Wave Overtopping Resilient 'Afsluitdijk': Design Procedures for Landward Slope Erosion Stability during Large Overtopping Volume Events

The Afsluitdijk is unable to withstand the future conditions that belong to an annual probability of occurrence of 1/10 000 per year for water level and wave conditions. Solution of Rijkswaterstaat is to strengthen the dike according to the principle of the wave overtopping resilient dike (NL: Overslagbestendige dijk). Therefore the Afsluitdijk has to be able to cope with large amounts of wave overtopping (>150 l/s/m). The current design methods are unsuitable to prove the resistance of the Afsluitdijk against these large amounts. The goal of this thesis is to develop design procedures in order to prove the landward slopes erosion resilience of the Afsluitdijk against large amounts of wave overtopping. Based on a theoretical study a new design procedure has been developed, this method consists of six sub-procedures, two choices and two results. The first step is to determine the wave overtopping discharge. Up to 30 l/s/m good quality grass can be applied. This average wave overtopping discharge appeared a good measure for the load below 30 l/s/m. Otherwise the load should be expressed as the front flow velocity per overtopping wave. It is important to express the velocity for each wave separately, because only waves resulting in a higher velocity than the critical velocity contribute to the damage. Damage only occurs if the critical velocity the landward slope is able to withstand is exceeded. These wave are the number of critical overtopping waves Ncow or the percentage of waves that contribute to the damage Pcow. The overtopping velocities are based on empirical relations between the overtopping velocity and volume. The volume has been determined using the probability of a certain wave volume to occur (a Weibull distribution with a freeboard dependent scale factor a and shape factor b). A categorization based on the discontinuities and objects that are present on the Afsluitdijk has been created. For each category an amplification factor for the front flow velocity has been derived which varied from 1 (no influence) to 2.1 (for holes). With the velocity distributions the required critical velocity can be calculated using the cumulative overload factor. In which each wave has contributes to the damage D. The influence of the storm duration and the allowable damage number D on the critical velocity has been analyzed. As well as the relation between the critical velocity, the number of critical overtopping waves and the percentage of critical overtopping waves. The results of the procedure are a required protective top layer or required investigations to complete the procedure. These investigations can be the development of the flow velocity on the berm, gradual transition between slope and berm, transition between dike section geometry, transition between different revetment types, influence of bushes, damage as an effect of large external structures and the resilience of revetments other than grass. The application of this procedure to the Afsluitdijk resulted in required critical velocities varying from 5.8 (smooth crest) to 12.4 m/s (rectangular structures). From tests in the past it appeared that the top layer of the Afsluitdijk is able to withstand 6.3 m/s. This is insufficient for the conditions considered in the thesis. The limits of grass are exceeded and another revetment type should be applied. For pulsating wave overtopping currently no method exists that is able to prove the resilience of other revetments than grass, this is due to the non steady state character of wave overtopping. With this knowledge and realizing that the whole outer revetment should be replaced as well, it can be questioned if the concept of an overtopping resilient dike is the best choice for the reinforcement of the Afsluitdijk.

Subject

overtopping
resilient
Afsluitdijk
large volumes
landward slope
erosion

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Embargo date

2014-01-18

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Student theses

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master thesis

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