The “Dam Safety Case” - Corps Risk Analysis Gateway
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Delivering Integrated, Sustainable, Water Resources Solutions Risk modeling “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Principles • What is probabilistic risk analysis (PRA)? – Structured approach to identifying failure modes and analyzing their effects – Accounting scheme for combining uncertainties – Approach to reasoning about uncertainties using the math of probability. • What it is not … –Inherently different from traditional practice “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Risk models Hazard Vulnerability Loss Inventory •Some of the hazards (perils) modeled: – – – – – – – – Earthquakes Floods Hurricanes Fire, wildfire, conflagration Tornados Tsunamis Landslides Extreme weather “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Exceedance Probability Annual exceedance probability (AEP) AEP = Annual probability of an event or consequence greater than a given level. p L “ Building Strong “ Loss Delivering Integrated, Sustainable, Water Resources Solutions History •The reason we use PRA is because we don’t have enough data. “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions History •The reason we use PRA is because we don’t have enough data. •The reason we use PRA is because people have (very) poor intuition about probability. “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Approaches to hazard modeling 1. Where most of the work is done for CAT models – – – – Seismic Hurricane Flood Fire Hazard Vulnerability Loss Inventory 2. Approach I. Spatial statistical model of occurrence II. Historical frequency model of severity III. Spatial model of effect at a particular site • (Note: there may by a difference between frequency of event and frequency of effect) “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Earthquake risk Source: Jones, K. 2003 Willis Limited “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Wind risk offset wind speed pressure the USAID Office of Foreign Disaster Assistance and the Caribbean Regional Program “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Wildfire outbreak risk Virginia Department of Forestry “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Approaches to inventory modeling • Geospatial databases (GIS) • Biggest advance in recent years; changing quickly • Mostly publicly available records (e.g., HAZUS) – Impacted by DHS concerns • Many web-based (free or cheap) tools Hazard Vulnerability Inventory “ Building Strong “ Loss Delivering Integrated, Sustainable, Water Resources Solutions Approaches to vulnerability (reliability) modeling • • • • Failure modes and effects analysis (FMEA) Event tree analysis Fault tree analysis Stochastic simulation (Monte Carlo) Hazard Vulnerability Inventory “ Building Strong “ Loss Delivering Integrated, Sustainable, Water Resources Solutions Principles •Levels of probabilistic reasoning Ideal: theoretical understanding with many data Model Completeness Purely theoretical, with few or no data Little understanding, few or no data Theoretical Assessment Model-based Assessment Guess Statistical Assessment Data Completeness “ Building Strong “ Little understanding, but many data Delivering Integrated, Sustainable, Water Resources Solutions FMEA •Failure Modes and Effects Analysis “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions FMEA “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions FMEA “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions FMEA “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Event trees upstream shell rip rap core filter downstream shell filter Simple representation of earth dam system “ Building Strong “ rip rap Delivering Integrated, Sustainable, Water Resources Solutions Dam Safety •Reasons for Dam Failures – OVERTOPPING ~ 35% of all failures • Inadequate Spillway Design • Debris Blockage of Spillway • Settlement of Dam Crest – FOUNDATION DEFECTS ~ 30% of all failures • • • • Differential Settlement Sliding and Slope Instability High Uplift Pressures Uncontrolled Foundation Seepage “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Dam Safety • Reason for Dam Failures – PIPING AND SEEPAGE ~ 20% of all failures • Internal Erosion Through Dam Caused by Seepage"Piping" • Seepage and Erosion Along Hydraulic Structures Such as Outlet • Conduits or Spillways, or Leakage Through Animal Burrows • Cracks in Dam – CONDUITS AND VALVES ~ 10% of all failures • Piping of Embankment Material Into Conduit Through Joints or Cracks – OTHER ~ 5% of all failures “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Event trees Generic event tree (US NRC 1975). Initiating Event S ys tem 1 S uc c es s S tate (S 1) Initiating Event (I) Failure S tate (F 1) S ys tem 2 A cc ident S equenc es S uc c es s S tate (S 2) (IS 1S 2) Failure S tate (F 2) (IS 1F 2) S uc c es s S tate (S 2) (IF 1S 2) Failure S tate (F 2) (IF 1F 2) “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Event trees Simple event tree for an embankment dam Initiating Event Foundation liquefaction None (S 1 ) Earthquake (I) Liquefiable soil (F 1 ) Embankment cracking Accident Sequences No cracking (S 2 ) (IS 1 S 2 ) Cracking (F 2 ) (IS 1 F 2 ) No cracking (S 2 ) (IF 1 S 2 ) Cracking (F 2 ) (IF 1 F 2 ) “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Event Trees Initiating Event Foundation liquefaction Embankment cracking No cracking 0.95 None Pr=0.9 Earthquake Pr=0.01 Liquefied soil Pr=0.1 Accident Sequence Earthquake - no liquefied soil - no cracking Pr = 0.01 x 0.9 x 0.95 = 8.55 E -03 Cracking 0.05 Earthquake - no liquefied soil - cracking Pr = 0.01 x 0.9 x 0.05 = 0.45 E -03 No cracking 0.67 Earthquake - liquefied soil - no cracking Pr = 0.01 x 0.1 x 0.67 = 0.67 E -03 Cracking 0.33 Earthquake - liquefied soil - cracking Pr = 0.01 x 0.1 x 0.33 = 0.33 E -03 Dependence of Probabilities on earlier events “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Event trees Annualized initiating ev ent Probability of failure Annualized initiating ev ent Ev ent Tree Annualized initiating event generates input Fragility curv e Load “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Event Trees 5 6 die 2 3 4 Sample space Event trees = sample space 1 2 Simple event die 1 1 2 3 4 5 6 earthquake magnitude > m soil liquefies earthquake of magnitude > m soil liquefies soil does not liquefy earthquake magnitude < m “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Event Trees Initiating Event Success State (S1) Leaf States of Nature Logic Tree System 1 Initiating Event (I) Failure State (F1) System 2 Success State (S2) (IS 1S2) Failure State (F2) (IS 1F2) Success State (S2) (IF1S2) Failure State (F2) (IF1F2) Event Tree “ Building Strong “ Accident Sequences Delivering Integrated, Sustainable, Water Resources Solutions Event Trees Event Tree Consequence Tree Logic Tree States of Nature Sys tem Events Cons equenc es “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Event Trees W ater level above c res t Overtopping E xtrem e s torm < 50% of c res t Failure P iping oc c urs S tringers exis t Duration >week No Failure No piping oc c urs S tringers do not exist 50%-100% of c res t Duration <week High pore pres s ure S oft s oil fill Duration >week Geotec hnic al Failure No Failure Low pore pres s ure Good s oil fill 50%-100% of c res t Duration <week Hydrologic, piping, and strength failure parts of the levee failure event trees “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Influence diagrams river stage (water height) Potential failure surface Lev ee Floodway sand boil flow path through lens possible sand lens “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Event Trees Ext reme R ainfall Piping Flood Duration T Influence diagram for levee failure Sand Stringer s Exist Loss of Containment Pore Pr essur e Peak Discharge Q Weak Soil Fill Static Strength Failure River Stage H Overtopping Levee Floodway sand boil “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Event Trees Extreme Rainfall Piping Flood Duration T Influence diagram for levee failure, including active decision node Sand Stringer s Exist Loss of Containment Pore Pr essur e Peak Discharge Q Weak Soil Fill River Stage H Static Strength Failure Overtopping Release Water Levee Floodway no failure sand boil “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Event Trees debris 0m dam overtopping 0-1m 1-2m stilling basin undermining scour under sheet piles dam breach none stilling basin collapse intact dam breach none slab uplift dam breach none no scour median inflow excess drain inflow spillway uplift Partial event tree for hydrologic failure at Alouette Dam (Salmon 1995) none no excess debris plugging creek creek scourng bank erosion splwy clps none none none sillway wall overtopping degris plugging spillway channel none overtoping, erodes slab none overtoping erodes slab none dam breach none dam breach none “ Building Strong “ dam breach none Delivering Integrated, Sustainable, Water Resources Solutions Dam Safety •Different approaches of the various owners and regulators –Bureau of Reclamation –US Army Corps of Engineers –New South Wales (Austl) –Canadian Electricity Association • Dam Safety Interest Group – BCHydro, OPG, HydroQuebec, Vattenfall, Scottish & Southern Energy • Ontario Ministry of Natural Resources & OPG –Department of Environment and Rural Affairs (DEFRAUK) “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Glenmaggie Dam “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Fault tree analysis TE Top Ev ent AND S1 Sub-sy stem S1 Sub-sy stem OR OR T1 Sub-sy stem A T2 Sub-sy stem C AND OR B C A “ Building Strong “ B Delivering Integrated, Sustainable, Water Resources Solutions Fault tree analysis “ Building Strong “ Delivering Integrated, Sustainable, Water Resources Solutions Fault trees Dam Breached AND Overtopping Factor Exceeded Dam Overtopped OR Water Levek too High OR Design Flood Exceeded Crest Settles Below Water Level Crest Eroded Below Water Level AND Spillway Fails Crest Eroded AND No A ction Taken Crest Settles OR OR Gates Closed Spillway Blocked OR Operator Error Equipment Malfunction No A ction Taken Foundation Settles Foundation Erodes OR Blocked by Food Debris Fill Consolidates OR Blocked Prior to Flood Foundation Erodes Fault tree of dam failure (Fry 2001) “ Building Strong “ Filter Inadequate Foundation Cracks Fill Erodes Delivering Integrated, Sustainable, Water Resources Solutions Approaches to consequence modeling • • • • Economics (NED) Loss of life/property Environmental Historical Hazard Vulnerability Inventory “ Building Strong “ Loss
