May 26, 2026

New high‑flow green infrastructure delivers successful urban stormwater treatment pilot

New high‑flow green infrastructure delivers successful urban stormwater treatment pilot

Pivoting midway through a project, a TerraMod™ high-flow green infrastructure system enabled San Pablo, CA to achieve a regulatory first for the Bay Area.

 

Background

The Sutherland Avenue Urban Greening Project in San Pablo, California, demonstrates how green infrastructure can succeed in one of the most challenging environments possible: dense urban infill with high groundwater, flood risk, heavy utility congestion, and limited right of way.

Through close collaboration between the City of San Pablo, regulators, engineers, and Oldcastle Infrastructure, the project became a first‑of‑its‑kind high‑flow green infrastructure pilot for the San Francisco Bay Area, showcasing how adaptive design and regulatory flexibility can dramatically improve stormwater outcomes without expanding footprint.

 

The site

San Pablo is located on the eastern edge of San Francisco Bay, just north of Berkeley and Oakland, and falls under the jurisdiction of the San Francisco Bay Regional Water Quality Control Board (Region 2).

The Sutherland Avenue site lies within the Wildcat Creek watershed, an urbanized, low‑lying basin that experiences frequent flooding. The surrounding neighborhood is characterized by high population density (>10,000 residents per square mile), mixed residential and commercial land use, limited open space, and older infrastructure.

Unlike previous projects, community impact and constructability within a lived‑in neighborhood were central considerations from the outset.

 

Challenges

This project presented an unusually dense concentration of constraints:

  • 35 acres of contributing drainage area
  • Extremely high groundwater, less than 3 ft below grade at the project site
  • Frequent surface flooding, including:
    • Up to 1 ft of water in a 2‑year storm event
  • Highly constrained right of way
    • Overhead power lines
    • Underground water, gas, sewer, and electrical utilities
    • On‑street parking and active traffic lanes
  • Dense urban infill that left virtually no room for footprint expansion

Traditional stormwater treatment approaches were insufficient under these conditions.

 

Original design approach

The original design consisted of 20 TerraMod™ precast concrete bioretention structures filled with standard bioretention soil media (BSM), with the system surrounded by geocellular spacing units to prevent soil compaction, increase subsurface storage capacity and expand the effective treatment footprint.

Each TerraMod unit measured approximately 4 ft x 21 ft and included horizontal slots that allowed stormwater to flow from the bioretention cell into the surrounding soil.

Modeling demonstrated that this system design would meet treatment goals at a smaller footprint than alternative treatment options.

 

Regulatory collaboration and mid‑project innovation

The turning point for the project came during construction.

Through ongoing coordination between The City of San Pablo, Geosyntec Consultants, Carex Engineering, Region 2 regulators, and our expert stormwater team, the project team leveraged MRP Provision 3.C.3, which allows for special pilot projects under challenging conditions.

This provision enabled something that had never been approved in the Bay Area before: the mid‑construction conversion of select bioretention cells to high‑flow biofiltration using StormMix™ media.

 

Solution: high-flow green infrastructure

Three TerraMod units situated at the most flood‑prone intersection along Sutherland Avenue were converted from standard BSM to incorporate StormMix high‑flow media.

This increased the media infiltration rate by 30x from ~5 in/hr to ~150 in/hr, and added orifice controls to regulate treatment flow without requiring any structural changes to the precast units.

By modifying just three of the 20 bioretention cells, the pilot project achieved outsized performance gains without expanding the system footprint.

 

Impact

The impact of high‑flow green infrastructure was significant:

  • 700 sq ft reduction in system footprint
  • >200% increase in treatment credit
  • 3.45 acres of added drainage area

Achieving equivalent performance with traditional bioretention trench would have required over 175 linear feet of trench.

Overall the pilot achieved ~80% runoff capture, meeting regulatory targets, and validated high‑flow green infrastructure as a viable strategy for ultra‑constrained urban environments.

 

Construction and installation

As an active urban infill project, construction required exceptional precision. Installations were carried out directly adjacent to property lines, sidewalks and parking lanes, while horizontal slot alignment between the TerraMod systems and the geocellular spacing units required millimetric accuracy.

However, as the TerraMod units were constructed from precast concrete, this enabled rapid excavation and backfill, preventing the need for slower and less precise cast-in-place procedures.

Once structural components were in place, our stormwater team returned to install underdrains, replace existing fill with clean drain rock, install the StormMix media, apply mulch and install cast‑iron access covers.

Despite the mid‑project pivot, construction stayed on schedule.

 

System performance and early observations

Performance data immediately following installation highlighted the value of the high‑flow system: during a storm in which 3.5 in of rain fell, the high-flow green infrastructure cells effectively filled with surface water, while surrounding areas experienced ponding.

One year post‑installation, inspections following ~3 in of rainfall showed controlled drawdown, water retained for treatment residence time, and continued inflows from residential sump pumps due to high groundwater.

The system proved particularly effective at intercepting persistent street‑level flows before they could accumulate or overwhelm downstream homes, businesses and infrastructure.

 

Community impact

Because this project was approved as a pilot, long‑term monitoring is a critical next step.

The City of San Pablo is pursuing EPA funding to support 3-5 years of performance monitoring and water quality sampling under Region 2 oversight.

Educational signage was installed throughout the corridor, explaining the local water cycle and educating residents about the role green infrastructure plays in building neighborhood resilience.

As a result, the project serves both an engineering and community education function.

 

Conclusion

The Sutherland Avenue project achieved successful deployment of high‑flow green infrastructure in a dense urban corridor, a regulatory first for Bay Area stormwater treatment.

It delivered effective stormwater treatment with major footprint reduction through targeted innovation, improved flood mitigation at a critical intersection, and a scalable pilot model for other urban municipalities.

 

Learn more