Project Summary A3 - Dike reliability analysis: Better methods for the assessment and design of dike systems

Hydraulic Structures and Flood Ris

The Weakest Link: Spatial Variability in the Piping Failure Mechanism of Dikes

Piping is an important failure mechanism of flood defense structures. A dike fails due to piping when a head difference causes first the uplift of an inland blanket layer, and subsequently soil erosion due to a ground water flow. Spatial variability of subsoil parameters causes the probability of piping failure to increase, often to unacceptable levels. The general research question is: How can we incorporate spatial variability in a flood defense system design dealing with the piping failure mechanism? The question in solved in three steps: first by quantifying the spatial variability in subsoil parameters, second by assessing the influence of this spatial variability on the piping mechanism and third by analyzing optimal decisions to deal with unacceptable situations. There are two new models presented in this thesis. The first model, is a simple design model that uses historical failures to assess the piping safety. The second model describes the formation of piping erosion paths in spatial variable soils. Especially the second model might potentially lead to improvements in piping modeling and potentially in cost reductions. The main conclusions from this thesis is that the piping mechanism and influence of spatial variability in the subsoil are a very significant threat to flood defenses. However, performing local soil measurements in combination with local dike improvements can be a cost-effective method to deal with unacceptable piping failure probabilities.Hydraulic EngineeringCivil Engineering and Geoscience

Analysis and influence of uncertainties on the reliability of flood defence systems

Uncertainties are introduced in probabilistic risk analysis when we deal with parameters that are not deterministic (exactly known) but that are unknown instead, hence uncertain. This report describes how uncertainties influence the reliability of flood defence systems. The purpose of the study is to identify all uncertainties that influence the reliability of dike ring systems, to determine which uncertainties contribute most to the probability of failure and how can be dealt with uncertainties.Floodsit

Safety format and calculation methodology slope stability of dikes

Civil Engineering and Geoscience

Updating piping probabilities with survived historical loads

Piping, also called under-seepage, is an internal erosion mechanism, which can cause the failure of dikes or other flood defence structures. The uncertainty in the resistance of a flood defence against piping is usually large, causing high probabilities of failure for this mechanism. A considerable part of this uncertainty is of epistemic nature, which can be reduced by incorporating extra information. It is shown how the knowledge of historically survived water levels, the main load factor, can be integrated into the probability distribution of the piping resistance variables by means of Bayesian Updating. The effects are demonstrated by means of a realistic numerical example.Hydraulic EngineeringCivil Engineering and Geoscience

Bayesian Inference of Piping Model Uncertainties Based on Field Observations

This paper presents a Bayesian model to determine the model uncertainty of a critical horizontal gradient model for piping for dikes, such a Lane and Bligh. A Bayesian model is needed for two reasons. First, there is a large overlap in cases that failed and survived. Second, the evidence of the failed cases is limited .The model consists of a non-informative prior that is combined with likelihood functions for failed and survived cases. This involves modeling the mean and standard deviation of the model uncertainty as random variables. For survived cases we know the limit state function was larger than 0 for the observed water level. For failed cases we know the limit state function was smaller than 0; or Z = 0; which is a less conservative assumption. This information is used to determine the likelihood functions for failed and survived cases. The prior and likelihoods are combined to find the posterior distributions of the mean and standard distribution of the model uncertainty. Using integration, this finally results in the (lognormal) distribution of the model uncertainty. The model is applied to the data of Bligh and Lane and shows both a high mean and high standard deviation of the model uncertainty, where the model of Lane performs better than Bligh. It is recommended to tailor the proposed model to dikes by making a different distinction between horizontal and vertical erosion. Furthermore, it is recommended to apply the model to more dike specific data since the Bligh data mainly consists of dams instead of dikes.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Hydraulic Structures and Flood Ris

How does the risk-based approach work?

