PRELIMINARY RISK ASSESSMENT RIDM PROJECT FOR A DAM WITH A VEGETATION- LINED SPILLWAY AND FERC PILOT

Transcription

1 PRELIMINARY RISK ASSESSMENT FOR A DAM WITH A VEGETATION- LINED SPILLWAY AND FERC PILOT RIDM PROJECT MAY 3, 2018 DAVID S. BOWLES AND LOREN R. ANDERSON RAC ENGINEERS AND ECONOMISTS, LLC AND UTAH STATE UNIVERSITY ADAM J. MONROE AND MICHAEL J. THELEN CONSUMERS ENERGY COMPANY Third Workshop on Case Histories in Dam Safety Risk-Informed Decision Making 2018 Annual Conference Workshop

2 Alcona Hydroelectric Project Embankment Dam Emergency Spillway Powerhouse & Service Spillway Emergency Spillway

3 Preliminary Risk Assessment (PRA) FERC regulations provide a choice for informing risk reduction decisions between: 1. Engineering standards/guidelines approach, OR 2. Risk-informed Decision Making (RIDM) approach Incorporates reference to Engineering standards/guidelines. Purpose of the PRA: To inform the owner s decision about requesting permission to be included as a pilot project in the FERC Risk-Informed Decision Making (RIDM) program. Focused on following potential failure modes (PFMs) related to flood capacity Excluded other less significant PFMs related to embankment core wall, or the absence of a core wall.

4 Alcona Hydroelectric Project Embankment Dam Emergency Spillway 4) Service Spillway Apron Breakup Powerhouse & Service Spillway 3) Embankment Overtopping 2) Toe Erosion 1) Emergency Spillway Erosion Emergency Spil Potential Failure Modes Included in Preliminary Risk Assessment

5 Steps in Risk Assessment Process 1. Define the purpose 2. Conduct site visit, engineering assessment and review previous potential failure modes analysis (PFMA) 3. Develop the risk model 4. Estimate flood loading probabilities 5. Estimate system response probabilities (SRP) 6. Perform breach analyses and estimate life-loss consequences 7. Calculate risk estimates for the existing dam and indicative RRMs, including sensitivity runs Workshop participants: 8. Evaluate the risk - what risk is tolerable? Regulator: FERC representative 9. Recommend and make the case for the RIDM decision Owner/Licensee: Consumers Energy Dam Safety Consultant: Barr Engineering Risk Assessment Facilitator (RAC Engineers and Economists) Subject Matter Experts (SMEs)

6 Depth of Erosion in Emergency Spillway (feet) 4 Postulated Ranges of Max Depths of Emergency Spillway Erosion Emergency Spillway Peak Flow (cfs) 0 2,000 4,000 6,000 8,000 10,000 12, No Erosion Depth Range Evidence 1 No erosion m (4 ft.) m (8 ft.) m (16 ft.) m (0 2 ft.) m (2 6 ft.) m (6 12 ft.) m (12 16 ft.) 2.2 Maximum 4 ft Erosion Maximum 8 ft Erosion 2.8 Maximum 16 ft Erosion 2.9 Figure 4.5. Sensitivity cases for emergency spillway maximum erosion depth (measured from the emergency spillway sill at Elevation of 832 feet NGVD29) conditioned on emergency spillway peak flow 1. Velocity and flow direction evidence from SITES modeling. Boring GCES B-5 indicating: Bottom of a brown loose sand layer at 1.1 (3.5 ft.) to 1.2 m (4 ft.) Peat, silty marl and medium fine sand to a depth of about 2.4 m (8 ft.) to 2.7 m (9 ft.) Underlain by a thick medium dense sandy gravel Boring GCES B-5 indicating a gravel layer starting at 2.4 m (8 ft.) m (9 ft.) Boring GCES B-5 indicating somewhat erosion resistant clay layer 0 ft. 4 ft. 8 ft. 16 ft. Emerg Spillway Discharge Conditional Probability of Maximum Erosion Likelihood of Max 1 Erosion Depth Scenario ,000 4,000 6,000 8,000 10,000 12,000 Emergency Spillway Peak Flow (cfs feet Maximum Erosion 2-6 feet Maximum Erosion 6-12 feet Maximum Erosion feet Maximum Erosion Emerg Spillway Discharge Figure Team likelihoods for four maximum erosion depth intervals for various emergency spillway peak flow rates.

