Stability of Single-Layer Cubes on Lee-Side of Breakwaters

Abstract

Breakwaters are hydraulic structures used to create calm and steady environments for moored ships in harbors. They dampen the wave by dissipating the energy through wave breaking. Most studies of breakwaters have only been carried out on the seaward slope of the structure. However, few have conducted research on the landward-, inner-, slope of breakwaters. The existing accounts
re all empirical design formulae, commonly denoted as applicable for conceptual design. Improvement of the design of the armour layer of the inner slope leads to economical advantages.This research focused on three main goals: (1) Presenting a theoretical methodology for the waves tested, (2) It veried the Wave Overtopping Simulator as built by Rietmeijer (2017) and (3) Performed stability tests for the inner slope by conducting physical model tests in the Fluid Mechanics Laboratory of Delft University of Technology.

The first goal of this research has been to develop a theoretical approach to overtopping waves in combination with a simulator. This has been done by linking certain storm characteristics, being the significant wave height Hs and peak period Tp to wave characteristics; water volume, water layer thickness and the front velocity of the water body. These three combined lead to an impinging wave on the rear slope causing damage. Subsequently the storms have been altered to only select the highest percentage of waves, since these were marked as extreme storm conditions. This resulted in dierent signicant wave heights which were tested.

To verify the Wave Overtopping Simulator tests have been performed and specic elements have been measured and analyzed. This has resulted in relationships between theoretical wave characteristics and simulator settings. The relationships are the required water level in the reservoir to obtain a certain front velocity of the water body. To adjust the water layer thickness the valve settings have been changed. For a specic volume the overtopping period has been altered. These three elements have been measured using wave gauges and a high speed camera after which the data was carefully analyzed.

The stability tests have been executed using a model of a breakwater in the Fluid Mechanics Laboratory of Delft University of Technology. Results of the preparatory calculations together with results obtained from former research have led to the design of the test set-up. A single armour layer of 20 mm cubes with an porosity of nearly 40% was built with an fixed toe. This configuration was tested with increasing storm intensities until failure. Subsequently the damage has been measured using photo camera's and laser technique.

This research has successfully verified the wave overtopping simulator by measuring individual waves and their specific discharge, water layer thickness and velocity. These measurements have been compared to an other method, using a high speed camera. The found values have been checked with found literature and it has been concluded that the Wave Overtopping Simulator together with the test set-up as built in the Fluid Mechanics Laboratory is useful for this research. By measuring the displacement of cubes by means of video camera and laser technique, the research has been able to identify damage and failure cases. The horizontal displacement has been divided into several classes ranging from settlement of the armour layer, to instant failure. In addition, this research has presented a possible link between the stability of a single cubic armour layer and the number of rows beneath the waterline.