Nearly all of Hawaii’s regulated dams are classified as “high hazard,” meaning their failure could result in loss of life — a designation that state officials say reflects downstream development, not the likelihood of collapse.

According to the state Department of Land and Natural Resources, 122 dams fall under state jurisdiction statewide, including 13 on Oahu. Of those, 115 are classified as high hazard, five as low hazard and two as significant hazard.

During the recent Kona-low storm, several Oahu dams came close to overtopping, raising concerns among residents even as officials maintained the structures “performed as designed.”

DLNR said that phrase refers to spillways — the overflow structures built into dams — successfully channeling storm runoff and preventing water from spilling over the top of the dam embankment, which can lead to failure.

However, officials acknowledged that some dams were pushed close to their limits due to undersized spillways and intense rainfall forecasts.

At Wahiawa Dam, water levels came within 4.7 feet of overtopping.

With the installation of a temporary water-filled barrier, or Aquadam, the margin increased to about 5.7 feet. At Helemano Reservoir 6, estimates varied, with DLNR measuring the water level as close as 6 inches below overtopping, while the dam’s owner reported as much as 2 feet of remaining capacity.

The near-capacity conditions prompted emergency discussions about potential evacuations in Wahiawa. DLNR said the “imminent failure” designation referenced in public alerts comes from standardized emergency action plan categories developed under federal dam safety guidelines — not necessarily an indication that structural collapse was imminent.

For Wahiawa Dam, the emergency plan defines “potential failure” at a water elevation of 84 feet and “imminent failure” at 86 feet.

DLNR staff coordinated with the City and County of Honolulu’s Department of Emergency Management, which ultimately holds authority over evacuation orders, weighing forecasts from the National Weather Service alongside logistical factors such as evacuation timing, shelter availability and exit routes.

The state’s two “significant hazard” dams are both on Maui — Waikamoi Reservoirs, located about six miles from Kailua, and Maui Reservoir 82, roughly 2-1/2 miles from Waiakoa Homesteads.

DLNR said both are currently rated in poor condition, with their operational status listed as “enforcement pending/ongoing,” indicating unresolved safety issues that require corrective action.

State officials declined to speculate on which dams might be most vulnerable in future storms, noting that each weather event presents different conditions.

Still, long-standing concerns about the condition of Hawaii’s dams persist.

DLNR reported that 87 of the state’s high-hazard dams are rated in poor or unsatisfactory condition, meaning deficiencies have been identified that require corrective action or further study.

Most of the state’s dams are aging infrastructure, many built during the plantation era. While the exact number is unclear, 86 dams are privately owned, many historically tied to sugar and pineapple operations.

All but one of Hawaii’s regulated dams are low-hazard structures. The exception is Waikamoi Dam, near the Waikamoi Reservoirs on Maui. Most dams in the state are earthen embankments, a design more vulnerable to erosion and potential failure if overtopped, while the low-hazard dams are primarily concrete.

The state owns 12 dams, while the remainder are largely under private ownership.

DLNR said it has enforcement authority, including the ability to issue fines, and is working with owners on compliance measures such as repair plans, engineering studies and water level restrictions.

If a dam reaches an emergency condition and an owner cannot act, the state could intervene to make necessary modifications, officials said.

DLNR said that oversight capacity remains limited as the state’s dam safety program currently has 4-1/2 full-time equivalent staff, including three dedicated inspectors and others who split duties with flood risk management.

DLNR said inspections go beyond visual checks and include engineering analysis and risk assessments, though the agency often relies on dam owners to hire consultants for detailed investigations and repairs.

Since the deadly 2006 Ka Loko Dam failure on Kauai, DLNR said the state has updated laws and rules to strengthen oversight, including requiring older dams to meet modern safety standards, improving maintenance practices and expanding emergency planning.

All but two regulated dams now have emergency action plans in place, and the state has developed dam-break modeling and inundation maps to guide emergency response. Officials said they are working on a public-facing map so residents can determine whether they live in a dam evacuation zone.

DLNR said it is aware of roughly 200 smaller, unregulated dams across the state that fall below its jurisdictional size criteria. Under state law, only dams that meet specific thresholds for height or capacity are regulated. Regulated dams are defined as those with artificial barriers 25 feet or taller or with an impounding capacity of 50 acre-feet — about 17 million gallons — or more.

Despite those efforts, DLNR acknowledged that risks remain.

“Wherever a dam exists, the potential for an incident or failure will always exist, as is the case with any manmade structure,” the agency said in an email to the Honolulu Star- Advertiser, citing factors such as extreme weather and owner actions as ongoing concerns.

State officials emphasized that a “high hazard” classification does not reflect a dam’s structural condition, but rather the potential consequences if it fails — a distinction that underscores the growing tension between aging infrastructure and expanding communities downstream.

The Garden Island

Message History

Delivery option(s)