There’s a dilemma at Ocean Beach, and it’s not the fog spoiling what would be a perfectly sunny day in other parts of the city. It’s the slow erosion of sand inching closer to a critical sewage treatment plant.

If pipes that carry 12.5 million gallons of wastewater daily are battered by waves, they could burst, releasing dookie into the surf.  

To avoid that disastrous scenario, San Francisco devised a solution. In 2024, the state’s coastal commission greenlit a $175 million plan to build one of the largest seawalls in California history at the south end of the beach. The concrete barrier would span the length of nine football fields and reach 55 feet in depth to protect against nature’s assault, with construction slated to start in late 2027.

The concerns stem from a lingering mystery: No one understands why sand has been rapidly accumulating at the beach’s north end since the 1980s, while the south has eroded. Making a major investment without that fundamental answer could mean squandering taxpayer dollars while permanently altering the beach.

One of the paper’s authors and the newest voice in a long line of researchers trying to make sense of the mystery is Chase Davenport, an Outer Sunset resident, surfer, and founder of the conservation nonprofit Ocean Beach Institute. He’s especially worried about the long-term impacts of a major infrastructure project.

“I’m sure we’ll go down a geoengineering path over the next 50 years, trying to keep our rigid coastlines,” Davenport said. “But over time, we need to accept that we can’t control a force like the Pacific Ocean.” 

One of Ocean Beach’s foremost sand sleuths is UC Santa Cruz researcher Patrick Barnard, who spent two decades analyzing San Francisco’s shoreline for the U.S. Geological Survey.

Barnard said humans have been wrestling with the sand for nearly two centuries. He cites data from as far back as the Gold Rush, when excessive mining injected a massive volume of sand into the bay.

But over the following 100 years, the tap was turned off. Dams trapped sand upstream, and tidal marshes were filled for development, weakening the currents that naturally transport sediment between the ocean and the bay.

Today, infrastructure blocks nearly all of the Sacramento-San Joaquin Delta’s sand from reaching the ocean, and companies continue to extract the bay’s sand for construction materials faster than local watersheds can replenish it.

Barnard says these changes have been collectively shrinking one of the city’s key sand features: a massive horseshoe-shaped sandbar at the mouth of the bay, arching five miles from central Ocean Beach to the Marin Headlands.

“Something like 25 million dump trucks of sediment were extracted from San Francisco Bay historically,” Barnard said. “It’s definitely not any one thing that’s driving these observed changes [at Ocean Beach]. It’s probably a combination.”

Bob Battalio, a coastal engineer who has studied and surfed Ocean Beach’s swells since the ’90s, offers another theory. He argues that sand displaced in the 1970s, when the U.S. Army Corps of Engineers deepened the shipping channel that cuts through the offshore bar, explains the extreme accumulation of sediment at the beach’s north end. 

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Battalio referenced photos from the ’70s showing surfers entering the water from the edge of the seawall steps and graffiti on parts of the wall now buried beneath feet of sand. “The beach was very narrow,” Battalio said. “Now it’s 800 feet wide.”

He explained how waves bend around the shallow offshore bar and deposit sand in the center of the beach, where currents transport it north and trap it on the rocky headland. However, the south end of the beach experiences weaker waves, and opposing currents move sand toward the south. 

With no natural barriers, the sand continues drifting. As a result, despite the city’s continual attempts to replenish sand on the south end, the ocean has consistently carried it away.

More than a decade ago, researchers collaborated on a fix for Ocean Beach’s erosion problem. The effort led to urbanist think tank SPUR’s 2012 Ocean Beach Master Plan (opens in new tab). Developed in partnership with local stakeholders, including Battalio and Barnard, it was presented as a viable compromise to maintain the beach south of Sloat Boulevard and protect the wastewater plant. 

The plan, which was projected (opens in new tab) to cost $350 million, recommended restoring dune ecosystems, relocating select infrastructure, and using a smaller, buried seawall to protect a vulnerable sewage overflow tunnel.

But in 2024, the California Coastal Commission approved a far more invasive solution: a 3,200-foot-long seawall to be covered in sand, built to a depth of 55 feet in the beach soil. Opponents blasted the decision and the precedent it could set for other erosion-prone areas in the state, arguing that waves reflecting off seawalls actually erode beaches in the long term. The city, however, deemed the wall necessary to protect the sewage plant and maintains (opens in new tab) that dune restoration on top of the seawall and continued sand replenishment will keep the beach in place.

