Chances are most Franciscans have never heard of the Lake Merced Tunnel, but it’s a historic piece of 19th-century infrastructure that’s critical to keeping the city’s sewers flowing without debris going straight into the Pacific .
There is also a risk of being destroyed by the same ocean as climate change continues to creep the tides inward, forcing the city into a more than $ 150 million protection plan.
According to the San Francisco Public Utilities Commission (SFPUC), the Lake Merced Tunnel is a 14-foot-wide drainage pipe that runs under the Great Highway and serves the Oceanside Treatment Plant and its pumping station on Ocean Beach.
The facility is one of three facilities in SF that are responsible for the rehabilitation of rainwater and wastewater discharges prior to disposal. It treats around 20 percent of the city’s wastewater.
If the facility can hold too much waste at one time, such as a large storm, the overflow flows into the Lake Merced Tunnel.
Being a giant sewer pipe may not sound very impressive. When it was built in 1896, the San Francisco Call described the tunnel as “one of the most difficult engineering work ever carried out in the state” and raved about its completion in just 17 months despite “many difficulties with turbulence in underground water.” (Note that the tunnel precedes the sewage treatment plant nearly 100 years. The Oceanside facility was not built until 1993 and the existing tunnel was adapted for its purposes.)
The tunnel was one of the crowning achievements of a so-called canal century in San Francisco.
“In the 1800s we built more sewers than anyone,” George Engel, operations manager at the SF Public Utilities Commission, told Curbed SF.
“We still have on the order of 100 miles of 19th-century sewers operating in San Francisco today,” adds Engels, including tunnels.
The Lake Merced Tunnel is a hidden gem, like the entire apparatus used to dispose of SF waste. In 2014, SPUR marveled at the “invisible infrastructure” that keeps the system in motion, but is also not visible to the public on the beach.
But this obscure piece of plumbing is important – and it’s in serious danger too. The planning department warns that erosion and rising sea levels are threatening to flood the tunnel and the associated infrastructure in the near future.
The city hasn’t said in so many words that the coastal water system itself is in danger – SFist describes the system as potentially “washed into the Pacific” – but there is important plumbing inland of the tunnel, and it makes sense that it is imagine more complex problems in the future.
To counter this aquatic encroachment, the city has a $ 151.3 million plan to push back the ocean.
In a November memo, SFPUC Director General Harlan Kelly set out the two critical initiatives: removing several thousand feet of the Great Highway between Sloat and Skyline Boulevard (a stretch of doomed consumption) and creating “multipurpose coastal protection.” / a multi-purpose restoration ”/ access system. ”
The latter is a broad term consisting of actions ranging from “managed coastal retreat” to “beach food” – that is, bringing in large amounts of sand to replace eroded material (which the city does every year).
The city hopes to start the actual construction work in 2023.
As the San Francisco Examiner notes, the current price is up roughly $ 60 million from previous estimates. The city blames “refinements” in the construction plan for rising costs.
In 2015, environmental group Surfrider advised the city to consider relocating the Merced Tunnel altogether, arguing that “sea level rise and climate change-induced storms will increase in the years to come,” leaving the tunnel only delaying where it is is inevitable, but SFPUC will still try to keep the tunnel in its original location.