Flooding Design Basis Reevaluation Challenges and Solutions

Transcription

1 Flooding Design Basis Reevaluation Challenges and Solutions Dave Schowalter, PhD Dan Gessler, PhD, PE, DWRE

2 Outline Introduction to Alden Flood Hazard Re-Evaluation Background Required Analysis Recommendations

3 Alden started as the hydraulics laboratory for Worcester Polytechnic Institute in Early projects focused on hydropower, but there were some other interesting applications

4 in the 21st Century, Alden is focused on fluid dynamics problems associated with the energy and environmental industries, and has been working on nuclear power plant applications since the 1960 s

5 Flood Hazard Re-Evaluation: Background March 11, 2011 Tohoku magnitude 9.0 earthquake June, 2011 Missouri River Flooding and Fort Calhoun March 12, 2012 NRC issued 50.54(f) letter The 50.54(f) transmittal defines a 10 step procedure (Attachment 1 to Recommendation 2.1: Flooding Enclosure 2) which "represent an acceptable approach to perform the reevaluation of the flood hazard and integrated assessment." This talk will focus on: Steps 1 through 6, which is what plants are doing now Details of Step 2

6 Required Analysis Step 1: Review information on current flooding hazard for which plant is designed. Design basis flood hazard Flood elevations and effects considered for all mechanisms Changes since licensing (drainage, watershed changes) New information (flood studies, PMP, dam operations) Flood protection mechanisms Pertinent features identified in the walkdown

7 Required Analysis Step 2: Reevaluate flood hazard based on present day regulatory guidance. "... the reevaluation apply present-day regulatory guidance and methodologies being used for Early Site Permits (ESP) and Combined license (COL) reviews..." NUREG/CR-7046 NUREG/CR-6966 (Tsunami hazard assessment) NUREG-0800 NUREG-0800 gives 10 flooding mechanisms to analyze

8 Required Analysis Step 3: Compare flood level for each causing mechanism to current design basis. Determine if current design basis flood bounds all reevaluated hazards If Yes: Proceed to Step 4 If No: Proceed to Step 6

9 Required Analysis Step 4: Submit report in accordance with Item 1 of 50.54(f) letter Step 5: Achieved when no further action is required Step 6: Submit a report in accordance with requested information item 1, Hazard Evaluation Report describing plans for further analysis. Step 7: Develop an Integrated Assessment showing whether existing protection mechanisms can mitigate the impacts of the re-evaluated hazard for the entire flood duration

10 10 Components of the Analysis (Step 2) Hydrologic Characterization Local Flooding Probable Maximum Flood Dam Failure Surge and Seiche Tsunami Ice Cooling Water Canals Channel Diversions Low Water Considerations

11 2.4.1 Hydrologic Characterization Develop a narrative characterization of the hydrosphere in which the plant exists. Characterize all nearby water features Characterize the source of cooling water Characterize how the plant affects the hydrosphere Characterization should include ground water Has likely not changed significantly since plant was permitted. Should require minimal effort.

12 2.4.2 Flooding Narrative of the historic flood events on all relevant water bodies. Local flooding due to Probable Maximum Precipitation Site drainage Historically and some COLA s use HEC-HMS and HEC-RAS Two dimensional runoff models may be better suited Precipitation data from NWS HMR Report 51 and 52 Required Modeling Data Topography Boundary conditions

13 HEC-HMS and HEC-RAS HEC-HMS (Hydrologic Modeling System) Designed to simulate the surface runoff, computing the streamflow hydrographs at desired locations in the river basin. HEC-RAS (River Analysis System) One-dimensional hydraulic model intended for calculating water surface profiles at cross-sections along a stream, for both steady and unsteady flow

14 2D runoff model Flooding Flow moves from cell to cell (elevation, slope, etc.) Accumulates in channels for routing. Consider as an alternative to HEC-HMS

