The U.S. Army Corps of Engineers has unveiled a “prospective plan” for how to deal with a potential flooding disaster facing 200,000 people in Waikiki and neighboring communities.

While the controversial concrete walls around the Ala Wai Canal remain in the plan, another contentious proposal has been dropped: the Manoa District Park detention basin.

“I’m overjoyed,” said Manoa Neighborhood Board member Elton Fukumoto.

The “prospective plan” was unveiled in a video released this week ahead of Monday’s 5:30 p.m. virtual public meeting and available at the project’s website, honolulu.gov/alawai.

The plan, designed for a storm size that would occur every 20 to 50 years, has been described as an early version of the “tentatively selected plan,” which is scheduled to appear in the Ala Wai Canal Flood Risk Management General Re-evaluation draft report scheduled to be released in June or July.

In the video, Ala Wai project manager Eric Merriam said the prospective plan features floodwalls on the Ala Wai Canal, Kaimuki High School, Woodlawn Drive and Koali Road, as well as a detention basin encompassing Ala Wai Golf Course.

These projects are not only economically justified, Merriam said, but will help to reduce the risk of flooding for residents living along Makiki, Manoa and Palolo streams and the Ala Wai Canal.

According to the plan, the Woodlawn and Koali Road floodwalls will reduce flooding risk to the Manoa area. The Kaimuki High School floodwall will reduce risk to communities east/southeast of Manoa-Palolo Canal. And the Ala Wai Golf Course detention basin and the Ala Wai Canal floodwalls will minimize the flooding risk to Waikiki and nearby communities.

Removed from consideration are detention basins at Manoa District Park and Makiki District Park, along with the proposed Kanaha floodwall.

Merriam, who is based in Pittsburgh, said that while these projects would have contributed positively to the plan, neither was determined to substantially reduce risk to lives or to benefit most socially vulnerable communities.

“They did have benefit,” he said, “but it wasn’t enough to warrant inclusion in the project.”

The project also received pressure from members of the Manoa Neighborhood Board, which voted to formally oppose the detention basin at Manoa District Park, and from area representatives, including state Rep. Andrew Takuya Garrett, state Sen. Carol Fukunaga and Council member Calvin Say.

Fukumoto said community members felt a detention basin at the heavily used park would not only negatively affect the experience for park users, but probably result in an earthen berm around the park that would destroy the viewshed for neighboring residents.

In the end, the neighborhood board wasn’t exactly sure how the park would be affected, Fukumoto said, in part because project officials turned down an invitation to make a presentation at their meetings.

As for the floodwalls and detention basin levees, Merriam said they are currently assumed to be 6 feet tall — although their ultimate height will be determined in the next study phase.

Also to be addressed in the coming weeks, he said, are the aesthetics of the floodwall facades and Ala Wai canoe launches, an effort that will aim to reduce their visual impacts.

“We’re also still considering an elevated path or walkway, running and bike path behind the floodwall. It might be elevated 3 feet so that you have a 3-foot elevated walkway and a 3-foot wall on top of that,” he said.

Sidney Lynch, president of Protect Our Ala Wai Watersheds, said the discord over the floodwalls likely will continue unless the project is able to soften the floodwalls’ impact on the viewshed.

“It depends on the final product and how they look,” she said.

Merriam said engineers also will consider installing pumps to help deal with drainage and recommending other methods to deal with flooding, including elevating residential structures on stilts and dry-floodproofing other types of buildings.

“Now that we have a better handle on what this plan looks like, we will be reaching out to those landowners potentially affected by the plan prior to the release of the draft report,” he said.

Earlier in the planning process, project engineers considered but rejected several proposed flood mitigation measures. They included creating a second outlet for the Ala Wai Canal and making Kapiolani Park a sizable detention basin.

They also rejected proposed green solutions such as removing invasive trees and planting native ones in the watershed, saying that while they would have an effect on runoff and filtration, their impact primarily fell on storm events that happen every one or two years. This project, they said, is aimed at the bigger, catastrophic storms.

By December, officials had narrowed the project to three potential action plans. One was focused on water storage, with detention basins to hold stormwater. Another was aimed primarily at water conveyance, featuring a variety of channels and floodwalls.

The third was a hybrid plan featuring a combination of both, and that is the one that forms the basis of the prospective plan that will be presented Monday.

Meanwhile, the public has until May 8 to comment on the project’s supplemental environmental impact statement.

Following the release of the draft plan in June or July, the project will hold two public workshops on the tentatively selected plan: one virtual and one in person.

More meetings will be held when the final draft comes out later this year or in early 2024, with the project expected to wrap up by June 2024.