1. 1 Scoggins Dam Geotechnical Analysis and Risk Analysis July 18, 2012

2. Overview and General Topics Facility Description Types of Reclamation Dam Safety Studies 2004 Comprehensive Facility Review 2008 Issue Evaluation 2010 Issue Evaluation Ongoing Corrective Action Study 2

3. Detailed Analysis Topics Geotechnical Analyses –Field Investigations –Embankment Analyses Risk Analysis in Reclamation Potential Failure Modes Estimation of Failure Consequences Estimation of Annual Probabilities of Failure Summary of Risks Conclusions SOD Recommendation Corrective Action Study 3

4. Facility Description 4

5. Scoggins Dam Scoggins Dam is an earthfill embankment located on Scoggins Creek about 25 miles west of Portland, Oregon Dam construction was completed in 1975 Reservoir (Henry Hagg Lake) has a capacity of 53,323 acre-ft at the top of joint use capacity, elev. 303.5 ft Structures at this facility include: –Embankment dam –Gated spillway –Tunnel outlet works

6. Location 6

7. Embankment Dam Dam has length of 2,700 ft Maximum structural height of 151 ft (Crest El. 313) Zoned embankment Due to presence of soft foundation soils, dam was designed with two foundation (cutoff) trenches

8. Dam Cross Section

9. Appurtenant Structures Outlet works consists of a tunnel through the left abutment, with a capacity of 400 ft 3 /s Spillway is a gated structure located on left abutment, with a capacity of ~14,000 ft 3 /s

10. Plan View

11. Dam Safety Studies 11

12. Types of Reclamation Dam Safety Studies Comprehensive Facility Review (CFR) –Every 6 years –“Screening” level Issue Evaluation Study (IE) –Detailed –Range in scope Corrective Action Study (CAS) Modification Final Design 12

13. Risk Analysis Used Throughout These Studies Potential Failure Mode (PFM) analysis conducted at all phases CFR risks are estimated by simple means, and typically use “best estimates” with limited uncertainty analysis –Include all loading conditions (static, hydrologic, seismic) For all higher level studies, risk analysis is accomplished by a facilitated team –Likely to focus only on specific loading conditions 13

14. Scoggins Dam - 2004 CFR Concluded that static and hydrologic risks do not exceed Reclamation Public Protection Guidelines (PPG) thresholds and thus provide decreasing justification for any additional actions –This finding verified in recent 2010 CFR Concluded that latest earthquake loadings were higher than used in previous engineering analyses, and that seismic risks may exceed guideline values, justifying additional actions to better define risks 14

15. Scoggins Dam - 2004 CFR Resulted in one new Safety of Dams (SOD) recommendation 2004-SOD-A: After the study to update the potential seismic hazards has been finalized, evaluate the need to perform additional investigations and dynamic analyses This led to IE studies 15

16. Scoggins Dam – 2008 IE Updated the PSHA, but did not develop site-specific ground motions No new explorations or investigations Simplified engineering analyses, including: –Screening-level spillway analyses with earth pressures –Foundation “triggering” analyses –Post-EQ stability analyses –Newmark analysis using ground motions from other sites 16

17. Scoggins Dam – 2008 IE Conducted team risk analysis Concluded that estimated risks from dam overtopping or internal erosion due to seismic loading justified further risk reduction actions Concluded that estimated risks from spillway wall failure or separation at embankment-structure interface due to seismic loading justified further risk reduction actions 17

18. March 2008 Decision Although risks appear to justify corrective actions, need additional study since findings were based on “preliminary” studies –Essentially, conclusion that a well-built dam could fail under subduction zone earthquake, with no assumed strength loss, is a critical conclusion, and needs verification To withstand scrutiny, perform a more detailed Issue Evaluation study to fully verify risks –Gather additional embankment/foundation data –Update seismic loading –Perform state-of-practice engineering analyses 18

19. Scoggins Dam – 2008 IE 2008-SOD-A: Develop and perform a field exploration program for Scoggins Dam that will obtain information that will better define the earthquake loading and the site’s response to large earthquakes. 2008-SOD-B: Perform detailed stability and numerical dynamic analyses using the information obtained in the field exploration program, updated (if necessary) seismic hazard analyses and ground motions. 19

