The Ala Wai Watershed spans about 19 square miles, extending from the Koolau Mountains to draining tributaries in Makiki, Manoa and Palolo valleys before reaching the Ala Wai Canal. Due to steep slope — starting at a 3,000-foot elevation and dropping to 300-foot levels — and considerable annual rainfall, its canal endpoint is designated as “high risk” of flash flooding.

There’s little argument that mitigation is needed. But the U.S. Army Corps of Engineers’ proposed fix, the Ala Wai Flood Risk Management Project, has touched off a lot of debate about acceptable parameters, forcing major revamp of initial plans.

Community input on the project’s latest draft will be collected at a series of virtual scoping meetings, starting today, 8-10 a.m., and over the next three Saturdays.

(Participants can log onto usace1.webex.com/meet/jeffrey.a.herzog or dial 1-844-800-2712; Access Code: 199 126 8738#; Security Code: 1234#.)

In addition to updating engineering models, it’s clear that grassroots opposition helped prod a welcome design switch from a system of six upstream basins to detain storm waters, to a less-intrusive conveyance to the canal, easing potential impact on residential properties and streams.

Among other elements likely to generate more sparring: a consolidation of two pump stations into a single four-story station at the Ala Wai Golf Course, and a floodwall on the canal. Instead of a wall, the Corps should consider opting for a green space alternative, which could be a better fit in a neighborhood that sees a steady flow of tourist and resident foot traffic.

The project is now expected to cost $376 million and includes a $48 million contingency, with federal funding in place to cover a large portion of construction. Given that the city will be asked to consider signing a partnership agreement obligating it to maintain the project’s infrastructure, Mayor-elect Rick Blangiardi and the City Council will need to take a close look at the mitigation plan.

A year ago, when some infrastructure parts prompted pushback from upstream residents and others, three Council members formed a permitted interaction group and hired Oceanit, a local engineering consultant, to pursue alternatives.

In August, Oceanit recommended a different take on conveyance: two tunnels — one for Manoa and one for Palolo — that could shuttle water from the watershed’s top to the ocean, bypassing the lower levels and the canal. Councilmember Ann Kobayashi said recently that the upshot of going underground is “about the same cost, and there won’t be all this ugliness.”

If that’s the case, this option, too, is worthy of a round of flood-control debate.