CS30B - Concurrent Session 30B: South Fork Dam: Understanding and Mitigating Complex Geologic and Seepage Issues

This paper presents a case study documenting the lessons learned while exploring alternatives to address seepage issues at South Fork Reservoir, the only dam currently owned by the State of Nevada Division of Water Resources. South Fork Dam is an 85-foot high, zoned earth embankment dam situated on an extremely complex geological formation. The site was once considered for development by the U.S. Army Corps of Engineers, however no work supported by the Corps was ever completed. The reason the site was passed over by the Corps is unknown, but in the mid-1980s, the Elko County Recreation board experienced firsthand how complex geological and seepage issues can complicate dam construction. The intersection of three, variably weathered geologic formations beneath the dam create conditions that require multiple means of seepage mitigation, including a multi-line grout curtain, multiple foundation aggregate drains, and relief wells. Additionally, a very flat floodplain prohibits the conveyance of collected seepage downstream from the dam. When ownership of the dam was transitioned to the State of Nevada shortly after completion of construction, establishing a new state park, much of the information documenting the subsurface conditions was lost. Despite the existing seepage mitigation systems, the State has battled a clogged toe drain, relief well inefficiency, and occurrences of uncontrolled, unfiltered seepage.

This presentation will explain the importance of understanding a dam’s seepage regime and mitigation systems. The method used to evaluate the seepage regime and develop supplemental mitigation will be discussed. The effectiveness of this approach will be illustrated by the South Fork Dam case study. Lessons learned from this project will be shared so that other engineers that encounter similar conditions may benefit.