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David S. Bowles, Loren R. Anderson, Terry F. Glover, Sanjay S. Chauhan, Ronn S. Rose

A risk assessment was performed for the Sacramento District of the U.S. Army Corps of Engineers to explore the justification for imposing an operating restriction on Lake Success to reduce the probability and consequences of an Earthquake-induced dam failure. The potential for both a sudden overtopping failure and a delayed “seepage erosion through cracks” failure were considered.

The risk assessment focused on the seismic performance of the dam, the potential life loss and economic consequences of Earthquake-induced dam failure, and the estimated residual risk and degree of risk-based justification for the Existing operating regime, a range of Potential Operating Restrictions, and an Indicative Improved Warning and Evacuation System. Risk assessment inputswere supported by seismic deformation analyses under various Earthquake loadings and pool elevations, dam break-inundation modelling, and reservoir simulation.

Evaluations against tolerable risk guidelines from the USBR, ANCOLD, and the UK HSE, together with insights into the relationship between pool elevation and dam failure risk, provided important inputs for the decision to implement an operating restriction.

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2005 Papers

2005 – Risk-Based Evaluation of Interim Operating Restrictions to Reduce the Risk of Earthquake Induced Dam Failure
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David S. Bowles, Loren R. Anderson, Terry F. Glover, Sanjay S. Chauhan, Ronn S. Rose

A risk assessment was performed for the Sacramento District of the U.S. Army Corps of Engineers to explore the justification for imposing an operating restriction on Lake Success to reduce the probability and consequences of an Earthquake-induced dam failure. The potential for both a sudden overtopping failure and a delayed “seepage erosion through cracks” failure were considered.

The risk assessment focused on the seismic performance of the dam, the potential life loss and economic consequences of Earthquake-induced dam failure, and the estimated residual risk and degree of risk-based justification for the Existing operating regime, a range of Potential Operating Restrictions, and an Indicative Improved Warning and Evacuation System. Risk assessment inputswere supported by seismic deformation analyses under various Earthquake loadings and pool elevations, dam break-inundation modelling, and reservoir simulation.

Evaluations against tolerable risk guidelines from the USBR, ANCOLD, and the UK HSE, together with insights into the relationship between pool elevation and dam failure risk, provided important inputs for the decision to implement an operating restriction.
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$15.00

2005 Papers

2005 – RISK-BASED EVALUATION OF INTERIM OPERATING RESTRICTIONS TO REDUCE THE RISK OF EARTHQUAKE-INDUCED DAM FAILURE
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A risk assessment was performed for the Sacramento District of the U.S. Army Corps of Engineers to explore the justification for imposing an operating restriction on Lake Success to reduce the
probability and consequences of an Earthquake-induced dam failure. The potential for both a sudden overtopping failure and a delayed “seepage erosion through cracks” failure were considered.
The risk assessment focused on the seismic performance of the dam, the potential life loss and
economic consequences of Earthquake-induced dam failure, and the estimated residual risk and
degree of risk-based justification for the Existing operating regime, a range of Potential Operating
Restrictions, and an Indicative Improved Warning and Evacuation System. Risk assessment inputs
were supported by seismic deformation analyses under various Earthquake loadings and pool
elevations, dam break-inundation modelling, and reservoir simulation.
Evaluations against tolerable risk guidelines from the USBR, ANCOLD, and the UK HSE, together
with insights into the relationship between pool elevation and dam failure risk, provided important
inputs for the decision to implement an operating restriction.
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$15.00

2013 Papers

2013 – DAMRAE-U: A tool for including uncertainty in dam safety risk assessment
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Anurag Srivastava, David S. Bowles and Sanjay S. Chauhan

Based on a generalized event tree algorithm, a deterministic model (DAMRAE) was developed for the US Army Corps of Engineers to support the dam safety risk assessment. With an objective to incorporate the uncertainty analysis functionality for the event tree based risk models, we extend the DAMRAE framework to develop a generic uncertainty analysis tool (DAMRAE-U) for dam safety risk assessment. DAMRAE-U provides a convenient way to efficiently characterize, propagate, and display the outcomes of uncertainty analysis. DAMRAE-U is structured to analyze knowledge uncertainty for the event tree variables and natural variability associated with flood and earthquake loadings. It also provides for separating the effects of uncertainty in the existing condition of the dam system on which the event tree model is dependent. In this paper, we present the details of the developed computational framework. Also, an example risk model to illustrate the inputs and outputs of the framework and the implementation of tolerable risk evaluation incorporating uncertainty in risk estimates is included.
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2012 Papers

2012 – Baseline Risk Assessment for Herbert Hoover Dike
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David S. Bowles, Sanjay S. Chauhan, Loren R. Anderson, Ryan C. Grove