Laura Walsh, an Outer Sunset resident and the former California policy manager at the nonprofit Surfrider Foundation, said those who invested years in the master plan felt blindsided by the decision to build a seawall of that scale. 

“Ocean Beach is iconic,” Walsh said. “It has tons of fishermen, surfers, and beach walkers, and you see how they use that beach when it’s low tide. That’s all lost recreation space if you’re going to put a wall there that’ll erode the beach.”

City officials disagree. San Francisco Public Utilities Commission spokesperson Nancy Crowley defended the departure from the master plan, saying the city has spent many years working to understand the dynamics of the coast. She noted that her agency’s core responsibility is ensuring “people can flush their toilets, take showers, and rely on wastewater being safely carried away for treatment every day.”

The project’s long-term cost to taxpayers, aside from the risk of beach loss, remains unclear. Costs would include adding sand when the beach narrows beyond set thresholds and repairing damage if waves expose the wall. The coastal commission estimates (opens in new tab) it would cost at least $1 million each time the city has to dump more sand — the last major sand replenishment (opens in new tab), in 2021, cost $13 million.

“We don’t want to permit a seawall. We are still searching for a long-term fix,” Davenport said. “But if there were a cut-and-dry solution to handle this, other than moving the entire sewage plant landward, we would be doing that right now.” 

While he recognizes that the city is grappling with a challenging problem, Barnard shares concerns about the seawall. He predicts that, within a decade or two, the beach could erode, putting the seawall on the front line against battering waves and forcing the city to revisit solutions.

In the late 19th and early 20th centuries, city engineers straightened the coast, extending Ocean Beach 200 feet seaward to accommodate new infrastructure, including the Great Highway and a wastewater tunnel built underneath it. Nearly all of the original coastal dunes were replaced with landfill.

“Half the city was dunes. You couldn’t even run a buggy or wagon without getting stuck,” Battalio said. “Now, somehow, we’ve managed to create an erosion problem.”

The area south of Sloat, where the San Francisco Zoo and the treatment plant currently sit, was pushed farther out, creating a bulge in the coastline. It has made the area especially prone to erosion, prompting the city to close the Great Highway between Sloat and Skyline Boulevard last year.

A 1986 SF Chronicle story (opens in new tab) quoted Coastal Commissioner Michael Wornum blaming Ocean Beach’s erosion on “miserable” city planning, while others argued (opens in new tab) that engineers had made a mistake by building infrastructure so close to an erosion-prone coast.

For years, the city has spent millions to combat the erosion with the simplest methods possible: dumping sand and rubble on the beach, trucking in sand from the north end, and pumping in offshore sediment.

In 2021, the city spent $13 million (opens in new tab) bringing in sand from offshore sandbars. But waves swept most of that sand away within months, according to Bill McLaughlin, a surfer and longtime Outer Sunset resident. As surfers constantly scour Ocean Beach’s ever-changing sand bars for the best spot, surf quality near Sloat has diminished over the years. McLaughlin blames the repeated sand interventions.

He opposes the city’s plan, saying that if the seawall causes the beach to disappear, waves crashing directly on the wall would create chaotic chop and perilous conditions. Surfers would be among the first affected.

“We were trying to make sure that the beach just doesn’t erode and go away,” McLaughlin said. “Then surfers are stuck jumping off of a vertical wall to get into the ocean, and that’s not safe.”

The master plan anticipates up to 55 inches of sea-level rise by 2100, which would add unprecedented pressure to beachfront development. In an email, San Francisco Zoo spokesperson Nancy Chan said there are no plans to relocate the attraction farther from the eroding coast, but it will continue to monitor environmental changes as the seawall takes shape across the street.

Even if scientists unlock the mystery behind Ocean Beach’s transformation, Davenport doesn’t think there’s an easy fix. Instead, in the paper he authored with Battalio, Barnard, and researcher Jeff Hansen, he argues that cracking the code could compel the city and state to stop treating erosion as an isolated problem and instead view it as part of a wider regional system — a framework that could apply to eroding beaches across California and beyond.

But he says the most realistic solution may be the most radical: accepting that the ocean will eventually win. Speaking from a Great Highway apartment overlooking Ocean Beach, Davenport said that, regardless of whether the sand mystery is solved, the city will have to take more drastic action in the future.

“Sea levels will rise to the point that blocks are going to get trimmed,” he said. “It’s just a question of whether we are going to trim them, or if the ocean is.”