Mission Bay may need tidal barriers, huge levees as sea rises

A conceptual design showing what Mission Creek might look like if it was turned into a fresh-water lake, with a levee closing the current tidal connection to San Francisco Bay. The design is among the conceptual options found in "Mission Creek Sea Level Rise Study," released on Sept. 26, 2016 and done with participation by five San Francisco city agencies as well as the Bay Conservation and Development Commission. less A conceptual design showing what Mission Creek might look like if it was turned into a fresh-water lake, with a levee closing the current tidal connection to San Francisco Bay. The design is among the ... more Photo: Arcadis/CallisonRTKL Photo: Arcadis/CallisonRTKL Image 1 of / 5 Caption Close Mission Bay may need tidal barriers, huge levees as sea rises 1 / 5 Back to Gallery

San Francisco’s Mission Bay neighborhood might need large levees or a tidal barrier to help protect it from the impacts of sea level rise in coming decades, a study released Monday and done with the city’s participation concludes.

The 80-page study is billed as “an imaginative exercise” and consists of design concepts rather than formal recommendations. Still, the emphasis on eventually altering the shoreline — one concept would turn Mission Creek into a lake — is a strong signal that local government sees the tidal aspects of climate change not as a distant possibility, but as a likelihood that needs to be planned for now.

“We want to help the public understand what protecting us from sea level rise might look like,” said Laura Tam of the planning advocacy nonprofit SPUR, which managed the project for the city.

Collaborators on the $200,000 study included five city agencies as well as the Bay Conservation and Development Commission, which has final say over any projects proposed along the bay’s shoreline.

“It’s important for our people to expand our own minds, and for people to understand they needn’t be afraid” of what the future might bring, said Larry Goldzband, the commission’s executive director. “That doesn’t mean the ideas will become reality tomorrow, or without a lot of community review and discussion.”

Three concepts focus on Mission Creek, which is lined with buildings planned during the 1990s and where there is a wastewater pump station at its west end. The other four look at the district’s southern shore, a low-lying stretch next to land where rail yards have been replaced by the likes of the UCSF-Mission Bay campus and where the Golden State Warriors seek to break ground next year on an 18,000-seat arena.

All are driven by the likelihood that if current sea level rise projections are accurate, and no protective measures are taken, a major storm during high tides could send water spilling down several streets in the neighborhood by 2050. By 2100 — when projections by the National Research Council set a 36-inch increase in tide levels as the most likely scenario — the same combination could flood the decks of Mission Creek’s historic bridges and cause as much as 4 feet of flooding of the city’s Public Safety Building, which opened last year and includes police headquarters.

In terms of the next half century, the report suggests that the main threats can be defused by relatively simple fixes. The public walkway along AT&T Park, for instance, could be guarded from occasional water surges by 3- to 4-foot glass barriers along the edge that wouldn’t block views. Raising the shoreline at two low points would minimize the risk of flooding.

As 2100 draws near, though, the report suggests that larger responses might be needed, including levees outside the current shore that might be large enough to hold buildings, creating a development opportunity that could help pay for the improvements.

Each design concept comes with pros and cons, as well as examples of locations where it already can be found.

The idea of a tidal barrier at the mouth of Mission Creek near AT&T Park that could be closed to repel storm surges might seem outlandish, for example, but a much larger one already exists on the River Thames outside London. Tokyo has begun to build “super levees” protecting it from the Sumida River, designed in such a way that they double as user-friendly waterfront landscapes.

As for the idea of turning Mission Creek into a freshwater lake, perhaps connected to the bay by a submerged culvert to allow for some tidal circulation, it’s a scenario similar to the current condition of Oakland’s Lake Merritt.

Such a step goes against a half century of public efforts to keep the bay from being filled, a grassroots advocacy effort that helped lead to the creation of the Bay Conservation and Development Commission in 1965. But at some point, study participants say, the public and regulatory groups need to rethink how they approach the bay.

“There are ways to keep the waterfront accessible and for people to enjoy the shoreline,” said Peter Wijsman of the local office of Arcadis, the Dutch engineering firm that prepared the report and was the study’s lead consultant. “The Bay Area would do well by not polarizing the discussion, but understanding all the options that might exist.”

The study released Monday has been in the works since 2014. It was to be released more than 18 months ago but was held up for review by various departments. It’s also likely that there was a desire not to make the concepts seem too provocative.

“It was not the intention of this process to select one preferred approach,” the report says. The goal, rather, was to show ways to “integrate flood protection into the urban fabric for an attractive and economically viable city.”

John King is The San Francisco Chronicle’s architecture critic. Email: jking@sfchronicle.com Twitter: @johnkingSFChron