A risk assessment (RA) was conducted for 27 miles of Herbert Hoover Dike to better understand and estimate the Baseline failure risk. Unique aspects of this risk assessment included the following: high stillwater levels persisting for almost a year; highly dynamic and spatially variable wind loading; short-duration wind setup that reduces likelihood of piping; dike length that increases probability of failure; and multiple breaches with overlapping inundation areas that affect failure probability and consequences and the risk evaluations.
A wide range of stillwater and wind loading combinations were considered. Following a potential failure modes analysis (PFMA), failure modes included were: piping through foundation, embankment piping, piping along conduits, piping along structures through embankment, embankment and flood wall instability, and overwash and overtopping. System response probabilities (SRPs) were estimated using toolboxes, analyses and expert judgment. Life-loss consequences were estimated using LIFESim. RA calculations were performed using DAMRAE-HHD, which includes length effects. Estimated risks were evaluated against the US Army Corps of Engineers (USACE) tolerable risk guidelines (TRG). Uncertainties were explored using sensitivity analyses.

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2008 Papers

2008 – Risk assessment with uncertainty analysis for an already-initiated failure mode
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David S Bowles, Sanjay S Chauhan, Loren R Anderson, Terry F Glover

Abstract: A nested model is presented for considering variability and knowledge uncertainty in a dam safety risk assessment of an existing dam and interim risk-reduction alternatives (operating restrictions) during the staged implementation of a permanent structural risk reduction measure. The effects of some important aspects of natural variabilities on estimated risks are represented as cumulative distributions of probability of failure, annualised life loss, economic risk cost, and an F-N representation of life loss. Many cumulative distributions are generated to represent the effects of some important aspects of knowledge uncertainties.

An important aspect of the knowledge uncertainty is the current level of development of an already-initiated piping failure mode. Also, an approach to conditioning the system response probabilities (SRPs) for the piping failure mode on the duration of reservoir pool exceedance is included in the failure event tree risk model.

ANCOLD and Reclamation tolerable risk guidelines are evaluated at selected percentiles of variability and percentiles (levels of confidence) of knowledge uncertainty. The incremental cost-per-statistical-life saved and benefit-cost ratio for interim risk-reduction alternatives are estimated and evaluated to examine the case for more-severe levels of operating restriction than the least-severe operating restriction that is estimated to satisfy, at a selected percentile of variability and a desired level of confidence, the limit values in all of the tolerable risk guidelines that were considered.

Keywords: Dam safety, risk analysis, risk assessment, uncertainty analysis, aleatory uncertainty, epistemic uncertainty.
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2009 Papers

2009 – Improvements to DAMRAE: A Tool for Dam Safety Risk Analysis Modelling
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Anurag Srivastava, David S. Bowles, Sanjay S. Chauhan

DAMRAE is a software tool for performing the event tree risk model computations for dam safety risk analysis. It is being applied by the US Army Corps of Engineers (USACE) and undergoing continued development and testing at Utah State University. DAMRAE is designed to overcome the limitations of existing business risk analysis software. It includes a generalized algorithm for constructing and calculating event trees. A generic project framework provides functionality for considering risk reduction alternatives or a staged implementation of risk reduction measures including obtaining estimates of their cost effectiveness of risk reduction. Evaluations against USACE tolerable risk guidelines are made. A flexible capability exists for obtaining tabular and graphical presentations of estimated risks at different levels of detail.

This paper provides an overview of the structure and capabilities of DAMRAE. It also includes an example screenshots to illustrate its capabilities. Plans for future improvements are summarized.

Keywords: Dam Safety Risk Assessment, Event Tree Analysis, Risk Reduction Measures.
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2010 Papers

2010 – A risk-based re-evaluation of reservoir operating restrictions to reduce the risk of failure from earthquake and piping
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David S. Bowles, Loren R. Anderson, Michael E. Ruthford, David C. Serafini, Sanjay S. Chauhan, Utah State University, Logan, Utah, U.S. Army Corps of Engineers, Sacramento, CA

In 2005 the Sacramento District of the US Army Corps of Engineers implemented an operating restriction to reduce the risk of an earthquake-induced failure of Success Dam, which could cause significant life loss and property damage. This paper describes an update of the 2004 risk-based evaluation of operating restrictions for Lake Success, which incorporated new information obtained by the District and enabled a re-evaluation of the level of the operating restriction and provided a basis for a possible modification of the restriction.

A RISK-BASED RE-EVALUATION OF RESERVOIR OPERATING RESTRICTIONS TO REDUCE THE RISK OF FAILURE FROM EARTHQUAKE AND PIPING
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