20. Scoggins Dam – 2008 IE 2008-SOD-C: Perform an issue evaluation risk analysis for Scoggins Dam using the information obtained from the field exploration program and dynamic analyses. 20

21. Scoggins Dam – 2008 IE 2008-SOD-D: Have all work done for this issue evaluation reviewed by a Consultant Review Board (CRB). 21

22. Scoggins Dam – 2010 IE This latest round of IE studies resulted from the preceding SOD recommendations, and included the following activities: –Updated PSHA and development of ground motions –Extensive field program and geologic/testing reports –Evaluation of in situ and laboratory testing of embankment and foundation soils –Detailed engineering analyses of embankment and spillway (strength loss triggering, post-EQ stability, Newmark deformations, FLAC deformations, LS-DYNA spillway analysis –Facilitated, team risk analysis 22

23. Scoggins Dam – 2010 IE Concluded that estimated mean seismic risks from dam overtopping or internal erosion brought about by earthquake-induced slope failures and cracking justify further risk reduction measures Concluded that estimated mean seismic risks from failure of spillway wall, which could lead to an erosional failure of the embankment, justify further risk reduction measures Concluded that seismic risks from spillway pier failure did not justify additional action 23

24. Scoggins Dam – 2010 IE 2010 IE led to one new SOD recommendation 2010-SOD-A: Initiate a Corrective Action Alternatives Study to evaluate potential alternatives to mitigate the high risks of seismic failure modes of the embankment and spillway at Scoggins Dam 24

25. Scoggins Dam – Ongoing CAS Initiated, based on findings of 2010 IE Conducted concurrently with finalization of IE studies, and convening of a Consultant Review Board In progress, with ongoing analyses and design work 25

26. Geotechnical Analyses 26

27. Field Investigations 27

28. Field Investigations Program CPT testing along downstream toe and beneath shell –Screening exploration to better locate additional borings –Determine peak/remolded undrained strength of clayey overburden –Strength loss potential of clays/silts SPT testing in basal sand/gravel unit –Liquefaction potential of sands/gravels Vane shear testing in overburden –Peak and remolded undrained strengths of clayey overburden Four shear wave velocity crossholes –Also have seismic cone downhole data –Plus a line of surface shear wave data at toe Undisturbed sampling holes –Used shear wave holes for undisturbed sampling 28

29. Fall 2008 Explorations 29

30. Observations from Field Investigations Strengths of foundation soils are generally higher than as measured in pre-construction explorations Foundation soils to the right of Scoggins Creek still appear to be of generally lower strength and also contain more sandy, silty materials Foundation soil strengths increase under the dam footprint There was relative agreement between strengths measured by the CPT and by vane shear tests, although the CPT values were typically lower Clay sensitivity, or ratio of peak undrained to remolded strength, typically ranges from 2 to 3 30

31. Laboratory Testing Contracted with then-URS lab in New Jersey Both embankment and foundation samples were tested, but focus was on strength of foundation clays Tests included: –1-D consolidation –U-U triaxial shear (peak undrained strength of clays) –C-U triaxial shear –DSS (both peak and remolded undrained clay strengths) –Lab vane shear (peak/remolded clay strength) –Cyclic triaxial –Cyclic DSS 31

32. Observations from Undisturbed Sampling and Lab Testing Foundation clays are lightly overconsolidated, with OCR typically around 2 or 3 Most foundation soils are plastic, with an average PI value of 22 Laboratory testing of undisturbed foundation soil samples confirmed the presence of low strength soils (similar to what was determined by CPT and vane shear testing) 32

33. Embankment Analyses 33

34. Embankment Analyses Strength loss triggering in foundation –CPT, SPT, V s, and vane shear test data –Looked at liquefaction in coarse-grained soils, strength loss in clayey overburden Limit equilibrium post-EQ stability “Squashed dam” analyses Newmark analyses –Used both DYNDSP and QUAKE/W FLAC analyses 34

