Identifying and managing risks arising from defence structure transitions
Providing consistency in risk assessment guidance associated with flood defence asset transitions.
Summary
This project developed guidance on how to identify, assess and manage the effects of transition zones on the performance of flood defences. This will help flood risk management authorities prioritise improvements to reduce flood risk.
Background
Transitions are the connection points between flood defences and other infrastructure (including other flood defences). They introduce irregularities which increase the chance of failure, as seen in many historic flood events.
Current guidance on visual asset inspection does not explicitly account for the potential effects of transitions on defence performance, which means that the associated risks can be missed from local, regional and national flood risk assessments.
Method
The study, carried out by international experts in this field, focused on 4 specific types of transition:
- longitudinal transitions
- cross-sectional transitions
- crossing infrastructure
- revetments
Through a literature review, stakeholder engagement and 2 onsite pilots, a range of potential transition issues were identified, including:
- altered geometries (shorter seepage paths, steeper slopes, or irregular geometry causing turbulence)
- differences in behaviour of materials (hydraulic separation, impeded grass root formation and shading, or gaps in filter structures)
- impediments to construction and maintenance (poor compaction around transition elements, or impeded maintenance due to poor access and visibility)
- preferred traffic paths causing deterioration (rutting and furrowing or animal burrows)
The following failure mechanisms were explored in terms of their relevance and significance to transition failures:
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external erosion (also called surface erosion)
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internal erosion
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slope and mass instabilities
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hard structure instability
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crest degradation
Findings and outputs
The project developed a range of tools and guidance, published in 4 reports described in the sections below. The has a short summary of the study and outputs.
Report 1 - Tiered inspection process
This contains improved guidance for identifying, inspecting and assessing transition zones.
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Tier 0 – Identification and initial prioritisation of asset transitions
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Tier 1 – Inspection by field teams
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Tier 2 – Assessment by a senior engineer
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Tier 3 – Assessment by a specialist
Report 2 - Prioritisation methods
Methods to support identifying and prioritising interventions at transitions based on risk analysis.
Report 3 - Fragility curves
New methods and tools for the reliability analysis (performance assessment) of flood defence transitions including:
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evaluation of the reliability (performance) of embankment assets
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an updated hrRELIABLE performance assessment tool for creating transitions specific fragility curves
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A user guide to the hrRELIABLE performance assessment tool for transitions.
Report 4 - Transitions remediation guidance
Guidance on the design of transition improvements and associated asset management to address the impacts of transitions on flood defence performance.
Project Information
Project manager: Dr Sarah Twohig
This project was commissioned by the Environment Agency’s FCRM Directorate, as part of the joint Flood and Coastal Erosion Risk Management Research and Development Programme.
Updates to this page
Last updated 20 July 2023 + show all updates
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Published appendices to Report 1 Corrected minor typos in Reports 1, 2 and 4
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Final project reports published and page contents updated to reflect completion.
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First published.