Hawaii’s most populous watershed encompasses 19 square miles, descending from the Koolau Mountains to the south shore by way of Makiki, Manoa and Palolo streams, with all draining into the Ala Wai Canal.

Constructed during the 1920s — not for flood control, but to drain wetlands and allow for development of Waikiki — the 2-mile-long canal’s capacity to funnel flood waters is now a key focus of a $345 million Army Corps of Engineers risk management project.

For much of two decades, the Corps of Engineers worked in tandem with the city and state in relative quiet to develop a plan to fortify the canal and upstream areas against mega-storm threat. Over the years, there have been opportunities for public input regarding a draft feasibility study and environmental impact statement.

But community debate — and push-back — was minimal until earlier this year, when the initial lineup for structural elements surfaced: six large detention basins in the watershed’s upper reaches; an in-stream debris catchment structure; and three multipurpose detention basins, and flood control features along the canal, including a 3- to 5-foot-tall wall.

The project’s framers also pointed out that the mitigation structures would directly affect — and in some cases, displace — more than three dozen private-property owners with land situated near the streams.

Since then, there has been a steady clamor for more public engagement.

The cranked-up volume is welcome as it has prompted months of additional Corps of Engineers study aimed at changes that could be less intrusive while still effective in addressing flood hazards.

Among the possibilities under consideration: Removing from the lineup detention basins — earthen dry dams — in Palolo and Makiki valleys. In their place, more structural elements would be eyed for installation on public land in Manoa Valley, where the fast-moving water threat is particularly strong.

This seems to be a sensible move to better preserve upstream environments and shield downstream sites, including portions of the University of Hawaii-Manoa campus. In October 2004, Manoa Stream overflowed during a flash flood, causing an estimated $80 million in damage to 32 buildings. Hamilton Library wreckage accounted for nearly half of the damage costs.

That storm rated as a once-in-25-years event, and flooding was exacerbated by debris clogging at stream bridges. The Ala Wai Canal Flood Risk Management Project was initiated to build a network of structures fit to handle a once-in-a-century storm. In such a case, the watershed’s torrent is forecast to spill over the canal’s brim, flooding Waikiki.

The reach of potential damage: some 3,000 structures, requiring more than $1 billion in repairs. This scenario is reason enough for the community to continue the conversation about large-scale flooding mitigation. Given tourism’s role as Hawaii’s economic engine, even a slim chance for massive flooding in Waikiki is a statewide problem.

In addition to zooming in on Manoa’s water volume, the Corps of Engineers is recommending project changes at Ala Wai Canal. Responding to complaints that a wall spanning its length would be a view-blocking eyesore, the Corps is now recommending earthen berms, which likely will be more palatable for both residents and tourists.

While Congress has approved the project, release of some $220 million in federal funds hinges on moving forward with a signed agreement at state and city levels. On that score, both have signed a memorandum of agreement that commits the state to paying the project’s balance money, $125 million, by late November while the city agrees to maintain the project.

The city’s forward motion was met with opposition from several neighborhood boards in areas slated for construction. Seven out of eight boards — Waikiki’s the exception — have passed resolutions urging government officials to delay the project.

Also seeking a pause button: Protect Our Ala Wai Watersheds, a recently formed community group of mostly Palolo and Manoa residents that filed a lawsuit in mid-September to stop the city and state from initiating the project, claiming it lacks the necessary state environmental impact reviews. The group favors “nature- based” watershed protection and restoration.

A third call to refrain from sealing a deal that locks in project elements comes from a recently formed City Council permitted interaction group involving Council members Carol Fukunaga, Ann Kobayashi and Tommy Waters. The group has hired Oceanit, an engineering consultant, to conduct community outreach and solicit mitigation alternatives.

Oceanit, which is weighing various suggestions such as retractable walls at the canal, wants to further vet ideas and then forward refined recommendations to the Corps of Engineers.

In response to the community clamor, government officials should tap the brakes and listen carefully. Brainstorming now in the works holds promise to help produce a flood-mitigation project that’s better suited for both the watershed and area residents.