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Nu'uanu Reservoir Water Level Spurs Emergency Response

Kirk Caldwell

At 10 am Thursday morning the Honolulu Board of Water Supply began notifying Oahu residents that one of its reservoirs was reaching dangerous levels. The reservoir in question is held in place by a 103-year old earthen dam that sits uphill of thousands of homes.

Nu’uanu Reservoir #1 sits just off the Ewa side of Pali Highway behind the Oahu Country Club. It’s not one of the larger reservoirs in the area, but it still holds millions of gallons of water above dense residential neighborhoods.

Using remote sensors and cameras, BWS officials were able to identify the issue before the water level had reached the dam's spillway. Had water reached the spillway a high volume of water would have been released into the Waolani Stream, creating a potential flood hazard. The spillway is an additional three feet below the upper lip of the dam.

Credit Department of Planning and Permitting / City and County of Honolulu
City and County of Honolulu
A map from the city Department of Permitting and Planning outlines the evacuation area for a Nuuanu Reservoir #1 spillover.

The problem began after several weeks of steady rain elevated the water level in the reservoir. The BWS began siphoning water out of the dam, but even heavier rains hit the Nuuanu watershed Wednesday night as Tropical Storm Olivia passed Oahu. The water level in Reservoir #1 rose 4 feet overnight. Board of Water Supply Chief Engineer Ernie Lau then made the decision to begin actively pumping water out of the reservoir. Crews from the Honolulu Fire Department augmented the effort.

By 11 a.m. on Thursday the water level was holding steady and city officials declared they had the situation under control. However, that declaration so soon after the 10 a.m. announcement that some 10,000 residents may need to evacuate generated consternation among some residents. 

A $7 million upgrade is already underway at the larger Nu'uanu #4 Reservoir. Plans are still in development to upgrade the #1 dam.

Nuuanu Reservoir #1 Evacuation Area Map.jpg by HPR News on Scribd

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