“Dworshak Dam is safe”
By Cloann McNall
Dworshak Dam’s new high risk safety rating reflects high consequences if dam were to fail though likelihood of failure is low. Corps official states, “At this time there is no problem. Potential for failure of Dworshak Dam is low. Dworshak Dam is safe.”
A new safety assessment of 610 dams nationwide was motivated by Katrina and the lessons learned, there according to Donna Street, chief engineering and construction division of the Walla Wall district of the U. S. Army Corp of Engineers.
Corps officials stressed last Wednesday in media briefings “that Dworshak Dam remains safe in spite of a new safety rating, resulting from the assessment, in which Dworshak was classified potentially unsafe."
The Corps has adopted a new system to judge the safety of its 610 dams. The system gives dams a safety rating of 1 to 5 with 5 being 100% safe and 1 being the most unsafe. Dworshak received a 2 rating.
Dworshak was given a low probability of failure but it earned a safety rating of 2 because the consequences are high if it should fail. Street said “many people and lots of property downstream would be severely damaged if the dam failed.” Ahsahka and portions of Orofino, Lewiston and Clarkston are downstream from the dam.
The new nationwide dam safety assessments are based on two criteria. First is the potential of a dam to fail and the second is the potential consequences if a dam were to fail.
Street said like all dams Dworshak has some seepage and the seepage, if left unchecked, could cause upward pressure in the concrete monolift and could cause the dam to fail. But Street said “The seepage and pressure are frequently monitored and the upward pressure remains within the dam’s design criteria and both drainage systems and groutings have been added to divert the seepage away from the toes of the dam.”
The seepage and pressure have not lead to any cracking of the dam’s monolift and the dam can also pass the maximum amount of water it was designed to pass when constructed 30 years ago Street said.
The new rating should help in securing additional funding for the corps to conduct more structural studies of the dam. Around $1 million to conduct an analysis is in President Bush’s current budget.
The new rating will lead to series of precautionary measures designed to reduce the risk of failure. These include updating the emergency action plan for the dam and surrounding communities should failure become imminent.
Corps officials met Wednesday in Orofino with county and city elected officials and emergency personnel.
Pictured: Dworshak Dam, located above Ahsahka, went into operation in 1972 and has a capacity to store 3,468,000 acre-feet of water. The dam blocks the North Fork of the Clearwater River.