The theories behind the erosion of Catalina Island’s South Beach explained

Avalon City Council receives the results of this year’s sedimentation study.

AVALON ― Two theories have been circulating in Avalon regarding the cause of sand disappearing from South Beach. One notion involves currents created by the Catalina Express running its engines in reverse while docked. Another explanation: waves are eroding the beach.

Dr. Scott Jenkins of Michael Baker International (MBI) presented the findings from the Avalon Harbor Sediment Study to the Avalon City Council on Sept. 19. The erosion occurring on South Beach is due to a combination of both of these factors, the expert explained to the council.

The prop wash from the Catalina Express does create a current, which loops along the seawall, Dr. Jenkins confirmed.

“When Catalina Express boats are berthed and running reverse to tension dock mooring lines for better sea keeping, its reverse thrust is creating a wake coming towards the seawall,” Dr. Jenkins explained. “When that prop wash strikes that seawall, it then follows the seawall as a boundary current but that’s not strong enough to erode the sand.”

However, a combination of the current created by the Catalina Express along with the occurrence of large waves reflecting off the seawall does move the sand off the beach. The sand then settles under Pleasure Pier along a very old concrete groin.

The city entered into a contract with Michael Baker International in October 2016 to conduct the sedimentation study. The survey conducted this year was compared to a survey done in 1994.

“We did determine about 5,000 cubic yards of sand have eroded off of South Beach and was deposited under Pleasure Pier,” Dr. Jenkins stated.

Dr. Jenkins recommended 5,000 cubic yards of sand be removed from under the Pleasure Pier and be distributed back onto South Beach.

The groin underneath Pleasure Pier should be demolished and removed, according to Dr. Jenkins.

The process of dredging and recycling the sand is just the first part of the solution. The second part would involve taking measures to prevent erosion from continuing at South Beach.

A couple options Dr. Jenkins presented to the council involved a wave absorber or a retaining wall to prevent or reduce wave reflection off the seawall.

“The problem with these two approaches,” Dr. Jenkins stated, “is…the footprint of [either] structure is going to take up the beach you are trying to restore.”

The other concern is the cost of building a wave absorber or a retaining wall, called a rock revetment. The total investment for a wave absorber is $13.6 million and for a rock revetment is $6.8 million.

Another option, according to Dr. Jenkins, is to build a “100 foot deflection wall to dissipate the loop current.” The cost of a deflection wall is $850,000.

The deflection wall brought up another concern of displacing half a dozen moorings.

“One other option would be a non-structural approach,” Dr. Jenkins suggested. “Make some modifications to the way of the operational procedures of the Catalina Express.

“I want to be very delicate with this, because they’ve brought over 25 million visitors to our island,” Dr. Jenkins added. “Catalina Express is just vital to this community.”

Makrom Shatila, project manager with MBI, said they can come up with other alternatives if they are allowed to investigate further.

Avalon City Council agreed to have them look into further solutions and have a proposal prepared for a future council meeting.

“My solution is a soft solution of building our beaches, not building walls,” Dr. Jenkins stated.

He also concluded with a bit of good news: Avalon has plenty of sand in its own harbor to replenish the beach.

city of Avalon photo

 

 

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