The Underground Flow Equalization System (UFES) project consists of:

• An underground, concrete holding structure capable of temporarily storing up to 5.3 million gallons of wastewater. It is located in the east corner of the Event Center property. The facility is approximately 200 feet by 150 feet, and 35 to 50 feet deep. Access hatches and manhole covers will be visible.
• Above ground electrical building
• Diversion System (diversion sewers, diversion structure, and force main) that routes wet weather flows to and from the existing Delaware sewer trunk line
• Odor Control System will continuously clean the air and vent the facility.

• During heavy rains, the Delaware sewer trunk line and other areas in the system reach capacity and cause sanitary sewer overflows (SSOs). The UFES will temporarily route diluted wastewater flows from the Delaware trunk line to the underground holding structure, where it can be stored for up to 24 hours.
• Once the downstream collection system and WWTP has available capacity, a pump station inside the holding structure will pump the wastewater back into the collection system.
• The facility can also be used to support operations and maintenance activities in other areas of the system, such as pipeline inspection and cleaning.
• After any use, self-cleaning mechanisms will flush and clean the facility. The interior is inspected after every use.
• Odor control systems will operate 24/7.
• The facility is expected to be used fewer than 20 times per year on average.

• During heavy rains, San Mateo’s sewer system is overwhelmed and causes SSOs that impact the health of the community, creeks, beaches, lagoon, and San Francisco Bay.
• In 2009, the San Francisco Bay Regional Water Quality Control Board (RWQCB) issued a Cease & Desist Order that requires the City to eliminate SSOs.
• Many of these SSOs occur on Delaware Street and contaminate Borel Creek, which flows through the Fiesta Gardens neighborhood.
• In conjunction with the other Clean Water Program improvements, the UFES enables the City to prevent SSOs from occurring and comply with State and Federal regulations.

• Staff initially evaluated 55 sites throughout the City to potentially locate UFES. As part of the evaluation process, City staff and members of the Council visited several underground flow equalization facilities in Seattle, Washington. These facilities were located underneath parks and parking lots, and were adjacent to various residential, school, and commercial properties. Based on the information learned from the Seattle systems, parks at Bay Meadows and Fiesta Gardens were determined to be preferred locations for UFES. However, due to opposition to these locations from local residents, staff focused on further evaluating the City’s Corporation Yard and the County Event Center as potential sites.
• After further evaluation, the County Event Center Corner site was preferred over the Corporation Yard due to various factors (such as impacts to operations, proximity to residents, permitting requirements, and longer construction schedule). Because the Corporation Yard is the essential hub for the City’s Public Works and Parks and Recreation operations, staff would have to be relocated for the duration of the UFES project. However, the City was unable to find suitable temporary sites for the operations. Additionally, there were zoning restrictions on the Corporation Yard.
• The County Event Center site offered a shorter construction schedule and a less complex permitting process. Initially, the City identified the three corners (northwestern, southern, and eastern corners) of the County Event Center. Because the northwestern and southern corners presented challenging conditions for the operations of the Event Center, these were eliminated from further consideration. As a result, the Event Center staff determined that the eastern corner would be preferred and agreed to negotiate an easement for the location of the UFES project.

Construction of the 5.3-million-gallon underground structure will be done in six phases: site preparation, shoring installation, excavation, micro-pile installation, structure, and site finishing. Some of the phases will overlap, and the diversion system will be constructed concurrently. More information on the phases of construction is available on our UFES page.

• Work may take place from 7 am to 7 pm, which is allowable per City ordinance, with most construction activities only requiring 8-10 hours per day.
• Weekend and evening construction is not allowed without specific approval. However, exceptions may be needed for certain activities, such as concrete pours. City approval and notice to the surrounding community would be required for weekend work.

Yes. The dedicated hotline number is 1-650-522-7250, and it is answered by a member of our construction and project management team.

• The City will require construction noise to comply within the City’s noise ordinance.
• The City has eliminated impact pile driving and is now using a quiet pile alternative called “micropiling” that generates one-quarter the amount of noise.
• Piling will be installed 50 feet below ground surface and will require approximately 6 to 8 weeks to complete.
• The City has developed a noise monitoring plan which includes monitoring devices in the vicinity of the work. The City will enforce noise violations through their independent construction manager who will be onsite during any work activity.

• Dust control measures will meet or exceed regulatory requirements.
• Dust will be visually monitored for compliance with Bay Area Air Quality Management District construction permit requirements. Dust particles will be monitored by instruments located on the construction site.
• Measures will be taken by the contractor, such as wet sweeping on surrounding streets and around the construction site, covered trucks, site watering, fence screening and tire wash facilities.
• The City’s construction manager will enforce the dust control measures and compliance with regulatory requirements.
• A soil investigation was performed for the project and no contamination of the soil or groundwater was found.

