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Increased flood safety due to time-dependent pipe growth
Dike safety assessments can be optimised when considering the time that the water flowing underneath the dike takes to erode its foundation.
Piping or the erosion process in the dike foundation takes time and may not evolve into a dike breach and flooding during a high-water event. Quantifying the time-dependent aspects of piping helps the responsible authorities and technical advisors to reduce the scope of reinforcement efforts and that way meet safety standards faster and cheaper
Piping is a gradual erosion process. It starts when water flowing through a sandy dike foundation erodes so much sand that it forms a small (mm size) channel or ‘pipe’ that may grow (or not) into a shortcut towards the river. Dike safety assessments and designs neglect the time piping takes to develop. Without considering the time development of piping, the dike failure calculations may be too conservative, particularly if:
- high-waters are relatively short, or
- the piping erosion can be stopped with timely flood fighting interventions.
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Illustration of the piping erosion processes.
Studying piping under short-lasting high water levels and with timely flood-fighting interventions
By using three complementary methods, I investigated the time-dependant aspects of piping:
- Lab experiments at small (50 cm) and large (10 m) scales increase our understanding of the piping processes in the pipe (mm size), reveal which factors influence the time scale of erosion, and help validate the below pipe development models.
- A simplified pipe development model helps predict piping for dike properties and water levels beyond the conditions of the lab experiments.
- Probabilistic methods allow to include uncertainties of, for example, the high-water duration, the soil properties and the seepage length to determine whether a particular dike meets the safety standards.
Experiments on pipe development. Top left: large-scale test in the Flood Proof Holland facility at the TU Delft. Top right: measurements during the large-scale test. Bottom right: sand boil in the large-scale test. Bottom left: small-scale experiments.
Lessons learned
Considering the time needed for piping development can significantly reduce the failure probability. Not only in coastal areas with short storms but under certain conditions, such as effective flood fighting, also in river areas.
The comparison between the six hypothetical cases with still some conservative assumptions already provide great insights under which conditions the time of piping development is a relevant factor, and by which order of magnitude it affects the failure probability.
Finally, including the factor time for piping requires a shift in thinking: there is not just a critical water level, but combinations of peak water level and high-water duration that gradually lead to failure.
Next steps
As this research is still ongoing, I currently use the experimental data to improve the physical basis of the simple pipe development model. Furthermore, I am studying the effects of pipe development under multiple high-water events. Those findings may allow to loosen the conservative assumption that there is a pipe present already up to 1/2 the seepage length.
A useful step for practice would be to use the research findings to derive simplified rules to include the effects of time in dike safety assessments. For instance, a reduction factor on the computed failure probability, depending on the governing high water duration, seepage length and foundation soil.
Since flood fighting appears to be an important factor in the river area, a discussion is needed when we want to use that information in dike design. If we want to include it, the effectiveness (detection accuracy and required time) of interventions needs to be quantified further and improved.
Interested to know more?
- Progression rate of backward erosion piping in laboratory experiments and reliability analysis (Publication on earlier version of the reliability analysis)
- Temporal Development of Backward Erosion Piping in a Large-Scale Experiment (Publication on large-scale test)
- Please contact me for more information; the work is still in progress and a publication under preparation.