Piece by piece, much of the flood-control plan for the Ala Wai watershed’s steep descent to Waikiki is changing. The U.S. Army Corps of Engineers, which has been working on a mitigation fix for two decades now, announced this week that recent modeling has revealed shortcomings in its original vision for the project.

The upshot appears to be a welcome switch from a system of basins to detain rushing storm waters to a less-intrusive one of conveyance. It would reach from the Koolau Range’s 3,000-foot elevation to 300-foot levels in Makiki, Manoa and Palolo valleys, down to sea level at the Ala Wai Canal, edging Waikiki.

In addition to engineering revision, it’s clear that grassroots opposition helped prod changes. In July 2018, when Congress appropriated $345 million toward the project and the Corps of Engineers outlined key elements — including upland detention basins slated to cross into some residential properties — neighborhoods responded with clamor for more input opportunity.

Then in November, in response to a lawsuit filed by community group Protect Our Ala Wai Watersheds, an Environmental Court judge ruled that the state cannot commit its proposed share of funding for the project in the absence of a completed environmental impact statement, to assess potential impact on land and water flow as well as historical, archaeological, cultural and social effects.

Amid this tapping of brakes, the Corps of Engineers updated its models, finding that, due to slope steepness, its plan for six total detention basins in the three valleys wouldn’t be as effective for flood control as initially thought. It’s a relief that the new plan scraps them, along with the threat of purchase or condemnation of private property.

Among the new plan’s features: a proposed bypass channel running underneath the Manoa Marketplace, which sensibly aims to divert waters from impacting the nearby University of Hawaii-Manoa campus, which was hard hit by flash-flooding in 2004. Among still-concerning plan elements is the Corps of Engineers’ intent to erect a controversial wall on the canal.

While Honolulu Hale has expressed general support for stepped-up mitigation along the canal, Mayor Kirk Caldwell and others have rightly questioned whether a concrete floodwall is a necessary eyesore along Waikiki’s border.

The new plan calls for installing a pump station at the Ala Wai Golf Course, instead of at the Waikiki-Kapahulu Public Library, which would allow the agency to remove about a half-mile of proposed wall along the Waikiki side of the Ala Wai Canal, from Kapahulu Avenue to Lewers Street. But a wall of some sort would be erected on that side of the canal from Lewers to Ala Moana Boulevard.

Due to the system switch from catch-basins to conveyance, waters funneling downhill from the valleys could be pumped onto the Ala Wai Golf Course and elsewhere near sea level, which seems to align with natural drainage flow.

In a recent commentary in the Honolulu Star-Advertiser, two UH professors — Chip Fletcher, associate dean in the School of Ocean and Earth Science and Technology, and Judith Stilgenbauer in the School of Architecture — suggested returning the golf course to its predevelopment state as a wetland, flooded-field agricultural system and natural retention basin to manage flooding.

Given growing climate-change threats, including long-range forecasts for sea level rise and more intense rain storms in Hawaii’s watershed areas, this pragmatic idea is worthy of thoughtful examination on the part of the Corps of Engineers, along with local and state agencies.

Continued brainstorming on project changes now in the works hold promise to help produce flood-mitigation that’s better suited for both the Ala Wai watershed and area residents.