35. Liquefaction Triggering in Coarse-Grained Foundation Soils Focused on those foundation soils that had a plasticity index (PI) of less than 7 Generally limited to the basal sand/gravel (Qalb) and the sandy soils to the right of Scoggins Creek Potential for liquefaction of these types of soils was evaluated using Standard Penetration Test (SPT) blow counts and by shear wave velocities Evaluated in accordance with state-of-the-practice procedures (Seed simplified method for SPT, Andrus and Stokoe for shear wave velocity) Looked at both earthquakes – local and subduction zone earthquake, as well as several different return periods, ranging from 500 years to 50,000 years 35

36. Liquefaction Triggering in Coarse-Grained Foundation Soils SPT blow count analysis and the shear wave analysis yielded similar findings Both indicated that the silty and sandy soils at the toe and beneath the downstream slope of the embankment immediately right of Scoggins Creek were potentially liquefiable Also, liquefaction could be triggered for the 500-year EQ Weak zone of low plasticity soils was in the vicinity of Station 7+00 and was generally from elevation 170 to 185 Liquefaction potential was limited in all other areas of the foundation. Basal sand/gravel appeared relatively dense based on both the SPT and shear wave tests, and there were no other continuous areas of coarse-grained soils. 36

37. Strength Loss Triggering in Fine-Grained Foundation Soils The potential for strength loss or cyclic failure of the fine- grained soils which comprise the majority of the foundation overburden was evaluated by 3 methods -Boulanger and Idriss, Seed et al, and Bray and Sancio The Boulanger and Idriss approach will indicate a potential for the soils to lose strength past the peak undrained strength, but not necessarily all the way to remolded strength Appears that the Seed et al and Bray and Sancio methods may assess the potential that the soils will go to remolded strengths 37

38. Strength Loss Triggering in Fine-Grained Foundation Soils Boulanger and Idriss – Utilized vane shear tests and CPT results to measure resistance to cyclic loading –Widespread cyclic failure across the entire valley –Strength loss would occur to some degree even during a 500-year earthquake –Most widespread during earthquakes with return periods of 5,000 years or more Seed et al –Moisture content and Atterberg limits –22 to 30 percent of all samples may be liquefiable Bray and Sancio –About 20 percent of the samples would be potentially liquefiable 38

39. Assignment of Foundation Strengths Considered strength results from field vane shear testing, CPT, and laboratory tests on undisturbed samples – focused on clay strengths Used test data to estimate both peak undrained and remolded undrained strengths for clayey soils Estimated strengths in terms of reasonable low, best estimate, and reasonable high values Used different strengths for left and right sides of Scoggins Creek, as well as for different areas under the embankment - e.g. beneath crest, under downstream (and upstream) slope, and at downstream toe 39

40. Assignment of Foundation Strengths 40 Peak Undrained StrengthsRemolded Undrained Strengths Low Estimate Su (psi) Best Estimate Su (psi) High Estimate Su (psi) Low Estimate Sur (psi) Best Estimate Sur (psi) High Estimate Sur (psi) Right Side of Valley (Approximate Dam Stations 6+00 to 10+00) Beneath Crest 1219255710 Beneath D/S Shell 812153.558 D/S Toe45101.52.55 Beneath U/S Shell 68.512.52.53.756.5 Center and Left Side of Valley (Approximate Dam Stations 11+00 to 22+00) Beneath Crest 1419255710 Beneath D/S Shell 91216358 Beneath U/S Shell 91216358

41. Post-Earthquake Stability Analyses SLOPE/W was used to assess post-earthquake stability Stability evaluated at three different embankment cross sections (stations 9+00, 15+00, and 21+00) Modeled several different assumptions of strength loss Different foundation strengths were used under various portions of the embankment Looked at deep-seated failure surfaces that would take out the crest (ignored shallow failure surfaces that may have lower factors of safety but would be less likely to lead to dam failure) 41

42. Post-Earthquake Stability Analyses – Typical Failure Surface 42

43. Post-Earthquake Stability Analyses – Results Analyses consistently indicated that upstream failure surfaces resulted in higher factors of safety; thus, downstream failures pose greater risk of failure Embankment is not stable if earthquake loading leads to remolded/residual strengths (s ur ) in either the fine- grained or coarse-grained foundation soils Greatest chance for instability appears to be in that portion of the embankment located to the right of Scoggins Creek 43

