Fragility Curves 1. What is a fragility curve (and how do
Transcription
Fragility Curves 1. What is a fragility curve (and how do
Fragility Curves Jonathan Simm Flood Management Group, HR Wallingford Practitioner workshop on asset management Questions/topics addressed in presentation 1. What is a fragility curve (and how do fragility curves relate to design thinking)? 2. Why are fragility curves important in FRM? 3. Generalised fragility curves (with ref. to asset types and condition grades) 4. Challenging accuracy of generic curves. 5. Generating bespoke fragility curves 6. Conclusions Page 2 1 1. What is a fragility curve? Answer: A curve which expresses the probability of failure of a defence as a function of the loading Page 3 Estimating fragility curves (Simm et al, 2008) Standard of protection provided by defence Probability of defence failure Difference relates to factor of safety in design 100% 1.0 Typically assumed fragility curve ‘True’ fragility 50% 5% to 10% 0 0 Severity of load event Page 4 2 Establishing fragility different from design Strength Rrep R Load S S rep partial load factors Statistical variability reflected by setting design values a certain number of standard deviations away from the mean: R k rep R R R S rep S k S S To generate fragility curves we must change approach and use the mean values. (Otherwise, for example, we would not get pf = 50% when FS=1) Page 5 Fragility curve - uncertainty Page 6 3 Better data/models reduce uncertainty Increasing certainty in defence performance Pf load Pf load Increased defence data and analysis Pf load Page 7 Questions/topics addressed in presentation 1. What is a fragility curve (and how do fragility curves relate to design thinking)? 2. Why are fragility curves important in FRM? 3. Generalised fragility curves (with ref. to asset types and condition grades) 4. Challenging accuracy of generic curves. 5. Generating bespoke fragility curves 6. Conclusions Page 8 4 Fragility curves in flood system analysis Pathway (e.g. beach, raised/non-raised Receptor defence and floodplain) (e.g. people and property) Source (River or sea) Page 9 Fragility curves importance in FRM • In flood systems analysis • In determining flood risk attributable to individual defences • In providing a rigorous discipline for understanding our flood defences Page 10 5 Questions/topics addressed in presentation 1. What is a fragility curve (and how do fragility curves relate to design thinking)? 2. Why are fragility curves important in FRM? 3. Generalised fragility curves (with ref. to asset types and condition grades) 4. Challenging accuracy of generic curves. 5. Generating bespoke fragility curves 6. Conclusions Page 11 Asset typology Asset typology: Flood defences Fluvial Type 1 Vertical wall Type 2 Slope or embankment Type 3 High ground Coastal Type 4 Culverts Type 5 Vertical seawall Type 6 Sloping seawall / dyke Type 7 Beach Within this framework there are 61 asset types. For each type, a generic fragility curves is available for each of 5 condition grades. Page 12 6 Example generic fragility curves (cf. presentation on condition assessment) HLM+ Fragility curve - Condition Grades 1 to 5 (Central estimates) 1.0 P (breaching¦feeboard) 0.9 0.8 0.7 Condition grade1 Condition grade2 Condition grade 3 Condition grade4 Condition grade 5 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Overflow head (Water level - crest level) WL < CL WL > CL Page 13 Example generic fragility curves (cf. presentation on deterioration) HLM+ Fragility curve - Condition Grades 1 to 5 (Central estimates) 1.0 P (breaching¦feeboard) 0.9 0.8 0.7 Condition grade1 Condition grade2 Condition grade 3 Condition grade4 Condition grade 5 0.6 0.5 Structure 0.4 Deterioration0.3 0.2 0.1 0.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Overflow head (Water level - crest level) WL < CL WL > CL Page 14 7 Questions/topics addressed in presentation 1. What is a fragility curve (and how do fragility curves relate to design thinking)? 2. Why are fragility curves important in FRM? 3. Generalised fragility curves (with ref. to asset types and condition grades) 4. Challenging accuracy of generic curves. 5. Generating bespoke fragility curves 6. Conclusions Page 15 New science to improve fragility curves • FloodProBE workpackage 3 addressing: − Internal erosion (piping etc) − External erosion of grass cover (riverine) - Asset Performance Tools − Extension of RELIABLE fragility curve generation tool to cover most asset types and most failure modes − Overtopping research to look at coastal grass cover erosion − Guidance for practitioners Page 16 8 Questions/topics addressed in presentation 1. What is a fragility curve (and how do fragility curves relate to design thinking)? 2. Why are fragility curves important in FRM? 3. Generalised fragility curves (with ref. to asset types and condition grades) 4. Challenging accuracy of generic curves. 5. Generating bespoke fragility curves 6. Conclusions Page 17 How to create a specific fragility curve? BASIC STEPS: 1. Identify and analyse all relevant failure modes (key issue emerging in production of International Levee Handbook) 2. Identify Limit State Equations (LSE) or models for all failure modes and recast them in reliability format: Z (reliability) = R (strength) – S (loading) 3. Prepare schedule of engineering parameters and their uncertainties. 