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617 Structures at risk from scour and erosion

Report ID: 617

Published: Newsletter 57 - January 2020

Report Overview

Following some recent structural failures, a reporter is concerned that asset owners may not fully realise risks associated with scour and erosion.

Report Content

Following the structural failure caused by scour at Lamington viaduct and the Whaley Bridge dam collapse, a reporter is concerned that asset owners may not fully realise risks associated with scour and erosion, have poor systems in place to identify vulnerable structures, and be unwilling to spend the money required to implement remedial work.

They are also concerned that embankment structures, not just dams, may not be adequately assessed for risk of scour and erosion because they are often assessed by engineers who may not have relevant experience.

They say that there are numerous examples of failure in this field and that this is likely to get worse with increased flooding due to climate change.

Comments

Scour has been the cause of many major collapses. The occurrence of scour cannot be easily identified by inspection and collapses as a result of scour might happen suddenly. These twin effects violate a structural safety principle that the onset of any failure should be detectable and should give ample warning. For those reasons, scour failure is not a preferred failure mode and design against scour should be robust.

The collapse of the Malahide viaduct in Ireland in 2009 was caused by scour and it was fortunate that no one was hurt.

This report is increasingly relevant with increases in floods and the possible consequential effects on bridge foundations, and also on some building foundations. Indeed, the risk of scour needs to be considered in a more holistic manner considering the influence of a range of factors including dredging/sand or gravel removal, removal of weirs near bridges and the influence of flood defences on the speed and flow of the water.

In 2018, Network Rail updated their Scour Assessment of Bridges, Culverts and Retaining Walls document which describes the procedures for safeguarding their structures from the risk of scour.

Highways England, who maintain the Strategic Road Network in England, have a process in place for the inspection, assessment and monitoring of existing highway structures for scour and other hydraulic actions. The process, including requirements and advice, is provided in the Design Manual for Roads and Bridges (DMRB) document BD 97/12, which includes detailed requirements and advice on risk management for scour and other hydraulic actions on highway structures.

Additionally, Highways England’s document for the design of highway structures for hydraulic action (CD 356) provides practical requirements and guidance for designers to determine hydraulic actions which are either not covered or covered in insufficient detail in the Eurocodes.

Highways England also conduct planned geotechnical inspections in accordance with the DMRB. These inspections are specifically required to observe/record the topography and condition of land adjacent to their assets. Where rivers, lakes or the sea abut or run close to their assets, the inspections necessarily include the recording of any land scour or risks of scour. Management plans are developed from the asset condition reports, and appropriate repairs or mitigations are routinely undertaken within a timescale governed by the risks presented.

The value of such plans has to be judged in the context of the predicted greatly increased rises in sea levels and the consequential effects on the world’s infrastructure. See for example reports from the Intergovernmental Panel on Climate Change.

The Reservoir Act 1975 puts certain duties on dam owners to appoint qualified Panel Engineers to supervise their construction, inspection and maintenance. The Environment Agency, and the Scottish Environment Protection Agency (SEPA) in Scotland, monitors and enforces compliance with the Act. However, there are many embankments and minor structures and buildings that do not come under a specific inspection regime.

 

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View other CROSS reports published in Newsletter 57


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