ML16102A330

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Presentation Slides for 04042016 Public Meeting on Resolution of TVA Flood Model Issue
ML16102A330
Person / Time
Site: Watts Bar, Sequoyah  Tennessee Valley Authority icon.png
Issue date: 04/04/2016
From: Wetzel B
Tennessee Valley Authority
To:
Office of Nuclear Reactor Regulation
References
Download: ML16102A330 (28)
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Text

Resolution of Flood Model Issue April 4, 2016

Agenda

  • Purpose
  • Background
  • Flood Model Issue
  • Path Forward/Resolution
  • Scope of Work
  • Schedule FLOOD MODEL ISSUE RESOLUTION l 2

Purpose The purpose of this meeting is to discuss:

  • Technical details of the recently discovered external flood model issue using the HEC-RAS code
  • Impacts to TVA nuclear sites current licensing basis
  • Impacts to NTTF Rec 2.1 Flood Hazard Reevaluation Reports
  • Resolution of Issue FLOOD MODEL ISSUE RESOLUTION l 3

Watershed Model FLOOD MODEL ISSUE RESOLUTION l 4

=

Background===

  • Watts Bar/Sequoyah LAR submittal -

July/August 2012

  • Watts Bar LAR supplement submittal -

September 30, 2014 HEC-RAS and updated dam stability criteria adopted for PMF NRC issues Safety Evaluation Report - January 28, 2015 FLOOD MODEL ISSUE RESOLUTION l 5

=

Background===

  • NTTF Recommendation 2.1 Flood Hazard Reevaluation Reports (FHRRs) submitted for Browns Ferry, Sequoyah and Watts Bar

- March 12, 2015 NRC issues Interim Staff Response to Reevaluated Flood Hazards for Browns Ferry, Sequoyah and Watts Bar - September 3, 2015 NRC issues Staff Assessment of Response to Reevaluated Flood Hazards for Watts Bar -

December 1, 2015 FLOOD MODEL ISSUE RESOLUTION l 6

=

Background===

  • Model Issue Identified - November 6, 2015 Condition Report 1101784
  • Model Issue Confirmed - December 16, 2015 Condition Reports 1116461 and 1116506 FLOOD MODEL ISSUE RESOLUTION l 7

TVA Current Hydraulic Model FLOOD MODEL ISSUE RESOLUTION l 8 Traceability of inputs from the SOCH model influenced the decision to maintain historical cross section locations in TVAs current hydraulic model.

TVA manually augmented ineffective flow areas to account for reach storage between cross sections to match published reservoir storage curves.

Volume Computation Differences FLOOD MODEL ISSUE RESOLUTION l 9 TVA Augmented Ineffective Flow Areas

& HEC-RAS Post Processor calculates volume for left and right overbanks separately.

HEC-RAS internal computations calculates volume using a simple arithmetic average of the right and left reach lengths.

Volume differences are compounded as left and right overbank length variance is increased.

Flood Model Issue Difference in volume computations between HEC-RAS internal processor

> computes a simple arithmetic average of left and right overbank lengths HEC-RAS post processor

> treats each overbank (and overbank length) separately providing a prismatic representation of the volume of each overbank Differences exacerbated by TVA approach in accounting for actual reservoir storage by augmenting ineffective flow areas at Cherokee and Douglas where there are significant differences in right and left bank lengths Result: TVAs HEC-RAS model overestimates the amount of reservoir storage volume available at Cherokee and Douglas FLOOD MODEL ISSUE RESOLUTION l 10

HEC-RAS Bug Report After discovering differences in volume checks, TVA submitted a HEC-RAS code Bug Report to USACE Hydrologic Engineering Center USACE confirmed HEC-RAS internal and post processors calculate volume differently. However, in typical applications of the HEC-RAS software, this is not expected to significantly change the results USACE concludes the issue is not a bug, but a misunderstanding of the HEC-RAS internal computations USACE expert agreed to revise the users manual to provide clarification on the volume equation and is considering a HEC-RAS code revision sometime in the future that would add an alternative solver algorithm which aligns the internal to the post processor USACE expert recognized adjustment of augmented ineffective flow areas as a viable approach FLOOD MODEL ISSUE RESOLUTION l 11

Cherokee Example - Worst Case FLOOD MODEL ISSUE RESOLUTION l 12 Due to TVAs placement of augmented ineffective flow areas on one side of the channel, the HEC-RAS internal averaging of storage volumes in the overbank areas and the biased sinuosity in these reservoirs, TVAs hydraulic model issue has the most pronounced impacts at Cherokee and Douglas.

