Public Works

OMP Stormwater Capture Project

Background

In 2015, the San Francisco Regional Water Quality Control Board (RWQCB) issued the countywide Municipal Regional Stormwater Permit (MRP) to regulate stormwater discharges in San Mateo County. The MRP requires all permittees to implement green infrastructure improvements to reduce the amount of mercury and polychlorinated biphenyl (PCB) in stormwater discharges to the San Francisco Bay. The San Mateo County Stormwater Resource Plan (SRP) was developed to identify projects that could help meet the requirements of the MRP. The SRP identified the proposed project as a high-priority regional project that can capture water from a large multi-jurisdictional drainage area.

Additional Details on the SRP

Colma Creek Water Shed

Rain falling east of the Santa Cruz Mountain range drains into the San Francisco Bay, while rain falling on the western side drains to the Pacific Ocean. This project helps clean water flowing from our cities to the Bay. The reach of Colma Creek running through Orange Memorial Park drains over 6,500 acres of land from six different municipalities, including the City of South San Francisco.

OMP site

This regional stormwater capture project, the first-of-its-kind in Northern California, diverts all dry-weather flow and the dirty first flush of urban stormwater runoff from Colma Creek into an underground system integrated within the park.

The project supports the City’s park Master Plan by co-locating stormwater capture, storage, and treatment facilities in Orange Memorial Park with the park renovations which will follow this project.

Orange Memorial Park Stormwater Capture Educational Placard(PDF, 9MB)

Project Summary

Orange Memorial Park Stormwater Capture project will provide water quality improvements to meet the requirements of the San Francisco Bay MRP. The project is designed to reduce discharges of PCBs and mercury to San Francisco Bay and is the first regional stormwater project of its kind in the Bay Area.

The project provides 1.8 million gallons in storage that is fully activated during a flood event and helps protect surrounding neighborhoods, portions of which are in the FEMA 100-year floodplain. By removing sediment, trash and debris upstream of the most flood-prone areas, the project helps prevent downstream clogging and maintain peak flood conveyance capacity during major storms.

The project further enhances regional resilience by recharging 240 acre-feet (80 Olympic swimming pools) into the underlying Westside Groundwater Basin every year, and by using 15 million gallons of treated stormwater to irrigate three local parks and reduce annual potable water consumption.

OMP Waterflow

The project will capture runoff through the installation of an instream diversion and pre-treatment structure (trash screen and sediment removal chamber) at the upper end of the Colma Creek flood control channel within the park. Pretreated water will then enter a diversion pipe leading to an underground stormwater storage reservoir. A portion of the storage facility will function as a cistern, holding water for eventual non-potable irrigation use in the park and for water trucks, and the remainder will function as an infiltration chamber to recharge groundwater in the Westside Basin. Construction of these storage facilities will occur underneath a portion of the existing baseball and softball fields. A flow splitter is located just upstream of the storage facilities to discharge treated overflow back into the channel when storage capacity is exceeded.

The Orange Memorial Park Stormwater Capture project

Status

The Orange Memorial Park Stormwater Capture project construction began in 2021 and remains on schedule. Completion of the project is anticipated in Spring, 2022. For additional information, please contact our Construction Hotline at (650) 829-4600, or email us at Construction@ssf.net.