Sevier Bridge Reservoir is the final major storage reservoir on the Sevier River, located in central Utah. At 92-ft tall and stretching 1,340-ft long, the earthen dam creates a 227,000 acre-foot reservoir, making Sevier Bridge Reservoir the largest privately owned reservoir in the state of Utah. With a 5,160 square-mile drainage basin the Sevier Bridge Dam is required to pass a probable maximum flood (PMF) of 53,200 cfs, significantly higher than the old spillway’s approximate 17,000 cfs capacity. To accommodate the full operational storage and PMF requirements, a full spillway replacement was necessary. Additionally, liquefaction potential and stability concerns at the downstream toe of the dam necessitated foundation stability improvements and filtration system upgrades. This rehabilitation effort to replace the spillway and address the geotechnical deficiencies commenced in the summer of 2022 and was completed in the spring of 2024.
Two site specific issues made the selection and design of the primary spillway challenging and played critical roles in developing two dimensional HEC-RAS models for the reservoir and river system. First, a highway bridge immediately upstream of the primary spillway constricted flows when routing the PMF. Second, a natural low spot at the north end of the reservoir acted as an auxiliary spillway during the PMF. The incorporation of these site specific elements in the models led to the final design of a gated primary spillway (three 25-foot by 20-foot stainless steel actuated roller gates), a 1-foot raise of the dam crest, and added erosion protection at the auxiliary spillway site. The HEC-RAS modeling also helped quantify the PMF attenuation in the reservoir and downstream river system, resulting in a reduced PMF requirement for this and other downstream dam rehabilitations.
A lesson learned and re-emphasized was the importance of creating and implementing a reservoir management and emergency action plan during construction. The project started in late 2022 with the removal of the spillway and downstream embankment excavation as the reservoir sat at record-low drought levels. Utah then experienced its wettest winter on record, placing the dam in a vulnerable position with the spring runoff season approaching. Execution of the reservoir management and emergency action plan helped fine tune inflow forecasts and the final cofferdam configuration. The extreme weather during construction highlighted both the strengths and weaknesses of the Sevier River forecasting system and can serve as a template for other reservoir and river systems needing similar improvements.
.jpg "Thayne Clark, P.E. photo")
