Influence of foreshore steepness on wave velocity and acceleration at the breakwater interface

Influence of foreshore steepness on wave velocity and acceleration at the breakwater interface

Oortman, N.J.

Stive, M.J.F. (mentor)
Verhagen, H.J. (mentor)
Fontijn, H.L. (mentor)
Reedijk, B. (mentor)

Civil Engineering and Geosciences

2006

This Masters Thesis is a pilot research project in order to investigate which parameters, other than the wave energy density spectrum, could play a role in breakwater damage on variable foreshore steepness. The research project is based up on laboratory research, were for equal wave spectra at the toe of a coastal structure significantly more damage (order of 30%) occurs to a steep foreshore in contrast to a mild slope. In order to investigate which parameters, other than the parameters included in the wave energy density spectrum could play a role in the increase in damage for a steep foreshore, an experiment was conducted. In the wave flume of the Fluid Mechanics Laboratory an experimental test set up was constructed. The test setup consisted of a breakwater build on variable foreshore steepnesses (1:30, 1:15 and 1:8). Tests were conducted with regular waves, were the wavelength and the wave steepness at the toe of the breakwater were kept constant per individual test. These tests were conducted for the three different foreshore steepnesses. Wave runs were completed for different wave heights and wave steepnesses. Each experiment was repeated in order to increase the accuracy. At the breakwater the wave velocity and acceleration was measured with zero buoyant particles. The particles were monitored with a video camera. From these video images the velocity and acceleration relative to the breakwater slope were calculated. In order to check the accuracy a reliability study of the experiment was performed. The faults in the experiment were estimated and assumed to be normal distributed. The reliability of the entire experiment was calculated for velocity and acceleration measurements. The velocities and accelerations for up-rush and down-rush were compared for equal wave conditions and different foreshore steepnesses. The experimental data shows that the velocities for up-rush and down-rush are higher for waves travelling over a steep foreshore (order of 10%). Also the obtained accelerations for both up-rush and down-rush are higher for waves travelling over a steep foreshore. When the wave force is calculated with a Morrison equation, the increase in force due to the increase in velocity for waves travelling over a steep foreshore is in the order of 20%. When considering that the acceleration is also larger in this case, this results an even larger total wave force. By linking the wave force to the level of damage, it can be explained that for equal wave spectra at the toe and for variable foreshore steepness the largest damage was measured for waves travelling over a steep foreshore.

http://resolver.tudelft.nl/uuid:5f8ed8f1-cb32-4c71-8dd3-a9ef89ef9267

TU Delft, Faculty of Civil Engineering and Geosciences, Hydraulic Engineering

https://doi.org/10.4121/uuid:76a99f73-b670-4df9-bc38-72477a1f30eb

Student theses

master thesis

(c) 2006 N.J. Oortman