The Virginia Lakes and Watershed Association (VLWA) named the Diascund Dam Principal Spillway Reconstruction Project the 2017 Best Major Rehabilitation Project for publicly owned dams. Gannett Fleming nominated the project on behalf of Newport News Waterworks (NNW) to recognize the provider’s proactive measures to improve the structural integrity of the high-hazard-potential dam.
NNW engaged Gannett Fleming to conduct a routine inspection in 2011 as part of a five-year contract that included dam inspection and observation services, design services, and construction administration for the project. The dam stretches across two counties and impounds the Diascund Reservoir, a water supply source for Newport News, Virginia, and surrounding communities.
Gannett Fleming’s Robert Kline Jr., PE, vice president and deputy manager for the Dams & Hydraulics Practice, identified an uplifted slab in the spillway chute during the initial inspection and conducted a subsequent review of NNW’s archives. This revealed several design deficiencies in the spillway slabs as well as a history of chute slab displacement incidents starting in 1963 during the initial reservoir filling. Previous spillway repairs repeated the deficiencies of the original 1960s design, including an insufficient underdrain system, independent concrete slabs, and a lack of waterstops in the slab joints to prevent high-velocity spillway flows from injecting water into the underdrain system.
Steve Davidheiser, PE, senior project engineer and project manager in the Dams & Hydraulics Practice, led the project team through implementation of the solutions, which involved installing a larger underdrain system as well as thicker, heavier concreate slabs with waterstops and steel dowels at slab joints to prevent slabs from becoming displaced. As a result, the improvements reduced the threat of chute slab displacement, ensuring the safety of the people, properties, and infrastructure located downstream.