44. “Squashed Dam” Analysis A “squashed dam” analysis refers to a progressive analysis of the failed embankment using a limit equilibrium procedure SLOPE/W was utilized to iteratively determine the stability of the dam by applying a pseudo-seismic load on the dam and deforming the dam along the resulting failure surfaces After each failure, the dam geometry was changed to represent the estimated deformed dam, and the stability reanalyzed This analysis suggested that progressive sliding under seismic load could ultimately lead to the embankment deforming to almost half its original height 44

45. “Squashed Dam” Results 45

46. Newmark Deformation Analyses Newmark displacements were calculated using two methods – using QUAKE/W and using DYNDSP Different time histories were used for the two types of earthquake (local and subduction zone) Two different embankment cross sections were analyzed – Station 9+00 and Station 21+00 Different foundation overburden strengths were modeled. Since Newmark analyses essentially require that the initial factor of safety must be above 1.0, not all strength assumptions (particularly the lower values) could be modeled Critical failure surfaces were selected from the post- earthquake stability analyses. The failure surfaces were deep-seated, and located in the downstream slope of the dam. 46

47. Newmark Deformation Analyses - Results Largest predicted Newmark deformations resulted from the lowest strength assumptions, which correspond to the lower safety factors and lower yield accelerations (Sta. 9+00) Predicted deformations are much larger due to the subduction zone earthquake than due to the local earthquake Predicted Newmark deformations were similar whether calculated by DYNDSP or by QUAKE/W During the 50,000-yr subduction zone earthquake, very significant crest loss (on the order of 40 feet) is predicted even for a drained strength scenario - this is due to the long duration of severe shaking that results in frequent exceedance of the yield acceleration and thus makes large embankment deformations likely 47

48. Newmark Analyses - Results 48 Predicted Newmark Vertical Deformations at Station 9+00 Local Earthquake with USBR time histories DYNDSP values shown first; QUAKE/W values follow in parentheses Loading Drained Strength Best s u Low s u High s ur 500-yr EQ0.01 ft (n/c)0.2 ft (0.2 ft)0.8 ft (1.3 ft)1.0 ft (1.6 ft) 1,000-yr EQ0.2 ft (n/c)0.9 ft (1.1 ft)2.6 ft (3.5 ft)3.0 ft (4.2 ft) 5,000-yr EQ1.4 ft (n/c)3.4 ft (3.6 ft)8.0 ft (10 ft)8.7 ft (12 ft) 10,000-yr EQ 1.9 ft (n/c)4.6 ft (4.5 ft)11 ft (13 ft)12 ft (15 ft) 50,000-yr EQ 4.4 ft (n/c)11 ft (9.4 ft)n/c (22 ft)n/c (27 ft)

49. Newmark Analyses - Results 49 Predicted Newmark Vertical Deformations at Station 9+00 Subduction Zone Earthquake with USBR time histories DYNDSP values shown first; QUAKE/W values follow in parentheses Loading Drained Strength Best s u Low s u High s ur 1,000-yr EQ0.3 ft (n/c)5.0 ft (4.2 ft)24 ft28 ft 5,000-yr EQ6.0 ft (n/c)32 ft (28 ft)n/c (n/c) 10,000-yr EQ 14 ft (n/c)52 ft (48 ft)n/c (n/c) 50,000-yr EQ 42 ft (n/c)122 ft (70 ft)n/c (n/c)

50. QUAKE/W Deformations 50

51. FLAC Analyses FLAC (theoretically) has the advantage of being able to estimate potential deformations using even the lowest assumed strength scenarios, while the Newmark analyses discussed above were limited to higher assumed strength scenarios As with the Newmark analyses, deformations were modeled at two stations – Station 9+00 and Station 21+00 In addition, both the local and the subduction earthquakes were evaluated, as well as a number of different strength scenarios for the foundation overburden FLAC model deformation results were driven by the value of the reduced strength assigned to the foundation soils, as well as the severity and duration of dynamic loading 51

52. FLAC Analyses Typical deformed mesh 52

53. FLAC Analyses - Results 53 Predicted Vertical Deformations (from FLAC) at Station 9+00 Local Earthquake (USBR time histories) Loading Drained Strength Best s u High s ur Low s ur Gravity onlynot calculated 8 ft 1,000-yr EQ1.5 ft1.7 ft3 ft33 ft 5,000-yr EQ4 ft5 ft7 ft34 ft 10,000-yr EQ 5 ft6 ft9 ft34 ft 50,000-yr EQ 9 ft11 ft15 ft36 ft

