By Thomas K. Pendergast
To help the beach between Sloat Boulevard and Fort Funston keep its sand from eroding away entirely, the U.S. Army Corps of Engineers has placed a sand berm next to the rocks and cliffs protecting a sewage facility from the ocean waves.
Officials say the berm rises 30 feet upward into the air from where they started and extends roughly 1,000 yards down the beach. At least 255,300 cubic yards of sand were dredged up from the main shipping channel leading into the bay, according to the Corps. The project started in the latter part of August and finishing up at the end of September.
“That’s what you see out there now. That’s the product of work that was ongoing for about a month and a half there,” said Peter Mull, project manager for the Corps. “So, now there’s a big buffer of sand to help protect the infrastructure there. The sand is always in motion. It’s like a river. The waves will drive it, typically in California, from north to south.
“If you don’t have enough sediment on the shore, these large waves will come in and there’s no sand to denude. They hit these rocks that were built hastily down there … the rock revetments that you see down there. That causes deeper scouring; the water depth inside of that rock structure goes deeper and deeper,” he said.
“So, when it’s deeper, those waves will come in totally unimpeded and spend all of their energy scouring out the front of the sea wall. The next thing you know, you have no beach.”
The project was managed by the Corps at the request of the City and in partnership with the Golden Gate National Recreation Area, with the $7-8 million cost split 65% to 35%, the latter coming from San Francisco.
Erosion of the beach in this area has been a problem and a risk going back to the early 1990s, according to Anne Roche, a project manager and climate advisor to the San Francisco Public Utilities Commission (SFPUC), which runs the Oceanside Water Pollution Control Plant that is now under threat.
The immediate concern, however, is the Lake Merced sewage tunnel, a 14-foot-diameter pipeline that carries sewage water from the residences of the Lake Merced area to the plant, where it gets treated, cleaned up and then released through another pipe going into the ocean at about four miles out to sea. The Lake Merced tunnel goes under the Great Highway along Ocean Beach, but well below the beach level. It is also the most seaward component of that facility.
At its northern end, Ocean Beach is actually getting wider, as sand collects there, so the SFPUC has been taking that extra sand and trucking it down to the southern end and dumping it on the beach south of Sloat Boulevard.
“The idea is to slow erosion while the long-term project design goes through environmental review and goes to construction,” Roche said.
The final phase of the project will be using more sand to eventually create a long-term coastal hiking and biking trail.
As for the dredging, a 375-foot ship name Stuyvesant — owned by the Corps and based out of Portland, Oregon – was used to suck up the sand from a shipping channel that gets dredged annually.
Mull said the channel is in what is known as the “potato patch,” a large horseshoe shaped “remnant” sand bar, that extends up to about four miles out around the Golden Gate, starting near Point Bonita, with the other end coming back to shore right about where Taraval Street ends at Ocean Beach.
This sand bar was formed during the last ice age, when lower sea levels made the San Francisco Bay a river delta. But now the sand bar is a submerged ebb-tidal delta. It remains there about 35 feet beneath the surface.
Mull said in the winter if you stand near the Cliff House restaurant when there are big winter swells, you can see waves breaking on those sand bars about three or four miles from shore.
Much of the ship traffic going in and out of the bay require a draft of more than 35 feet. So, every year they dredge a channel in that sand bar and take it down to 55 feet below the surface to make sure the big ships clear the sand bar, usually during high tide.
That requires dredging between 300,000 to 500,000 cubic yards of sand out of that channel every year, using a “hopper” dredge ship like the Stuyvesant, which can hold up to 9,000 cubic yards of sand before it must be emptied. It has two long “drag arms” that go along the bottom and big hydraulic pumps to suck up the sand.
Then the ship goes to where it can dispose of that sediment.
Since 2005, to help fight the erosion on Ocean Beach south of Sloat Boulevard. they have been dumping it offshore there at about a depth of 40 feet because it builds up the offshore sand bars, causing the waves to break further out and diminish the erosion that happens during the winter months.
This year, instead of just dumping it offshore, they put a mile-long 30-inch-diameter pipeline along the bottom and ran it from about one mile offshore up to the beach. Then the sand they dumped onto the beach was “de-watered” to create a big stockpile for a sand dune.
“This methodology offers the opportunity to maintain a natural beach system,” Mull said. “We’ve now developed a system whereby we will support the natural sand transport processes in the area of this erosion hot spot that is threatening vital infrastructure.
“The Army Corps will continue to dispose of the material we dredge annually in that designated zone offshore of Sloat Boulevard. That will help to support the natural sand processes. As the beach recedes and the infrastructure gets threatened again … we can come back and renourish the shoreline on an as-needed basis like this year.”
“This is a huge undertaking and the City is working diligently to make sure this project progresses,” Roche said. “We are taking climate change seriously and doing efforts like this. So, I think that that’s good news for San Francisco.”
Categories: ocean beach