Methods have been developed to predict how hydrodynamic loads acting on nearly saturated porous media are transmitted to the subsoil. In line with the effective stress principle of Terzaghi, these methods apply the boundary conditions that the effective stresses at the surface of a porous medium are zero, and that the pore water pressures carry the full load. Here, a new approach is presented which is based on defining a stress and a stress gradient as boundary conditions. The stress gradient follows from the momentum balance equation, thereby assuring that the solution abides by d'Alembert's principle of minimization of virtual work. The corresponding solution is in full accordance with the volume and momentum balance equations of the linear elastic soil matrix and the volume and momentum balance equations of the pore water across the computational domain. The new method is thereby able to correctly reproduce measurements of pore pressure changes due to hydrodynamic loads under the assumption of a porous medium consisting of incompressible particles and pore water which could either be compressible or incompressible. The advantage of the proposed method is that it requires one less boundary condition at the surface of the porous medium. The method is therefore able to predict the magnitude of the effective stresses on a soil surface. Due to the ability to retain the assumption of incompressible water, the method has also become independent on a calibration parameter. The results of the method induce questions with respect to the validity of Terzaghi's principle of effective stress at the boundary when porous media are subjected to hydrodynamic loads.

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When soil surfaces are exposed to high shear stresses induced by high-velocity water flows, they erode. Erosion consists of the detachment, entrainment and transport of soil particles. When exposed to high shear stresses the process of soil detachment is no longer given by the pick-up of individual particles but by the continuous shear failure of layers of soil. The understanding of how soil properties influence the detachment rate of soil is lacking. This paper presents how the bed shear stress, initial and critical porosity, hydraulic conductivity, and density influence the detachment rate of dilatant soils which are subjected to high-velocity flows. Based on the constitutive mass and momentum balance equations it is hypothesized that the exchange of mass and momentum during soil detachment corresponds with that situation for which the dilatancy induced shear resistance of the soil is maximum. The hypothesis is validated against high-velocity erosion experiments on sand.@en

An accurate prediction of the breach widening rate after the onset of a levee failure is essential for flood risk assessments. Current state-of-the-art analytical breach growth relations are empirical in nature. The large variety in loading conditions, levee design, and levee construction material, combined with the limited amount of accurate measurements of breach growth, make the development of accurate empirical breach growth relations challenging. In this paper, a process-based breach widening relation is presented for levees constructed of dilatant soils. The process-based relation is derived from the weir flow equation and a process-based erosion equation. The breach widening relation can account for the effects of variations in soil parameters. For those cases for which soil parameters are unknown, a calibrated catch-all-coefficient is provided. The relation is benchmarked against the state-of-the-art empirical breach growth relation used in the Netherlands and validated against data on historical levee failures and experimental data.@en

Dit rapport geeft een samenvatting van de belangrijkste resultaten van het SAFElevee project. In het project is op basis van informatie over daadwerkelijk opgetreden dijkdoorbraken en grootschalige experimenten kennis ontwikkeld over de veiligheid van dijken. De rapportage is geschreven voor een technisch geïnteresseerde lezer, en kan relevant zijn voor beleidsmakers, waterkeringbeheerders, experts, aannemers en adviseurs en andere geïnteresseerden. Dit onderzoek is uitgevoerd als onderdeel van het NWO TTW project SAFElevee (Project nummer 13861). Het projectteam wil graag NWO en de eindgebruikers bedanken voor hun inzet en steun bij de uitvoering van het SAFElevee project. @en

Apart from the soil erodibility parameter, the critical shear stress is the most important parameter in predicting erosion rates. On the basis of experiments several empirical formulas have already been de-veloped which relate the critical shear stress to soil properties. Based on these findings and supported by new large scale experiments, a new predictive relation between the critical shear stress and soil properties is proposed here. In support of this study, Delft University of Technology collaborated with Saitama University in the preparation and execution of a large scale levee erosion experiment in Janu-ary 2019. The erosion experiments were performed in the on a 1.8m high levee with a sand core and respectively clay and loam cover types. The cover types were subjected to a constant overflow dis-charge of approximately 70 l/m/s. The test levee was constructed in the Flood Proof Holland test pol-der in Delft, The Netherlands. During the experiment, time lapse measurements of the erosion depth were obtained at 15 locations along the landside slope. Before and after overflow tests were performed on each cover type, soil samples were collected along the landside slope at 8 locations. This paper out-lines how these large experiments were used to evaluate the effectiveness and application limit of the new predictive equation for the critical shear stress. A comparison between the predicted and meas-ured erosion rates shows that by applying the new empirical relation for the critical shear stress, meas-ured erosion rates could be predicted around ±30 % errors.@en

A high quality safety assessment of levee systems requires a good prediction of when the grass cover of levees fail. Current methods relate the onset of failure to the peak in momentum or energy of the flow, instead of the peak in momentum transfer or energy transfer to the grass cover. The critical velocity necessary in the current methods is thereby difficult to quantify. In line with determining the peak in momentum transfer to the grass, here is shown that the onset of damage of the grass cover can be related to the peak normal stresses acting on the grass cover during wave overtopping. The peak in momentum transfer is thereby assumed to be located at the point of reattachment of the flow with the landside slope. The method is validated against the results of two wave overtopping experiments and benchmarked against the cumulative overload method. An advantage of this method is thereby that both the time and location of the onset of damage can be predicted.