54. FLAC Analyses - Results 54 Predicted Vertical Deformations (from FLAC) at Station 9+00 Subduction Zone Earthquake (USBR time histories) Loading Drained Strength Best s u High s ur Low s ur Gravity onlynot calculated 8 ft 1,000-yr EQ5 ft6 ft13 ft43 ft 5,000-yr EQ15 ft21 ft31 ftnot calculated 10,000-yr EQ 21 ft30 ft40 ftnot calculated 50,000-yr EQ 34 ft46 ftnot calculated

55. Risk Analysis in Reclamation 55

56. Risk Analysis Overview Reclamation uses quantitative risk analysis to aid in making risk-informed dam safety decisions For Issue Evaluation (and higher level) studies, risk analyses involve a team of “experts” led by a facilitator Steps include PFMA, creation of event trees, discussion of factors influencing nodal probabilities, consensus assignment of probabilities and distributions, Monte Carlo analysis, team discussion of risk results, and ultimately portrayal of risk Risk numbers a key part of decision, but not the sole factor 56

57. Measures of “Risk” Reclamation’s Public Protection Guidelines define two measures of acceptable performance for our dams The Annual Probability of Failure (APF) is the probability that the dam will fail in a given year, and is expressed as (Prob. of Loading) x (Prob. of Structural Response) The Annualized Life Loss (ALL) combines the probability of failure and the consequences. It is expressed by the equation (Prob. of Loading) x (Prob. Of Structural Response) x (Consequences) For Reclamation dam safety studies, “consequences” refer solely to loss of life

58. Potential Failure Modes 58

59. Embankment Seismic Failure Modes Risk team brainstormed potential seismic failure modes for embankment; 12 mechanisms were identified Most were judged to pose low risk, or at least risks substantially below that posed by more critical failure modes Four failure modes were judged to pose potentially significant risks, and each of these was carried into the risk analysis and evaluated 59

60. Brainstormed Failure Modes Overtopping due to foundation liquefaction Overtopping due to foundation strength loss in clays Overtopping from Newmark displacements (no strength loss) Internal erosion from cracking from Newmark displacements Internal erosion from cracking from foundation liquefaction Internal erosion from cracking from clay strength loss Internal erosion from embankment/spillway separation Internal erosion from cracking from left abutment landslide Internal erosion from cracking from foundation fault offset Overtopping from seiche wave – reservoir landslide Overtopping from seiche wave – fault offset in reservoir Internal erosion from differential settlement cracking 60

61. Most Plausible/Critical Embankment PFMs PFM A - Dam overtopping (deformation > freeboard) due to slope failures caused by significant strength loss in foundation soils PFM B - Dam overtopping (deformation > freeboard) due to Newmark-type displacements (without significant strength loss in foundation soils) PFM C - Internal erosion resulting from cracking due to partial slope failures (and associated extensive cracking) caused by significant strength loss in foundation soils PFM D - Internal erosion due to cracking caused by Newmark-type displacements (without significant strength loss in foundation soils) 61

62. Potential Failure Mode A Large earthquake causes strength loss in foundation soils, either due to liquefaction in coarse-grained soils or cyclic failure in fine-grained soils Strength loss leads to deep seated failure surface After initial slide, progressive sliding possible due to long duration of subduction zone earthquake Slope failures result in a remnant of remaining embankment that is lower than the reservoir level Reservoir flows over the top of the remnant, resulting in a fairly rapid breach by erosion 62

63. Potential Failure Mode B Large and prolonged earthquake shaking leads to a Newmark-type slope failure in the embankment (due to accelerations repeatedly exceeding the yield acceleration) Progressive sliding (due to long duration of subduction zone earthquake) occurs along a deep-seated failure plane Slope failures result in a remnant of remaining embankment that is lower than the reservoir level Reservoir flows over the top of the remnant, resulting in a fairly rapid breach by erosion 63