• The UFES project will utilize a shoring system that acts as a cutoff wall during construction, preventing groundwater from entering the excavation and isolates the excavation from the surrounding groundwater. This creates a closed “bathtub” effect that allows work to proceed inside the structure without impacting the surrounding groundwater table and eliminating soil instability and settlement nearby.
• The City has conducted a geotechnical evaluation of the project to help develop prevention measures for settlement. The results showed that groundwater is 3’-5’ below ground, no contamination was found in the soil or groundwater and historical baselines of groundwater level fluctuations were obtained in wells installed around the site. Based on the subsurface conditions, the following measures will be taken to ensure settlement does not occur:
  • Additional observation wells will be installed near the site before construction to monitor and establish baseline groundwater level measurements.
  • Dewatering wells will be placed within the cutoff wall, and will be prohibited from being placed outside the excavation (this significantly reduces the potential for settlement).
  • Dewatering drawdown will be minimized to 2 feet below the excavation.
  • The groundwater cutoff wall will extend at least 15 feet below the excavation and watertight shoring will be required.
  • Enforceable drawdown stop-action thresholds and corrective actions will be required.
  • Groundwater levels will be measured during and after construction and compared to the baseline measurements.
  • Properties close to the project site will be assessed and documented before, during, and after construction to differentiate between construction related impacts and pre-existing conditions. Home condition assessments were performed on homes in a designated area if the homeowner entered into a written right-of-entry agreement with the City.
• By implementing the actions above, the results will lead to a proven groundwater dewatering approach that will minimize the groundwater “cone of influence” to less than 10 feet from the edge of the UFES basin. This will significantly reduce the potential issues that sometimes impact surrounding community structures.
• Vibration monitoring will occur during the use of heavy equipment. The contract documents specify maximum vibration limits and enforceable actions if they are exceeded. The City has a monitoring plan which includes vibration monitoring devices in the vicinity of the work. However, vibration is not anticipated to be a significant impact to the project with both driven sheet piles and driven foundation piles being excluded from consideration.

• The City has approved a traffic management plan submitted by the contractor for delivering equipment and materials as well as hauling away soil from the work site.
• The plan attempts to minimize the traffic impacts to the surrounding community as much as possible.
• 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.
• Lanes on Saratoga Drive and Delaware St. will need to be closed for some of the pipeline work.
• The increase in traffic due to UFES construction activities will vary based on construction phase, day to day activities (i.e., construction pours), type and quantity of truck contents, and local conditions, among other factors. The heaviest anticipated truck traffic will occur during excavation at the Event Center site.
• More detailed information regarding traffic can be found in the slides from our December 21, 2020 Virtual Community Meeting. The City will continue to evaluate alternate routes and traffic control options and will coordinate with other construction projects in the area.

As part of the traffic management plan provided by the contractor and approved by the City, safe pathways will be provided for pedestrians and bicyclists attempting to navigate the construction area.

• The force main along Saratoga Dr., from the 5.3-million-gallon underground structure to Delaware St., will be constructed using trenchless methods for most of the alignment. Most of the work will be injecting new lining into an existing 18-inch sewer pipe. A portion of the work to install a segment of new 18-inch sewer pipe will utilize a trenchless method called horizontal direction drilling.
• The 36-inch gravity diversion sewer will be installed by a combination of open trench and trenchless (microtunneling) methods. Microtunneling was selected because it would involve the least impact and be the least disruptive.
• The City has been using open cut trenching throughout the City for many decades and is currently using open cut to numerous sewer projects throughout the City.

The Final Environmental Impact Report was adopted by the San Mateo City Council in October 2019. It provides specific environmental requirements for wildlife protection. For example, Mitigation Measure 5-2 requires biological surveys be performed prior to construction to ensure the protection of any native bird nests and the establishment of protective buffers based on standards established by the California Department of Fish and Wildlife.

• The odor control system will continuously operate and will dry out the interior after usage
• Proven carbon scrubbing technology will be utilized, similar to the UFES projects the Council visited in the Seattle area in 2016.
• After each use, the underground structure will be automatically pumped out and flushed clean with water, which will remove material that could cause odors.

The public can call the UFES hotline at 1-650-522-7250, or email info@CleanWaterProgramSanMateo.org.

The final design is required to meet seismic safety thresholds and standard building codes. The system will be designed to withstand the U.S. Geological Survey seismic acceleration requirements specific to San Mateo. The design will also comply with governing codes, such as Reinforced Concrete ACI 350, California Building Code 2013, and Design Loads for Buildings and Other Structures (ASCE 7-10).