7 No Erosion 1 Maximum 4 ft Erosion Conditional Probability of Maximum Erosion ,000 4,000 6,000 8,000 10,000 12,000 Emergency Spillway Peak Flow (cfs 0-2 feet Maximum Erosion 2-6 feet Maximum Erosion 6-12 feet Maximum Erosion feet Maximum Erosion Maximum 8 ft Erosion Maximum 16 ft Erosion Figure Team likelihoods for four maximum erosion depth intervals for various emergency spillway peak flow rates. Event Tree Risk Model for the 4 postulated range of Emergency Spillway maximum erosion depths

8 Event tree showing dependencies between branches 100,000 Level 1 Inflow Flood Frequency No Erosion Peak Inflow (cfs) 10,000 1, Annual Exceedance Probability intervals of flood loading Level 11 Failure Modes Level 12 Day-Night Exposure Factors Level 13 Life-Loss Event Tree Risk Model

9 Event tree showing dependencies between branches Peak Inflow (cfs) Failure Mode 100,000 1 Emergency spillway erosion 10,000 1,000 1 Level 1 Inflow Flood Frequency 2 Embankment toe erosion 3 Service spillway apron breakup Embankment overtopping Conditioned on Emergency Spillway maximum erosion depth (Level 4) AND: Emergency spillway peak flow rate (Level 6) Emergency spillway peak flow rate (Level 6) 1. High flow causes a breakup of the tailrace slab as a function of service spillway peak flow rate (Level 9) 2. Initiates erosion which works its way under the training wall Annual Exceedance Probability Level 11 Failure Modes 3. Loss of confinement activates uplift forces 4. Collapse of the training wall as a function of duration of spill tube flow exceeding 5,000 cfs (Level 7) 5. Continued erosion undercuts the toe of the embankment adjacent to the power house as a function of duration of spill tube flow exceeding 5,000 cfs (Level 7) 6. A breach (uncontrolled release) of the embankment dam occurs as a function of peak reservoir stage (Level 5) Peak embankment overtopping depth (Level 10) Level 12 Day-Night Exposure Factors Event Tree Risk Model Level 13 Life-Loss

10 Emergency Spillway Max Depth of Erosion PFM Interdependencies 843 Peak Reservoir Stage 841 Peak Reservoir Stage (feet NGVD29) Revised PMF Peak Reservoir Stage - 2D Modeling 2015 Crest Survey Lowest Elevation Current PMF Peak Reservoir Stage - PMF Report Emergency Spillway Threshold for Erosion Emergency Spillway Sill 0 ft. 16 ft. 30,000 Emerg. Spillway 25,000 Peak Discharge Emergency Spillway Discharge (cfs) 20,000 15,000 10,000 5, ft. 0 ft ,000 10,000 15,000 20,000 25,000 30,000 70,000 Peak Total 60,000 Discharge 50,000 Peak Inflow (cfs) No Erosion 4-feet Maximum Erosion 8-feet Maximum Erosion 16-feet Maximum Erosion Peak Total Outflow (cfs) 80,000 40,000 30,000 20,000 10,000 Peak Inflow Rate Figure Peak reservoir stage peak inflow for emergency spillway erosion sensitivity cases ,000 10,000 15,000 20,000 25,000 30,000 Peak Inflow (cfs) No Erosion 4-feet Maximum Erosion 8-feet Maximum Erosion 16-feet Maximum Erosion Peak Inflow Rate Service spillway apron breaks up Toe erosion Overtopping breach Figure Peak total outflow vs. peak inflow for emergency spillway erosion sensitivity cases for breach and non-breach cases. 16 ft. 0 ft. 30 Duration of Service 25 Spillway Flow Duration of Spilltube Flow Exceeding 5,000 cfs (days) 20 >5,000 cfs No Erosion 8-feet Maximum Erosion Reservoir Stage (feet NGVD29) 4-feet Maximum Erosion 16-feet Maximum Erosion Peak Reservoir Stage Figure Emergency spillway peak discharge - peak stage for emergency spillway erosion sensitivity cases ,000 10,000 15,000 20,000 25,000 30,000 Peak Inflow (cfs) 0 ft. 16 ft. Emerg. Spillway Peak Discharge No Erosion 4-feet Maximum Erosion 8-feet Maximum Erosion 16-feet Maximum Erosion Figure Duration of spill tube flow exceeding 5,000 cfs - peak inflow for emergency spillway erosion cases.

