SAN DIEGO — A group of Oceanside residents are pushing for what they say is the best solution to the city’s shrinking beaches — building rock structures known as “groins” — but it’s likely to be an uphill battle.

Groins are lines of boulders or concrete placed on the beach that stick out like fingers into the surf to catch and hold the sand, preventing it from eroding.

“This is a logical, feasible ... thing to do,” said Nick Ricci, a member of Save Oceanside Sand. The SOS group is trying to persuade city officials to back the proposal, which would require the approval of the California Coastal Commission and other state and federal agencies.

It’s an old idea that’s fallen from favor with environmentalists and regulatory agencies. The Coastal Commission generally frowns upon seawalls, revetments and groins. Numerous studies have shown the structures can contribute to erosion and cause more problems than they solve.

“They are definitely not a panacea,” said Sarah Christie, legislative director at the California Coastal Commission.

Ocean currents flow from north to south along the Southern California coast, carrying sand with them. Building a groin may stop some of the sand at one spot, but may cause less sand to flow onto beaches south of that point.

“It’s a dynamic system,” Christie said. “Anytime you try to make things stationary, you are going to have problems.”

Part of Oceanside’s sand problem is that the city’s harbor, built in the 1960s, shares an entrance with the harbor constructed by the federal government at Camp Pendleton. A long jetty and breakwater protect the north side of the entrance and there’s a shorter jetty on the south side. Those barriers stop much of the sand that ocean currents otherwise would carry south, collecting it in and around the harbor’s mouth.

Another groin at the San Luis Rey River helps hold sand on the beach between the harbor and the river. But south of the river, especially south of the municipal pier, the beaches are mostly bare except for a narrow line of cobblestones.

Every spring, the U.S. Army Corps of Engineers dredges the harbor entrance and pumps the sand onto beaches south of the river. Still, that’s not enough and it constantly washes away. Also, there’s less sand flowing down rivers, such as the San Luis Rey, because of extensive upstream development. And other factors such as sea-level rise contribute to coastal erosion.

The corps launched a three-year shoreline feasibility study in March 2016 to document the effects the Camp Pendleton harbor, built in 1942, has had on Oceanside beaches. The results of that study could be used to apply for federal grant money to aid the city’s beach preservation efforts, which could include groins or seawalls but would probably go to sand replenishment.

However, work on the study stopped after about a year when the corps ran out of money and asked the city to pay $1.5 million to $2 million, or about half the cost needed to finish the study. Originally, the corps said it would pay the entire cost, and city officials have said they don’t have the money.

City and state officials are urging the corps to finish its shoreline feasibility study and shoulder the entire cost.

“The study will determine how to mitigate the erosion of Oceanside’s beaches,” Kiel Koger, the city’s public works director, said in an email last week.

“In my opinion, a permanent structure would be needed to keep sand on our beaches,” Koger said. “The sand replenishment projects from dredging the harbor entrance or offshore borrow sites are only a temporary solution.”

Coastal Commission staffers believe building a groin at Oceanside would harm beaches to the south the same way the harbor breakwater has done, he said.

Still, an engineering study of the entire San Diego County coast done in 2001 for the San Diego Assn. of Governments, the area’s regional planning agency, suggested that Oceanside would be a good spot to do a pilot groin project to study the effects of sand retention.

“It has been discussed in the past, but I’m not sure how much serious consideration was given,” Koger said.

Two high-profile elected state officials, U.S. Sen. Kamala Harris and Rep. Mike Levin (D-San Juan Capistrano), recently stepped in to ask the Corps to complete the study and pay all the costs.

“Oceanside has a long and well-documented history of beach erosion resulting from Camp Pendleton harbor construction in 1942,” states the June 26 letter signed by Harris and Levin. “This erosion has raised safety issues for beachgoers and resulted in lost tourism revenues. It also has reduced the value of affected properties, exposed them to storm damages, and complicated area redevelopment plans.”

Ricci of SOS and others say groins are the simplest and best solution to Oceanside’s beach erosion problem.

“The slam against groins has always been that it starves the southerly progression of sand to the next beach city,” Ricci said. “We believe ours is a very different situation.”

SOS is proposing a series of five groins be built at intervals from Tyson Street south to St. Malo. There’s no official cost estimate, but it could be more than $10 million, he said.

Money could be available from the federal government, as it is for other beach stabilization projects, Ricci said.

The group’s idea is to model Oceanside’s groins after the eight built in the 1960s at Newport Beach, where the rocks have helped widen and stabilize the beach.

Those structures were built without the approval of the Coastal Commission, which was established in 1972 by a voter initiative.

Diehl writes for the San Diego Union-Tribune.