It looks like a second ocean outlet for the Ala Wai Canal is off the table in the ongoing planning process aimed at preventing a flooding disaster in Waikiki and neighboring communities.

So is making Kapiolani Park a sizable detention basin, as well as underwriting proposed green solutions such as removing invasive trees and planting native ones in the watershed.

Still under consideration: those controversial concrete flood walls around the Ala Wai Canal, now tentatively planned to be 6 feet tall.

The U.S. Army Corps of Engineers and the city last week unveiled their latest flood risk management planning efforts during a virtual online workshop. The project has been narrowed to three action proposals.

A fourth proposal would have authorities doing nothing, which would leave Waikiki and surrounding communities exposed to potential disaster in the form of flooding from a large storm.

As for the action plans, one focuses on water storage and is dominated by detention basins to hold stormwater. Another aims for water conveyance, featuring channels and flood walls. The third is a combination of the two, offering what appears to be the best of both plans.

Project manager Eric Merriam said the Ala Wai flood walls aren’t yet a sure thing.

In fact, the storage alternative plan leaves them out altogether, while relying on detention basins at Makiki District Park, Manoa Valley District Park and Ala Wai Golf Course, plus additional measures to buffer the floodwaters.

However, the plan would only be able to take care of 10- to 20-year storms, Merriam said.

By contrast, modeling shows that the conveyance and hybrid plans would be able to handle 20- to 50-year events.

The conveyance proposal includes flood walls around the Ala Wai Canal, at Ka­naha, Woodlawn Bridge and Koali Road, plus some drainage structures and other features. The hybrid proposal includes the same flood walls plus detention basins at Manoa Valley, Makiki and Ala Wai Golf Course.

Numerous measures were still in the running during the last round of public meetings in July but have since been eliminated.

One of them was a second Ala Wai outlet to the ocean on the Diamond Head side of the canal. Some were cheering for this measure not only for stormwater drainage, but for improving the canal’s notoriously poor water quality.

Merriam said the project’s team modeled the outlet proposal to see how it would affect the hydraulics.

“Unsurprisingly, it did reduce water surface elevation to some extent,” he said.

But then they looked at the costs, including the price of land and construction, and it became clear it wasn’t going to be worth it — “especially considering other measures would be equally effective at far less cost,” Merriam said. There are environmental concerns about the second outlet as well, he added.

Using Kapiolani Park as a detention basin was also screened out, in part because floodwaters would have to be pumped all the way to the park, an expensive proposition.

In July one proposal under consideration described “nature-based” measures as a cornerstone, including watershed management, invasive species control and reforestation, but that was eliminated, too.

Merriam said that while hydrology models found “the green solutions” did have an effect on runoff and filtration, their impact primarily fell on storm events that happen every one or two years. This project, he said, is aimed at the bigger, catastrophic storms.

At last week’s virtual meeting, there were plenty of complaints and fears about how the concrete flood walls would affect the viewshed around the Ala Wai.

But Merriam said there are ways to soften the walls’ impact.

“A 6-foot wall doesn’t necessarily mean a 6-foot wall,” he said. “We could look at something like a 3-foot walking path and a 3-foot wall on top of that. Or we could look at berms in certain areas.”

Haku Milles, city Department of Design and Construction deputy director, said there would be a concerted effort to make the flood walls more aesthetically pleasing.

“The city is absolutely committed to doing everything we can to make these walls anything but plain concrete walls,” he said.

Officials said the detention basins would be created by excavating and lowering the elevation of the parks to allow for water storage while maintaining the existing recreational uses.

Ellen Watson, Manoa Neighborhood Board member, objected to making Manoa Valley District Park a detention basin.

“This is our home. This is our sanctuary and we love this valley,” Watson said. “And you guys just want to make us a swimming pool to protect tourists in the Waikiki area.”

Merriam countered that the detention basin likely would be in use only every 20 to 50 years. “In reality, the impact to the community would be fairly minimal,” he said.

The Ala Wai flood risk management project dates back to 1999, but it wasn’t authorized and funded for design and construction until 2018.

By 2021, however, costs had nearly doubled to $651 million amid prolonged community dissension, much of it over the concrete wall planned around the Ala Wai.

In a new agreement with the city, the federal government agreed to pay up to $3 million for a general evaluation study that draws on previous knowledge to find a new scope for the plan that balances risk and cost.

No price tag has been estimated for the total cost of the project at this point, officials said.

Officials said they plan to further evaluate the various proposals and release a “Tentatively Selected Plan” in May or June.