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Understanding levee failures can be significantly improved by analysing historical failures, experiments and performance observations. Individual efforts have been undertaken to document flood defence failures but no systematically gathered large scale, open access dataset is currently available for thorough scientific research. Here, we introduce an efficiently structured, global database, called International Levee Performance Database (ILPD), which aims to become a valuable knowledge platform in the field of levee safety. It comprises information on levee characteristics, failure mechanisms, geotechnical investigations and breach processes for more than 1500 cases (October 2019). We provide a macro-scale analysis of the available data, aiming to provide insights on levee behaviour based on historical records. We outline common failure mechanisms of which external erosion is identified as the most frequent for levees. As an example, we investigate flood events occurred in Germany (2002, 2013) and examine breach characteristics of hundreds of failures. It is found that initial failure mechanisms have an influence on breach characteristics and that failures due to instability and internal erosion are less frequent but lead to larger breaches. Moreover, a relation between the return period and the expected breach density during a flood event is identified. These insights could complement flood risk assessments.@en

During infiltration of water in soil, menisci form at the interface of water, grains, and air in the pores, inducing suction due to surface tension. Due to the random distribution of interconnected pores of different sizes, characteristic of porous media, differences in suction and friction inside pores give a diffusing infiltration front. The process of infiltration is often simulated by solving Richards’ equation in which the water flux is calculated with Darcy’s law. Underlying Darcy’s law is the assumption that the gradients in flow potential and the flow resistance due to viscous forces are independent from each other. This paper shows that these parameters are dependent and negatively correlated. A new method for calculating flows in unsaturated porous media has been developed to evaluate the impact of the covariance on infiltration predictions. The results show that the impact is significant and leads to a reduction in infiltration rate and mean friction experienced during infiltration. The method thereby provides a physical explanation for the subdiffusion observed during water infiltration in soil and is consequently expected to provide more insights into the processes of infiltration. @en

In breaching of levees, sediment erosion is induced by high flow velocities. Due to the often continuously acceler-ating flow, no equilibrium transport conditions are reached. The erosion rate is often described by the erosion equation which linearly relates the erosion rate to the excess shear stress by means of a soil erodibility coefficient. The soil erodibility is thereby often determined by means of the JET test. In the field of Dredging Engineering the study of the behaviour of sand under high flow velocities has also been an area of interest. At Delft University, the department of Dredging Engineering has performed several experiments on the behaviour of non-cohesive material when subjected to high flow velocities. It was noted that the initial porosity, and permeability of the material are important parameters to account for in the erosion process. This corresponds with observations that soil erodibility is sensitive to variations in material texture, compaction moisture content and compaction energy. It was further-more noted that soil no longer erodes due to the pick-up of individual particles but fails as entire layers when sub-jected to high flow velocities. For shear failure to occur in non-cohesive soils the soil needs to dilate. The associat-ed increase in pore volume causes an inflow of water into the soil. Based on the fundamental mass and momentum balance equations that describe the process of dilation of a bed when subjected to shear, a new process based ero-sion equation has been derived, which when validated against erosion measurements from the dredging industry shows promising results. This paper compares the prediction of soil erodibility by means of a JET test to the out-comes of the process based erosion equation. The paper highlights important differences and attempts are made to explain these.@en

Flood defense failures are rare events but when they do occur lead to significant amounts of damage. The defenses are usually designed for rather low-frequency hydraulic loading and as such typically at least high enough to prevent overflow. When they fail, flood defenses like levees built with modern design codes usually either fail due to wave overtopping or geotechnical failure mechanisms such as instability or internal erosion. Subsequently geotechnical failures could trigger an overflow leading for the breach to grow in size Not only the conditions relevant for these failure mechanisms are highly uncertain, also the model uncertainty in geomechanical, internal erosion models, or breach models are high compared to other structural models. Hence, there is a need for better validation and calibration of models or, in other words, better insight in model uncertainty. As scale effects typically play an important role and full-scale testing is challenging and costly, historic flood defense failures can be used to provide insights into the real failure processes and conditions. The recently initiated SAFElevee project at Delft University of Technology aims to exploit this source of information by performing back analysis of levee failures at different level of detail. Besides detailed process based analyses, the project aims to investigate spatial and temporal patterns in deformation as a function of the hydrodynamic loading using satellite radar interferometry (i.e. PS-InSAR) in order to examine its relation with levee failure mechanisms. The project aims to combine probabilistic approaches with the mechanics of the various relevant failure mechanisms to reduce model uncertainty and propose improvements to assessment and design models. This paper describes the approach of the study to levee breach analysis and the use of satellites for breach initiation analysis, both adopted within the SAFElevee project.@en

