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Dam Weak Spots

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Posted: Thursday, May 5, 2011 8:45 am

"A weak spot in the dam was revealed. That's why the DSOD (Division of Safety of Dams) has delayed putting the boards back in," stated Maureen Snelgrove, deputy director of Regional Parks. That startling fact was revealed at last Thursday's Crest Forest MAC (Municipal Advisory Council) meeting.

Testing of the boring samples indicated there are three weak spots in the dam. Tetra Tech, an engineering and architecture service hired by the county, and the DSOD are still conducting tests on more samples.

"We located a couple of weak spots on the east side of the dam near the road into Camp Switzerland," explained Jim Oravets, Special Districts' division manager of engineering. "The weak spots are due to unclassified materials used in the original dam construction. This material would tend to liquefy during a significant seismic event resulting in a loss of strength.

"The result of the liquefaction," he continued, "is that the crest of the dam may tend to slough off or deform above the weak spots. The severity of this deformation would depend on the size of the earthquake and how much deformation occurred. It would probably take an earthquake of 6 magnitude to liquefy the weak spots."

Oravets said the weak spots are "one fairly good sized and two small" spots. When asked how big the weak spots are, Oravets said, "It is impossible to know the full extent. In cross section, the weak spots take up less than 5 percent of the dam mass."

Snelgrove stated there are three areas that could be affected if the dam failed and the lake's full capacity was released. She said, "Camp Switzerland, Pilot Rock, and down to the facilities at Lake Silverwood. Those are the only three areas that are populated that would have damage if the lake were to release its full capacity."

MAC board member Connie Bracher asked what would happen if the lake were to lose its full capacity and how deep would it be when it reached Silverwood Lake.

Charles Rangel, Supervisor Rutherford's field representative, explained that the DSOD requires an inundation map of every dam and body of water in the state. "If there's a significant seismic event we can get to safety. Fortunately, the areas aren't too populated."

Oravets said that Rich Tocher, Tetra Tech's vice president of engineering and architecture services, and his crew have created an inundation map that shows the time and the depth and volume of water if there was complete failure at the Lake Gregory Dam.

"To get to Silverwood Lake, it would be 15 feet deep in 13 minutes," he said. "At Camp Switzerland, the impact would be very minor there. The depth of the water just past Camp Switzerland would be 29 feet with a full breach of the dam-with complete failure. It would take six minutes to get to Pilot Rock and would be 12 feet deep."

Oravets explained that the water wouldn't be that deep throughout the wave that would be released. He said the numbers indicate how deep it would be at its deepest point. He also said at one point the wave could be traveling at 30 miles per hour.


Snelgrove announced to the group at the meeting that an emergency action plan (EAP) has been developed, which was one of the requirements made by the DSOD to complete before the spillway boards could be replaced in the dam.

"What you'll notice on the EAP is there are three levels of emergency notification; level one is an unusual and slow developing issue; level two would be a potential dam failure rapidly developing; level three means dam failure is imminent or is in progress," Snelgrove said.

Snelgrove explained that even though it would take 13 minutes to reach Lake Silverwood, people could have more than that amount of time to get to safety because they would be notified that dam failure was possible, allowing them to prepare-unless complete dam failure happened quickly.


Tocher said they made the boring sites into observation wells so they could monitor the water level inside the dam. He also said there are a lot of trees around the dam and that isn't good for it. The roots can create a path in the dam, and when the tree dies it becomes an open pipe through the dam.

"Another thing we have is seepage between the base of the dam and the natural slope of the ground, and we would have to collect that," Tocher said. "We would put a gravel drain in so if some of the material of the dam came piping through there would be a gravel drain that would stop the material from moving and it would collect the water and run down to Huston Creek. The solution with the trees is to remove them and compact the holes."

Tocher said another solution would be to create a buttress. "We would use large rocks and boulders on the downstream face and we'd come about halfway up the dam, flatten the slope and rise up to hold it in place."

"We're hoping we don't have to go that far," Oravets commented.

When asked what kind of timeline there would be with the analysis of the remaining core samples and conducting a meeting with the DSOD to make some sort of decision on the solution, Tocher said it would probably be in the next four to six weeks.

"I met with the head of the DSOD a couple of weeks ago and he's very cognizant about what the issues are and what we need to do," Tocher said.

"They have not told us yet it will be that long before we could put the boards in," Snelgrove pointed out. "Now that they have all of the samples and are beginning their analysis, Keith and I will be speaking to them Monday (May 2)."

Snelgrove also said the DSOD has been timely is responding to their questions.

"When you're dealing with a state entity, it's unprecedented how quickly we've done this," Oravets noted. "I know you anxiously want to get the boards in, and I deal with them routinely, and I'm really happy. I feel the DSOD has gone extremely out of their way in dealing with this community."

The next question was who would finance the repairs? Oravets said it would probably be financed by a number of different entities. He said this type of work is conducive to grant funding.

"Would one source we hope we could rely on be FEMA (Federal Emergency Management Agency)?" Rangel asked.

"Absolutely," replied Oravets. "The Department of Water Resources, which is the parent company of the DSOD, is also a very good source for grant money. One of the big qualifiers with grant funding is it has to come as some sort of a compliance order. The thing that drives funding of these types of projects is when a regulatory agency puts a mandate in. We're going to have to wait, and it will probably only be four to six weeks, to see what the DSOD says."

Oravets said the bore holes reveal how much water is moving through the dam and the more data that is collected from those instruments, providing the data is good, the higher the comfort level the DSOD will have.

"We have a better handle on the dam now than anyone probably has since 1936," Oravets concluded.


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