Half Moon Bay Wet Weather Storage Facility Built

It’s no secret that in recent decades California’s climate has grown more extreme. Epic droughts, raging wildfires, and—when it does rain—heavier storms often inundate the state year round. While the impact and response to such newsworthy events is front and center, countless other weather related situations are playing out behind the scenes.

In El Granada, California, just south of San Francisco, heavy rain events were overwhelming the Portola Pumping Station, leading to sanitary sewer overflows in the environmentally rich area in and around Half Moon Bay. The Sewer Authority Mid-Coastside (SAM), which serves the communities of Montara, Moss Beach, and Princeton in addition to the cities of El Granada and Half Moon Bay, decided to take action and approved the construction of the new underground Wet Weather Storage Facility with up to 600,000 gallons of capacity.

Phase One of the project featured a 200,000 gallon chamber, which was completed in 2012. The design called for five interconnected precast concrete box culvert runs. Jensen Precast provided 60 three sided box culverts, each 120”W x 72’H x 96”L, for the five runs of 15 boxes each. The boxes were capped onsite and buried.

Phase Two, a nearly identical 200,000 gallon chamber, was completed in April 2021, with Jensen Precast again supplying the box culverts. The main difference in Phase Two was the use of monolithic four sided structures. Bay Pacific Pipelines installed the new system alongside the Phase One storage and connected the two, providing 400,000 gallons of storage. The gravity system accepts peak stormwater and sewage and holds it temporarily until the pumps at the Portola Pumping Station can catch up.

Seismic Ready & Watertight

To protect this sensitive ecosystem, the San Francisco firm engineering firm SRT Consultants designed the storage facility with seismic considerations and watertightness at the forefront. The Jensen Precast team understood the stakes.

“We had to make sure the entire system was watertight,” says Borjana Savic, Jensen Precast Engineering Project Manager. “The Pacific Ocean is close by, so we had to make sure that nothing would get out of the boxes and nothing would get in.”

Specifications called for gaskets between the boxes and joint wraps outside to ensure decades of watertightness. Arshad Vali, Jensen Precast Engineering Projects Director, worked on the calculations for the boxes, which were manufactured at the Jensen Precast Fontana facility. “The seismic loading specified in the design required additional reinforcement,” Vali says. The result was 60 heavily fortified box culverts headed up the highway to Half Moon Bay earlier this year.

As a historic drought tightened its grip on California, the storage facility came online in the spring. Even as the drought continued through the summer, the additional storage is paying dividends. The added capacity enables SAM to shut down the pumping station to perform maintenance if needed. And some day, it’s likely that those flooding rains will return.

400,000 Gallons of Storage

Either way, this slice of paradise, just off Highway 1 and across the road from public beaches and the Pacific Ocean, will be protected by the Jensen Precast boxes buried underground. The 400,000 gallons of storage should be enough for the long run. The land originally set aside for Phase Three of the project could be converted into a public park instead.

“We were absolutely very happy with the way it went,” Prathivadi says. “We were extremely happy with the timeliness, and we didn’t have any hiccups during the entire project. Everything went well. The watertightness testing showed that everything was in compliance.”

That’s a key consideration for the SAM, which relied on Jensen Precast for a system that is “absolutely leak tight,” Prathivadi says. “The beaches are close by, just across the highway. We have to make sure that we take proper care of the environment.”

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