How to Determine the Phreatic Surface in a Dike During Storm Conditions with Wave Overtopping: A Method Applied to the Afsluitdijk

The most famous dam of the Netherlands, the Afsluitdijk, is in need of renovation. Hydraulic conditions for the year 2050, for some aspects even 2100, must be considered during the renovation of the dam originally constructed 1927-1933. A combination of sea level rise due to climate change and a change in the applied statistical methods result in design conditions that are, at some locations, meters higher than those used a century ago. In a mission to retain the required level of safety while keeping expenses at a minimum, several innovative designs are considered; one of which is to allow very large overtopping discharges by making the dam resilient to erosion. However, calculations have shown that the overtopping values can be over 10 litres per second per meter dam, and the effect of this overtopping on the Afsluitdijk is unknown. In Dutch national design guidelines for dikes, schematizations are proposed for the phreatic surface in dikes during storm conditions. Wave overtopping is mentioned to have a rising effect on the phreatic surface, but clear methods to determine the magnitude of this effect are not prescribed. The guidelines suggest assuming full saturation of the dam in severe overtopping conditions, which has a detrimental effect on the slope stability. Recent studies and large scale infiltration tests on the Afsluitdijk show that only a very limited amount of water infiltrates in the dam and that the assumption of full saturation is much too conservative in case of the Afsluitdijk. In the present study, a finite-element model (Plaxflow) is used to model the phreatic surface in the Afsluitdijk during storm conditions, taking into account the effect of wave overtopping on the phreatic line. Parameters were determined by in-situ and laboratory investigations. To verify the model, calculations were calibrated on standpipe measurements. Wave overtopping is modelled by defining infiltration boundaries based on experiments and sensitivity analyses are conducted. The result of the study is a phreatic surface that can safely be used for the calculation of the slope stability of the levee during predicted storm conditions for the year 2050.