64. Potential Failure Mode C Large earthquake causes strength loss in foundation soils, either due to liquefaction in coarse-grained soils or cyclic failure in fine-grained soils Strength loss leads to deep seated failure surface, but not one that leads to overtopping Shearing and associated extensive cracking resulting from the slope failure create continuous seepage paths in dam Seepage begins to erode embankment materials If no self-healing or intervention, dam breaches by gross enlargement of seepage/erosion path or from progressive sloughing of downstream slope 64

65. Potential Failure Mode D Large and prolonged earthquake shaking leads to a Newmark-type slope failure in the embankment (due to accelerations repeatedly exceeding the yield acceleration) Progressive sliding (due to long duration of subduction zone earthquake) occurs along a deep-seated failure plane, resulting in extensive shearing and cracking but no overtopping Seepage through the continuous shearing/cracking begins to erode embankment materials If no self-healing or intervention, dam breaches by gross enlargement of seepage/erosion path or from progressive sloughing of downstream slope 65

66. 66 Estimation of Failure Consequences

67. 67 Consequences Failure of Scoggins Dam would be expected to cause life-threatening flooding and significant property damage along Scoggins Creek and the Tualatin River Expected peak breach flow is about 675,000 ft 3 /s Inundation area includes a lumber mill and portions of several towns Quantitative approach used

68. 68 Estimation of Annual Probabilities of Failure

69. 69 Risk Analysis Process Team effort, with multi-disciplined teams & facilitators Evaluated embankment and spillway seismic failure modes separately Developed event trees to model failure modes Estimated probabilities for each node/branch of tree –Based on newly gathered data and latest analysis methods –Involved thorough team discussions –Used judgment (degree-of-belief estimates) Performed Monte Carlo analysis (10,000 iterations) to multiply nodal probabilities and estimate mean annual failure probabilities Reviewed estimates for reasonableness

70. 70 Embankment Risk Analysis Decided to combine the four potential failure mechanisms into two basic failure modes First: Dam overtopping resulting from seismic-induced deformations that exceed available freeboard –with or without significant strength loss in the foundation –includes Newmark-type deformations as well as flow slides Second: Internal erosion resulting from cracking in the embankment due to slope failures or Newmark-type displacement –with or without significant strength loss in the foundation

71. 71 Dam Overtopping Resulting from Large Deformations Probability of failure was estimated by using an event tree that included: type of ground motion model, probability of the earthquake loading, probability of widespread foundation strength loss, and probability that deformations would exceed freeboard Two event trees were considered: one for a subduction zone earthquake, and one for a local earthquake. The most severe loading condition, or the one that generated the highest risk, was used to represent the risks of this failure mode

72. Main Trunk of Event Tree 72

73. Branches for each Loading Increment 73

74. Ground Motion Model Weights Several methods were used to de-aggregate the ground motions – these included the “USBR method,” the CMS method, and a hybrid approach termed USBR-1 Seismotectonic group suggested a weighting of 33% to each model (all potentially viable) Our screening analyses indicated little differences between USBR and USBR-1 approaches, but did show noticeable differences between USBR and CMS models (CMS ground motions were smaller and resulted in somewhat smaller deformations) Team decided to weight the models as 60% for the USBR and 40% for the CMS 74

75. EQ Loading Increments Increments were chosen to bracket the 5 return periods developed in the ground motion study No failures assumed for EQ smaller than 300-yr event 75 Basic Return Period Loading IncrementProbability of Load Approximate Ground Motion Range < 300-yr99.667 %<.23g 500-yr300- to 800-yr0.208 %.23 to.42g 1,000-yr800- to 3,000-yr0.092 %.42 to.76g 5,000-yr3,000- to 8,000-yr0.021 %.76 to 1.05g 10,000-yr8,000- to 25,000-yr0.008 %1.05 to 1.42g 50,000-yr> 25,000-yr0.004 %> 1.42g

76. EQ Loading Example 76

77. Strength Scenarios Key question is how the EQ loading will affect the strength of the foundation soils Our analyses assumed a number of different strength scenarios including drained strengths, low/best/high peak undrained strengths, and low/best/high residual/remolded strengths For efficiency in event tree, 3 strength scenarios were developed –SS1 (lowest) – reasonable low to best estimate remolded undrained –SS2 (intermediate) – reasonable high remolded undrained to reasonable low peak undrained –SS3 (highest) – best estimate to reasonable high peak undrained 77

