The decades-long erosion of the shoreline just outside the city’s Sand Island Wastewater Treatment Plant leaves the facility’s outfall pipeline in a precarious situation where “failure or damage to the outfall could have catastrophic health, environmental and economic consequences,” according to an environmental report made public by the city last month.

To shield the land-side portion of the outfall pipe from damage, the city Department of Environmental Services wants to rebuild a sloping, bermlike barrier “to armor the shoreline” and to shield the land-side portion of the outfall pipe from damage.

City officials insisted that the planned structure, technically called a “revetment,” not be called a wall. It uses materials and a construction form similar to the Kapahulu Groin and the berms that protect the inner portion of Magic Island and the Ko Olina lagoons.

Ross Tanimoto, deputy city environmental services director, told the Honolulu Star-Advertiser on Wednesday that a sea wall is designed to protect the shoreline while the revetment being proposed is aimed at shielding the outfall pipe. “A revetment is usually angled, too, so the waves come up and expend their energy along the inclined area,” he said.

The city needs federal, state and city permits to build the structure, and the draft Environmental Assessment, completed by SSFM International and filed with the state Office of Environmental Quality Control on May 23, concludes the project would not cause any adverse environmental impacts.

Photos and maps included with the report show how the shoreline fronting the plant has eroded dramatically over the past few decades. The study estimates the shoreline receded about 150 feet from 1955 to 2015. While the erosion continues, it appears to be doing so at a slower pace than in the past.

The goal of the $3.7 million project is to protect the 84-inch ocean outfall pipe, the only one of its kind on Oahu.

It extends about 1,450 feet on land before it bends and stretches about 12,500 feet offshore — buried in a trench excavated into the fossil reef. It’s there, about 2.3 miles offshore, that sewage that has been given primary treatment enters the ocean about 200 feet below the surface.

But it’s a 400-foot shoreline section of the pipe that is the source of concern because it is currently only 2.2 feet below grade. “Therefore, the land portion of the outfall is relatively unarmored and vulnerable to damage should the section become exposed to wave activity,” the report said.

Tanimoto said the project should be done sooner rather than later. “To wait to see what sea level is doing to it wouldn’t be prudent,” he said.

During storms, “lots of stuff moves around — rocks, some boulders — that could ram into the pipe if that’s exposed,” Tanimoto said. If left exposed and unprotected, the outfall pipe could even rupture. “Say for example, a boat got run aground along the shoreline, it could ram into the pipe if the pipe was exposed … to prevent that occurrence, we put the revetment over it,” he said.

The outfall is deemed “critical infrastructure” because it discharges 66 million gallons of sewage daily. “The progressive shoreline erosion is a serious potential threat that could eventually expose and compromise the outfall pipeline,” the report said.

The city is proposing to put up a 450-foot revetment, defined in the report as “a sloping, un-cemented structure constructed of wave-resistant materials.” Specifically, there would be a sloping structure consisting of boulder-size rocks sitting atop a mound of smaller rocks and rubble, the report said. It would stretch from near where the outfall extends into the ocean northward along the shoreline to a dredged channel, the report said.

An original revetment was constructed when the outfall was first built in 1975. A “stop gate,” which houses a type of shutoff valve for the pipeline in case of an emergency, was installed more than 150 feet inshore of the shoreline but is now located near the waterline due to erosion, the report said.

“There is concern that continued shoreline erosion, particularly in conjunction with a potential tropical storm, may begin to remove cover material and expose the trench and possibly damage the outfall,” the report said.

The gate itself also is being relocated 20 feet inland, the report said.

The city is hoping to obtain all necessary approvals by June 2019 and have the project completed within six months of that. Estimated cost of construction is $3.7 million.

Jodi Malinoski, Oahu coordinator for the Sierra Club of Hawaii, said her organization is reviewing the draft Environment Assessment and has yet to come up with a formal position.

“We do recognize that this project may be inevitable,” Malinoski said. “But we would encourage our state to do more to prevent climate change and sea level rise.”

If nothing else, she said, the need for the project reinforces the importance for the city to have a fully staffed Office of Climate Change, Sustainability and Resiliency. After initially cutting seven staff positions for the newly formed office, the Honolulu City Council Budget Committee on May 16 restored the positions. Oahu voters in 2016 approved an amendment to the Honolulu Charter establishing the new agency.