Federal officials propose spending $200 million to erect walls around the Ala Wai Canal and build upland detention ponds to protect Waikiki and other areas from a major rainstorm that could cause catastrophic flooding.

The plan includes 3- to 5-foot walls around the canal, and detention basins that span five upland streams that feed into the canal. The Ala Wai Golf Course, Hausten Ditch, and land owned by Woodlawn Cemetery would be turned into repositories for floodwater.

The plan is designed to “reduce flooding in and around the Ala Wai and to save lives,” Athline Clark, a project manager for the U.S. Army Corps of Engineers, told a small group at Stevenson Middle School on Wednesday during the second of two community meetings to discuss the proposal.


Federal officials predict major flooding if the Ala Wai Canal isn’t fixed

U.S. Army Corps Of Engineers

But the estimated cost has soared, and federal and state funding is far from certain. As of last year, Army Corps officials had pegged the cost of design and construction at about $100 million. The cost has since doubled, said Clark.

The earliest construction would begin is likely 2020, she said.

Civil Beat reported extensively last year on the history of the Ala Wai Canal and the major flood risk it poses to Waikiki, the state’s major economic engine and tourist haven, in the series, Ala Wai Canal, Hawaii’s Biggest Mistake?

U.S Army Corps

Proposed flood mitigation measures

The canal was built in the 1920s to drain agricultural wetlands and make room for development in Waikiki and the surrounding area. But over the decades, the canal has become an engineering quagmire for government officials. Not only has it become a repository for pollutants in one of the country’s most densely populated urban regions, but it’s also a flood hazard.

Streams flowing from Manoa, Palolo and Maikiki used to percolate through wetlands leading toward the ocean. But the streams were diverted into the canal, which can’t handle major rain events.

Clark said that a homeowner would have about a one-in-four chance of being hit by such a storm during the course of a 30-year mortgage.

“It’s pretty startling when you think about it from that perspective,” she said.

The fast-moving waters, which could reach the canal in as little as 30 minutes from the mountains, could cause almost $400 million worth of property damage, the Corps estimates. And that doesn’t include cleanup costs or the costs of businesses being shut down.

The Corps of Engineers has been studying flood mitigation plans for more than a decade at a cost of $9.1 million. The agency started out studying several options before selecting the one unveiled this week.

Slowing the Floodwater

The selected plan can still be tweaked based on public input and as the design work is completed, said Clark.

Its overall aim is to slow floodwaters in the upper watershed areas. The detention basins, composed of walls with a pipe in the middle, would extend the amount of time it takes for water to descend from the upper watersheds to the canal to between seven and 10 hours, said Clark.

U.S. Army Corps

Rendering of a detention basin at Waiakeakua
Street, Manoa

Without such measures, walls as high as 16 feet would have to be built around the canal to control flooding, according to Carty Chang, chief engineer at the Hawaii Department of Land and Natural Resources, which is the Corps’ state partner on the project.

Instead, the Corps is proposing 3- to 5-foot walls around much of the canal, leaving room for recreational access — namely the hundreds of canoe paddlers that use the canal every year.

The proposed measures would not prevent all the damage, however. There is little that can be done for Makiki, much of which would still be underwater, flood maps show. And the massive amount of water coming down from the mountains still needs to go somewhere.

The Ala Wai Golf Course, which Clark says will flood no matter what, would be used as a depository for flood waters. An earthen berm — similar to a small hill — would be built around the golf course. A detention basin would also be built around the Hausten Ditch, which sits mauka of the canal. Land owned by Woodlawn Cemetery would also be turned into a detention basin. The land is currently being leased to a tropical flower farm, said Clark.

A Question of Funding

Government officials have spent years deliberating on a flood control plan and it could still be a number of years before anything is executed. For the Army Corps, this is just the first phase of the project. In the Fall, the Army Corps plans to release its overall project plans and environmental impact statement, which will go out for public review.

U.S. Army Corps

Rendering of a wall around the Ala Wai Canal

If the plan is ultimately approved by the head of the Army Corps, as well as state and city agencies, it would go to Congress for approval and funding. The federal government would need to cover 65 percent of the estimated $200 million cost. The state would pick up the rest. It would also be up to the state or city to cover the ongoing maintenance costs, estimated at $1 million annually.

Chang said that the funding could be a problem. Construction costs could be budgeted over a number of years, but state and city officials still need to decide who would pay for the ongoing upkeep.

“I think operating and maintenance is a big issue,” he said.

Ultimately, the cost may be worth it given the potential loss of life and injuries from a major flood. About 160,000 people live in the watershed area and Waikiki attracts more than 70,000 visitors daily, according to the Corps.

Clark said that if there is enough notice of a flood, residents and visitors in Waikiki could quickly move into the upper floors of high-rises and out of harm’s way. But the speed of the water coming down from the mountains could be too fast to provide adequate warning.

“The bad news is because it is so flashy and would happen so fast, it is just how much of a warning can you give beforehand?” she told Civil Beat. “Will there be massive drowning? That’s unlikely. Would people and property get swept away? Absolutely, because the flashy nature of the streams and the velocity of the waters would move stuff along pretty quickly — faster than you could stand up against it. If you happen to be near the canal, you could get knocked down and dragged in.”

The Ala Wai Canal has flooded in the past, with 25- to 40-year floods sweeping through Waikiki in 1965 and 1967. Major rains in 2004 also caused about $80 million in damage to Manoa.

Adding to concerns, major rainfall events in Hawaii have increased by 12 percent in the past 30 years, according to information from the Army Corps, which predicts that this trend will continue with climate change.

You can view more renderings and slides from the U.S. Army Corps’ presentation here.

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