A study on the feasibility to raise the Lake Farmington dam led to the discovery of a failing conduit installed in the 1960s.
The 36-inch, lined steel pipe is not only outdated but there is significant corrosion and leaking inside the conduit, said John Sikora, a water engineer with AECOM, the company tasked with completing the feasibility study of a 25-foot dam raise at the lake.
Sikora recommended replacing the conduit pipe and building a new intake tower to address the issue.
Embankment particles are being pushed into the pipeline, which is causing corrosion. Further study led AECOM to discover the soil is corrosive – something that is native to the region, Sikora said.
The dam at this time is stable. The only “major concern” is the conduit drain, which “has to come out in the long term,” Sikora said.
“The reality is this isn’t going to fail tomorrow. We are not in immediate danger,” Councilor Linda Rodgers said, adding the city doesn’t have to start the project tomorrow.
“I can’t tell you how long that conduit is going to last,” Sikora said, adding the city needs “to be pretty deliberate” in replacing the conduit with an updated design.
Not replacing the conduit could cause a sinkhole in the front of the dam. There are structures within the flood zone should there be a breach in the dam, Sikora said.
There was a report in 1970 that recommended the city install a failsafe, which was never installed, according to Sikora, who said there also was a buttress installed on the downstream face of the dam in the 1980s.
The conduit has a “burrito” design for the drain, which has a geosynthetic wrap that with age ceases functioning as a drain because of sediment and bacteria, Sikora explained.
This type of piping is common for municipal pipelines, but it is not something that should be done in a dam, Sikora said.
“In modern dam design, we take those pipelines and encase them in concrete. That wasn’t done here,” Sikora said.
“The most common failure of a dam is due to piping,” Sikora said, adding the entire buttress must be excavated and the conduit will be replaced with “a sand drain in our new embankment.”
Sikora is proposing a 6-foot diameter tunnel with a 5-foot diameter pipe inside. That would increase the capacity of water the city is able to get out of the dam and delivered to the city.
The only issue with the construction is the need to build a coffer dam to hold 15-acre feet of water so the lake can be drained down to 5 feet for access to the conduit, Sikora said.
“Essentially, we’ve got to drain the dam down to 5 feet, and then increase by 10 feet when the coffer dam is in place. This could take four to five months,” Sikora said.
“People would be on pins and needles during that time. We’ve got such a low-storage amount there,” Sikora said, because during construction, the city would only have 15-acre feet of water stored for the entire community’s needs.
“The term pins and needles was really important to me, as we sit in the middle of drought right now,” said Mayor Nate Duckett. “We are already on pins and needles.”
Rodgers asked if the proposed plan was using “best practices available in today’s world. We’re not just band-aiding it, right?”
“We’re bringing the dam up to current safety standards and would be up-to-date for the next 100 years,” Sikora said. “Unfortunately, this conduit replacement – you’ve got to get it done.”
The engineering design is done, and with funding the city could potentially start tunneling late next summer and then drop the lake levels in mid-November, when the city’s water needs drop off, Sikora said.
This phase of updating the dam would include a new intake tower at a cost of $10 million, the tunnel at a cost of $10 million and 5-foot raise of the dam at a cost of $4 million, according to Farmington Public Works Director Angelique Maldonado.
The entire project of raising the dam by 25 feet could cost upward of $200 million and would include installing a new labyrinth spillway, raising embankments, extending the boat ramp and elevating the boat dock, updating the access road, raising the embankments of the settlement ponds, relocating the Farmer’s Ditch pump station and adding two additional saddle dams, among other things.
It would increase water storage by 90%,” Sikora said.
“I think we definitely saw the 25-foot raise as a multi-generational project,” Duckett said. “There are immediate things we need to do to protect the integrity of the dam. You can only eat an elephant one bite at a time.”
Maldonado pointed out that infrastructure ages, and in order to be proactive the city is conducting these studies and planning construction on the conduit.
“This work really does set the foundation of how we can prioritize things,” Maldonado said. “Grant funding is not something unattainable. We’re in the position to start applying for grants.”