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What if it rained so hard that the normally gentle streams flowing down the mountain valleys above Waikiki swelled to an angry swirling roar?

Frothy brown water filled with the sediment and trash from hundreds of homes and businesses would surge into the Ala Wai Canal. The canal would soon top its banks and a flood of major proportion would bury Waikiki in a wall of water that could reach five feet high.

The water would move so quickly that people would be knocked off their feet. Basements, parking garages and the lower floors of homes, hotels and high-rises would be deep in water and muck. Power lines, sewers and water service would be out of commission, likely for days if not longer.

Floodwaters would cover the 1.5 square miles that make up Waikiki, from Diamond Head to Ala Moana and from the shoreline up to Moiliili.

It’s a tourist mecca that brings in $10.6 billion annually for the state, often described as Hawaii’s economic engine.

So it’s hard to imagine the destruction a flood like that would cause and the economic devastation to a state that depends on the tourism industry, especially in Waikiki, for its very life.

But the scenario is very real, and one that government officials like Derek Chow of the U.S. Army Corp of Engineers have been worrying about for years. Led by the Corps, government engineers and planners have been working — some say too slowly — to pull together a plan to re-engineer the Ala Wai Canal in an effort to prevent massive destruction.

The flooding would be “catastrophic,” said Chow, and people would likely have little time to prepare.

“It’s not unusual for us to see in Hawaii’s flashy systems, flashy drainage systems, where it’s raining in the mountains, and half an hour to an hour later the streams are flooded,” he said. “So that gives you an idea of how little time people have to react.”

The “100-year-flood” is so named because it’s not supposed to happen very often. But 25- to 40-year floods swept through Waikiki in 1965 and again in 1967. And climate change has everyone worried that a catastrophic weather event is a distinct possibility.

A century ago, Hawaii’s territorial government and business leaders thought that diverting the streams flowing from Manoa, Palolo and Makiki into a single canal that emptied out into the ocean at the present day Ala Wai Boat Harbor was a good way to control flooding in Waikiki. Engineers decided at the last minute not to make a second outlet near Kapiolani Park, fearing currents would carry pollution back to the west and right on to Waikiki beaches.

The construction of the canal eliminated vital wetlands that absorbed and filtered the runoff and allowed it to flow relatively gently into the ocean. In place of wetlands, farms and fields, homes, hotels and high-rises have taken shape over the decades along paved streets and asphalt parking lots.

Now, there are few places for the water to go, and the Ala Wai Canal has left Waikiki in a precarious position in the event of an extreme storm.

“This is in effect a tsunami coming in from the sky,” said Karen Ah Mai, executive director of the Ala Wai Watershed Association, a community group involved in flooding and pollution issues. “The fact is we have become so developed, I don’t think we could stop the damage. We don’t have anywhere to put the water.”

The decisions made in the 1920s have turned out to be a costly mistake, too, one that engineers are only fully beginning to realize as they brainstorm expensive ideas to make it better against budgets that are growing thinner, especially as federal dollars run short. New flood control measures are expected to cost at least $100 million.

Government leaders have been aware of the flooding problems since at least the 1960s, when heavy rains twice overtopped the canal’s banks leaving cars stranded and pedestrians wading through knee-deep water.

In the 1970s, flood analyses for Waikiki, completed under the National Flood Insurance Program, added to the worries about the flood risk.

In the late 1990s, local officials reached out to the U.S. Army Corps of Engineers for help. Federal, state and city officials have been working since at least 2001 on a solution.

But while it’s been a concern for decades, the agencies have been slow to accomplish anything. And activists and business leaders say the intensive, decade-long collaboration of governments to fix the canal seems to have stalled despite all that’s at stake.

“We were very interested in working with the various government agencies on a solution, but then the effort seemed to lose momentum,” said Rick Egged, president of the Waikiki Improvement Association.

“There is definitely a problem in that there are a lot of fingers in the pie and yet nobody wants to own the pie, because there is a lot of money and responsibility that goes with that.”

Federal officials acknowledge delays, but say the overall planning process is still moving forward.

A Growing Threat

The flood risk is gradually getting worse, officials say.

Erosion, particularly in Manoa, where there is limited plant-canopy cover to slow the absorption of rainwater, has exacerbated runoff.

In the 1800s, the land was used for taro farming and growing various crops. But dairy farms moved in around the turn of the century and hoofed animals decimated native plants. In the 1920s, Manoa was reforested, but instead of native plants, fast-growing trees like albizia were planted.

As the trees have matured, their expansive leafy branches have choked out lower level plants that helped absorb water and slow erosion. Albizia trees’ upward sloping branches and smooth trunks also act as a .

“Water comes down in a sheet,” said Ah Mai.

