New assessment framework for investment in dike upgrades

Published: 8 July 2014

When is a dike safe enough? ‘Monitoring and/or upgrading?’ was the question facing a range of experts on 4 July looking at how to tackle flood defences that don't comply with safety standards. An assessment framework allows decision-makers to balance investments and failure risks.

The meeting was organised to mark the completion of the doctoral research conducted by Timo Schweckendiek (Deltares/Delft University of Technology), who studied ways of reducing uncertainties associated with piping. This is a process in which water seepage erodes dikes from below and it is one of the geotechnical mechanisms that can weaken dikes and cause them to fail.

Issue and approach

In the auditorium of Delft University of Technology, host Bas Jonkman (of the University) opened proceedings by sketching the current situation with respect to flood protection in the Netherlands. There is a challenge that needs to be addressed: the dike assessment that takes place every four years has shown that 1200 kilometres of dike fail to comply with the prevailing safety standards. Secondly, a recent study has shown that the failure mechanism of piping constitutes a serious safety threat. Given the new flood protection standards in the context of the 2015 Delta Programme, extra measures that are both sustainable and efficient will be required, particularly for dikes in the rivers area of the Netherlands. In other words, which cost-effective approach will provide adequate protection? Model results generally involve uncertainties, not least because of gaps in our information about the subsurface. Bas Jonkman emphasised that applied research like Timo Schweckendiek’s can contribute to the establishment of a well-founded approach.

Risk-based governance

In the pursuit of improvements in infrastructural design and the reduction of management and maintenance costs, a risk-based approach has been adopted in a range of sectors: this involves striking a balance between failure probabilities, measures and costs. Michael Faber (from Denmark Technical University) explained how the offshore industry has to cope with corrosion and metal fatigue. On the basis of those failure mechanisms, he looked at how it is possible to invest effectively in inspection and maintenance. Raphaël Steenbergen (TNO/Ghent University) then described how monitoring and test loads can be used in safety assessments for bridges to reduce uncertainties in traffic load model outcomes. Demonstrably safe bridges do not need replacing, and that saves money.

Trade-offs

Although maximum test loading is not an option with dikes, Timo Schweckendiek thinks it is entirely feasible to collect on-site information, data from the field, about failure mechanisms affecting flood defences. During and after a period of high water, it is possible to study landside seepage flows, or piping. Or to establish that they are not a feature, despite expectations. Soil surveys establish a picture of the local soil conditions. “The main question is how much information you think you need to determine whether the flood risk is acceptable.” He points out that this question is in line with the probabilistic approach advocated by the Delta Programme: decision-making on the basis of acceptable flood probabilities. Timo Schweckendiek established an assessment framework for this purpose. Can investments in soil surveys or monitoring be earned back through savings on dike upgrades? The examples in his research suggests that this is indeed possible. And so an important instrument for decision-making would appear to be available.

Click here for the video made during the meeting.

Nieuw afwegingskader voor investeren in dijkversterking