by Eric Barker
Lewiston Tribune, October 5, 2008
Despite a seemingly worrisome rating, officials say Dworshak Dam is in no danger of falling down,
but that doesn't mean the 30-year-old structure has no problems at all
DWORSHAK DAM - It's made up of 6.5 million cubic yards of concrete, weighs more than 26 billion pounds and holds back more than 1 trillion gallons of water.
At 30 years old it leaks a little bit and has been declared "unsafe or potentially unsafe."
That might sound scary but officials at the U.S. Army Corps of Engineers say there is little to worry about - Dworshak Dam is solid and not in danger of failing.
"It's as safe now as when it was built," said Dworshak Operations Manager Greg Parker.
Then why the rating with such alarming language? It's all a matter of assessing risk. Because Dworshak is so large and holds back so much water, a failure would wreak massive damage to life and property. (See related story)
And while the chances of Dworshak failing are deemed low, the consequences are massive. So when corps officials completed a risk assessment of the dam last February, they rated Dworshak a 2 on a scale of 1 to 5, with 5 being the safest.
"If the economic consequences are there, it's going to be rated a 2," Parker said.
So does that mean Dworshak has no problems and its negative rating is only because of its location? Not exactly. The dam that blocks the North Fork of the Clearwater River near its mouth does have some issues.
Namely, water is squeezing through flex joints, known as water stops, on the front of the dam and even though that leakage doesn't immediately threaten the dam, it needs to be fixed.
Many of the drains used to monitor water slipping beneath the foot of the dam are clogged and need to be cleaned so potentially dangerous uplift pressures can be monitored.
An emergency action plan that would be employed if the dam were to fail is more than 25 years old and is in the process of being updated. And other programs used to assess the safety of the dam need to be completed. There is tentatively $1 million in the 2009 federal budget to continue the work corps officials say is needed. The money would pay for things like uplift drain cleaning and coming up with a way to stop the leaking.
The current leakage does not threaten the structural integrity of the dam, Parker said.
"If you ever go into a dam that doesn't leak at all, I'd head for the hills because they are all designed to leak."
But the leaking does make it more difficult to carry out monitoring of pressures that give engineers a clue as to what is happening beneath the massive structure. When the dam was built, engineers included drains to pick up any water being pushed beneath it. Water slipping beneath dams can cause tremendous uplift pressure. If that pressure is high enough it can cause a dam to fail.
The water that enters the uplift drains is piped through the center of the dam. Engineers monitor the amount of water carried by the pipes and if there is any sediment in the water that might indicate dangerous erosion of the dam's concrete or sub-dam bedrock.
Many of those drains are clogged with mineral deposits. A scheduled drilling program will address that. But in some cases there is so much water leaking through the front of the dam, 500 to 600 gallons per minute at full pool and half that at the current reservoir level, that it is mixing with the water from the uplift drains. This makes it difficult for corps officials to tell how much of the water is coming from underneath the dam and how much from the front. Again, water from the front of the dam is not a problem, but water coming from beneath the dam could be.
To make matters more complicated, corps officials used sandbags to try to control the water and keep the two sources separate. Over time those bags have broken down and the rushing water has picked up the sand. That means they sometimes find sediment in water. They believe that sediment is largely, if not solely, from the sandbags. But they can't be sure.
"We need to clean it all out of there and monitor it and see if there are any concerns," Parker said.
The dam is made of 51 concrete monoliths. But the dam is not totally solid. There are a series of walkways, known as galleries, that crisscross the structure. The walkways allow corps officials to move through the interior of the dam to monitor leaking. The floors of the walkways have gutters on their upstream and downstream sides. Those gutters are designed to transport leaking water through the dam.
The gutters on the upstream side carry water leaking through water stops on the front of the dam. Gutters on the downstream side carry water from the uplift pipes.
At places the upstream gutters are white with rushing water. The water is leaking through failed copper water stops placed between the dam's monoliths. The water stops are designed to prevent water from leaking and to expand and contract with the dam because of changes in temperature.
Parker said the corps is working to come up with a way to fix the failed water stops. Fixes under consideration include using a chemical grout to keep the water out or installing a curtain on the front of the dam, a sort of tarp, that would also block the water.
In August, corps officials joined with local emergency managers to run through a drill simulating what would happen in the event of a failure. They also completed an analysis of the ways the dam might fail.
At the Tribune's request, Jim Milligan, a retired University of Idaho professor of civil engineering, toured the dam and talked to Parker and others about its safety assessment, leakage problems and what is being done to address them. Milligan said he was impressed.
"My overall impression is the dam seems to be more carefully attended to than many I have visited in the past," he said. "It appears to be under sound management with planning for dealing with situations as they unfold. The issues that have arisen there are not unexpected or out of the ordinary. They are fairly typical."
He was not alarmed at the amount of water leaking through the dam.
"I'd have to say, out of all of them I have been in, this one is pretty dry."
Nor was he troubled by the rating the corps gave the dam. He said engineers are conservative and they expect other engineers to challenge their findings. He comes from a long line of engineers and has two sons who work for the corps as engineers. But he says that does not cloud his judgment.
"The engineers' first obligation is to public safety. Only after that comes the obligation to the client. With that public safety concern being paramount they have a tendency to be conservative by nature and that conservatism is certainly reflected in the way (corps officials) deal with maintenance issues and operational issues there at Dworshak Dam."
The safety assessment of Dworshak was conducted by a corps team from outside of its Walla Walla district. Members of the team did not visit the dam. Instead they looked at reams of data produced by local engineers.
"It's an outside set of eyes," said Bob Hollenbeck, chief of structural design for the Walla Walla district.
He said members of the team did debate rating Dworshak a 1 on the 1-to-5 scale. Such a rating would have meant the dam would not be able to operate at full pool. In the end, Hollenbeck said a rating of 2 was arrived at because the uplift pressures beneath the dam were not out of line with design criteria.
"I believe it's a very appropriate rating," he said. "We need to be cognizant of everything that is downstream of us."
But since the rating was made public earlier this year, the corps has fielded numerous calls from concerned citizens. Joe Saxon, a spokesman for the corps' Walla Walla district, said the agency is aware of the concerns and is trying to be as transparent as possible.
"If folks are concerned we'd like to know about it because we'd like to address those concerns," Saxon said. "If we need to talk to service clubs or through the media, we need to do that. We want to be up front as we can to assure the people of the safety here at Dworshak."
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