Hydraulic Structures and Flood Ris

A 3D time-dependent backward erosion piping model

Backward erosion piping (BEP) is a failure mechanism of hydraulic structures like dams and levees on cohesionless foundations subjected to seepage flows. This article models the time-dependent development of BEP using numerical simulation of the erosion process. A 3-dimensional finite element equilibrium BEP model is extended with a formulation for the sediment transport rate. The model is compared to and calibrated with small- and large-scale experiments. Finally, a large set of simulations is analyzed to study the effects of factors such as grain size, scale (seepage length) and overloading on the rate of pipe progression. The results show that the development of BEP in the small-scale experiments is predicted well. Challenges remain for the prediction at larger scales, as calibration and validation is hard due to limited large-scale experiments with sufficiently accurate measurements. The results show that the progression rate increases with grain size and degree of overloading and decreases with seepage length, which is consistent with experimental observations. The model results provide a better physical basis for incorporating time-dependent development in the risk assessment and design of levees.Hydraulic Structures and Flood Ris

Cost optimal river dike design using probabilistic methods

This research focuses on the optimization of river dikes using probabilistic methods. Its aim is to develop a generic method that automatically estimates the failure probabilities of many river dike cross-sections and gives the one with the least cost, taking into account the boundary conditions and the requirements that are set by the user. Even though there are many ways that may provoke the dike failure, the literature study showed that the failure mechanisms that contribute most to the failure of the typical Dutch river dikes are overflowing, piping and inner slope stability. Based on these, the most important design variables of the dike cross-section dimensions are set and following probabilistic design methods, the probability of failure of many different dike cross-sections is estimated taking into account the abovementioned failure mechanisms. Different cross-section configurations may all comply with a set target probability of failure. Of these, the cross-section that results in the lowest cost is considered the optimal. This approach is applied to several representative dikes, each of which gives a different optimal design, depending on the local boundary conditions. The method shows that the use of probabilistic optimization gives more cost-efficient designs than the traditional partial safety factor designs.Hydraulic EngineeringCivil Engineering and Geoscience

Safety standards of flood defenses

Current design codes like the Eurocode use safety or reliability classes to assign target reliabilities to different types of structures or structural members according to the potential consequences of failure. That, in essence, is a risk-based criterion. A wide range of structures is designed with such codes, and distinction is made between reliability classes. These reliability classes are not necessarily well suited for flood defense systems, neither are the design rules and partial safety factors, which are calibrated for a wide range of standard applications. For a flood defense system protecting a large area from flooding, on the other hand, it is worthwhile to base the design and safety assessment standards on a risk assessment - a tailor-made solution. The investments can be considerable and the stakes are high, especially for low-lying delta areas, where the consequences of flooding can be devastating. In order to answer the question “How safe is safe enough?” a framework for acceptable risk is required. Subsequently, from acceptable risk we can deduce target reliabilities for the protection system as a whole as well as for its elements. For practical application, these target reliabilities can then be translated into design and assessment rules; for example, using LRFD (load and resistance factor design) to derive partial safety factors. This paper describes how to define safety standards for flood defenses, in particular dikes, step-by step. An important aspect in translating high-level requirements into specific (low-level) design rules that apply to specific failure modes for specific flood protection elements is the so-called “length effect”. This is especially relevant for long-linear structures like dikes, where usually the length is much larger than the scale of fluctuation of dominant load or resistance properties. The longer the structure, the higher the chance to encounter either and extreme load or a weak spot (i.e., low resistance) – hence the word “length-effect”. The effect is that the probability of failure increases with the length of the dike. The implication for design and assessment rules is that the reliability requirements to a cross section (“zero length”) need to be stricter (i.e., higher target reliability) than for the whole reach. This paper attempts to demonstrate how tailor-made safety standards for large scale flood defense systems can be derived in a risk-based fashion. Since flood defenses differ from smaller scale geotechnical structures in many aspects and given the volume of investments in such large-scale engineering systems, it is very attractive to deviate from the standard design codes. That is not deviating conceptually, but rather deriving safety factors for the specific application to better account for the characteristics and uncertainties involved. The authors strive to show that safety levels and partial safety factors in the presented approach are far from arbitrary. They are part of an overall consistent flood risk framework, a framework that provides a link between geotechnical engineers and other disciplines involved in providing safety from flooding.Hydraulic EngineeringCivil Engineering and Geoscience