Managing Safety of Flood Defences in the Netherlands: Study and Assessment

1. May 2002 LTV & VNK Managing the Safety of Flood Defences in the Netherlands Fola Ogunyoye Presented on Friday 17th May 2002 at the IMPACT Project Workshop, HR Wallingford, UK

2. May 2002 Managing the safety of flood defences in the Netherlands Manual for safety assessment of flood defences (LTV) • Background to study • Methodology • Assessment process • Example Quantifying safety against flooding (VNK) • Management philosophies – Present – Future • Current status of project

3. May 2002 LTV: Framework & assignment • LTV: Manual for Safety Assessment of flood defences ( L eidraad T oetsen op V eiligheid) • Law on Flood Defence: – Safety level prescribed for each of the 53 “flood defence rings” (1/1250 - 1/10.000) – Water boards are responsible and have to report in a 5 year cycle whether safety is according to the legally prescribed safety level – For each section of defence, Department of Public Works (‘Rijkswaterstaat’) provides hydraulic conditions with required probability of exceedance – ‘Rijkswaterstaat’ provides guidelines to determine if the safety level is sufficient (‘safe’ or ‘unsafe’)  LTV • Present assignment: update, incorporation of new knowledge, finish end 2002

4. May 2002 LTV: Method • For each failure mechanism, LTV contains a set of rules in the form of a flow chart that can be used to assess whether safety is sufficient. • For each failure mechanism and each section of flood defence, the flow chart assessment is ‘safe’ or ‘unsafe’. • The worst scoring mechanism determines the final score of each section (‘safe’ or ‘unsafe’) • The worst scoring section determines the final score of each flood defence ring (‘safe’ or ‘unsafe’)

5. May 2002 LTV: Scope • Each type of flood defence is treated separately: The three principal types are dikes, dunes, structures • For each type each relevant failure mechanism is treated separately: e.g. overtopping, geotechnical stability, piping, dune erosion, reliability of closure operations, etc. • Basic principle: determine ‘safe’ or ‘not safe’ as efficiently as possible  flow charts with different assessment levels: – simple : easy methods, little data, but often insufficient for determination of a final score (‘uncertain’)  next level – detailed : equivalent to design methods, more data required, in most cases sufficient for a final score (‘safe’ or ‘unsafe’), in rare cases still ‘uncertain’  next level – advanced : specialised methods, advanced data collection, expert judgement

6. May 2002 LTV: Piping for dikes as an example (1): Simple calculation methods such as Bligh (2): A check if the methods that were used for the design are compatible with current design manuals and if all relevant parameters are equal or more favourable to the ones used in design (3): Calculation using the design methods from the current design manuals, guidelines for data collection (4): Examples: unsteady, 3D or probabilistic calculations, including expert judgement (5): Result ‘safe’ can only be given if the failure mechanism has not occurred within the section

7. May 2002 Main improvements from the update • Clearer and more consistent flow charts and accompanying text • Incorporation of new Knowledge, based on recent advances in design and management methods • Incorporation of improvements due to feedback from use of existing document • Increased scope for conclusive assessments made at simple or detailed stages

8. May 2002 VNK Project: Framework • VNK: The Safety against flooding in the Netherlands quantified ( V eiligheid van N ederland in K aart) • Philosophies in flood defence management: – Present: Deterministic Section Method: Each section of flood defence ring must be sufficiently strong to withstand hydraulic conditions with the legally prescribed frequency  Safety assessments, LTV – Near Future: Probabilistic Ring Method: Probability of inundation of flood defence ring must be less than a whole range of legally prescribed safety conditions  VNK Project – Possible future: Ring Risk Method: Optimisation of safety levels on the basis of the expected damage level after inundation

9. May 2002 VNK Project: Current status • Co-operation between Rijkswaterstaat, water boards, large technological institutes’ and consultants • Development of calculation methods and tools • Developments of methods and guidelines for data collection, especially for (old) structures • Analysis of first sets of flood defence rings (pilot studies underway) • Next couple of years: – calculation of safety level for all 53 rings – based on safety levels: prioritising of improvement works – possible development towards inundation risk-based methods