Rocking Revisited 2

Abstract

Rocking behavior of breakwater research was conducted by Centre for Civil Engineering Research and Codes workgroup CUR C70 (1989) and after that no further validation of the research work has been done. In order to understand the rocking behavior and validate the previously conducted research CUR C70 (1989) wave flume test has been conducted in similar setup. One of the important parameter of rocking behavior is the magnitude of impact velocity and the distribution of this impact velocity stochastically and spatially. This research investigates the order of the magnitude of the impact velocity and distribution of the impact velocity comparing, validating and concluding new findings those were not incorporated in CUR C70 (1989) research.

During this research a new type of sensor Tinyduino has been used inside a cube to measure the acceleration and angular velocity of the cubes. Eight of these sensors were used in eight different cubes. All the sensors were tested properly in Deltares to check the sensors working properly and better understanding the sensors. Before this research two test programs were performed by Deltares in tetrapod in double layer using this Tinyduino sensor in stand alone mode but the data were not analyzed. This research analyzed the data for the tetrapods provided by Deltares and compared with CUR 70 (1989). During this analysis and the testing of the sensors it is found that angular velocity measurement can be much more reliable than acceleration hence the angular velocity measurement is used during data processing of instrumented cubes in double layer.

For experiments eight instrumented cubes were placed using very flexible wire in a randomly placed double layer cubes with same size and almost similar density over a permeable filter layer. The slope 1:1.5 is used for the experiment same as the CUR C70 (1989) research conducted. The cubes were placed in one constant level but the water level is varied in order to demonstrate different slope position. Three slope position Y/Dn=0, Y/ Dn =-2 and Y/ Dn =-4 is used during the research program. Three different wave heights and two wave steepness were used during this test program. Due to time limitation 18 test setups within two days were performed. The data were collected real-time using wire and saved in text file simultaneously with eight instrumented cubes stored in a laptop provided by Deltares.

After the test program all the data is processed with matlab script and analyzed. The result of the analysis showed that the order of the magnitude of impact velocities is same as the CUR C70 (1989) research. It has also been found that the impact velocities are also dependent on the wave steepness which was not included in the CUR C70 (1989) research. So it is recommended for future work to update the equation incorporating wave steepness. Another important parameter is number of collision which was assumed to be 3 times is CUR C70 (1989) but found incorrect during this thesis. It is concluded that number of collision is dependent wave height, wave steepness, position over slope and also exposure to wave attack. It is also found that the number of collision is continuous during wave attack after a certain impact velocity.

One of the finding of CUR 70 (1989) was the location of maximum impact velocity over slope. It was concluded in CUR 70 (1989) that the maximum impact velocity lies on Y/Dn=0 meaning on the water level but during this research all the sample those worked observed that the maximum impact velocity is located on Y/Dn =-2 over the slope under the water level.

For future research work it is recommended to use different types of armor unit in single layer to understand the rocking behavior and also it is recommended using the Tinyduino sensors which provides accurate data on movement of the armor units.