The bulbout stormwater gardens along Ingleside’s stretch of Holloway Avenue are designed to make a safer and more attractive street while directing water away from the city’s aged sewer system and into Lake Merced’s watershed.
So how well does the new green infrastructure work?
Two years of post-construction monitoring reports show that the bioretention planters and permeable pavement along Holloway Avenue from Ashton Avenue to Lee Avenue direct more water than expected back into the aquifer.
The project was informed by a pilot block installed on Sunset Boulevard, part of the Sunset Boulevard Greenway project completed in September. Both were SFPUC Early Implementation Projects designed to test what green infrastructure works best for the city’s neighborhoods.
“Overall, the project has slightly exceeded the expected performance level for the volume of stormwater removed from the combined sewer system,” SFPUC spokesperson Will Reisman told the Ingleside Light. “Specifically, the western blocks of the project performed exceptionally well due to the highly permeable soils near Jules Avenue, Faxon Avenue and Capitol Avenue, while the eastern blocks experienced a lower peak flow and volume reduction due to lower soil permeability.”
Two reports represent the full extent of SFPUC’s monitoring efforts for Holloway Avenue, Reisman said.
Baseline flows — stormwater runoff under pre-construction conditions — were established by monitoring a nearby unimproved block of Holloway Avenue.
The eight blocks of green infrastructure are estimated to have reduced the total volume of stormwater entering the sewer system by 77%, or 655,000 gallons, during the 2017-18 wet weather season, and by 78%, 764,000 gallons, during the 2018-19 wet weather season.
There was one significant lesson learned from the Holloway Green Street project that ought to be included in other green infrastructure projects, according to the 201819 report.
Permeable concrete installed within the 7-foot-wide parking lanes on both sides of the street can be installed up to the curb without a formal gutter, which saves money. However the street cleaning vehicles aren’t able to remove sediment.
“The rotating brushes do push a good portion of the sediment to where the vacuum component can capture it, but the scouring action also pushes some of the sediment down into pervious concrete’s pore spaces,” the report states.
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