Structural flood protection systems such as levees are an important component in flood risk reduction strategies. Levees can fail through various failure mechanisms; this thesis focuses on the mechanism Backward Erosion Piping (BEP) which occurs when a sandy levee foundation is eroded by groundwater flow. To assess whether a levee's reliability complies with safety standards, authorities use models which describe the levee properties and failure mechanisms.

This thesis aims to extend the current failure model by considering piping as a time-dependent erosion process instead of the current assumption of immediate failure once a critical threshold is exceeded. Therefore, it is shown how time-dependent development of backward erosion piping can be quantified and how it affects levee reliability analyses. This is achieved by a combination of literature review, analysis of previous experiments, additional experiments on different scales, numerical modeling and probabilistic modeling.

The following key findings were established. Analysis of historical levee failures due to BEP and previous experiments indicates that there can be significant time between initiation and breach, highlighting the importance of time-dependence for piping. The rate of pipe progression in experiments can be explained by the sediment transport rate, which is shown to depend on the pipe flow conditions. A numerical groundwater flow model which includes this sediment transport process can predict the pipe development in small-scale experiments. Relations between the progression rate and levee properties and hydraulic loads as derived with this numerical model can be used efficiently in reliability analyses. These analyses show that including time-dependent pipe development in BEP analyses has a significant impact on the levee failure probability, both in coastal and riverine water systems.
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With the shift of the safety standards for levees from exceedance probability (standard 1996) to flooding probability (new standard), the part of the levee failure process after the initial occurrence of a failure mechanism becomes more important for the calculation of the strength and thus, reliability of the levee cross-section. Interactions between failure mechanisms can take place in this part of the levee failure process. An interaction refers to one failure mechanism influencing the probability of another failure mechanism, where positive interaction is defined as a reduction and a negative interaction as an increase of the failure probability. The main problem is a lack of knowledge about the interactions of failure mechanisms and their influence on the failure probabilities of levees. Several studies (Calle, 2002; ’t Hart et al., 2016; Kok et al., 2017) suggest that there is an interaction between different failure mechanisms. However, only the effects of overtopping on slope instability (de Visser et al., 2018) and a reduction of the shear strength in slope instability due to uplift (Kanning and van der Krogt, 2016) is currently included in the statutory assessments of levees (Rijkswaterstaat WVL, 2017a,d). This research aims to quantify the influence of the interaction between the failure mechanisms backward erosion piping (BEP) and slope instability (SI) on the safety of a levee. The parametric study of this research assesses whether the occurrence of a failure mechanism (BEP or SI) affects the parameters' present states in the limit state functions of the other failure mechanism. A model was created to calculate this interaction between SI with BEP. After a slope instability, the remaining profile is assumed to contain a berm. This change in levee profile, does not result in the immediate breaching of the levee, it can however influence the probability of BEP. The model sequentially executes a stability analysis, a displacement analysis of the sliding plane and a BEP analysis. The degree of interaction is defined as the difference between the safety factor of BEP of the original profile (prior to sliding) and the safety factor of BEP of the remaining profile (after sliding). The modelling study shows that the degree of interaction from SI to BEP is significantly affected by the presence of cracks in the berm. A consequence of these cracks is that the exit point moves and is now located in the berm. The degree of interaction also depends on the shape and size of the sliding plane, because it determines the size of the berm of the remaining profile. A vast majority (50\% to over 75\%) of the considered scenarios of the seven considered levee profile cases result in a positive interaction. However, in all the considered cases, negative interactions were found that give a reduction of 50\% to 2\% on the original safety factor, which could lead to an overestimation of the safety in the assessment of the levee. All of these negative interactions result from the assumption that large cracks can form in the soil after sliding. ... Read more

