Benefits of Hazard Assessment using SQRA for Rural Dams
Decades-old dams often become higher risk as they age, but owners might have limited resources for assessing the actual risks. A semi-quantitative risk assessment (SQRA) offers a solution. South River Dam #19 (SR-19), a flood control structure built in 1956 by the USDA-Soil Conservation Service near Waynesboro, Virginia, was designed as a low hazard structure but the Virginia Dam Safety Program now classifies it as high hazard. The dam is underlain by competent residual and soft epi-karstic residual soils that overlie potentially karstic Elbrook formation. During design for modifications to the embankment dam and auxiliary spillway, seepage and slope stability evaluations were conducted to demonstrate that computed factors of safety (FSs) comply with state and NRCS dam safety guidelines. The FSs met NRCS requirements for steady-state seepage and rapid drawdown conditions but not for seismic and post-earthquake conditions because the epi-karstic residual soils provide inadequate foundation support. Karstic bedrock was identified to potentially develop unstable conditions within the embankment dam foundation. A potential failure mode analysis (PMFA) was followed by a semi-quantitative risk assessment (SQRA) to estimate the risk associated with a karst- or seismic-induced dam breach to address these failure mechanisms in lieu of engineered mitigation measures. The annual probability of failure and potential loss of life were estimated for the Sunny Day, 1/3 PMF, and PMF dam breaches. Risks were quantified and plotted for comparison with acceptable societal limits (1.0E-4) within a risk (f-N’) diagram. The SQRA for karst-induced breach indicated that the risk level for each evaluated PFM remains below USACE tolerable societal limits; as such, only routine dam inspections are needed. Dam owners can look to this project for lessons on the value of SQRA to manage risks without costly investigative or remedial measures, particularly for sites that have changed hazard classification levels.