The U.S. Army Corps of Engineers is developing new plans for heightened flood control in one of Hawaii’s most populous and economically important areas – the Ala Wai watershed – after the Corps scrapped its previous, contentious plan when the costs nearly doubled.

The new “prospective” plan, which Corps officials presented at a public meeting this week, doesn’t include the retention basins and other features in the mauka (inland), upper reaches of Makiki, Manoa and Palolo that had galvanized so much community opposition against the previous iteration. 

The Corps has made “fairly significant changes in how the overall system functions,” Project Manager Eric Merriam said at Monday’s virtual meeting. Notably, the project no longer has to be designed to protect against an event as severe as a so-called 100-year storm, according to the Corps’ presentation.

Nonetheless, the plan still calls for 6-foot-tall flood walls around the Ala Wai Canal, Kaimuki High School, Woodlawn Drive and Koali Road. It would also include a detention basin at the Ala Wai Golf Course. 

The latest early designs spurred an array of questions from residents and community members on Monday.

“Why’s the Army Corps still stuck on the wall?” asked Palolo resident Dave Watase, who was outspoken in his opposition to the previous plan. 

Other community members pressed Corps officials for more details on how exactly they arrived at their latest prospective approach.

Winston Welch, executive director of The Outdoor Circle, a longtime Honolulu environmental group, called the approach presented by the federal agency “disjointed.” 

“It’s hard for someone to understand how the planning was done. Why not (use) walls that would descend” into the ground, he asked project officials at the meeting. “We’ve got some work to do. Let’s not rush this. There’s a lot of unanswered questions about how this looks.”

Welch and other meeting participants wanted more specifics on how the Corps came up with the designs, instead of the agency just saying that it had conducted modeling. The agency’s technical lead wasn’t available to attend Monday’s meeting, however, Corps officials said.

However, the Corps has said that it considered 224 measures to better protect the approximately 200,000 residents and businesses across the Ala Wai watershed from so-called “riverine,” or stream flooding for the latest plan. Its engineers then came up with nine potential plans before landing on the approach that uses extensive floodwalls and one major basin at the golf course.

Some residents called the scope of the project too limited because it doesn’t address the impacts of sea level rise or storm surge on the watershed as climate change intensifies.

Corps officials noted that the scope was set by Congress. “That is the scope we live in,” Merriam said of the riverine flooding. The project can consider how sea level rise and storm surge impact the watershed’s riverine flooding during heavy storms, however, he added.

The prospective plan will likely see some changes before the Corps finalizes it, according to Merriam. The next public meeting on the plan is scheduled for May 15. 

The previous plan was scrapped in January 2021, when the agency announced its cost estimate had nearly doubled from $345 million to $651 million, largely due to design changes including a proposed pumping station where the Ala Wai and Manoa-Palolo canals meet.