Conceptually, it seems simple enough: Heavy rains fall on the mountains, 3,000 feet up at its highest point, and stream down the slopes. If the intention is to prevent flooding along the way, some means of slowing things down and diverting the water must be found.

But it’s complicated — by private ownership of abutting parcels, by the importance of how a flood-control project ending at Waikiki might look, by worries about the cost and liability for keeping it all in good shape. Further, there can be justifiable concerns that the community has not been given enough of an opening to weigh in.

That latter issue is evident in the Ala Wai Flood Risk Management Project, which is coming up against a deadline of sorts this week. On Wednesday, according to the U.S. Army Corps of Engineers, the project was to have executed an agreement on the local matching funds for the $345 million in federal money approved by Congress a year ago.

July 31 is noted more officially as a “milestone” date, and if it is not extended, that funding could be withdrawn. A decision on a possible extension is left up to the assistant secretary of the Army for civil works, said Jeff Herzog, the Army Corps project manager.

Meanwhile, he said, officials are still negotiating the local funding and the respective responsibility of city and state agencies for project maintenance. That is a critical consideration.

They should come to terms — clearly government must control flooding risk— but there still needs to be room for adjustment to the plans, and more opportunity for the public to suggest them.

Herzog said there is still time for that: The project has completed its feasibility phase and is now moving to design and construction. A geotechnical survey contractor has been hired to determine whether the proposed elements actually can do the job.

“I guarantee, there will be changes,” he added. Good; that door should be open.

The Corps is still “making the rounds” of neighborhood boards, he said, but anyone who has been sitting on the sidelines up to this point bears some responsibility to step up now.

The community is invited to visit the project website (808ne.ws/ala-wai), where there are points of contact, status reports and a link to the feasibility report and environmental impact statement.

The plan examines the course of the rainfall, which can reach up to 150 inches annually, across four miles through the watershed to the ocean. What makes regional flooding such an acute risk is the immense economic damage a flood would cause at the Waikiki end.

The fear is that storm runoff into the Ala Wai would overtop the canal and begin to flood the low-lying properties of Waikiki. One proposal in the mitigation plan under consideration is constructing a 4-foot concrete floodwall along the canal.

How would that look? Not very appealing, is the general conclusion. Mayor Kirk Caldwell has suggested a landscaped berm to help contain the overflow; that’s an alternative that should remain on the table.

Caldwell tried unsuccessfully to secure the local matching funds through the state Legislature; despite support from Gov. David Ige, that route was closed off by lawmakers. Senate Bill 77, which sought to finance $125 million of project costs, died in the state House. The governor has restated his commitment to help secure the local match, a plan that should come into focus this week.

It probably didn’t help that, in addition to community testimony citing insufficient public input, the legislative session coincided with some vocal opposition aired by regional neighborhood boards.

The Waikiki Neighborhood Board is one that has steadfastly supported the project, which is logical, given its location at the receiving end of all that water.

Even if the probability of the tourist district’s inundation is only estimated at 1 percent in any given year, it’s impossible to know whether the heavier storms brought by climate change may have nudged the flooding odds upward. And the losses from such an event would be devastating.

But some residents living in the upstream communities — such as in Palolo — have concerns about the plan’s element of “detention basins” to collect storm waters. The basins’ construction could require that private property along streambeds be taken.

Further, other residents have called for a more environmentally sensitive approach to mitigation. One Palolo resident, Sidney Lynch, said these include reducing the impermeable surfaces paved over the soil that increase runoff. And existing sites for detention basins, such as the Ala Wai Golf Course property, could be considered, she said.

The Corps should be listening.

It is planning annual inspections to identify maintenance priorities, which is welcome news. But ultimately, managing the Ala Wai Watershed will rest with the city, which does not have the best record of facilities upkeep. This project hands the mayor the imperative to change all that.