Wave overtopping aspects of the Crest Drainage Dike

Abstract

In the framework of the ComCoast project, the concept of the Crest Drainage Dike has been studied regarding the reduction of wave overtopping. This study only focuses on the average wave overtopping discharge. The basic concept of the Crest Drainage Dike is a basin, integrated in the crest of the dike, that collects overtopping water and thus reduces the load on the inner slope of the dike. The collected water in the crest basin is drained landward or seaward through pipes. The main goal of this report is to identify the physical background of the concept of the Crest Drainage Dike and to predict the wave overtopping discharge as a function of hydraulic and geometric boundary conditions. Therefore two different types of theoretical studies have been executed. The first study is process-based and serves as a basis for the numerical program that has been developed. Since this model is partly based on several assumptions, several physical model tests have been executed to verify or reject the stated hypotheses. In the physical model tests, several hydraulic and geometric boundary conditions, such as the wave height, the crest freeboard, the use of berms, the wave spectra, the wave steepness and the drain layouts, have been varied. Since the predictions of the numerical program are well in line with the measured wave overtopping discharges, the numerical program is used to investigate the use of a Crest Drainage Dike in two case studies. The case studies are the Hondsbossche Sea Defence and the Perkpolder Sea Defence. Both dikes are located in the Netherlands The use of this numerical program gives a better insight in the physical background of the Crest Drainage Dike. The description of these physics is the second part of the theoretical study. For dikes with severe wave attack, such as the Hondsbossche Sea Defence, only a small fraction of the waves is reaching the crest of the dike. However, the waves that do reach the crest of the dike have a relatively large volume and the buffer capacity of the Crest Drainage Dike limits the effectiveness of the Crest Drainage Dike. Besides this, there is a high statistical uncertainty since the average wave overtopping discharges are determined by only a couple of waves. For dikes with a lower wave attack, such as the Perkpolder Sea Defence, more waves with a lower volume per wave are overtopping and therefore the concept of the Crest Drainage Dike works well. However, the crest freeboard reduction with the use of the Crest Drainage Dike is in these specific cases is only minor. Based on the numerical studies and the current Dutch overtopping criteria, the reduction of the crest freeboard with the use of the Crest Drainage Dike is determined and is significantly lower then the assumed reductions in earlier studies.