15 2.4.3 PMF on Streams and Rivers Probable Maximum Flood (PMF) on rivers near the plant due to Probable Maximum Precipitation (PMP) in tributary basin. Typically involves HEC-HMS and HEC-RAS Reservoirs and Dams are handled separately Precipitation data from NWS HMR Report 51 and 52 Required Modeling Data Precipitation information Basin characteristics River and flood plain bathymetry HEC-HMS HEC-RAS 2D models

16 2.4.3 PMF on Streams and Rivers Important to consider debris and sediment Debris or sediment could overwhelm systems Could cause river diversion

17 2.4.4 Potential Dam Failures Two possible scenarios: Instantaneous and simultaneous failure of all upstream dams. Flood waves can be routed with HEC-RAS or other 1D model Domino failure starting at most upstream dam Storage in all upstream reservoirs is added to most downstream reservoir, and flood routed to plant Coincident with 2 year design wind and 500 year or ½ PMF per ANSI/ANS-2.8 Required Modeling Data River and flood plain bathymetry Impoundment characteristics HEC-RAS 2D models

18 2.4.4 Potential Dam Failures Consider debris, sedimentation, and scour, modes of failure Dam failure is also a source of debris

19 2.4.5 Surge and Seiche Flooding Flooding due to probable maximum hurricane (PMH) Most COLAs have been using SURGE by NOAA Met parameters from NOAA TR NWS 23 Required Modeling Data More sophisticated models exist Recent preference for ADCIRC Flooding due to probable maximum wind storm (PMWS) Use USACE Coastal Engineering Manual to compute run-up Flooding due to Seiche Atmospheric conditions that cause sloshing at the resonance frequency of the water body.

20 2.4.6 Tsunami Flooding Tsunami flood is a complex phenomena Identify tsunamigenic source mechanisms Atlantic and Gulf of Mexico Tsunami Hazard Assessment Group report Propagate the tsunami to near the coast Use 2D model such as MIKE21 to propagate the tsunami on shore accounting for local bathymetry

21 2.4.7 Ice Effects Consider how ice may affect any of the flooding predictions. Ice dams which fail Ice dams that cause a low water condition

22 2.4.8 Cooling Water Canals Some plants have cooling water canals that transfer water to the plant. Consider how the canal will be affected by: Flood Wind blow out Sedimentation Debris Cut off

23 2.4.9 Channel Diversions Channel diversion can occur at riverine plants. During a flood there is increased potential for a river to cutoff Plant becomes isolated from river. Very complex analysis, most plants won t require Analysis Geomorphic studies to understand historic meanders Sedimentation studies to understand erosion and lateral migration potential 1 and 2D modeling is likely required

24 2.4.9 Channel Diversions

25 Low Water Consideration Primarily consider downstream dam failures, causing loss of pond. Also consider Sedimentation that could obstruct intake Debris that could obstruct intake Requires multidimensional modeling Required Modeling Data 1 and 2D modeling may be required

26 Walkdowns With that described modeling effort in mind: What to look for during the walkdown: Hydrologic Characterization Local Flooding Probable Maximum Flood Dam Failure Surge and Seiche Tsunami Ice Cooling Water Canals Channel Diversions Low Water Considerations

27 Summary 10 analysis components cover potential scenarios Hydrologic Characterization Local Flooding Probable Maximum Flood Dam Failure Surge and Seiche Tsunami Ice Cooling Water Canals Channel Diversions Low Water Considerations Studies completed under 10 CFR Part 50 Appendix B Challenges you will face: Data Resources Schedule

28 Start soon Recommendations (Owners) 1 st year plants should be well underway 2 nd and 3 rd year plants are expect to have complex analysis Balance with finalization of guidance Obtain multiple proposals Clever engineering or modeling can affect cost Ensure the bidders have depth and expertise in all areas relevant to your plant. Tsunami experts are not the same as sedimentation experts Monitor schedules and progress Some plants may require field work that is best done during a 2 or 3 summer months per year.

29 Recommendations (Bidders) Assemble teams with true expertise in each area required. Schedules are tight for all plants, allocate sufficient resources. Monitor progress and schedules Anticipate field data needs some sites have very short field work windows.