The National Weather Service has issued a flash flood watch for all of the Hawaiian Islands as Hurricane Lane moves closer.

And while Honolulu has done some mitigation to make flood-prone areas safer, Mayor Kirk Caldwell said valleys and coastal areas are still prone to flooding.

City crews have dug ditches in many valleys, Caldwell said during a Wednesday press conference. But he said they could not reach areas in the backs of valleys where most of the water collects.

Manoa Valley river near Manoa marketplace mall, same location of the UH Library flood.

A crew works on flood mitigation in the Manoa Valley. Mayor Kirk Caldwell said the efforts should help but probably won’t be enough to prevent flooding in many areas.

Cory Lum/Civil Beat

As for neighborhoods near shorelines, runoff ditches can only be dug so deep before hitting seawater.

“No matter how deep you dig, they’re filled with saltwater all the time,” Caldwell said.

And the city often can’t dig wider ditches, the mayor said, without infringing on private property.

Hurricane Lane is expected to produce heavy rain. While the National Weather Service is warning there may be major flooding on all parts of the island, some areas are more flood-prone than others.

All coastal areas are prone to flooding from heavy rains, according to the state Department of Land and Natural Resources. Certain areas in valleys extending from Aina Haina to Waiawa may be especially vulnerable, along with areas close to streams and any other body of water, according to a flood hazard map from DLNR.

But while the department keeps track of areas that are flood prone, it did not have information available about the number of homes or people in those areas, a department spokesperson said.

Managers who deal with floods for Honolulu’s Department of Planing and Permitting did not return calls Wednesday. A representative with the Honolulu Department of Emergency Management also did not return calls.

With heavy rains, the city’s sewers may also be taxed, said Lori Kahikina, the county’s environmental services director.

For example, the water treatment plant at Sand Island has a normal capacity of about 67 million gallons. During storms, the plant could become flooded with up to 250 million gallons of water that must be treated, Kahikina said.

“We plug the pukas,” Caldwell said. “We’ll have people standing by to pump the sewers.”

The city is advising Oahu residents to conserve water, starting immediately.

“Don’t wash dishes. Don’t wash clothes. Don’t flush more than you have to,” Kahikina said.

Live Around Water

Caldwell said people in flood-prone areas need to “change the way they live with water.”

He used Kauai as an example. The island experienced major flooding caused by a storm earlier this year.

Caldwell said homes built on stilts generally fared better than those built at grade. However, while some homes built on stilts may have avoided flood waters, they may be endangered by the shifting ground on which they stand.

At least three homes along Hanalei Bay’s shoreline were pushed off their stilts by Kauai flood waters .

Anthony Quintano/Civil Beat

Oahu, like Kauai, has also developed neighborhoods around coastlines and streams and in valleys.

Landslides, mudslides, rockslides and flash flooding are ever-present dangers to the nearly 80,000 residents who live in valleys on Oahu, according to census data.

During his press conference, Caldwell also said he’d like to see changes in building codes to make homes better equipped to deal with floods. He didn’t provide specifics on what those changes might entail or if they would hep to defend homes from unsteady land.

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