In the Netherlands, a new assessment method for the required safety level of primary flood defences is introduced in the Water Act. It is estimated that currently roughly 1900 kilometres of flood protection does not meet the safety requirements for the future (Jonkman, Jorissen, Schweckendieck & Van den Bos, 2017). Past flood defence projects show that integrating reinforcement measures is a difficult assignment. It is specifically focused on dike reinforcement projects in this thesis. A deficiency in the design process is identified in the role that the influence on the existing situation has on the evaluation and selection of design alternatives that are input for the Environmental Impact Assessment. The difficulty for improving the importance of the existing situation in the surroundings, in the design process is that the subject experts on these themes are typically assessing the consequences of proposed interventions (Aalberts, 2018). The research objective is set to develop a method that allows to include the influence on the existing situation within the evaluation and selection of design alternatives in dike reinforcement projects. This method aims for providing information to compare the spatial integration of reinforcement designs in early stages of reinforcement projects. Subject experts should assess the provided information, and determine whether a reinforcement design provides a satisfying influence on the project area. It is focused on dike reinforcement projects in rural area. A research through design study is used to develop the method to compare design alternatives. The study included the identification of technical design measures to increase the strength of dikes in rural area, and the decomposition of rural areas in functional characteristics that can be valued. A conceptual evaluation model is developed to express the relation between functional characteristics and design alternatives for dike reinforcements. It is studied how a method can be provided to compare the outcome of the conceptual evaluation model for a large variety of design alternatives. The value of the developed method is assessed in a case study, which is the dike reinforcement project “Wolferen-Sprok”. The resulting method is able to compare design alternatives on the influence on the existing situation by following seven steps. It is providing design measures for the identified safety problem of the dike in a certain project area. Simultaneously, the characteristics of the project area should be identified and visualized on maps. This provides the required information to apply the conceptual evaluation model that identifies effects of design alternatives on the existing situation. The retrieved data is visualized to compare design alternatives. It is concluded that the developed method focusses mainly on natural, cultural, and social economic functions in rural project areas. This means that influence of design alternatives on costs, maintainability, and landscape values should be addressed separately, in the evaluation and selection of design alternatives. It is recommended to study how the influence on these topics can evaluated in a similar method. The developed method can be applied to include the existing situation in the project area in the evaluation and selection of design alternatives in dike reinforcement projects in rural areas. It is expected that application of the method in design teams provides new input for further improvement of the method. ... Read more

In this thesis, the effects of the foreshore and heterogeneities of the subsoil on the piping safety analysis are determined. The effects are searched for by use of the numerical model D-Geo Flow. The results of this research give more insights in when and how a foreshore has to be included in the safety analysis. It is also concluded that heterogeneous soils are more resistant to piping than homogeneous soils, which are currently used in the safety analysis. ... Read more

Levees are earthen structures that are designed to protect land from flooding and are commonly composed of impervious soils and built on sandy foundations. They are sensitive to an erosion process that is known as piping. After several years of investigation into piping, Sellmeijer proposed design rules by curve fitting results of a numerical model. The design rule are currently applied in the assessment of levees, despite being obtained under the assumption of uniform and homogenous subsoils. Consequently, the design rules seem to be fine for standard consultancy, but it is insufficient for heterogeneous and anisotropic. In this thesis, the effects of anisotropy and heterogeneity are analysed in a elementary and a realistic numerical model study in order to improve the design rule of Sellmeijer. In the elementary study, several aquifers compositions are created by adding an artificial element with a different permeability to the body or by varying the permeability of the body depending on direction. The realistic study is a modification of the elementary study since stratified aquifer compositions have been implemented. After each simulation, the critical head computed in the elementary or realistic composition is compared to the critical head computed with Sellmeijer design rules which requires one calculation value of the bulk permeability. To improve the design rules an alternative method to determine the bulk permeability is proposed that assumes that there is curved or radial flow towards the exit point instead of the Dupuit method which assumes that there is only horizontal flow. As a consequence, the bulk permeability is calculated as the geometric or logarithmic mean of the vertical and horizontal permeability. Based on both studies, it can be concluded that both anisotropy and heterogeneity in the aquifer greatly increase the critical head so that heterogeneous and anisotropic aquifers are significantly more resistant to piping. In some cases, the aquifer can withstand a 45% higher water level. Furthermore, Sellmeijer’s design rules more accurately approach the heterogeneous and anisotropic aquifer, if the bulk permeability is determined with the new approach. ... Read more