Time-dependent development of Backward Erosion Piping

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.Hydraulic Structures and Flood Ris

Hydrograph shape variability on the river Meuse

Design water levels are a basic concept in flood risk management practice. These water levels with a specified return period are used for the design of dikes and other flood protection measures along the river Meuse. Current practice is that these water levels are determined by hydrodynamic simulation of a standard design hydrograph at the upstream gauging station Borgharen. The peak discharge of this standard design hydrograph is based on a frequency analysis and its shape is determined by scaling and averaging all flood hydrographs in the dataset. Then it is assumed that the simulated water levels have the same return period as the peak discharge at Borgharen. Until now, this method was not validated. The aim of this thesis is to investigate the influence of hydrograph shape on design water levels on the river Meuse, and to evaluate current and alternative methods to take this shape into account. The five alternative methods are (1) hydrodynamic simulation of all floods in the dataset and apply frequency analysis afterwards on the simulated water levels (the reference), (2) the standard method extended with dependence between the hydrograph shape and the peak discharge, (3) vertical averaging, and two probabilistic methods which combine hydrograph shape statistics at Borgharen with a transformation function that relates local water levels to these hydrograph shape statistics. Within the probabilistic methods one can distinguish explicit (4), which expresses the statistics in probability distribution functions, and implicit (5), which does not use these functions. The influence of hydrograph shape variables on the downstream water level was investigated by means of a correlation analysis. To evaluate the different methods, all methods were applied to a GRADE dataset of 50,000 years of generated discharge at Borgharen, and the resulting design water levels compared to the reference. In addition to the ability to estimate design water levels, the methods were evaluated on the ability to estimate the design water level reduction of a retention basin (Lob van Gennep). Peak discharge combined with peak curvature were found to be good predictors of the downstream water levels, and were used in the probabilistic methods. The evaluation of methods shows that the currently used standard hydrograph method overestimates the design water levels up to 37 cm with respect to the reference. The present research shows that the current method to determine design water levels can be improved significantly. A simple improvement is to use the vertically averaged design hydrograph with a modified selection interval. More advanced probabilistic methods also improve the estimates, and are potentially valuable in case of retention basins, of which the effectiveness is sensitive to the hydrograph shape.Hydraulic EngineeringCivil Engineering and Geoscience

Influence of erosion on piping in terms of field conditions

The Shields–Darcy (SD) model by Hoffmans and Van Rijn (Citation2018) describes the resistance of hydraulic structures to backward erosion piping, which is a form of internal erosion. In the article being discussed, Hoffmans compares the SD model to the model by Sellmeijer et al. (Citation2011), focusing on field scales. This Discussion presents finite element simulations that deviate from Hoffmans’ conclusions that the model by Sellmeijer et al. (Citation2011) results in an unrealistically low critical gradient. As both the SD and Sellmeijer models fit reasonably well to laboratory experiments (Hoffmans & Van Rijn, Citation2018), extrapolation to field scales (say aquifer thickness D > 5 m, seepage length L > 10 m) is important, particularly since these models are used for the design of flood defences. Hoffmans addresses this issue by analysing the resistance as function of aquifer depth D. Hoffmans recommends checking the outcomes of the SD model with a mathematical piping model like that of Van Esch et al. (Citation2013).Hydraulic Structures and Flood Ris

Project Summary D3 - Time-dependent piping and interactions: A framework for safety assessment with time-dependent failure processes

Hydraulic Structures and Flood Ris

Shields-Darcy pipingmodel. Verschilanalyse met Sellmeijer en D-GeoFlow.

Hydraulic Structures and Flood Ris

Numerical interpretation of regressive localized internal erosion in a real-scale levee physical model

This paper presents the numerical interpretation of a recent experiment on a real-scale levee physical model, in order to investigate the process of Backward Erosion Piping (BEP) and validate a recently proposed finite element formulation able to model both the simultaneous processes observed in backward erosion piping, i.e. the propagation of the pipe tip and the enlargement of the conduit cross-section, as well as the time-dependent effects. In previous papers, the numerical formulation already demonstrated its ability in reproducing available experimental data of full-scale physical models of levees, e.g. for the IJkdijk and for the Delta Flume tests. In the present work, as a further validation for the aforementioned formulation, we consider the numerical interpretation of the regressive localized internal erosion observed in the newly constructed real-scale levee at the Flood Proof Holland facility test site in Delft, The Netherlands. This test was mainly focused on the experimental evaluation of the time-dependent effects typically observed in these phenomena. To this purpose the levee foundation was equipped with an effective and accurate pore water pressure monitoring system. The aforementioned formulation was considered for the numerical interpretation of the test, in view of its ability in modeling the time-dependent effects in backward erosion piping. Indeed, a good agreement between calculated and measured piezometric heads and pipe tip propagations was obtained.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

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

Temporal Development of Backward Erosion Piping in a Large-Scale Experiment

This paper presents a large-scale backward erosion piping experiment aimed at studying the erosion rate. This temporal aspect of piping complements previous research that focused on the critical head. To study the progression rate in realistic conditions, an experiment was carried out on a 1.8 m high levee with a cohesive blanket on a sandy foundation. The pipe was guided along a row of pore pressure transducers in order to measure its temporal development. Pipe development in space and time was successfully derived from pore pressure changes, showing an average progression rate of 8  m/day during the progressive erosion phase. The results show a relation between upstream gradient and progression rate. Furthermore, analysis of the eroded sand mass shows a relatively large pipe volume compared to existing lab tests, and an approximately linear relation between pipe length and volume. The results and insights from this study can be used to validate and improve transient piping models, leading to more accurate dam and levee safety assessments.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

Verification of a Predictive Formula for Critical Shear Stress with Large Scale Levee Erosion Experiment

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.Hydraulic Structures and Flood Ris