As part of the SAFElevee project Delft University of Technology collabored with Flanders Hydraulics Research, and Infram B.V. in the preperation and execution of a full scale embankment breach experiment in November 2015. This breach experiment was performed on an 3.5m high embankment with a sand core and clay outer layer situated along the tidal river Scheldt in Belgium near Schellebelle. During the experiment a wave overtopping simulator and overflow simulator were used to initiate a breach. Both simulators were placed near the top of the waterside slope. The use of the simulators facilitated comparison between the effects of continueous overflow and the effects of intermittent wave overtopping. This paper presents the data collected during the experiment, describe the development of hypotheses on the failure processes using the latest insights, and comment on the failure initiation process of a grass covered flood embankment with a clay outer layer and a sandy core.@en

Within the frame work of the realisation of the ‘Sigmaplan’ for the river Schelde in Flanders (Belgium), a large-scale dike breaching experiment following overflow was held at Lillo (Antwerp) in 2012. The outcomes of the breach test serve to unveil the impact of a chosen breach growth model, to set application limits, to come up with guidelines for proper selection and usage of the model to be applied.

Breach growth models are used to predict the breach dimensions and to estimate the flow through the breach. All assessed models pretty well succeed in this. However, starting from various premises and taking into account a (limited) set of different breaching mechanisms, the use of today’s state-of-the-art breach growth models is not entirely trouble free
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Emergence of light detection and ranging (LiDAR) technology provides new tools for geomorphologic studies improving spatial and temporal resolution of data sampling hydrogeological instability phenomena. Specifically, terrestrial laser scanning (TLS) collects high resolution 3D point clouds allowing more accurate monitoring of erosion rates and processes, and thus, quantify the geomorphologic change on vertical landforms like dike landside slopes. Even so, TLS captures observations rapidly and automatically but unselectively. In this research, we demonstrate the potential of TLS for morphological change detection, profile creation and time series analysis in an emergency simulation for characterizing and monitoring slope movements in a dike. The experiment was performed near Schellebelle (Belgium) in November 2015, using a Leica Scan Station C10. Wave overtopping and overflow over a dike were simulated whereby the loading conditions were incrementally increased and 14 successful scans were performed. The aim of the present study is to analyse short-Term morphological dynamic processes and the spatial distribution of erosion and deposition areas along a dike landside slope. As a result, we are able to quantify the eroded material coming from the impact on the terrain induced by wave overtopping which caused the dike failure in a few minutes in normal storm scenarios (Q = 25 l/s/m) as 1.24 m3. As this shows that the amount of erosion is measurable using close range techniques; the amount and rate of erosion could be monitored to predict dike collapse in emergency situation. The results confirm the feasibility of the proposed methodology, providing scalability to a comprehensive analysis over a large extension of a dike (tens of meters).@en

A process based assessment of the probability of failure of a flood embankment, as well as an assessment of the consequences of failure of an embankment require insights into the stresses on the landside slope of an embankment. These assessments are hindered by the empirical nature of the wave overtopping parameters. Failure initiation is often linked to an allowable mean overtopping discharge which forms the input for the overtopping volumes distribution. The high level of uncertainty associated with predicting the mean overtopping discharge therefore leads to high levels of uncertainty in predicting wave overtopping volumes. The mean overtopping discharge is thereby not directly related to run-up parameters. This paper addresses these issues by presenting new distributions for the velocity, discharge, depth, volume, and shear stresses at the crest for those waves that overtop which have been derived from the wave run-up parameters. The proposed distributions are independent on the mean wave overtopping discharge and the large inaccuracies associated with predicting this. The proposed method has the added benefit of being able to express overtopping parameters in terms of each other. The paper also provides a method for determining the change in these random overtopping values along the landside slope, thereby facilitating a direct comparison between wave overtopping events and overflow events.@en

Deriving the bed shear stresses from hydrodynamic models in breach models is challenging due to the continuous changing hydraulic head over the breach in combination with horizontal and vertical flow contractions, and the continuous rapidly changing breach geometry. Three stages can be distinguished in breach flows. Stage 1 initiates when the embankment starts to overflow and is characterized by flows over the embankment crest and down the landside slope. Stage 2 initiates when the landside slope has retreated towards the waterside slope. The hydraulic head increases rapidly and the flow contracts both horizontally and vertically resulting in a fully 3-dimensional flow. During Stage 3 a full breach has developed and the flow contracts mainly horizontally. This paper presents a SWOT analysis of flow modelling methods applied in breach models to derive the bed shear stresses. The paper shows that for a number of cases analytical methods are more accurate than numerical methods due to the fact that they give a more accurate description of the shear stresses on the embankment surface, whereas for some numerical methods errors are found of the order of 50%.@en