University of Twente Student Theses


Quantification of grass erosion due to wave overtopping at the Afsluitdijk

Kriebel, Martijn (2019) Quantification of grass erosion due to wave overtopping at the Afsluitdijk.

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Abstract:Grass erosion on the crest and inner slope due to wave overtopping is one of the mechanisms that can cause a flood defense to fail according to the Dutch WBI 2017 safety standards. The probability of failure of the grass cover layer for a certain grass quality is determined based on the combination of significant wave height and average overtopping discharge. However, this relationship is only established for wave heights up to 3 meter. At the time of writing, a new design is being finalized for the Afsluitdijk, which is one of the primary flood defenses in the Netherlands. The normative significant wave height for this new design is 3.38 meter, thus exceeding the maximum wave height for which the probability of failure due to grass erosion is defined in the WBI 2017. Previous studies have used the cumulative overload method to quantify the combinations of wave height and critical overtopping discharge for wave heights up to 4 meter, but the effects of the geometrical and cover material transitions present on the Afsluitdijk have not been studied in detail. The goal of this study is to find the relationship between the significant wave height and the critical average overtopping discharge for wave heights larger than 3 meter. This is done using two approaches: (1) the cumulative overload method (COM) and (2) a combination of the coupled crest-inner slope velocity equations (VE) and the transition model (TM). However, the basic modelling approach that is used in this study could be applied to other models that can predict the amount of erosion due to wave overtopping. Simulations are carried out with significant wave heights up to 4 meter for the new design of dike section 17a of the Afsluitdijk. Several different cross-sectional locations are included in order to study the effects of transitions. Additionally, sensitivity analyses are performed to study towards which parameters the COM and the VE-TM are most sensitive and which could cause the largest variation in the found relationships.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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