Reservoir Extent of Condition Review FLOOD MODEL ISSUE RESOLUTION l 13 Extent of condition review performed for all reservoirs by comparing internal computed to published reservoir storage curves.

All produced less than 2%

difference, except Cherokee and Douglas.

Impacts

  • Watts Bar Current Licensing Basis
  • No Impact to Sequoyah and Browns Ferry Current Licensing Basis (SOCH code based)
  • Sequoyah HEC-RAS code based LAR Supplement
  • Watts Bar, Sequoyah and Browns Ferry FHRRs FLOOD MODEL ISSUE RESOLUTION l 14

Technical Approach for Resolution

  • Updated Precipitation Data Similar methods as HMR with an update to data set and orographic effects along with use of current technology
  • Revise Hydraulic Model For the most impacted reservoirs (Cherokee and Douglas) use a more standardized modeling approach, i.e. additional cross sections with storage nodes for large embayments For the remaining reservoirs, adjust the augmented ineffective flow areas using the volume averaging equations consistent with HEC-RAS internal processor FLOOD MODEL ISSUE RESOLUTION l 15

Hydraulic Model Revision -

Cherokee/Douglas FLOOD MODEL ISSUE RESOLUTION l 16 Add additional cross sections between existing cross-sections and use storage nodes for large embayments Provides a more standardized modeling approach utilizing current industry practices More realistically represents geography of storage volume in the reservoir Storagenodesused forlargeembayment

Hydraulic Model Revision -

Other Areas FLOOD MODEL ISSUE RESOLUTION l 17

  • Adjust the augmented ineffective flow areas using the volume averaging equations from HEC-RAS internal processor
  • Apply to entire model with the exception of Cherokee and Douglas reservoirs

Issue Resolution TVA has confidence in the model issue resolution

  • Alternate method produces comparable results
  • Calibration shows stage and discharge match well to historic storms
  • USACE expert recognized adjustment of augmented ineffective flow areas as a viable approach
  • Additional external reviews will be utilized FLOOD MODEL ISSUE RESOLUTION l 18

Issue Resolution

  • Precipitation update Preliminary studies show margin between current HMR 41 and updated data
  • TVA has high confidence that:

Model issue is corrected with planned resolution Final PMF simulations with use of the updated precipitation and update to the hydraulic model will produce elevations less than those shown in current FHRRs FLOOD MODEL ISSUE RESOLUTION l 19

Scope of Work

  • Mitigating Strategies Assessment TVA plans to use March 12, 2015 FHRR flood model and results to evaluate mitigation strategies

> Elevation and flow results are bounding with use of the conservative precipitation data in HMR 41

> NRC interim staff review has been completed for Watts Bar, Sequoyah and Browns Ferry with no open issues MSAs will be completed by December 2016 FLOOD MODEL ISSUE RESOLUTION l 20

Scope of Work

  • New report/calculations Precipitation data to replace HMR-41 PMP development to determine most critical inflows
  • Calculation revisions Main Stem and Tributary Geometry

> Cherokee and Douglas hydraulic model geometry revised for additional cross-sections with storage nodes

> Adjust augmented ineffective flow areas for the balance of the model using HEC-RAS internal processor equations Main Stem and Tributary Calibrations FLOOD MODEL ISSUE RESOLUTION l 21

Scope of Work, continued

  • Calculation revisions, continued Model Setup Probable Maximum Flood Simulations

> Significant increase in number of events to review to determine controlling simulation Warning Time Controlling FHRR Seismic Combination Simulation Uncertainties FLOOD MODEL ISSUE RESOLUTION l 22

Scope of Work, continued FLOOD MODEL ISSUE RESOLUTION l 23 Totalest.>25calculations InputDataDevelopment (est.4calculations)