78. Strength Scenarios (continued) Since analyses did not show dramatic differences between the USBR and CMS ground motions, strength scenarios were assumed to be the same for each Large differences in the intensity of subduction versus local earthquakes justified different strength scenarios A key factor influencing the assignment of probability estimates to the strength scenarios is that the deformation analyses indicated appreciable deformations during the larger subduction zone earthquakes, even with drained strengths. The potentially large straining of the soils suggested to the team that remolded strengths would be expected under those conditions. 78

79. Team Estimate 79 Probability of Strength Loss Scenarios Load IncrementStrength Scenario Probability during Subduction EQ Probability during Local EQ 300-yr to 800-yr Low (SS1).01 Intermediate (SS2).09 High (SS3).90 800-yr to 3,000-yr Low.32.05 Intermediate.37.35 High.31.60 3,000-yr to 8,000-yr Low.65.15 Intermediate.27.45 High.08.40 8,000-yr to 25,000-yr Low.78.40 Intermediate.19.50 High.03.10 > 25,000-yr Low.94.60 Intermediate.06.35 High0.05

80. Predicted Deformations For each type of EQ, each ground motion model (for the subduction zone EQ), each loading increment, and each strength scenario, the team estimated the reasonable lower bound, best estimate, and reasonable upper bound of deformations that might be expected The estimates were based primarily upon the deformation analyses, which included both FLAC and two different Newmark approaches The estimated deformations were developed into probability functions 80

81. Predicted Deformations (CSZ EQ) 81 Expected Deformation (feet of Vertical Crest Loss) - Cascadia Subduction Zone Earthquake using USBR approach Load IncrementType of EstimateSS1SS2SS3 300-yr to 800-yr Upper Bound60*5*4* Best Estimate33*3*2* Lower Bound10*1* 800-yr to 3,000-yr Upper Bound703512 Best Estimate40205 Lower Bound1052 3,000-yr to 8,000-yr Upper Bound755035 Best Estimate453020 Lower Bound20105 8,000-yr to 25,000-yr Upper Bound756050 Best Estimate453525 Lower Bound20157 > 25,000-yr Upper Bound756560 Best Estimate454030 Lower Bound201512

82. Predicted Deformations (Local EQ) 82 Expected Deformation (feet of Vertical Crest Loss) - Local Earthquake (identical for USBR and CMS approaches) Load IncrementType of EstimateSS1SS2SS3 300-yr to 800-yr Upper Bound6054 Best Estimate3332 Lower Bound1011 800-yr to 3,000-yr Upper Bound6086 Best Estimate3343 Lower Bound1021 3,000-yr to 8,000-yr Upper Bound65158 Best Estimate3484 Lower Bound1542 8,000-yr to 25,000-yr Upper Bound652010 Best Estimate34105 Lower Bound1552 > 25,000-yr Upper Bound653015 Best Estimate34158 Lower Bound1583

83. Probability of Dam Failure During the Monte Carlo simulation, the probability of deformation was sampled 10,000 times, as was the probability of the reservoir elevation (taken from the reservoir exceedance curve) The result of each sampling was a value of “residual freeboard,” which is the difference between the amount of deformation and the amount of pre-existing freeboard The risk team then developed a fragility curve that estimated the likelihood of dam failure for given amounts of residual freeboard Factors considered in developing the curve included the erodibility of the embankment, the severity of damage and expected configuration of the remnant, the filter compatibility of embankment zones, and whether materials could sustain a crack 83

84. Probability of Dam Failure Two fragility curves were developed, to differentiate between very large and smaller amounts of deformation 84

85. Overtopping versus Internal Erosion Failures The fragility curves just shown account for either a sudden (overtopping-type) failure, or a failure resulting from internal erosion through the damaged embankment A sudden (overtopping-type) failure was assumed to result whenever the residual freeboard was less than 0.1 feet. When residual freeboard is greater than 0.1 feet, an internal erosion failure due to EQ-induced cracking was considered possible 85

86. Resulting Embankment Risks Dam overtopping failure mode –Annual probability of failure estimated at 6x10 -4 Internal erosion failure mode –Annual probability of failure estimated at 1x10 -4 Subduction zone earthquake controlled the risks 86

