The Board of Water Supply said the water level rose four to five feet overnight, and by Wednesday morning, the reservoir was about 1.5 feet below the spillway.
Video: Heavy rain from former Olivia sent water level up at Nuuanu Dam No. 1
No Danger Of Flooding At Nuuanu Dam, City Says
The utility had been monitoring and siphoning excess water from the reservoir since the beginning of the week, however, officials say the passing of Olivia brought a deluge of rain that exceeded siphoning capacity.
"Out of an abundance of caution," BWS and Honolulu Fire Department personnel were deployed at the dam with extra water pumps Thursday to lower the reservoir level and prevent risk of overflowing.
"We're taking preemptive actions. This is not a reactive one," said Roy Amemiya, city managing director. "Additionally, the reservoir that we are working on is a much smaller dam than what you may be accustomed to, the one that everybody goes fishing for catfish, which is Reservoir #4."
"The water has not even reached the spillway, and the spillway is a normal part of a reservoir like this. It allows the safe release of water from the reservoir when the levels behind the dam reach a certain point," explained Ernest Lau, BWS manager and chief engineer. "You don't want to have a dam like this, which is an earthen dam, where the water levels go above top of the dam itself and start to go over the dam.
"This is not a dam breach situation right now. We're nowhere close to that," Lau stressed. "We're just trying to keep the water levels lower than spillway, because once water goes over the spillway, then you'll start to experience more flooding downstream, because water has to go someplace and it'll go makai downstream."
BWS is working with the Mayor's Office and the Department of Emergency Management to coordinate the operations plan, which includes public evacuation notification and sheltering if needed.
Lau says if the water level rose within a foot from the top, BWS would have to notify the state which would then notify some 10,000 residents that they might have to evacuate.
"There was no need to put notification out to community because there was no need to evacuate because it wasn't in danger of failing," Lau said.
"We have responders go around neighborhood and work through media to conduct notification and any means we have to get that out," said Hirokazu Toiya, deputy director of the city Department of Emergency Management.
"All these years, we haven't had any threat or high levels of flooding, and I know the back terrain has changed a lot, so the water comes down differently, but that reservoir has been dormant for a long time, so it's been low, and the Board of Water has always been draining or siphoning before storms to keep the level low," said resident Craig Hayashi.
For those living close to the dam, the Board of Water Supply released an evacuation zone map Thursday.
There is no longer a danger of flooding or a need for possible evacuations near a 21-million-gallon Nuuanu reservoir that filled close to capacity with the recent heavy rain from Tropical Storm Olivia, city officials said Thursday afternoon.
Earlier there had been speculation that 10,000 people might need to be evacuated, but that is no longer the case, Ernest Lau, manager and chief engineer for the Honolulu Board of Water Supply, said at a press conference.
Dam No. 1, an earthen dam built in 1905, filled with rainwater to about 1.5 feet below the dam’s lip. A mandatory evacuation would only be necessary if the water actually breached the dam’s lip.
In an earlier statement the BWS said it “has been monitoring and siphoning the excess water from the reservoir since the beginning of the week to keep the water level below the spillway. However, with the passing of Olivia, the rain exceeded the siphoning capacity.”
The Honolulu Fire Department had about 16 firefighters at the scene with about three trucks and a helicopter to provide aerial surveillance Thursday morning. Fire and BWS crews parked their vehicles and worked along the reservoir’s makai bank, as police and city officials observed.
By 11:30 a.m. those crews appeared to have the flood threat under control. The water had receded to about 5 feet below the dam lip as the pumps sent it into nearby Waolani Street.
Officials knew the reservoir was rising before Olivia even approached Oahu, Acting Mayor Roy Amemiya said.
BWS was in contact with the state Department of Land and Natural Resources, which requires all dam owners to report concerns to the department.
If an evacuation had to occur, the evacuation area could stretch from lower Nuuanu near the dam all the way down to Honolulu Harbor, according to a map provided by BWS.
“We would actually start the evacuation before a failure occurs,” said Hiro Toiya, deputy director of Honolulu’s Department of Emergency Management.
Any such evacuation should be completed in an hour, Toiya said. A city incident management team was evaluating how to approach potential evacuation conditions in the future.
The Legislature has provided funding to plan for dam renovations, but there is no construction money available yet, Lau said.
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