Labyrinth Weir Spillway Design: Lessons Learned on the DMAD Dam Rehabilitation Project

ABSTRACT ONLY - DMAD Reservoir is the penultimate reservoir on the Sevier River, located near the town of Delta, Utah. With a storage capacity of approximately 11,000 acre-ft, it primarily serves as intermediate storage prior to conveying irrigation water to the greater Delta area and cooling water to the Intermountain Power Plant. The 7,270 square-mile drainage basin upstream of the DMAD Dam required the spillway to pass a probable maximum flood (PMF) of 48,300 cfs, a significantly larger flow than that permitted by the old spillway’s approximate 18,000 cfs capacity. Insufficient capacity of the old spillway coupled with geotechnical concerns regarding liquefaction of the foundation soils underneath the spillway and continual scour of the immediate downstream spillway channel resulted in the replacement of the DMAD Dam spillway. This rehabilitation work commenced in the summer of 2021 and was completed in the spring of 2022. Various spillway configurations were evaluated to pass the required PMF, including an auxiliary fuse plug of varying layouts and controlled failure alternatives. In order to address the challenges associated with the potential uncontrolled downstream erosion and the need to pass the PMF flows, a new labyrinth weir spillway design was ultimately selected and constructed. The 8-cycle labyrinth spillway increased the effective weir length to 1,200 ft but was contained within a 300 ft span. CFD and 2D river models were produced in order to optimize the spillway configuration. The deep downstream tailwater shown through the modeling process allowed for a significantly reduced stilling basin design, thus saving the project stakeholders cost and time in construction. The earthen channel downstream of the stilling basin was also equipped with a deep riprap cutoff wall to prevent headcut erosion from progressing up to the stilling basin structure.One lesson learned and re-emphasized during the construction of this project was the importance of planning ahead for emergency reservoir control and the implementation of a reservoir management plan. A local 1,000-year 24-hr flood event nearly compromised the project site when the overexcavation was at its deepest levels. Quick action to follow the Reservoir Management Plan procedures and construct an emergency upstream cofferdam ultimately saved the owner and contractor time and money while preventing extensive downstream damages from what could have been an uncontrolled failure of the dam through the project site.