Officials think they know why Priest Rapids Dam above Hanford is leaking

Officials have an idea of what likely is causing leaking at Priest Rapids Dam, the Columbia River dam above the Hanford nuclear reservation.

The dam, which was built in the 1950s, remains stable, according to the Grant Public Utility District. No movement has been detected in the spillway structure associated with the leak.

In late March, Grant PUD declared a “non-failure emergency” out of what it described as an abundance of caution.

Checks were being done at the dam, following the detection and repair of a large crack at the Grant PUD’s Wanapum Dam upstream.

Vertical holes were drilled into the spillway monoliths, the large concrete structures that support the piers holding up the spillway gates stretching 1,142 feet across the dam.

Water seeped into some of the holes, indicating a leak in the dam.

Further investigation has shown that four of the dam’s 22 spillway monoliths have leaks at the same elevation, said Grant PUD spokeswoman Christine Pratt.

The water appears to be infiltrating the dam where one pour of concrete was completed and then another one added on top during construction of the dam.

Some concrete core samples have found a break in the concrete at the joint between the concrete pours, she said. The grout used to bond the horizontal layers of concrete is disintegrating.

The joint between the concrete layers is at 405 feet above sea level.

The water in the reservoir has been lowered since the leaks were detected by about three feet. It now is between 484.5 to 481.5 feet above sea level, which is at the lower end of the normal operating range.

Drilling has been done on about half of the spillway and is continuing on the remaining monoliths, with crews drilling inspection holes from an internal passageway that runs the length of the spillway.

The drilling could take four or five more weeks at a cost of about $30,000 per day, and then Grant PUD will determine if repairs are needed.

The drilling is reducing pressure from the water inflow through the joint, according to Grant PUD.

In 2014, a 65-foot-long crack was discovered in the spillway of the Wanapum Dam, causing water to be lowered to 26 feet and cutting power generation in half.

Analysts determined that a math error during design of dam in the late 1950s caused the spillway to be built without enough reinforcing steel. In a $69 million project, the spillway was anchored to bedrock.

Grant PUD officials said then that the crack was serious, but that there was not a threat of downstream flooding even in a worst-case scenario.

This story was originally published April 10, 2018 at 6:12 PM with the headline "Officials think they know why Priest Rapids Dam above Hanford is leaking."