Ocean Beach is one of San Francisco’s most uncertain recreational spots, its permanence in question as shifting sands and sea level rise threaten to change its dimensions, and even existence.

City and federal officials are discussing whether to take a more proactive approach towards the sea, rather than let natural forces take their course. They have launched the Ocean Beach Master Plan, a blueprint that will lay out a long term strategy for the area.

Two of the  most discussed options? Build up an “armament” to bolster the land from the sea by using sea walls or large stone piles call revetments. Or continuously dump sand along the shoreline to put back what nature is taking away.

The decision isn’t an easy one, since the cost and effectiveness of these approaches over time are uncertain. Some problem areas need immediate attention, notably the stretch of the Great Highway south of Sloat Blvd., which despite a revetment is eroding at a quick clip and washing away the road and endangering the Lake Merced sewage pipeline.

Other areas, including most of the northern part of Ocean Beach, present long term challenges. A sea level rise of four feet by 2100, within current estimates under state guidelines, would subsume 280 feet of shoreline, dumping water into low-lying neighborhoods in the Sunset district, said Bob Battalio, a coastal engineer with the northern California-based firm ESA-PWA.

“If we build a sea wall or a revetment we will lose the beach completely,” he said. “Kind of like what you have down (south of Sloat Blvd.) now.”

Hold the line
Permanent structures like sea walls and revetments offer a “hold the line” approach to the sea’s advancement, but do little to create beach habitat, Battalio said. What the public ends up with is solid walls that buffer the sea and protect infrastructure, but virtually no recreational setting.

That’s why officials are considering adding sand to the beach from the material picked up from the biennial dredging of an off-shore shipping channel that’s used for ship navigation into the San Francisco Bay. But even that option has its detractors.

“It’s something worth trying but I’m skeptical it will last long,” said Patrick Barnard, a coastal geologist with the USGS who has studied Ocean Beach for the past seven years. “Given the location and wave energy, it’s something you would have to do every year and probably in greater amounts than they’re proposing because the shoreline just doesn’t want to be there.”

Natural forces at play make human engineering of the beach difficult.

Independent of sea level rise, Barnard explained that Ocean Beach has been tilting with the northern part of the beach growing in size as the southern Sloat Blvd. area has receded. This is due to a shrinking of the San Francisco Bay Delta, which he said has lost “an extraordinary amount of sediment” over the last 50 years.

“That affects the way the waves approach the beach and the movement of sands up and down the beach,” Barnard said. “Where you once may have had wave energy being reduced by shoals (in the southern section of beach), they have since migrated to the North.”

An artificial shoreline

Another problem is that the shoreline around the Great Highway isn’t natural. Starting in the 1800s, the shore was artificially filled by 200 feet to construct the Great Highway for “pleasure driving” and the San Francisco Zoo. Since the 1980s, the city has been trying to maintain a sand dune barrier to buffer waves and prevent erosion. But it may be that even under the best of circumstances, the shoreline simply doesn’t want to be there.

“The erosion problem was one that was created,” said Battalio. “The closer we are to where it wants to be, the less costly it will be to maintain it.”

But ceding stretches of beach to the sea, however sensible from the standpoint of long term accommodation to the forces of nature, is a choice city residents may never accept.

Where sand dunes once stretched across the western edge of the city is now highly urbanized: homes and commercial areas, a pipeline carrying one-third of the city’s sewage, a waste water treatment plant, and a major recreational spot for San Francisco’s outdoorsy populace.

You can comment on the Ocean Beach master plan, contact SPUR (the San Francisco Planning and Urban Research Association), which leading the process on behalf of city and state agencies.

The Ocean Beach Bulletin has also been regularly covering the master plan process.

Tags: erosion, great highway, ocean beach, ocean beach master plan, sea level rise, yahoo