management will be in place for the duration of the UFES and pipeline work. Construction traffic to and from the Event Center site is planned to be routed from Delaware Street through the access easement on the Event Center Property. Construction traffic from pipeline construction on Delaware Street and Saratoga Drive will be routed to the nearest designated and approved City truck haul routes: Delaware Street or Hillsdale Boulevard.

Map of construction traffic routes.

Dust Control

Dust control measures will be implemented throughout construction. This includes wet sweeping on surrounding streets and around the construction site, covering any truck beds hauling off dirt, site watering, tire washing facilities, and screening the site. A dust monitoring program will be implemented to ensure dust control measures are effective, and regular updates will be provided.

Future Operations and Maintenance

The UFES will be used during heavy storms, as well as for some collection system maintenance and inspection activities. It is expected to be used fewer than 20 times per year on average. During use, it will temporarily hold flows for up to 24 hours.

Dry Weather Conditions

During dry weather, the flow equalization system will rarely be used, except for maintenance and diversion, because the wastewater collection system has enough capacity.

UFES Dry Weather Conditions


The equalization system will also provide temporary storage, or holding, to facilitate future sewer construction projects and operations and maintenance activities, such as pipeline inspection and cleaning.

UFES Maintenance/Diversion

During Heavy Rains

During storms, rainwater gets into the wastewater collection system and overwhelms its capacity. The excess, rain-diluted wastewater will be redirected to the temporary holding area to prevent overflows and to allow the system to better manage the large volumes of water in the system.

UFES During Heavy Rains

After Heavy Rains or Maintenance/Diversion

After the storm subsides, the diluted wastewater is released from the equalization system into the main wastewater collection system and sent to the treatment plant. When the system has been used for maintenance, the wastewater returned to the main collection system will not be diluted. Self-cleaning mechanisms are activated to flush and clean the equalization system, all while odor control is in effect.


UFES After Heavy Rains or Maintenance/Diversion

Continuous Odor Control

After each use, the holding structure will be emptied and cleaned via large tipping buckets. Odor control facilities will be in continuous operation. The UFES will be inspected by the City after each use.


UFES Self Cleaning Mechanisms

Odor Control: Air Tight Vaults, Self Cleaning Tipping Buckets, Carbon Odor Scrubber. Highly efficient technology: carbon odor scrubbers. Large pore surface area adsorbs large mass of odors. 3 grams has surface of a football field!

The CEQA Process

The Final Environmental Impact Report for this project is now available at the links below. Hard copies are also be available at City Hall and the San Mateo Main Library.

The Final EIR incorporates written responses to comments received during the public comment period, which ran from March 6, 2019 through May 31, 2019. The Final EIR will be presented to the Planning Commission and City Council for consideration of adoption.

Schedule of the CEQA Process, expected timeline, and opportunities for public involvement.