Ask Lookout: What was that construction near the trestle bridge, and is it done?

lagoon
(Via City of Santa Cruz Public Works Department)

The mouth of the San Lorenzo River was packed with heavy machinery during the late summer and fall, but that construction has since halted. What was the project, and has it wrapped up?

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Ask Lookout: San Lorenzo River Culvert Project
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Construction in cities isn’t exactly uncommon, as there’s always something (or lots of things) to be worked on. Though many of those projects probably don’t get much attention, Santa Cruz locals and beachgoers in the late summer and fall might have noticed the fenced-off area with excavators and large piping just in front of the trestle bridge that connects the Beach Boardwalk area to Murray Street.

That construction project is the culmination of eight years of planning and collaboration among a number of government agencies, including the California Department of Fish and Wildlife, the U.S. Army Corps of Engineers, the Regional Water Quality Control Board and the California Coastal Commission.

If you find yourself on Murray Street today, you won’t see much more than a small section of pipe poking out from the sand beside the eastern edge of the Boardwalk that holds rides like the whirling, perhaps nausea-inducing Cyclone and the classic Cave Train Adventure. But equipment and crews will indeed be back in the spring.

The San Lorenzo River Lagoon Culvert Project seeks to set up a water height-control system for the lower portion of the river where it empties out to the ocean. That includes installing two long pipes that run from the trestle bridge to the river’s mouth. A $2,215,000 grant from the California Wildlife Conservation Board’s Stream Flow Enhancement Program and $658,000 from the City of Santa Cruz’s storm water, general and liability funds are financing the project.

Associate Engineer Ryan Haley said there are three reasons for the work. The first is to prevent flooding due to high waters. Those elevated water levels typically occur in the summer when waves that come from the south push sand against the mouth of the river and create a sand bank. That can cause the river to fill up and eventually overflow, as the water no longer has anywhere to go.

“When the water is very high, the Boardwalk can flood, Ocean Street can flood and basements can flood on Front Street,” he said.

Though waves pushing sand against lagoons is natural and to be expected, here engineers are grappling with a problem that is 50 years in the making.

The situation has been exacerbated by the construction of the harbor under the Murray Street Bridge in the 1960s, Haley said. Sand that used to flow downshore began getting trapped by the harbor’s jetty and accumulating at the rivermouth and the surrounding beaches.

A diagram showing the natural flow of sand blocked by the harbor jetties, resulting in sand buildup.
(Via City of Santa Cruz Public Works Department)

The second reason for the project is public safety. A lagoon breach can sometimes be deadly. Unsuspecting beachgoers have been swept into hazardous waters. That’s particularly nerve-wracking in the summer, when loads of tourists rush to the sand, Haley said, “so it’s a very dangerous situation when it’s not controlled.”

Last, the project is designed to protect wildlife. A lagoon breach can contaminate the largely freshwater river with the ocean’s saltwater, thus altering the habitats of endangered species like the tidewater goby, said Haley.

Further, if the lagoon breaches, species within the lagoon could be swept into the ocean — an unsuitable environment for brackish water species.

So is the project done? Not quite, but it’s very close. A post on the City of Santa Cruz Public Works Facebook page said it’s “99% complete.”

However, an intake structure — the component designed to siphon the excess saltwater to protect the river species — requires workers to dig deep into the ground to install it. That’s typically hard, or impossible, to do if the sand level is too high, said Haley.

“We describe it as a tuna can. It’s a big, round cylinder that needs to get installed very deep,” said Haley. He added that the sand level is high right now, which is common at this time of year. As a result, the sand would collapse on itself as contractors attempted to dig, preventing further progress.

No worries, though. The project should reach completion in the spring just fine.

Haley explained that sand levels are lower in the spring because the river will naturally purge some of that extra built-up sediment preventing the dig: “We’re going to use Mother Nature to help us rather than use force that seems like it won’t work.”

Further, the mechanism isn’t intended to be functional in the winter anyway. Though Haley said that the water is high right now, the summer swells are the main culprit for the phenomena the project hopes to deter.

Watch the public works department’s video on the project below for an overview.

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