Heavy rain from Tropical Storm Olivia filled Nuuanu Reservoir #1 to dangerous levels Thursday.

Built more than 100 years ago, the reservoir is capable of holding 21 million gallons of water.

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.

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.

But he says the level was still about four to five feet from the top.

“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.

The city says if an evacuation is necessary residents would be notified several hours in advance.

“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.

One resident who lives next to the reservoir tells us he’s not too worried.

“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.