SeasonalityAnalysis AntecedentSoilMoisture(seasonallyadjusted)

TemporalDistribution(early/median/late)

PMPRainfall(est.>60events)

Inflow&PMFDevelopment (est.>20calculations)

ApplyProjectCandidateCriticalBasin(s)Rainfall ApplyCandidateTemporalDistributions ApplyAntecedentEvent CalculateSurfaceRunoff RoutetoModelLimits DistributeInflows RunHECRAS(est.>100eventsruns)

SummarizeResultsforSite(s)WorstCaseWSE WarningTime (est.2calculations)

ReviewallPreviousRunsforWarningTime DevelopAnyAdditionalLocalInflows RunHECRAS SummarizeResultsforSite(s)WorstCaseWT Uncertainties (est.1calculations)

DevelopandApplyNonlinearUH AdjustmenttoControllingEvent(s)

DevelopGateOperabilityRuns (est.6HECRASruns)

Scope of Work, continued

  • Deliverables:

License Amendment Request

> Sequoyah

> Watts Bar FHRR report revisions Flooding Integrated Assessment FLOOD MODEL ISSUE RESOLUTION l 24

Additional Licensing Basis Scope

  • Update FSAR Section 2.4.4 Potential Dam Failures (Seismically Induced)

Simulations in HEC-RAS vs. SOCH Use JLD-ISG-2013-01 guidance

> Update seismic dam stability evaluations to current methods

> Use dam specific probabilistic earthquake vs. nuclear site deterministic earthquake

>> 10-4 annual exceedance seismic hazard in place of SSE

>> Half the 10-4 ground motion in place of OBE

> Use deaggregation to evaluate the potential for multiple dam failures FLOOD MODEL ISSUE RESOLUTION l 25

Additional Licensing Basis Scope

  • Update FSAR Section 2.4.11 Low Water Considerations Simulations in HEC-RAS vs. SOCH
  • With this additional change to the licensing basis, legacy SOCH code use will be eliminated for Sequoyah and Watts Bar.

FLOOD MODEL ISSUE RESOLUTION l 26

Draft Schedule FLOOD MODEL ISSUE RESOLUTION l 27

Questions FLOOD MODEL ISSUE RESOLUTION l 28

Resolution of Flood Model Issue April 4, 2016

Agenda

  • Purpose
  • Background
  • Flood Model Issue
  • Path Forward/Resolution
  • Scope of Work
  • Schedule FLOOD MODEL ISSUE RESOLUTION l 2

Purpose The purpose of this meeting is to discuss:

  • Technical details of the recently discovered external flood model issue using the HEC-RAS code
  • Impacts to TVA nuclear sites current licensing basis
  • Impacts to NTTF Rec 2.1 Flood Hazard Reevaluation Reports
  • Resolution of Issue FLOOD MODEL ISSUE RESOLUTION l 3

Watershed Model FLOOD MODEL ISSUE RESOLUTION l 4

=

Background===

  • Watts Bar/Sequoyah LAR submittal -

July/August 2012

  • Watts Bar LAR supplement submittal -

September 30, 2014 HEC-RAS and updated dam stability criteria adopted for PMF NRC issues Safety Evaluation Report - January 28, 2015 FLOOD MODEL ISSUE RESOLUTION l 5

=

Background===

  • NTTF Recommendation 2.1 Flood Hazard Reevaluation Reports (FHRRs) submitted for Browns Ferry, Sequoyah and Watts Bar

- March 12, 2015 NRC issues Interim Staff Response to Reevaluated Flood Hazards for Browns Ferry, Sequoyah and Watts Bar - September 3, 2015 NRC issues Staff Assessment of Response to Reevaluated Flood Hazards for Watts Bar -

December 1, 2015 FLOOD MODEL ISSUE RESOLUTION l 6

=

Background===

  • Model Issue Identified - November 6, 2015 Condition Report 1101784
  • Model Issue Confirmed - December 16, 2015 Condition Reports 1116461 and 1116506 FLOOD MODEL ISSUE RESOLUTION l 7

TVA Current Hydraulic Model FLOOD MODEL ISSUE RESOLUTION l 8 Traceability of inputs from the SOCH model influenced the decision to maintain historical cross section locations in TVAs current hydraulic model.

