The Ala Wai watershed’s highest elevation reaches about 3,000 feet, before dropping to 300 feet at valley floors and rolling to sea level — all over the span of about four miles.

Due to its sheer slope, along with the combination of considerable rainfall — up to 150 inches a year in the Koolau Mountain Ridge — Waikiki’s dense population and climate-change concerns, it’s pegged as “high risk” for flash flooding. And officials say it’s only a matter of time before the Ala Wai Canal overflows during a powerful storm.

Since 2001, the U.S. Army Corps of Engineers and state Department of Land and Natural Resources have been working on a mitigation plan. In July, Congress appropriated $345 million toward the proposed Ala Wai Flood Control Project; and last month, the Corps of Engineers secured a right-of-entry permit to conduct due diligence related to affected state land.

Despite the push toward needed upgrades, it’s time to tap the brakes on the project as its scoping and re-scoping efforts are touching off valid concerns in neighborhoods targeted for fortifications. Further, City Councilwoman Ann Kobayashi has raised red flags about transparency and adherence to proper protocol.

The project calls for installation of six in-stream debris and detention basins in the upper reaches of Makiki, Manoa and Palolo to address the watershed flow into the canal. The basins would cross into various properties edging the watershed, including residential land as well as private and public school boundaries.

In a recent letter to the Army Corps of Engineers’ Honolulu District, Kobayashi said affected residents “were not made aware of the project, nor properly notified.” If that’s the case, they should get more opportunity to weigh in on the matter.

Tonight, the Manoa Neighborhood Board is expected to join a growing call for a chance to ask more questions, and provide additional input that generates government feedback. The scale of this perceived communication lapse is reason enough to prompt a pause.

The project’s framers point to public participation and outreach, from 2004 to 2015, to meet Federal National Environmental Policy Act (NEPA) requirements and comply with state environmental policies. But, clearly, more outreach is needed.

A better-known element of the mitigation plan would install a 4-foot concrete floodwall along the canal. That, too, is raising eyebrows. Mayor Kirk Caldwell, who has expressed general support for stepping up mitigation, is right to question a wall’s curb appeal along Waikiki’s border.

As Caldwell has suggested, a park-like berm would be more aesthetically appealing — and could serve the same public-safety function. Or, perhaps, rather than a permanent wall, anchor points could be installed along the canal to hold a barrier erected in advance of severe weather.

The Ala Wai Canal last topped its banks during Hurricane Iniki, 27 years ago. Upstream, 15 years ago, an October storm that flooded Manoa Valley caused $85 million in damage.

The Corps of Engineers has tagged the threat tied to August’s Hurricane Lane as a near miss, and warns that amid forecasts of increasing storm intensity in years to come, the watershed is due for more protections.

Flooding so severe that it encompasses Waikiki and the canal’s tributaries — a 100-year event, meaning there’s a 1 percent probability of it occurring in any given year — could damage an estimated 3,000 structures requiring more than $1 billion in repairs. Despite the very long odds, such a nerve-racking scenario must be addressed with more than fingers-crossed.

However, before final sign-off on this major construction project — now approaching two decades in the making — the Corps of Engineers, along with Caldwell and Gov. David Ige, should strive to secure community support. Of course, before that’s a possibility, they must first clearly articulate and justify the full range of this project.