Controversy over a $345 million project to lessen flood risks at the Ala Wai Canal by building a wall around the canal and huge flood control structures in the upper reaches of the watershed drew hundreds to a Tuesday town hall meeting in Manoa.

There was mostly opposition to the U.S. Army Corps of Engineers plan — including sign waving from students whose schools will be affected by the corps’ plans. They’ll be affected along with 37 private property owners, four of which will likely lose their entire properties.

Proponents say there is a need to shore up the canal, which was built in the 1920s to create land for Waikiki development. Waikiki, the epicenter of the state’s tourist economy, is at risk if the canal overflows.

Army Corps of Engineers Lt. Col. Kathryn Sanborn said, “Our goal is to get the project designed and constructed and in place before the next storm. This planned project is not just about Waikiki — the entire watershed will be afforded protections.”

The corps says the project’s design phase is 35 percent complete and so far calls for building a 4-foot concrete wall around the canal and placing six in-stream debris and detention basins in the upper reaches of the Ala Wai watershed that flows into Waikiki. The project also includes pump stations, a stand-alone debris catchment and three multipurpose detention areas in open spaces throughout the watershed.

Proponents fear delays could cause Oahu to be caught flat-footed in the wake of future storms that threaten Waikiki and communities in Manoa, Palolo, Makiki and Moiliili. They also fear that a slowdown could cost the state $220 million in federal funding that was approved last year based on the corps’ preliminary plans.

Charles “Chip” Fletcher, an associate dean at the University of Hawaii at Manoa and vice chairman of the Honolulu Climate Change Commission, warned the crowd that weather events could worsen.

“Bottom line, we are looking at continued increases in extreme rainfall,” Fletcher said.

Still, critics said they want the project to pause long enough to allow the corps and its city and state partners time to gather more community feedback, consider lower-footprint alternatives and develop an adequate maintenance plan to address future concerns. Six neighborhood boards have passed resolutions asking the corps to temporarily halt the project and state lawmakers to deny funding. Concerned residents even have a website, stopalawaiproject.com.

Sidney Lynch said she’s concerned about maintenance for the upstream concrete structures, which include a 30-foot-tall by 75-foot-wide by 100-foot-thick basin that will tear up about two football fields upstream.

“What will happen downstream if these basins fail?” Lynch said. “There’s a basin in Wailupe Stream in Aina Haina, and it failed and it was the responsibility of the city. These are 50-year basins, and they’ll need to be maintained on a regular basis.”

George Lee, 92, said he’s in in favor of protecting Waikiki but thinks “there are alternative ways to handle the flooding which do not require debris catchments and retention basins in our precious valleys.”

“It’s a 1920s technology. All over America the dams are failing,” he said.

Jeff Herzog, corps project manager for the Ala Wai project, said Congress authorized specific design features, including the Ala Wai wall and the retention and detention basins, based on “engineering, topography and modeling.”

The plan was funded through the same mechanism that funded recovery efforts after Hurricane Katrina, Herzog said.

“This is just one design; this is not the final design,” he said. “I need your feedback. Tonight is the start of a partnership.”

To continue advancing, the project needs legislators to approve a bill to fund a $125 million federal cost-share requirement. An amended version of that bill moved out of two house committees Thursday but still must clear the House Finance Committee.

The corps also needs the city and state or both entities to agree to become its nonfederal partner.

Robert Kroning, director of the city Department of Design and Construction, told the mostly hostile crowd that the city wasn’t currently in a position to say whether the project would advance. But he said the city expects that the project will move forward because the city must provide protection based on the information about the “storms coming our way.”