4. Prepare fault trees that specify the logical sequence of all possible mechanisms leading to defence failure. 5. Perform many reliability analyses, for a single hydraulic loading across range of parameter uncertainties (MonteCarlo sampling). For each loading analysed, probability of failure is proportion of times that Z < 1 . Repeat for other hydraulic loadings and draw the resulting fragility curve. Page 18 9 Variation in Ave Factor of Safety with River Level. TE2100 embankment example (SLOPE-W) Average Factor of Safety 2.9 2.4 1.9 1.4 0.9 0.4 Variation in Probability of Failure with River Level 3.0 4.0 5.0 Probability of Failure (%) 90.0 80.0 6.0 7.0 8.0 9.0 10.0 River Level (mOD) 100.0 With Surcharge Without Surcharge Existing Crest Level Critical PWP under Virgin Marsh Design Water Level Deterministic With Surcharge Critical PWP under Berm Deterministic Without Surcharge 70.0 60.0 50% failure probability for FOS = 1.0 50.0 40.0 30.0 20.0 10% failure probability at design extreme event 10.0 0.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 River Level (mOD) Page 19 With Surcharge Without Surcharge Existing Crest Level Critical PWP under Virgin Marsh Design Water Level Deterministic Analysis Critical PWP under Berm Reliability analysis – multiple failure modes Can combine using De Morgan’s Law Pr( f ) 1 {[1 Pr( f r )] [1 Pr( f s )]} if failure modes are independent Example: embankment High water breach If not, better to use RELIABLE tool generated under FLOODsite and UK FRMRC projects Erosion of embankment surface by overflow (Ba 1.1) SLOPEW Page 20 10 Combined fragility curve High water breach EX8 All condition Grades 1.2 Erosion of embankment surface by overflow (Ba 1.1) CG1 CG2 CG3 CG4 CG5 1 SLOPEW P(failure¦freeboard) 0.8 0.6 0.4 0.2 0 -1.5 -1 -0.5 0 0.5 1 Freeboard (m) Page 21 Intermediate way to generate fragility curves • Thames estuary (TE2100) studies adopted this approach rather than using generic curves: − Higher than national average defences − Composite structural defence forms − Complex geology with artesian pressures • Adaptation of ‘exemplar’ curves to other sites based on: − Comparisons of forms of defences around estuary − National defence categories for exemplar and other defences Elevation (mOD) 30 25 20 15 10 5 10 0 1 9 4 5 -5 2 7 3 6 8 -10 -15 Page 22 80 100 120 140 160 180 200 220 240 260 280 11 Questions/topics addressed in presentation 1. What is a fragility curve (and how do fragility curves relate to design thinking)? 2. Why are fragility curves important in FRM? 3. Generalised fragility curves (with ref. to asset types and condition grades) 4. Challenging accuracy of generic curves. 5. Generating bespoke fragility curves 6. Conclusions Page 23 Who can use fragility curves? • Engineers and researchers − who create generic and bespoke fragility curves − who use the curves in flood risk systems modelling − who want to enhance conventional analysis of asset failure modes by providing information about uncertainty in deterministic factors of safety (Duncan, 2000) • Operational flood risk management personnel − need to understand the basic concept − will use the curves indirectly because of their embedment within operational tools (UK example ‘RAFT’) – next presentation by Marta Roca will explain this − and within systems analysis tools – presentations in final session will explain this Page 24 12 Conclusions • Fragility curves are a fundamental tool in flood risk management. They − express the varying probability of breach of levees with load − allow flood systems analysis to properly include the effect of defences − can be generalised for broad scale analysis of bespoke for local system or project analysis • Demystifying the basic concept for non-experts is worthwhile so that they can be used in operational tools • Contributions of shared experiences on fragility and more generally on the performance, design, and management of levees are sought for the International Levee Handbook www.leveehandbook.net Page 25 References • Simm, J., Gouldby, B., Sayers, P., Flikweert, J, Wersching, S. and Bramley, M. (2008) Representing fragility of flood and coastal defences: getting into the detail. Proc. Eur, Conf. on Flood Risk Management: Research into Practice (FLOODrisk 2008). London: Taylor & Francis, 621-631. • Flikweert, J. and Simm J. (2008) Improving performance targets for flood defence assets. J Flood Risk Management 1, 201–212. • Simm, J. and Flikweert, J. (2009) PAMS (Performance-based Asset Management System) – Phase 2: Development of fragility curves for use in management of flood defence assets. Report SC040018/SR5. Environment Agency • Schultz, M., Gouldby, B., Simm, J. and Wibowo, J. (2010) Beyond the factor of safety: developing fragility curves to characterize system reliability. USACE ‘white paper’ ERDC SR-10-1. Page 26 13 The Use of Fragility Curves for UK Flood Defences and their Implications for Flood Risk Management Jonathan Simm Flood Management Group, HR Wallingford 14