TVA manually augmented ineffective flow areas to account for reach storage between cross sections to match published reservoir storage curves.

Volume Computation Differences FLOOD MODEL ISSUE RESOLUTION l 9 TVA Augmented Ineffective Flow Areas

& HEC-RAS Post Processor calculates volume for left and right overbanks separately.

HEC-RAS internal computations calculates volume using a simple arithmetic average of the right and left reach lengths.

Volume differences are compounded as left and right overbank length variance is increased.

Flood Model Issue Difference in volume computations between HEC-RAS internal processor

> computes a simple arithmetic average of left and right overbank lengths HEC-RAS post processor

> treats each overbank (and overbank length) separately providing a prismatic representation of the volume of each overbank Differences exacerbated by TVA approach in accounting for actual reservoir storage by augmenting ineffective flow areas at Cherokee and Douglas where there are significant differences in right and left bank lengths Result: TVAs HEC-RAS model overestimates the amount of reservoir storage volume available at Cherokee and Douglas FLOOD MODEL ISSUE RESOLUTION l 10

HEC-RAS Bug Report After discovering differences in volume checks, TVA submitted a HEC-RAS code Bug Report to USACE Hydrologic Engineering Center USACE confirmed HEC-RAS internal and post processors calculate volume differently. However, in typical applications of the HEC-RAS software, this is not expected to significantly change the results USACE concludes the issue is not a bug, but a misunderstanding of the HEC-RAS internal computations USACE expert agreed to revise the users manual to provide clarification on the volume equation and is considering a HEC-RAS code revision sometime in the future that would add an alternative solver algorithm which aligns the internal to the post processor USACE expert recognized adjustment of augmented ineffective flow areas as a viable approach FLOOD MODEL ISSUE RESOLUTION l 11

Cherokee Example - Worst Case FLOOD MODEL ISSUE RESOLUTION l 12 Due to TVAs placement of augmented ineffective flow areas on one side of the channel, the HEC-RAS internal averaging of storage volumes in the overbank areas and the biased sinuosity in these reservoirs, TVAs hydraulic model issue has the most pronounced impacts at Cherokee and Douglas.

Reservoir Extent of Condition Review FLOOD MODEL ISSUE RESOLUTION l 13 Extent of condition review performed for all reservoirs by comparing internal computed to published reservoir storage curves.

All produced less than 2%

difference, except Cherokee and Douglas.

Impacts

  • Watts Bar Current Licensing Basis
  • No Impact to Sequoyah and Browns Ferry Current Licensing Basis (SOCH code based)
  • Sequoyah HEC-RAS code based LAR Supplement
  • Watts Bar, Sequoyah and Browns Ferry FHRRs FLOOD MODEL ISSUE RESOLUTION l 14

Technical Approach for Resolution

  • Updated Precipitation Data Similar methods as HMR with an update to data set and orographic effects along with use of current technology
  • Revise Hydraulic Model For the most impacted reservoirs (Cherokee and Douglas) use a more standardized modeling approach, i.e. additional cross sections with storage nodes for large embayments For the remaining reservoirs, adjust the augmented ineffective flow areas using the volume averaging equations consistent with HEC-RAS internal processor FLOOD MODEL ISSUE RESOLUTION l 15

Hydraulic Model Revision -

Cherokee/Douglas FLOOD MODEL ISSUE RESOLUTION l 16 Add additional cross sections between existing cross-sections and use storage nodes for large embayments Provides a more standardized modeling approach utilizing current industry practices More realistically represents geography of storage volume in the reservoir Storagenodesused forlargeembayment

Hydraulic Model Revision -

Other Areas FLOOD MODEL ISSUE RESOLUTION l 17

  • Adjust the augmented ineffective flow areas using the volume averaging equations from HEC-RAS internal processor
  • Apply to entire model with the exception of Cherokee and Douglas reservoirs

