Workshop on Case Histories in Dam Safety Risk-Informed Decision Making Husein Hasan, Senior Program Manager Caleb Douglas, Manager Dam Safety Geotechnics USSD Meeting – May 3, 2018 - Miami, Florida
Agenda • TVA Dam Safety Risk-Informed Decision Making Program • Tellico Project Semi Quantitative Risk Assessment (SQRA) – Project Background – Risk status prior to SQRA − Risk Driving Failure Modes − Results – SQRA Team Recommendations • Lessons Learned Tellico Dam SQRA | 2
1. Purpose and Scope of the Risk Program TVA Dam Safety Risk-Informed Decision Making Program | 3
TVA Risk Management Program • In March 2016, TVA adopted risk informed decision making as a tool for ensuring and improving the safety of TVA’s dams. • Since 2016, TVA has completed eleven Semi-Quantitative Risk Assessments (SQRAs) and two limited scope Quantitative Risk Assessments (QRAs). • The order of risk assessments is primarily dependent upon which dams are thought to have higher risks, based on PFMAs and Screening Level Risk Assessment (SRA) completed between 2009 and 2012. • Most risk assessments to date have been performed for dams that have one or more issues (Tellico is an example). | 4
Risk Management Program Context • TVA’s Dam Safety Program is governed by procedures which are designed to comply with the Federal Guidelines for Dam Safety and FERC guidelines. • TVA’s Dam Safety Program is self regulated and includes an internal governance and oversight group. | 5
Advantages of Risk Informed Decision Making (RIDM) • Understand each dam - Analyze and assess risks associated with TVA dams to inform the decision making process. Risks include loss of life, environmental, reputational, and economic. - Includes assessment of non-standards based failure modes (erosion through karst features, operational errors). • Ensure appropriate level of action and investment - Create a structure to assess the risk of dam assets, formulate plans for risk reduction actions, establish guidelines for risk tolerance and evaluate the cost effectiveness of incremental risk reductions. • Invest at the right time in the right asset - Prioritize dam safety actions. | 6
Tolerable Risk Guidelines • TVA follows risk guidelines that are patterned after the approach of other federal agencies. | 7
TVA Dam Safety Risk-Informed Analysis Continuum Routine Non-Routine SQRA IES/QRA PPD w/Mod Study SRA Project Planning Doc w/ Mod Focused Semi‐quantitative Risk Issue Evaluation Study/ Screening Level Study Assessment IES Quantitative Risk Assessment Risk Assessment Purpose Purpose Purpose Purpose • • • • Aids in initial portfolio Aids in portfolio Justify action to proceed Justify action to proceed prioritization prioritization with Mod Study or re‐ with dam safety • • Identify potential project Identify project prioritize modification • vulnerabilities vulnerabilities Aids in portfolio Scope • • • Identify and prioritize any Identify and prioritize any prioritization Update critical project info data collection, analyses, data collection, analyses, Scope and review (e.g. loading) • • needs (O&M and DSAP). needs (O&M and DSAP). Study vulnerabilities to Address life safety, Scope Scope reduce uncertainty economic, and • • • Review available Review all background info Update background info environmental • background info PFMA with SQRA and review new data consequences • • • • PFMA + SME‐based RA Risk‐informed QRA on risk‐driving FMs QRA on risk‐driving FMs • • Risk‐informed recommendations Risk‐informed and modification Recommendations Uncertainty: High recommendations alternatives • Uncertainty: Very High Cost/Duration: $250k/6 mos Uncertainty: Med Compare and select risk Cost/Duration: $30k/2 mos Cost/Duration: $500k/2 yrs management plan Uncertainty: Low Cost/Duration: $1M/2 yrs | 8
TVA Portfolio Risk Management Framework - Issues are typically identified by a routine dam safety activity (blue) - Issues then enter a non-routine risk assessment process (red) Abbreviations: DS – Dam Safety DSAR – Dam Safety Action Rating OMR – Operations, Maintenance & Repair PFMA – Potential Failure Modes Analysis SRA – Screening-level Risk Assessment | 9
