Graduation of Dimitrios Dermentzoglou

Crownwalls with a fully curved face. An experimental study.

• Professor of graduation: Dr. ir. A. Antonini

• Supervisors of graduation: Dr.ir. B. Hofland (TU Delft), Ir. E. de Almeida (TU Delft), Dr.ir. P. de Girolamo (Sapienza University of Rome)

This work studies crownwalls with a fully curved face versus a vertical wall in non-breaking wave conditions. Its aim is to provide appropriate tools that can be used to compare the performance of the two structures in terms of loading and overtopping. To this end, small scaled experiments of the two structures are carried in a flume with a flat bottom. These are performed in intermediate water depths and consist of different combinations of wave height and wave steepness. A box is positioned behind the superstructures to collect overtopping volumes. Pressure signals are retrieved from pressure sensors positioned on the surface of the crownwall and the vertical wall, while water elevation signals are retrieved from wave gauges in the flume and in the overtopping box. Several analyses are performed in order to retrieve results of individual waves regarding the wave characteristics, reflection coefficient, overtopping volume, maximum pressure, impulse, impulse duration, maximum force, force angle and point of application of the force. The effectiveness of the crownwall in reducing overtopping volumes is found to decrease for more severe wave attacks with larger wave height and wave length. The lowest loading cases result in a quasi-static pressure signal which has approximately the same maximum value for both structures. More severe wave attacks generate impacts at the seaward top outer edge of the crownwall, which result in an increase of maximum pressure up to 1100%. For these cases, impulses are calculated and the results are found to be less uncertain compared to maximum pressure, while their duration shortens for higher and longer waves. The variables describing force are retrieved by integrating the pressure signals on the surface of the superstructures and the results of maximum force are found to follow a similar trend to these of maximum pressure. Corresponding equations from curve fitting are derived for each of the mentioned variables regarding the crownwall. In lack of design overtopping formulas for regular waves, an equation is derived for the prediction of overtopping for the vertical wall in order to allow comparison between the two shapes. The formula of Goda is evaluated for the case of the vertical wall and is found to accurately predict pressure at still water level, while it underestimates pressure at higher positions. Nevertheless, using this formula to estimate loading on a vertical wall is suggested due to its range of applicability and high accuracy at still water level. These equations can serve as a tool to compare solutions of a crownwall and a vertical wall, and assess which of the two is the optimal solution.

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