Every type of flood defence structure reacts differently under load. The factors that affect structure performance are equally varied. In order to undertake an effective assessment of the overall reliability of a defence it is essential to have a thorough knowledge and understanding of all key potential failure modes. Over recent years considerable effort has been devoted to improving our knowledge of how defences fail. However, gaps in knowledge still remain and differing approaches and levels of detail may be found in different national approaches.

The objective of this research was to provide a definitive listing of reliability equations for failure mechanisms of flood defence assets for use in flow system modelling. Modelling of flow systems is required as part of an effective approach for flood risk management. This requires that the behaviour of the flood defence structures is understood for different load conditions and flood defence asset types.

FLOODsite report T04-06-01 provides details of reliability equations representing different failure mechanisms of various flood defence assets under differing load conditions. As a definitive source of such information, this document may be updated and extended in the future as knowledge of structure performance and types increases.

Three principal hydraulic load types are considered:

  1. Water level difference across a structure
  2. Wave loading
  3. Flow velocity
  4. Additionally
  5. Structure impact
  6. Operational failure are also included

Flood defence assets are categorised under four main headings:

  1. Foreshores, dunes and banks;
  2. Embankments and revetments;
  3. Walls;
  4. Point structures.

A 'signposting' matrix presents load type and asset type to reference summary templates within which details of the particular asset, failure mechanism and reliability equation(s) are provided.

The information presented within the report is intended for use in system modelling of flood risk. The information presented within the failure mode templates offers potential solutions for modelling defence structure behaviour under various load conditions. Since, in reality, many flood defence structure types vary in design, construction and condition, users should recognise the importance of understanding how the structure being assessed may behave in relation to the ‘generic’ solutions to failure modes presented here. In order to model system risk, the user will need to appreciate the failure process, the applicability to field conditions, how to integrate the failure mode within a fault tree structure for modelling and how to deal with any uncertainties within parameters used to model the failure process. Guidance on fault tree structures for different defences and uncertainties within modelling parameters may be found under FLOODsite Task 7 reports.

Click here to access a full (PDF) copy of FLOODsite report T04-06-01

Click here or the Failure Modes link above to access an online, interactive version of the report.

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