Millsite Dam to undergo modification and rehabilitation

By PATSY STODDARD Editor

Millsite Reservoir Dam will undergo some serious reconstruction work to bring it up to dam safety requirements as required by the Utah Division of Dam Safety. The Natural Resources Conservation Service has been looking at Millsite since 2006 when the planning and investigation started. Since then a variety of alternatives have been evaluated. The three alternatives now being considered include: 1. No action where the dam would continue to operate and owners would maintain the dam as is, but this would lead to eventual decommission of the dam due to dam safety standards not being met.
2. The rehabilitation alternative which is the preferred alternative being considered and moving forward at this time and includes the rehabilitation and modification of the dam to meet current dam safety performance standards. Three separate drilling programs were needed to adequately assess the foundation of the dam for seismic/earthquake risk and to determine feasible rehabilitation design elements. A time table has been identified considering the time needed for safe construction and to minimize the impact to shareholders and other stakeholders while the dam is under construction.
The third alternative considered was the decommission alternative which would include the designed removal of the embankment in accordance with established NRCS standards and would eliminate the catastrophic flood hazard associated with a potential failure of the dam and restore the geomorphology of the stream channel.
Public comments are being taken at this time and any concerns or comments relating to the proposed rehabilitation of the Millsite dam are being accepted.
Representatives from the NRCS came to an information meeting in Ferron to take comments and address concerns from citizens and water users. The time table outlined would begin in August/September of 2014 with downstream slope excavation. Foundation excavation and backfill in the fall of 2014 as well as the current spillway removal and reconstruction. The outlet works extension would take place in winter 2014- through spring 2015. The downstream slope reconstruction would take place summer and fall of 2015 depending on available funding.
The Millsite dam is owned and operated by the Ferron Canal and Reservoir Company and provides 18,000 acre feet of storage for sediment, culinary, recreation, power generation and irrigation.
More than 11,000 flood control dams have been build in 2,000 watersheds across the nation since 1948. These dams were built with funding and technical help through the United States Department of Agriculture. Local units of government have assumed operation and maintenance of the dams after they were constructed. The Millsite Dam is one of these 11,000 dams built between 1948 and now.
NRCS personnel at the meeting included Norm Evensted and Matt Call (Utah Division of Water Resources) and they advised attendees that much has changed in the development of dams since the early days, technology has come a long way, but many things are still done the same. The rehabilitation of Millsite will be a mixture of the old that works and the new technology that provides for greater safety for residents living below a dam.
The draft plan for the rehabilitation has been written and involved in an internal review and it is now out for public comments. The project has a price tag of $24 million for the preferred alternative which is the rehabilitation. The funding will come through the NRCS for $15 million and the remainder will come from local entities as well as the State of Utah.
Evensted said the rule is that dam safety will be responsible for 95 percent of the remaining funds needed not covered by the NRCS. The remaining 5 percent of the responsibility for the upgrade will lie with the state and local entities. In-kind matches with equipment, fill dirt and other required items can be used for the remaining money needed. It’s a partnership and those involved viewed it as a good deal.
Call gave an overview of the project. He said Millsite was state of the art when it was constructed. The dam is made up of some coarse materials that can erode and cause dam failure. “We have looked at removing two/thirds of that material. We will leave the dam itself intact and will strip off the existing rock face in late summer of 2014. The water level will be drained to the intake structure. It will be quite low. The upper 125 feet of softer soils will be removed from the dam.”
Call said the excavation work will come in the vicinity of where the golf course maintenance sheds are currently located and they will have to be relocated. After the removal of the softer soils, back fill will be added which will confine and hold and put pressure on the first excavation. The widening of the dam will displace hole number three of the Millsite golf course. The added materials will bring the width of the dam out to where the current green is on hole three. The dam/berm will be 100 feet wide and 30 feet high. This will hold the soil in case of an earthquake event. Amy water that may move through the dam will hit the filter material and be stopped. There may be minor sink holes in the protective filter material which Call said was OK as they can be grouted in. There is a waste pile of material on the golf course which will be used for fill. Some of the materials that come from the reservoir basin will not be used for fill as well as a gravel hill owned by the irrigation company because the quality is not what it needs to be.
In the fall of 2014 the old spillway will be torn out and replaced. The new spillway will be able to handle a potential flood event of 31,000 cfs. There will be a significant increase in the size of the spillway. A labyrinth weir system with a fork extending into the reservoir will be used. The spillway will be roughly the same length but will be 120 feet wide and it is 80 feet wide now. This work will be completed throughout the winter of 2014 and must be done by spring 2015.
There will be an extension of the outlet works out about where the maintenance sheds are now. To keep interruptions to service at a minimum. By passes will be used to keep the city water flowing as well as stock watering. There will be an interruption during this time to power plant water. The power plant personnel at the meeting were instructed to have an alternative plan in place during this interruption of water to the plant.
During the spring and summer of 2015 the dam will be raised four feet. This will increase the capacity of the dam that’s been lost to sedimentation over the years. This additional four feet will add 60 years life span to the dam.
The draw down in water in the reservoir will occur in the summer of 2014. One of the big reasons for leaving the dam in place is there won’t be an interruption of irrigation waters while the rehabilitation takes place. The irrigation season will be shortened at the end of the season of 2014. In 2015, they will try to keep the water levels as high as possible. There will be some interruptions in water service.
There will be some impacts to the Millsite Golf Course. When the current spillway is taken out, the access from hole one to hole two will be interrupted.
Call said, “One of the big things we recognize is the impact to the golf course.” Hole number three will need to be relocated and possibly temporary holes constructed elsewhere to keep golfers out of the construction zones.
Call accepted questions from the audience. One question involved the risk to the dam while materials are removed from it. Call said yes, there is a small risk and that’s why the water is lowered down to reduce the risk. Once the backfill takes place it will be much better than it is now.
The new spillway will need 2,700 yards of concrete. One cement truck can hold nine yards of cement, so the truck traffic during pours will have to be dealt with and a temporary access road constructed.
There were questions asked about the silt in the reservoir. Tracy Behling from the Ferron Canal Company said they are exploring new technologies for removing silt from the reservoir. Behling said the options right now with the funding for the dam rehabilitation are a better opportunity than has ever been presented before. They are excited for this opportunity to move forward with the dam rehabilitation. It is mandated by the State of Utah that Millsite dam be brought up to code. He feels they have looked at all the scenarios and the best design has been selected which will accommodate everyone, “It’s a good thing,” said Behling.
Call did mention again the potential for sink holes and if a sink hole occurs on the upstream side they would put in a grout pipe and pump it full of concrete. The chimney drain design is still being used in dams, the concept was right, but the particle size is important and adjustments have been made there.
There will be additional meetings in the future to discuss the project as it draws closer to construction.
Jordan Leonard is the golf pro for Millsite Golf course. He said they are concerned about the impacts to the golf course and he expects the access to holes two, three, four, and five to be restricted during the construction phase. The safety of the golfers is an issue as well as access to the lower holes of the course. The bright side is the new back nine which won’t be impacted at all. “We really need the community support during the construction phase to help overcome the impacts. We are looking at having reducing green fees during the dam construction for the front nine,” said Leonard.