Issue Resolution TVA has confidence in the model issue resolution

  • Alternate method produces comparable results
  • Calibration shows stage and discharge match well to historic storms
  • USACE expert recognized adjustment of augmented ineffective flow areas as a viable approach
  • Additional external reviews will be utilized FLOOD MODEL ISSUE RESOLUTION l 18

Issue Resolution

  • Precipitation update Preliminary studies show margin between current HMR 41 and updated data
  • TVA has high confidence that:

Model issue is corrected with planned resolution Final PMF simulations with use of the updated precipitation and update to the hydraulic model will produce elevations less than those shown in current FHRRs FLOOD MODEL ISSUE RESOLUTION l 19

Scope of Work

  • Mitigating Strategies Assessment TVA plans to use March 12, 2015 FHRR flood model and results to evaluate mitigation strategies

> Elevation and flow results are bounding with use of the conservative precipitation data in HMR 41

> NRC interim staff review has been completed for Watts Bar, Sequoyah and Browns Ferry with no open issues MSAs will be completed by December 2016 FLOOD MODEL ISSUE RESOLUTION l 20

Scope of Work

  • New report/calculations Precipitation data to replace HMR-41 PMP development to determine most critical inflows
  • Calculation revisions Main Stem and Tributary Geometry

> Cherokee and Douglas hydraulic model geometry revised for additional cross-sections with storage nodes

> Adjust augmented ineffective flow areas for the balance of the model using HEC-RAS internal processor equations Main Stem and Tributary Calibrations FLOOD MODEL ISSUE RESOLUTION l 21

Scope of Work, continued

  • Calculation revisions, continued Model Setup Probable Maximum Flood Simulations

> Significant increase in number of events to review to determine controlling simulation Warning Time Controlling FHRR Seismic Combination Simulation Uncertainties FLOOD MODEL ISSUE RESOLUTION l 22

Scope of Work, continued FLOOD MODEL ISSUE RESOLUTION l 23 Totalest.>25calculations InputDataDevelopment (est.4calculations)

SeasonalityAnalysis AntecedentSoilMoisture(seasonallyadjusted)

TemporalDistribution(early/median/late)

PMPRainfall(est.>60events)

Inflow&PMFDevelopment (est.>20calculations)

ApplyProjectCandidateCriticalBasin(s)Rainfall ApplyCandidateTemporalDistributions ApplyAntecedentEvent CalculateSurfaceRunoff RoutetoModelLimits DistributeInflows RunHECRAS(est.>100eventsruns)

SummarizeResultsforSite(s)WorstCaseWSE WarningTime (est.2calculations)

ReviewallPreviousRunsforWarningTime DevelopAnyAdditionalLocalInflows RunHECRAS SummarizeResultsforSite(s)WorstCaseWT Uncertainties (est.1calculations)

DevelopandApplyNonlinearUH AdjustmenttoControllingEvent(s)

DevelopGateOperabilityRuns (est.6HECRASruns)

Scope of Work, continued

  • Deliverables:

License Amendment Request

> Sequoyah

> Watts Bar FHRR report revisions Flooding Integrated Assessment FLOOD MODEL ISSUE RESOLUTION l 24

Additional Licensing Basis Scope

  • Update FSAR Section 2.4.4 Potential Dam Failures (Seismically Induced)

Simulations in HEC-RAS vs. SOCH Use JLD-ISG-2013-01 guidance

> Update seismic dam stability evaluations to current methods

> Use dam specific probabilistic earthquake vs. nuclear site deterministic earthquake

>> 10-4 annual exceedance seismic hazard in place of SSE

>> Half the 10-4 ground motion in place of OBE

> Use deaggregation to evaluate the potential for multiple dam failures FLOOD MODEL ISSUE RESOLUTION l 25

Additional Licensing Basis Scope

  • Update FSAR Section 2.4.11 Low Water Considerations Simulations in HEC-RAS vs. SOCH
  • With this additional change to the licensing basis, legacy SOCH code use will be eliminated for Sequoyah and Watts Bar.

FLOOD MODEL ISSUE RESOLUTION l 26

Draft Schedule FLOOD MODEL ISSUE RESOLUTION l 27

Questions FLOOD MODEL ISSUE RESOLUTION l 28

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