The U.S. Army Corps of Engineers’ reevaluation report on a proposed Ala Wai Canal flood control project contains one big surprise: The project’s full estimated cost has ballooned above $1 billion — more than 50% higher than a $651 million estimate made just two years ago that caused the city to flinch and call for re-evaluation, and three times the estimated cost in 2017.

Other aspects of the reevaluation were telegraphed earlier, with a draft document released in July. The Corps concluded that only physical barriers and containment would be sufficient to prevent damaging floods, disappointing many critics who argued that upland watershed restoration and other “green” methods could be effective. That is indeed disappointing, but must be accepted as reality.

It’s perhaps sad, but true, that urban development of most land that is not almost vertical and the infill of former wetlands that held water near Waikiki’s shore have permanently changed the nature and risks posed by storms and rainfall in this man-made era.

Despite the sticker shock, Honolulu has no “do nothing” option — the city must act. The costs will continue to rise, for one thing: The cost of flood control was estimated at a mere $200 million in 2014, and that was double the estimate of $100 million in 2013.

It must be recognized that the Ala Wai watershed, which stretches from peaks of the Koolau through portions of Manoa, Palolo, Kaimuki, McCully and finally Waikiki, is a flood disaster time bomb. Honolulu’s leaders cannot afford to ignore this prospect.

Waikiki is most vulnerable to disaster from flooding that overwhelms the canal’s capacity, potentially causing loss of life and massive damage to property and businesses, contaminating the coast, harming land and marine life, and delivering a major gut-punch to the economy.

In 2022, the Army Corps of Engineers, which is partnered with the city to plan a flood control project, estimated that a major flood in the watershed could damage 3,200 structures and cost more than $720 million. That estimate, too, is likely far less than the actual toll of a flood, which would contaminate Waikiki Beach with debris and runoff, and keep tourists away. A 2016 report on the economic value of the shoreline of Waikiki alone estimated it at about $2.22 billion annually — this in revenue from tourists who come specifically to enjoy the beach and ocean.

It also clearly would be irresponsible to gamble with the health and safety of Waikiki residents and visitors. Add in the economic gamble inherent in failing to protect Waikiki as a tourism attraction and an oceanside jewel, and $1 billion, paid for largely by the federal government, no longer seems such an unjustifiable number.

Under the latest proposal, the U.S. government would be responsible for 65% of the total $1.075 billion cost, while Honolulu would pay 35%, or about $376 million.

This is not to say that the city should pay whatever it costs, without a cost-benefit analysis or protections against overspending. These must be incorporated. There may also be other sources of federal funding that can be applied for, connected to the additional flood risk that climate change poses as well as infrastructure support.

A public comment period is now underway, and two public meetings will be held to accept comments: 5:30-8:30 p.m. Dec. 12 at the Mission Memorial Auditorium, 650 S. King St.; and 5:30 to 8:30 p.m. Dec. 13, online. For more information and links, go to honolulu.gov/alawai. It’s time to get real about protecting Waikiki from a catastrophic flood.