Hawaii flooding overwhelms drainage systems, triggers dam safety concerns

Back-to-back Kona low storm systems struck Hawaii last week, producing the state's worst flooding in more than 20 years and straining stormwater and dam infrastructure across the island chain.

A second Kona low compounded damage from already saturated ground across Oahu's North Shore, the Hawaii Emergency Management Agency reported. Flood-impacted areas were more vulnerable to significant flooding, even with less rain than the first storm. What was forecast as 2 to 3 inches became 10 inches in three hours, Honolulu Mayor Rick Blangiardi told Hawaii News Now.

Gov. Josh Green called it "the largest flood that we've had in Hawaii in 20 years.” He estimated potential damages exceed $1 billion across public and private sectors, including airports, a Maui hospital, schools, roads and homes. First responders conducted 233 rescues during the storm, with no reported fatalities.

The flooding overwhelmed wastewater infrastructure across multiple islands. The City and County of Honolulu's Department of Environmental Services reported spills at two facilities on March 20 caused by heavy rain and system inflow and infiltration. At the Wahiawa Wastewater Treatment Plant, storage tanks reached capacity at 2:56 a.m., resulting in a spill into Lake Wilson. At the Laie Wastewater Reclamation Facility, elevated flows caused the equalization basin to overflow into Kahawainui Stream. On Maui, about 200,000 gallons of partially treated wastewater overflowed at the Lahaina Wastewater Reclamation Facility on March 21, when a manhole became surcharged due to excessive stormwater entering the sewer system, the County of Maui Department of Environmental Management reported. Officials said the discharge likely reached the ocean. On Kauai, almost 286,000 gallons spilled from the Waimea Wastewater Treatment Plant between Friday night and Saturday morning due to high flows and infiltration, Hawaii Public Radio reported.

The Honolulu Board of Water Supply issued a boil water notice for the entire North Shore water system from Mokuleia through Turtle Bay after the water system lost power and drained during the flooding. The Hawaii Department of Health issued a brown water advisory for waters surrounding Oahu, with Honolulu Ocean Safety Chief Kurt Lager urging residents at a press conference to stay out of the ocean for at least 72 hours.

The storm also raised questions about aging dam infrastructure. Officials had been closely watching the Wahiawa Dam, saying it was "at risk of imminent failure," according to NPR. According to a Department of Land and Natural Resources board document, the earthen structure is owned by Sustainable Hawaii, Inc. and Dole Food Company Hawaii, was built in 1906 and reconstructed following a collapse in 1921, and is classified as a high hazard potential dam whose failure "will result in probable loss of human life."

Following the storm, Green said the state would reassess its overall approach to dam safety, including adding visual monitoring such as cameras and accelerating structural improvements like spillway projects, Hawaii News Now reported. Hawaii has 122 dams statewide monitored by the Department of Land and Natural Resources' Dam Safety Program.

The Hawaii National Guard activated 195 soldiers and airmen and staged 21 high-water vehicle teams statewide, Green said at a news conference. The Guard and Honolulu Fire Department airlifted 72 children and adults from a spring break youth camp on Oahu's west coast, CNN reported. Green said the White House reached out to him and described the federal response as supportive as teams prepared to assess the damage, Hawaii Public Radio reported. 

According to preliminary state Department of Transportation damage assessments reported by DisasterAWARE, highway damage alone exceeded $23 million statewide, including $14 million on Oahu, $7 million on Maui and $2 million on the Big Island. Figures are subject to change as damage assessments continue.

Road infrastructure sustained significant damage across the islands as well. The storm's first Kona low, which struck earlier this month, washed out a road on Maui.