87. Key Factors Influencing Risk Numbers Very long duration and high peak accelerations associated with the Cascadia Subduction Zone earthquake Presence of silts and clays beneath dam that will likely experience some strength loss Large deformations predicted by different analysis techniques, even without strength loss Reservoir operations that result in a minimum freeboard of about 10 feet, and less than 20 feet approximately 50 percent of the time 87

88. Sensitivity Studies Key observation is that most of the risk comes from the smaller earthquakes – the 1,000- and 5,000-year events 88

89. Sensitivity Studies (continued) A number of sensitivity studies were conducted in which key variables were adjusted to determine the effect on the summary probability estimates Different scenarios included considering only USBR or only CMS ground motion models, looking solely at subduction or at local earthquakes, and evaluating risks for different strength assumptions, including only high strengths throughout For all these variations, the summary resulting annual probability of failure was always above 1x10 -4 89

90. Sensitivity Studies (continued) Some of the key sensitivity models and results 90 Results from Select Sensitivity Runs Sensitivity Condition Annual Probability of Failure Baseline Risk (no changes to team estimates)7.5x10 -4 Ground motions with only USBR method7.6x10 -4 Ground motions with only CMS method7.4x10 -4 Only local earthquake (no subduction zone)2.2x10 -4 Assuming peak undrained strengths during 1,000- and 5,000-year loading increments 4.0x10 -4 Note: Results are the total values, or the sum of both the overtopping and internal erosion failure modes

91. Summary of Risks 91

92. 92 Summary of Mean Embankment Seismic Risks FAILURE MODE ANNUAL PROBABILITY OF FAILURE Overtopping due to excessive deformations6x10 -4 Internal erosion due to EQ- induced cracking1x10 -4

93. All Risks Portrayed on f-N Plot 93

94. Risk Analysis Conclusions 94

95. 95 Risk Analysis Conclusions The estimated mean seismic risks from dam overtopping or internal erosion brought about by deformations resulting from earthquake shaking exceed Reclamation guidelines and provide justification to take risk reduction measures. The uncertainty with regard to these estimated risks does not suggest a need for additional studies.

96. 96 Key Considerations The seismic hazard at Scoggins Dam is among the most severe earthquake loadings within Reclamation’s inventory of dams. The principal seismic source of concern is the Cascadia Subduction Zone, which has the potential for very large earthquakes with very long durations of strong shaking, and with relatively frequent return periods. Foundation soils within the footprint of Scoggins Dam are comprised largely of low density and low strength silts and clays, which have the potential to lose strength during earthquake shaking.

97. 97 Key Considerations However, it is not necessarily the foundation strength loss that poses the greatest concern. Current, state-of- the-practice analyses indicate that even without any strength loss, large embankment deformations are predicted. This is most likely due to Newmark-type displacements resulting from amplification of the bedrock accelerations within the embankment and resulting frequent exceedance of the yield acceleration.

98. New SOD Recommendation 2010-SOD-A Initiate a Corrective Action Alternatives Study to evaluate potential alternatives to mitigate the high risks of seismic failure modes of the embankment and spillway at Scoggins Dam. 98

99. Corrective Action Study 99

100. 100 Input Parameters Loadings revised based on CRB input and recent events Material properties revised based on CRB input and additional data analysis PFMs remain the same Several alternatives considered

101. 101 Modification Alternatives Add downstream berm and shear key composed of sandstone or rhyolite/basalt rockfill Add downstream berm composed of rockfill with a soil cement shear key Secant piles near centerline and middle of downstream slope for additional strength New dam with concrete core and rockfill shells New dam with concrete core and sandstone shells Crest realignment with concrete core and rockfill shells Crest realignment with concrete core and sandstone shells

102. Preferred Alternative Add downstream berm and shear key composed of sandstone or rockfill

103. 103 Downstream Berm Significant modification to existing dam, with associated cost and schedule Deformation is still significant, but much smaller than for existing dam Risks are significantly reduced, and meet Reclamation guidelines

104. 104 Summary Loadings associated with Cascadia Subduction Zone present a risk to Scoggins Dam This is true even without strength loss in the foundation Risk is driven by relatively short return period loads, not “extreme” events Risk mitigation alternatives are large and complex No easy solution

105. Questions ? 105