Dam Quantitative Risk Assessment
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The dam is an earthen embankment constructed in the 1920s located in the Southeast United States. The embankment dam is approximately 1,000-ft long and 175-ft high and retains a useful storage volume of over 150,000 acre-feet within the upstream reservoir.

Geosyntec's Scope of Services

A gated, concrete, gravity overfall-type spillway with eight bays (five bays constructed in 1930 and three additional bays constructed in 1985) serves as the primary spillway for the dam. An ogee crest and chute spillway with six bays was built in 1995 to serve as the auxiliary spillway for the dam. Geosyntec Consultants, Inc. participated in a Potential Failure Mode Analysis (PFMA) and a Semi-Quantitative Risk Assessment (SQRA) conducted by the client to identify potential failure modes (PFMs) related to seismic, internal erosion, and hydrologic (spillway) events. The objective of the Quantitative Risk Assessment (QRA) for the dam and its spillways is to refine the estimates of risk associated with the seismic, internal erosion, and hydrologic PFMs.

Geosyntec was contracted by the client to be part of a Joint Project Team (JPT) with the client and perform the QRA for the dam. The development of the PFMs (i.e., from initiation to failure) was modeled by Geosyntec using logic and event trees, allowing for a structured approach to quantifying the annual probability of failure and associated uncertainty for each PFM. Probabilities were estimated for branches in the logic and event trees based on elicitation of the JPT and supporting analyses of the embankment dam and spillways, including seepage analyses, filter compatibility analyses, erodibility analyses, numerical seismic deformation analyses, structural analyses of spillway slabs, and hydraulic analyses of the spillways. As part of the hydraulic analyses, breach hydrographs were calculated and used to estimate downstream consequences. The annual probability of failure and probability of exceedance of life loss associated with each PFM were evaluated by the JPT to identify those PFMs that were risk drivers in the total project risk. Total project risk was compared to tolerable risk guidelines established by the client as part of their risk-informed decision-making (RIDM) process.

Notable Accomplishments

The QRA has aided the client in identifying the PFMs driving the total risk for the dam. The use of logic and event trees to model the PFMs allowed the JPT to identify sources of uncertainty in the risk estimates and take measures to reduce this uncertainty. For example, a field investigation was performed on the primary spillway to better understand the condition of the spillway slabs and foundations. As a result of the primary spillway field investigation, the risk estimates associated with the primary spillway PFMs were refined. Similarly, the hydraulic analyses and breach hydrographs provided refined estimates of the downstream consequences.