Graduation of Lars Stenveld

Stability assessment of the landside slope cover on clay flood embankments in the Netherlands

• Professor of graduation: dr. ir. J.P. Aguilar-Lopez (TU Delft)

• Supervisors of graduation: Dr. ir. M. van Damme (TU Delft & Rijkswaterstaat), Ir. S.J.H. Rikkert (TU Delft), Ir. J. Van Mechelen (RPS advies- en ingenieursbureau)

Large parts of the Netherlands are vulnerable to flooding. Due to the great impact that a flood would have for the Dutch society, The flood safety standards are very strict, which means that the required failure probability of flood defences is very low. To keep the flood risk within acceptable limits of the hinterland, flood embankments are assessed on whether the probability of failure matches the stringent norms using the methods and rules which are stated in the legal assessment instrument (WBI). Within the WBI the probability of failure of the flood embankment equals the sum of the probability of occurrence of individual failure mechanism. This thesis specifically focuses on the failure mechanism: sliding of the landside slope cover due to wave overtopping of clay flood embankments. Due to wave overtopping the relative permeable top layer of the clay embankment becomes saturated. This leads to a shallow subsurface flow parallel to the landside slope cover which is unfavourable for the geotechnical stability of the landside slope cover.  The stability assessment of this mechanism is currently assessed with the Edelman & Joustra formula. This formula method is not based on a probabilistic or semi-probabilistic analysis and therefore does not give an failure probability. Instead it uses safety factors. Levee managers therefore believe that the outcome is overly conservative (KKP: grasbekledingen, 2020). This thesis addresses the question: How can the stability assessment of the Edelman & Joustra formula can be optimized for clay flood embankments in the Netherlands within the legal assessment instrument? To answer this question, new semi-probabilistic safety factors were derived based on a full probabilistic assessment of the Edelman and Joustra formula. The outcomes show that the current approach is not always as conservative as is often assumed. Application of the newly derived safety factor allows levee managers to be more flexible in how to account for the contribution of this failure mechanism to the total levee failure probability. The newly developed method therefore prevents unnecessary rejection and over-dimensioning of flood embankments, so the available resources for flood embankment improvements can be used better.