SQRA Purpose and Scope Purpose • Aids in portfolio prioritization • Identify project vulnerabilities • Identify and prioritize any data collection, analyses, needs (O&M and DSAP) Scope • Review all background info • PFMA with SQRA • Risk-informed recommendations | 10
Advantages of SQRAs • Balance between SRA and QRA. • Completed with information available at time of workshop. | 11
TVA SQRA Process • Risk Assessments start with facilitated Potential Failure Modes brainstorming. • Consider potential failure mode for each major project feature. • Failure modes that are deemed to be risk-driving are fully developed and failure likelihood and life loss consequences are assigned. • The process is to determine the “Risk Driving Failure Modes.” • For TVA, the focus to date has been on failure modes that could lead to life loss. • Hydrologic loading is considered for flood events with annual chance of exceedance as small as 1 in 1,000,000. • Seismic loading is considered for ground motions with annual chance of exceedance as small as 1 in 100,000. | 12
Consequences • Consequence assessments focus on downstream life safety impacts. • Direct economic damages are computed, but do not explicitly influence the risk assessment outcome at this time. | 13
SQRA Outcome • Each risk-driving PFM placed in “box” that represents one order of magnitude. 1x10 -2 High Very High Annual Probability of Failure Annual Probability of Failure 1x10 -3 Historical Failure Rate for Dams, 1x10 -4 1x10 -4 Moderate PFM # 1x10 -5 Low 1x10 -6 Very Low Very Low Probability, High Consequenc e Events 1x10 -7 10 100 1000 1 Average Incremental Life Loss | 14
2. Baseline Risk Assessment Tellico Project SQRA | 15
TVA Watershed Map Power & Non-Power Dams | 16
Team Members Performed mostly by internal staff Consistency review was performed by the USACE Team Members: 2 Facilitators 3 Structural Engineers 4 Geotechnical Engineers 1 Geologist/Seismologist 1 Hydrologist 1 Instrumentation Engineer 3 Dam Safety Inspectors (Civil, Electrical, and Mechanical) 1 Water Resources Engineer (TVA River Forecast Center) 2 Former TVA employees who were involved in the design and the construction of Tellico Dam | 17
Vicinity Map | 18
Project Description Location of Project: Tennessee River near Lenoir City, Tennessee. Purpose of Project: Power generation and flood control Brief Physical Description: embankment (2760 ft.), gated spillway (190 ft.), non-overflow sections (300 ft.). In addition, there are 2 saddle dams: one is 350 feet and the other is 470 feet long. One emergency spillway, 2000 feet long. Ft. Loudoun Dam | 19
Project Features Emergency Spillway Saddle Dams 2 and 3 Saddle Dam #2 Emergency Spillway Concrete Dam 3 Radial Gates Main Embankment Right Rim Extension Saddle Dam #3 | 20
Project Features Concrete Dam Length: approx. 540 feet; Maximum Height: 129 feet PT Anchors EL. 834.9 PT Anchors: 92 anchors; depth: 65-97 feet 20' (6 m) BASE LINE CREST OF DAM EL 830 RIPRAP 2.5 DOWNSTREAM BERM 2.5 1 HEADWATER, EL 813 MAXIMUM TAILWATER EL 800 1 EL 799 10' 3 5' (1.5 m) (3.1m) 1 UPSTREAM 3 BERM, EL 795 1 INTERIOR RIPRAP ROLLER COMPACTED FILL 3 CHIMNEY TOE DRAIN 1 GROUND LINE DRAIN EL 760+/- CUTOFF TRENCH MINIMUM TAILWATER EL 735 OUTLET DRAIN OVERBURDEN ROCK, EL 725+/- TRENCH IN ROCK Embankment: Compacted impervious-rolled earth fill GROUT CURTAIN Length: 2760 feet; Maximum Height: 105 feet; Crest Elevation: El. 830 | 21
Project Features Emergency spillway: RCC structure Saddle Dam #2 Maximum Height: 13.5 ft. Compacted impervious-rolled earthfill Embankment Crest Elevation: El. 817 Length: 470 ft.; Height: 23 ft. EL. 834.9 Saddle Dam #3 Compacted impervious-rolled earthfill Embankment Length: 350 ft.; Height: 32 ft. | 22
Risk status prior to SQRA • PFMA (December 2010) • Screening Risk Assessment (May 2012) – Suggested “DSAC 4” • FY13 - FY15 Stability Analysis and Field Modifications in support of TVA Nuclear Program • Initial DSAR Assigned (September 2015) – Performance during earthquakes (Concrete, Embankment, and Foundation) – Piping through the foundation – Potential cracking at the Concrete-Rock- Interface (CRI) during normal operating condition – No SQRA. Baselined based on SRA. – DSAR 3 assigned | 23
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