Editorial: Take hard look at Ala Wai flood plan

Last week, when gusty winds signaled Hurricane Lane’s proximity, government officials coordinated preparations while urging residents and visitors to brace for a powerful storm that could inflict long-lasting damage.

The joint county-state-federal effort spotlighted some efficient and thoughtful moves such as offering free bus service to anyone wanting to go to a nearby shelter, and the loading of food and supplies on Hawaii-bound ships. However, the effort to address possible trouble spots also shined a light on shortfalls that were attended to with little more than fingers-crossed.

Looming large among them on Oahu: flood mitigation along the Ala Wai Canal and its draining tributaries, which cut steep paths through Makiki, Manoa and Palolo valleys. A major storm along the canal route could deliver a devastating blow to Waikiki, the heart of Hawaii’s tourism industry.

Mitigation plans have been on the drawing board for several years. And last month, amid mounting evidence that we cannot afford to continue to make do with bare-bones tactics — largely limited to trapping and clearing debris in the waterway — Congress appropriated $345 million toward robust fortifications. They include detention basins in the upper reaches of Makiki, Manoa and Palolo streams and installation of floodwalls along the canal.

It’s unclear precisely how high the floodwalls would stand on the canal banks, which are now offset by an unwalled promenade. But Mayor Kirk Caldwell, who with Gov. David Ige has expressed support for stepping up mitigation, is right to raise a question about curb appeal.

Caldwell maintains that a park-like berm would be more aesthetically appealing than a wall — and could serve the same public-safety function. He has a point, and the possibility of refining that mitigation move should be vetted.

A risk management study proposed by the U.S. Army Corps of Engineers, in tandem with the state Department of Land and Natural Resources, calls for an average wall height of 4 feet, and an earthen levee — averaging 7 feet — at the perimeter of adjacent Ala Wai Golf Course, which would also serve as a detention basin.

Underscoring a sense of urgency is flood damage done by April’s intense rainfall on Kauai and Oahu, followed by Lane’s apparent record- setting measurement — slightly more than 52 inches of rain in a Hawaii island area over a four-day period ending Sunday morning.

The Ala Wai Canal last topped its banks during Hurricane Iniki, 26 years ago. While that might suggest a remote chance of flooding, the Corps of Engineers’ Honolulu District points out that flash-flooding conditions can materialize within an hour in the upper portion of the Ala Wai watershed.

Its highest elevation is about 3,000 feet, before dropping to 300 feet at valley floors and rolling to sea level — all over the span of about four miles. Given the combination of sheer slope, considerable rainfall — up to 150 inches a year in the Koolau Mountain Ridge — as well as the dense Waikiki population and growing climate-change concerns, the waterway is pegged as “high risk” for flash flooding.

An October 2004 storm that flooded Manoa Valley, described as a 25-year event, caused $85 million in damage.

The likelihood of flooding so severe that it encompasses all of Waikiki and the canal’s tributaries is pegged at 1 percent (a 100-year event), with damage to some 3,000 structures requiring more than $1 billion in repairs. Despite the long odds, such a scenario is an eye-opener.

Before final sign-offs on mitigation plans and agreements on future local costs, government stakeholders need to further vet envisioned upgrades by offering the public a look and the chance to weigh in. Such scrutiny on safeguards and community trade-offs should get underway quickly as this project is critical to preventing flooding that holds potential to imperil Waikiki and neighboring areas.