A community association that owns the private marina in Hawaii Kai wants to more permanently inhibit sand from migrating off Portlock Beach into the boat entry channel by constructing a $2 million rock groin.

The Hawaii Kai Marina Community Association believes the groin, which would replace a row of giant sandbags first installed in 2004 on the east side of the entryway, will keep the channel clear for longer and be less costly in the long term compared with maintaining the sandbags.

However, some nearby residents and other community members are concerned about potential negative impacts on beach erosion and ocean currents.

The association anticipates no significant impacts, according to a draft environmental assessment published last week by the state Office of Environmental Quality Control.

Waimanalo-based firm Sea Engineering Inc. prepared the report that details the association’s plan.

According to the report, several layouts for the groin made from boulder-size rocks stacked 5 to 6 feet high have been analyzed. The preferred groin has an L shape and is 240 feet long with the base of the L running another 50 feet.

Alternatives include straight groins of 200 and 240 feet as well as L-shaped groins of 200 and 280 feet.

The existing row of sandbags is 200 feet long.

Developer Henry Kaiser created the marina channel in 1959 as part of building the Hawaii Kai community around what was once a fishpond. To keep the channel deep enough for boats, more dredging was done in 1981, 1985, 1988, 2004 and 2013, the report said.

In 2004 as part of dredging work, an experimental sandbag groin was installed to reduce sand moving from Portlock Beach into the channel. The row of sandbags was rebuilt in 2013 after significant deterioration.

The report said the temporary groin slowed the flow of sand toward the channel and reduced the need for dredging.

Replacing the sandbags with a bigger rock groin would hold back more sand and further reduce the frequency for channel dredging, according to the report.

The report also said that beachfront homeowners in the area recently expressed interest to Sea Engineering to replace sand that has eroded in front of their property. As part of the rock groin plan, sand that accumulates on the beach side of the groin would be redistributed farther up the beach to counteract erosion.

“The L-head groin retains enough sand for this to be a viable option,” the report said. “As the groin approaches its maximum sediment trapping capacity, an excavator and small off-road capable trucks could be used to carry the sand up the beach. This process could be repeated as needed every three to five years to ensure that sand doesn’t bypass the groin into the channel.”

An initial movement of 400 cubic yards of sand up the beach would be done during construction of the rock groin.

Public comments on the plan are being accepted through Jan. 23. Comments can be sent to Natalie Farinholt with the state Department of Land and Natural Resources at Natalie.a. farinholt@hawaii.gov. The report detailing the groin plan is available at health.hawaii.gov/oeqc.

Some early responses to the plan include concerns raised by Marshall and Barbara Rosa. They live next to the channel and said its original construction eroded their property and Portlock Beach.

In written comments the Rosas said the beach was stable for many years afterward with two boat passageways open. The passageways connect beach erosion with the last two dredging projects that included placement of the sandbags.

“The last two dredging projects, when the sandbag groins were placed and the third bridge bay was dredged, caused Portlock Beach and the beach park to erode,” they said.

Another nearby owner also expressed concern over how a rock groin would affect the existing shoreline.

Conservation organization Malama Maunalua said it recognizes the need for a safe marina channel but urged a careful study of impacts from a rock groin before any action is taken.

To build the groin, the association needs several government approvals including a Department of the Army permit, a state conservation district use permit and a city special management area permit. If approvals are obtained without unexpected delay, the association anticipates being able to build the groin in 60 to 90 days starting in the fall of 2018.