The reservoir water levels will be slowly drawn down starting in early November so that it is 10 feet below the current water surface level on November 15, 2015. The water surface level will stay low through December 15, 2015, to allow investigations for the dam rehabilitation project design (See Information Below).
The gate will be closed after December 15, 2015, and the water surface level will be raised back to typical level by the end of December.
It is anticipated that the reservoir will be drained in April of 2016 for construction of the dam rehabilitation project, which is expected to a year to complete.
Tibble Fork Recommended Partial Draining Plan for Test Holes December 1-10, 2015
Drill 6 test holes 100’ deep across face of existing dam to determine soil types/permeability.
Water elevation drawn down 10’ vertical (approximately 50 acre-feet) Dec. 1-10, 2015.
Accomplish this without producing excessive sedimentation in AF River killing deposited brown trout eggs and sending sediment all the way out AF Canyon with an immediate draw down of the reservoir.
Slower release without significant instantaneous flow changes in the river which would flush eggs out of the gravels or leave them dry or out of the water with reduced flows.
How and Timeline:
Current inflow and outflow is 8 cfs. Open gate valve to flow 2 cfs (equals 4 acre-feet per day) until reservoir elevation goes below trash rack (spillway approximately 1’ vertical). Reservoir is approximately 8 surface acres currently so it will take 2 days to go below spillway elevation. This will result in 8 cfs (spillway) plus 2 cfs from gate valve to equal 10 cfs in river.
When reservoir elevation goes below the spillway elevation gate valve will need to be opened to allow 10 cfs to continue to flow down the river. To remove the estimated 50 acre-feet of water (10’ vertical) the gate valve would need to flow at 10 cfs for approximately 10.5 more days, river is flowing at 10 cfs.
Then to keep lake at this elevation for trench work for 10 days (Dec. 1-10) the gate valve would need to flow at 10 cfs to maintain lake elevation (inflow is estimated at 8 cfs), river is flowing at 10 cfs.
On Dec. 10 gate valve flow would be decreased to 6 cfs to allow filling of reservoir at a rate of 2 cfs or 4 acre-feet per day for 12.5 days (Dec. 22), river would be flowing at 6 cfs until the reservoir would start to spill over spillway again at a rate of 8 cfs at which time the gate valve flow would be shut off, river flowing at 8 cfs again.
It is estimated that 10’vertical feet equals 50 acre-feet of water. It is also estimated that the max depth of the reservoir at this time may only be 7’ deep.
Observations of water depth in front of the trash rack or spillway appears to be very shallow for several feet horizontally. Perhaps 10’ vertical is more than necessary for equipment and needed work to be accomplished. As reservoir reaches this elevation engineers will be contacted to determine if water elevation drop can be stopped at less than 10’.
Continual monitoring of flows, filling and draw-down will be important prior, during and after drilling.
It is beneficial for the fish survival in the reservoir to remain as deep as possible.
The Natural Resources Conservation Service (NRCS) is analyzing alternatives to rehabilitate Tibble Fork Dam located on United States Forest Service Uinta-Wasatch-Cache National Forest property. NRCS performed an assessment of Tibble Fork Dam in 2004. The 2004 assessment concluded that Tibble Fork Dam does not meet current NRCS and Utah State Dam Safety regulations and engineering standards for a high hazard dam (potential “Loss of Life”). The purpose and need of this project is for Tibble Fork Dam to meet current USDA-NRCS and Utah State Dam Safety regulations and current engineering standards. It would also continue to provide current benefits for the primary authorized purposes of flood prevention and sediment retention with secondary benefits of recreation and irrigation water storage. Stabilizing the existing dam structures would address the risk of loss-of-life and flooding associated with a dam failure because the dam is not meeting current safety criteria.
This project is being partially funded by the NRCS Small Watershed Rehabilitation Amendments (PL 106-472) which authorizes funding and technical assistance to rehabilitate aging flood control dams built under the Small Watershed Program (PL 83-566). NRCS, as the lead federal agency, is initiating NEPA analysis in the form of a Supplemental Watershed Plan and Environmental Assessment to analyze impacts to the natural and human environment from this project. The Environmental Assessment will comply with the Council on Environmental Quality’s regulations at 40 CFR Parts 1500-1508 which require an evaluation of potential environmental impacts associated with federal projects and actions. This project is located within the boundary of the Uinta-Wasatch-Cache National Forest and the United States Forest Service is a cooperating agency.
The NRCS has completed the Final Supplemental Watershed Plan-Environmental Assessment (Final Plan-EA) describing the proposed rehabilitation of the Tibble Fork Dam. The Final Plan-EA presents the alternatives analyzed, potential impacts to the environment, and the Preferred Alternative for the project. The NRCS Utah State Conservationist has decided that a Finding of No Significant Impact is approved for the Preferred Alternative (Dam Rehabilitation Alternative).
For further project information please contact:
Norm Evenstad - NRCS
Wallace F. Bennett Federal Building
125 S. State Street – Room 4010
Salt Lake City, UT 84138-1100
Fax: 801-524-4403 firstname.lastname@example.org