Oceansides annual harbor dredging and the occasional regional sand replenishment projects are not enough to save the citys eroding beaches, a new study shows.

A proposal to build rock groins on the beach appears to be the best way to stop or at least slow the steady erosion that has been chewing away the citys coastline since the 1940s, according to the study prepared for Oceanside by the Long Beach-based consulting firm GHD.

Sand from this (harbor dredging) program does little to really benefit the city beaches, said Brian Leslie, a senior coastal scientist and project manager with GHD, in a June 30 community Zoom meeting.

There are several reasons for that, Leslie said. One is that the sand dredged from the harbor is too fine-grained to linger on the beach and is quickly washed away by waves and tides.

Another factor is the distribution method used by the U.S. Army Corps of Engineers, the federal agency responsible for the job. The sand slurry pumped from the harbor is deposited directly into the tidal zone, instead of higher up the beach where it would last longer.

The consultants study looked at four long-term alternatives for replenishing and retaining enough sand to keep a dry sand beach between Wisconsin Street and the harbor:

1. Pumping 300,000 cubic yards of sand from offshore deposits every five years for 20 years, without building any hard structures such as groins, jetties or reefs. Estimates show that alternative would cost about $28 million over the life of the project.

2. Building four 600-foot-long rock groins and replenishing the beach initially with 300,000 cubic yards of sand, then with 150,000 cubic yards of sand at five-year intervals for 20 years. That would cost $51 million over the life of the project.

3. Extend the harbors south jetty offshore by 350 feet, and deposit 300,000 cubic yards of sand on the beach initially and again at five-year intervals. This alternative would include the construction of a sand bypass system to pipe sand from Camp Pendleton across the harbor to Oceanside, and would cost a total of $36 million.

4. Build two artificial reefs with submerged edges as breakwaters just outside the surf zone, and replenish the beach with 300,000 cubic yards of sand initially, then 150,000 cubic yards every five years. The reef alternative is the most expensive, $148 million.

All of the options need sand, and lots of it, Leslie said.

Two types of sand bypass systems, fixed and semi-fixed were considered, he said. However, both would be expensive to build and operate and would require the cooperation of Camp Pendleton, which has refused in the past.

Another possibility to get more sand is to piggyback onto the annual Corps of Engineers contract, which Oceanside has done several times in years past.

Groins and reefs hold on the beach sand longer than with replenishment alone and provide a more stable environment for coastal marine life, said Aaron Holloway, also with the firm GHD.

Groins certainly look like the better value, Holloway said.

The meeting included a question-and-answer session. One question was why are the sand retention devices being proposed north of Wisconsin Avenue, when some of the citys most eroded beaches are south of there.

The consultants responded that the location is not final and will be subject to the wishes of city officials and residents. But another factor is the distance that sand would have to be pumped, and the rights of property owners that would have to be crossed.

Resident Bruce Parker said the rock revetments that line the citys beachfront homes are unsafe, and that a solution needs to be found quickly.

If we wait too long on this, we might even lose The Strand, because the street itself is developing cracks, Parker said. Im worried about our future if we dont get something started.

The report will be presented to the Oceanside City Council at its Aug. 11 meeting and, if the council approves, the next step would be to find funding for the final design and permits needed for the alternative supported by the council, said Public Works Director Kiel Koger.

All of the options ... are very expensive, so we are going to have to figure out how we are going to pay for this, Koger said.

The results here are not really new ideas, Koger said. Groins have been considered a viable option for Oceanside in several previous reports and publications over the years.

The city has always counted on the U.S. Army Corps of Engineers for large beach replenishment projects in the past. However, there is growing public support for the city to get involved and to push for something that could be a more permanent solution.

Concerns are often raised that the California Coastal Commission would not approve the groins. The agency, which oversees all coastal development, generally opposes any hard structures such as seawalls, jetties and groins on the beach because of the far-reaching environmental effects.

That could be changing, said Russ Cunningham, the citys principal planner.

My experience tells me that the Coastal Commission remains reluctant to approve or consider these types of improvements, Cunningham said. However ... the discourse is evolving.

What we are beginning to hear is that each stretch of coastline is unique ... and that unique solutions may be required, he said. We are uniquely impacted by the Camp Pendleton boat basin.

Camp Pendletons small harbor, also called a boat basin, just north of Oceanside was built during World War II. Oceansides harbor, built in the 1960s, shares its entrance. Together the harbors create a barrier that blocks the course-grained sand that ocean currents carry south along the coast, starving beaches in Oceanside and, some say, as far away as La Jolla.

The recent study showed that the harbor jetties deflect larger and denser grains of sand into the deeper ocean water, while the more fine-grained sand flows into the mouth of the harbor.

Annual dredging clears the harbor channel and pipes the sand onto Oceanside beaches, but its only the finer-grained sand and not the denser, heavier grains that accumulated on the beach before the harbors were created.

Regional replenishment projects in 2001 and 2012 pulled the larger-grained sand from nearshore deposits in the ocean, and the denser material stayed on beaches longer. Studies show some of that sand still remains in Carlsbad, protected by the jetties at the entrance to the Agua Hedionda Lagoon.

But its not just the North County harbors that are starving the beaches.

One hundred-plus years of development on our beaches and bluffs have fixed in place our coastline, Cunningham said. That effectively presents the processes of bluff erosion and beach retreat from occurring.

Sea-level rise compounds the problem and is something all coastal cities must keep in mind, he said.

More:

Oceanside searches for ways to keep sand on its eroding beaches - The San Diego Union-Tribune