The streams of the Ala Wai watershed drain from its 3,000-foot summit in the Koolaus down to sea level. Its lower sub-basin includes Waikiki, the Ala Wai Canal and several schools, and will bear the brunt of a catastro­phic flood if upstream flood management measures fail, Eric Merriam, a planner with the U.S. Army Corps of Engineers, said Friday in a public meeting.

Therefore, “we’re trying to store more water in the upper watershed, at the source,” he said, and “to understand how much water we can store in the headwaters.”

It was the latest in a series of public meetings the federal agency and the City and County of Honolulu have hosted since November to develop a new flood-risk management plan. Previous plans were abandoned following protests over lack of public input and the withdrawal of funding by the federal government.

After collecting a little over 200 suggestions in a management measure-­tracking spreadsheet on the project website, “We’re beginning to develop alternative plans,” said Cindy Acpal, a project manager with the corps.

The corps has screened out about a quarter of the suggestions, with the remainder still under consideration and grouped into three tiers, Merriam said.

Screening criteria include whether a measure falls within the study’s authority; is technically feasible; its effectiveness at reducing risk of harm to people and the economy; its efficiency, including cost; and environmental impacts.

Suggestions under consideration include opening a second outlet to the sea at the east end of the Ala Wai Canal, near the Waikiki-­Kapahulu Library, in two ways: extending the canal underground beneath Kapahulu or Monsarrat avenues, or extending the canal in an open channel flowing down Paki Avenue and makai through Kapiolani Park.

Merriam said the corps was prioritizing looking at existing storm sewer infrastructure and providing an outfall in front of the Tahitienne condominiums on Diamond Head’s Gold Coast, bypassing “the highly traveled beach and tourist area of Waikiki.”

This, he said, would be less costly and disruptive than digging microtunnels under Waikiki’s network of avenues and streets, such as Ohua Avenue and Lewers Street, to take water from the canal straight out to the ocean, although that is still under consideration.

Community member Paul Fujii asked whether, given possible upcoming water use restrictions, “are there any considerations for designs that utilize portions of the stormwater runoff for potable water recharge, or reuse in some way, rather than discharging directly to ocean outfalls?”

“If we’re storing (stormwater) up in the headwaters, we can use it for another purpose,” Merriam replied. “Right now we are talking about flood risk but wouldn’t say this is not within our authority on some level.”

Also under consideration is dredging the canal, adding raised walls along its banks and adding earthen/silt berms to divert floodwaters into Ala Wai Park, the golf course and neighboring school grounds, partially restoring the area’s former wetlands and kalo paddies.

“I do not believe there should be flood walls — just berms around the schools,” said community member Laura Ruby, adding, “For 40 years I’ve been watching how water overflows (from the canal) into open park areas; ducks, fish like it and then it goes away.”

Canal-side walking paths along elevated berms rather than walls would be more pleasant, she said.

“We have to consider flood walls because in lots of areas there’s not enough space to build berms,” Merriam replied, adding, “We could make them decorative.”

Rejected as outside the scope of the current study were community volunteer actions such as regular cleanups of the canal and dropping in genki balls containing microorganisms that remove sludge. Merriam said studies show removing sludge could provide “some benefits but not enough.”

Still under consideration, he added, were such popular themes as changing some impervious surfaces to absorptive surfaces; forest and stream maintenance and management; and underground water detention.

The previous study focused on a 100-year flood, while the new study considers flooding that could occur within five to 500 years, Acpal said.

Matthew Gonser, the city’s chief climate-change resilience officer, said 100-year flood “means in any given year there’s a 1% chance, and over 30 years a 26% chance, such an event could happen.”

Meetings on April 1 and 8 focused on the upper watershed sub-basins of Makiki, Palolo and Manoa valleys. The final workshop on Friday will comprise “continued discussions,” according to the city website for the project.

In the Manoa meeting, people expressed concern about a proposed development for 88 units of senior housing near the Chinese Cemetery, where there is currently a grassy field.

To join Friday’s meeting, view video and minutes from the sub-basin meetings, read the spreadsheet and contribute comments, visit honolulu.gov/alawai and click on “public engagement.”