Removing just a single tree costs between $6,000 to $12,000, she said.

Increasing erosion and growing urbanization means more water flows downstream into the canal every time it rains, said Chow.

Climate change is also adding urgency to the need for flood control measures. As the sea level rises, there is throughout Waikiki, which could exacerbate the damage in the event of a flood.

Underground parking lots in Waikiki are already experiencing increased flooding during high tides, said Chow.

By 2050, sea level is expected to rise by one foot, according to John Marra, the Honolulu-based climate services director for the National Oceanic and Atmospheric Administration. He said that the result would be more intense flooding, where water would pond quicker and longer.

“It doesn’t take a whole lot,” said Marra. “One or two feet doesn’t seem like a lot, but one or two feet is going to be huge in terms of the potential impacts.”

Tackling the Problem

When the Corps, the Hawaii Department of Land and Natural Resources and city officials began collaborating to fix the canal in 2001, the focus was on tackling the flood problem as well as trying to restore the waterway’s impaired ecosystem.

But it’s been a slow and expensive process that’s already cost taxpayers more than $9 million just to study the problem.

An initial 2001 report concluded that in order to control floodwaters, engineers would need to build walls up to 13 feet high around the canal. There were also plans to widen the canal from the McCully Bridge to the Ala Wai Boat Harbor and turn the Ala Wai Golf Course into a basin to catch floodwaters.

In 2004, the Corps of Engineers projected that flood prevention measures would cost $60 million. A final report was anticipated to be submitted to Congress in 2006.

But then a flood hit Manoa on Halloween Eve 2004, causing $85 million in damages to the University of Hawaii. And government officials decided to extend flood studies to include the upper Ala Wai watershed area.

Since then, millions have been spent on engineering, environmental and economic studies that have yet to be released, as well as public outreach.

Last year, officials decided to scrap work on ecosystem restoration despite all of the studies. It would cost too much and take too long, said Chow, and there were no species of national significance, such as endangered plants, that needed saving.

Currently, government officials are evaluating the flood control options, which have evolved from stopgap measures constructed around the canal to potential fixes in the upper watershed. These include building a dam in upper Manoa and small water detention basins throughout the watershed. Creating a rain basin at the Ala Wai Golf Course is still a consideration, as are levees around the canal. But the 13-foot-high walls were not politically popular, so the vision is for lower walls that could also double as bike paths.

Officials are also looking at developing a flood warning system and flood-proofing some homes, businesses and public buildings.

Critics, like Ah Mai, have accused the Corps of Engineers of taking too long to come up with a solution.

She is frustrated because there hasn’t been a public meeting for two years.

“It seems to have gone into an utter funk,” she said of the planning process.

Athline Clark, who has recently taken over management of the project for the Corps, acknowledged that there was some down time due to budget and logistical issues. But she said that things were moving forward on schedule, with studies expected to be completed by 2015, followed by design plans.

Clark said while there may not have been public meetings, people who are interested in the situation have been kept up to speed via email and other communication.

Nothing Is Certain

Ultimately, there is no assurance that the flood plan will ever be implemented.

Not all flood projects end up making the cut or they can be delayed for decades, said Joseph Bonfiglio, a spokesman for the Corps of Engineers.

The plan has to be endorsed by numerous federal agencies and then goes to Congress for authorization.

Congress must also appropriate funds, which Chow said could be more difficult to justify within the current fiscal environment. The state will have to match 35 percent of the flood plan costs. He said that approvals will take the support of Hawaii’s congressional delegation.

Hawaii Sen. Brian Schatz says he met with officials from the Corps of Engineers in March and stressed to them the importance of the project.

“With a central Honolulu location and a watershed stretching from Manoa up makua down to Waikiki, it is hard to overstate the importance of the Ala Wai watershed’s health,” he told Civil Beat in an email. “The Ala Wai needs comprehensive planning and cooperative management to protect the UH campus and other communities from flooding as well as to safeguard hotels and visitor attractions.”

Richard Rapoza, a spokesman for Hawaii Rep. Colleen Hanabusa, said that it was premature to predict what type of support the plan will have in coming years, noting that it’s still in the study phase. He said that Hanabusa “will provide support where appropriate.”

Carty Chang, chief engineer for DLNR, said that the federal process could be Hawaii’s only chance at fixing the canal. He said it’s unlikely that the state and city could afford the canal improvements on their own.

Meanwhile, most people give little thought to what some residents call the “New Orleans of floods.”

It could happen tomorrow. Or it might happen 100 years from now.

“It’s something that we are concerned about,” said Egged of the Waikiki Improvement Association. “But it’s hard to get anyone excited about it because there just hasn’t been many problems in recent history.

“That is not a good reason because we know at some point we are going to have a problem. The question is when.”