11 CONSEQUENCES Inundation modelling: Non-breach/no erosion for range of flow rates Range of erosion depths and flow rates Other PFMs Life Loss estimated by Graham Method Warning Time = Travel Time + Notification Time - Agency Response Time Conservatively applied for PRA Estimated incremental life loss is very small for all PFMs except for embankment overtopping for which it is still small relative to many dams. Small because of slow rate of emergency spillway erosion & very long travel time Expected that a more detailed evaluation of life-loss will support even lower life-loss estimates. Depth of Erosion in Emergency Spillway (feet) Emergency Spillway Peak Flow (cfs) 0 2,000 4,000 6,000 8,000 10,000 12, No Erosion Maximum 4 ft Erosion Maximum 8 ft Erosion 2.8 Maximum 16 ft Erosion 2.9

12 Tolerability of Risk Existing Dam & RRMs 1.E-1 f-nbar Plot with USACE APF and AALL Guidelines Societal Risk (SR) for Existing Dam Annual Failure Probability, f (/year) 1.E-2 1.E-3 1.E-4 1.E-5 1.E-6 1.E-7 2) 1) 1) 3) Total Risk with emergency spillway erosion failure mode Total Risk without emergency spillway erosion failure mode 1 in 10,000/year 4) 4) 3) 2) 1.E , ,000.0 Weighted Average Life Loss Estimate, Nbar

13 Interdependencies between PFMs Existing Dam RRM 1) Service SW & Toe Eros Fix RRM 2) + 1 in 10,000 ES Stabilization RRM 3) + PMF ES Stabilization Increase in APF & AALL of embankment overtopping due to: Survival effect of addressing other failure modes Higher reservoir levels if emergency spillway erosion is addressed Higher life loss for Embankment Overtopping Is < PMF RRM acceptable given that life safety is paramount? Likely would meet IR & SR but not APF is ES erosion considered a PFM Both Emergency Spillway Stabilization alternatives are extremely disproportionate More detailed RA needed to evaluate this option APF Annual Probability of Failure 1E-2 1E-3 1E-4 1E-5 1E-6 1E-7 AALL Annual Probability of Failure 1E-2 1E-3 1E-4 1E-5 1E-6 1E-7 Total Service SW Apron Breakup Emergency SW Erosion Emb Toe Erosion 1 in 10,000 per year lives per year Embankment Overtopping Estimated Annual Probabilities of Failure 1) Existing Dam 2) Service SW & Toe Eros Fix Estimated Average Annual Life Loss Total Service SW Apron Breakup Emergency SW Erosion Emb Toe Erosion Emb Overtopping RIDM? 1) Existing Dam 2) Service SW & Toe Eros Fix 3) 2) + 1 in 10,000 ES Stabilization 3) 2) + 1 in 10,000 ES Stabilization Eng Stds: PMF 4) 2) + PMF ES Stabilization 4) 2) + PMF ES Stabilization

14 PRA Recommendation Recommended Owner to propose the Alcona Project to the FERC to be included in the FERC pilot RIDM program PRA provides a robust case for this recommendation: PRA indicates that RIDM might justify smaller or no emergency spillway remediation except for protecting the toe of the dam. Avoids increase in APF & AALL for overtopping failure with higher life loss. Avoids/greatly reduces environmental concerns regarding elimination of existing wetlands emergency spillway stabilization. Potential reduced cost. Additional expected benefits of RIDM include: A strong assurance that all significant dam safety issues identified and understood A strong assurance that all options for addressing dam safety issues are identified An improved risk-informed basis for communicating with the FERC and other stakeholders; A more robust risk-informed basis for the FERC's regulatory decisions and for Owner s corporate governance of dam safety at the Alcona Project; and A sound basis for allocating resources for future investigations and to reduce the risk of dam failure.

15 Current Status FERC has accepted the Alcona Project into the FERC RIDM pilot program Kickoff meeting scheduled for next week Overall objective for the pilot risk assessment: To make a defensible risk-informed case for a decision on risk reduction measures for Alcona Dam. Level 3 Semi-Quantitative Risk Assessment (SQRA): Screen potential failure modes (PFMs) Formulate and justify supporting studies A Level 4 Quantitative Risk Assessment (QRA): All credible and significant PFMs (including associated with the core wall) In-depth emergency spillway erosion studies Expert elicitation using subject matter experts covering all PFMs Life-loss estimation using simulation Uncertainty analysis A sensitivity analysis for uncertainty in the flood hazard relationship Evaluate Risk Reduction Alternatives Participative Risk Review Board

16 QUESTIONS? HOME PAGE (INCLUDING LINKS TO SELECTED PAPERS): Third Workshop on Case Histories in Dam Safety Risk-Informed Decision Making 2018 Annual Conference Workshop