The state will begin emergency repair work Monday on the Kailua Reservoir in Waimanalo after recent inspections determined the reservoir was unsafe.

During heavy rains on April 9, the reservoir overflowed and forced the evacuation of 12 families. No one was injured and there were no reports of serious damage.

State officials inspected the reservoir and said remedial work needs to be done quickly to prevent the reservoir from filling up again.

The Kailua Reservoir was built in the 1930s for agricultural use and can hold about 5 million gallons of water, but it's been out of operation since 1993.

On Monday, crews and equipment from the Hawai'i National Guard will begin filling a 20-foot-wide sinkhole on the crest of the reservoir's dam. Later, the crews will create an additional outlet in the dam so the reservoir does not retain water.

"My assessment is that the structural integrity of the Kailua Reservoir dam is not sound," said Maj. Gen. Robert Lee, the state adjutant general.

"The priority now is to make sure that the reservoir drains whenever it rains so the reservoir does not endanger residents downstream."

Janelle Saneishi, Department of Agriculture spokeswoman, said the work will not involve tearing down the entire dam, as was reported earlier this week.

"We're just removing part of the dam so that the water can run," Saneishi said. "For now, we just want to make it safe in case there's future heavy rain."

She said it was not known how long the work would take. The department has requested $2.5 million from the Legislature to cover the costs of the work at the reservoir.

Reach Curtis Lum at culum@honoluluadvertiser.com.