The Army Corps of Engineers has overhauled its flood-control plans for the Ala Wai watershed, scrapping the upland detention basins that previously sparked much of the local opposition to the project.

In their place, the Corps looks to install culverts and bypass channels to guide fast-moving flood waters swiftly out to sea. It’s no longer looking to hold back their torrential flow in the upland valleys.

It’s an abrupt shift in strategy that local Corps officials outlined in a new “engineering documentation report” released Monday. The changes were necessary, the Corps said, largely due to shortcomings it found in its original modeling compared to the more recent work.

“Significant differences were observed between the two modeling results,” the report states.

That has local critics expressing more concerns about the flood-control effort. They question how the original, $345 million design could proceed so far based on insufficient data and modeling.

“That’s a huge, huge, huge error,” said Sidney Lynch, president of Protect Our Ala Wai Watershed. The grassroots group sued last year to stop the flood control plans from advancing and succeeded in getting an injunction pending further environmental review.

“If they didn’t do the engineering properly on the first project … what about what’s left of the project?” Lynch said Tuesday. The grassroots group still has not had a chance to thoroughly review the revised plan, however.

The Corps had already announced last year its intent to eliminate the basins in the Palolo and Makiki valleys. Under the new revisions, it would scrap the remaining three upland detention basins and a debris catchment structure in the Manoa Valley.

The report follows nearly a yearlong process in which the agency updated its models and found that the detention basins wouldn’t be as effective as it originally thought largely due to the steepness of the terrain, said Jeff Herzog, the federal agency’s project director.

“We’ve gone from a system of detention to a system of conveyance,” he said Monday.

It also follows strong, widespread criticism of the project’s earlier plans for a flood-control system that included the basins.

“I personally feel the public outcry forced these changes, as much as the engineering design. It forced them to look at these things seriously,” Lynch said.

She said Herzog has been “very sincere in his efforts to get community input,” but he and his Corps colleagues remain limited in what they can change because of constraints tied to the federal funding.

Indeed, a project centered on habitat restoration — something that multiple community groups said they would rather see  — was previously scrapped. Now, the Corps’ sole focus is flood control, prompting complaints that it’s advancing with “half a project.” The Corps says it might still pursue smaller, isolated restoration projects within the state’s most densely populated watershed.

Herzog said the Corps has briefed community groups and neighborhood boards in the area. So far, the project’s staunchest opponents remain “absolutely concerned” despite the changes, he said.

“But I believe we are starting to build a relationship where at least they feel now that they’re part of the process and we’re not trying to sneak anything by them,” he said. “They care about the community, and they want to make sure we’re not going in and making things worse.”

The new designs feature a 1,500-foot long culvert that would run along the Ala Wai Canal, starting at the Makiki Stream and then heading west along the Hawaii Convention Center promenade. The added channel would help keep the Makiki Stream from backing up in a heavy storm, Herzog said.

It’s not clear yet whether that channel would be above or below the surface.

In Manoa, the designs would add a bypass channel running underneath the Manoa Marketplace to redirect fast moving flood waters instead of holding them back with the detention basins.

The Corps’ new plans also call for installing a pump station at the Ala Wai Golf Course instead of at the Waikiki-Kapahulu Public Library. The move would allow the agency to remove about a half-mile of wall along the Waikiki side of the Ala Wai Canal, from Lewers Street to Kapahulu Avenue, Herzog said.

Plans for the wall on that side of the canal remain in place for the rest of the way to Ala Moana Boulevard, Herzog added.

Thanks to the Ala Wai Canal, a severe storm could leave much of Waikiki and its surrounding neighborhoods underwater, experts say. The Corps has estimated that each year the popular tourist hub, along with its 54,000 residents and nearly 80,000 daily visitors, faces a 1 percent chance of a major flood that would cause $1.14 billion in damage to more than 3,000 structures.

Under its previous designs, the flood control project was expected to cost some $345 million, with the state agreeing to pay $125 million and federal dollars covering the rest.

The Corps is not estimating how much the new plan will cost, although the report gives an early projection of $376 million. That price includes a $48 million contingency.

The new plans still have to go through state and federal environmental reviews. Herzog said he expected that process to run through sometime next summer.

The Corps has also yet to sign a so-called project participation agreement with the city to maintain the flood control features. All of the design plans out of the Corps’ local office so far have been recommendations. Top brass at the agency’s headquarters in Washington will have to give final approval. Those officials have been kept in the loop of the changes and support them, Herzog said Monday.

David Kimo Frankel, an attorney representing Protect Our Ala Wai Watershed, said the Corps should consider a proposal by University of Hawaii Manoa professors Chip Fletcher — one of Hawaii’s preeminent experts on sea level rise — and Judith Stilgenbaur to restore the Ala Wai golf course into a natural wetland and flooded-field agricultural system that could better absorb flood waters.