A framework for taking into account time dependency in the design of flood defences and a model describing the development of piping over time.
Top: The Waal river dike at Beuningen during the 1993 flood (source: Rijkswaterstaat / Bart van Eyck). Bottom: Piping in the laboratory (source: Sam Rentmeester Fotografie with limited use due to copyrights) and in the field (Photo by Joost Pool).
Motivation and practical challenge
Current safety assessments result in unexpected high failure probabilities for some failure mechanisms, such as piping. This conservative estimates for the probability of failure may result in high costs for dike reinforcement projects in the Netherlands. To optimize investments, it is of great societal importance to improve safety estimates. In case of high water levels (top photo), including the flood duration in the analyses is one of the aspects that can contribute to lower assessed failure probabilities and more efficient reinforcements.
Related to piping, the failure occurs when water flowing through a sandy dike foundation erodes so much sand that it forms a small (mm size) channel or ‘pipe’ (innovative components figure). Therefore, an important challenge is the interpretation of field observations during floods. For example, sand boils are the only visual manifestation of piping, but most do not result in a dike breach. A better understanding of the erosion process in the laboratory and the field (bottom photos) is important to estimate the likelihood of a dike breach during a flood to plan emergency responses.
The research aims to show how to quantify failure probabilities including (uncertain) flood durations and time-dependent failure processes. The assessment framework is elaborated for piping, for which a model shows the development over time.
Including time dependent information in dike safety assessments requires to understand the development of failure mechanisms such as piping at different levels over several flood events. To do so, I use the following innovative components:
- Better understanding of the development of the piping erosion process over time, using small scale and large scale experiments.
- Modelling of the piping erosion process for deriving a simplified model that complements the current practice (Sellmeijer model).
- Exploring when interactions (causal dependencies) between failure mechanisms are relevant.
Based on this improved understanding, I am developing a probabilistic framework to integrate uncertainties and time aspects in hydraulic loads, dike properties and failure processes, both over single and several flood events. The framework allows to quantify the effect of time dependence in several type of water systems such as rivers, coasts and lakes. The project focuses on the piping failure, but similar probabilistic methods can be used for other failure mechanisms.
Piping schemes adapted from van Beek (2015) and dike reliability graphs sketched by Joost Pool.
Relevant for whom and where?
Technical managers of flood defences, especially in areas with short flood durations. They can use the research to improve reliability estimates in assessment, design and operational phases. The research can also help in the planning of emergency measures.
Findings of this research are developed in the laboratory and a field test site and will be applicable to dike reinforcement projects in the Netherlands.
Progress and practical application
The results so far indicate that a time-dependent analysis can considerably reduce calculated piping failure probabilities. It can decrease the probability of piping failure with several orders of magnitude in areas with short, storm-dominated floods. This effect is smaller in case of prolonged river floods, but results also show that a breach often occurs after the flood peak; in riverine areas, this delay can be several days, which is important for operational flood management.
Status for day-to-day practice
Currently, the research moves from studying the piping erosion processes towards implementation in engineering models and case studies to demonstrate how this research can be applied in reliability assessments.
In the coming period, we may need several partner organizations who are actively involved in a case study, to improve the applicability of the methods.
Last modified: 19/10/2020
Delft University of Technology
- Pol, J.C., van Beek, V.M., Kanning, W., and Jonkman, S.N. Progression rate of backward erosion piping in laboratory experiments and reliability analysis. Proceedings Conference ISGSR2019. 7th International Symposium on Geotechnical Safety and Risk (ISGSR2019), J. Ching, D. Li, and J. Zhang, eds. Research Publishing, Singapore, 764–769.