Location: Watershed Physical Processes ResearchTitle: Experimental and Numerical Investigations of Floating Breakwater Performance.) Author
Submitted to: Laboratory Publication
Publication Type: Government publication
Publication Acceptance Date: 7/24/2009
Publication Date: 7/24/2009
Citation: Ozeren, Y., Wren, D.G., Alonso, C.V. 2009. Experimental and Numerical Investigations of Floating Breakwater Performance. Laboratory Publication. #65 Interpretive Summary: The earth levees commonly used for irrigation reservoirs are subjected to significant embankment erosion due to wind-generated waves. Large seasonal fluctuations in water level make vegetative bank protection impractical, and other stabilization methods, such as the use of stone or discarded tires, are not acceptable due to ecological or economic concerns. Here, a floating wave barrier made of polyethylene irrigation tubing is designed through a laboratory model study and subjected to a short-term prototype-scale field test. Based on wave characteristics measured in an irrigation pond near Carlisle, Arkansas, a laboratory scale wave generating flume was designed, constructed, and used to test multiple wave barrier configurations for regular waves in deep and transitional water depths. Several different configurations were used in the laboratory testing. In the end, a composite design made of two sizes of cylinders joined at the top was chosen for field testing. A short full-size section was constructed and deployed in an irrigation reservoir. After data analysis, the field test showed that wave heights were reduced by an average of approximately 50%, which translates into a 75% reduction in wave energy. Numerical investigations were also used to investigate the mechanisms responsible for wave energy dissipation. The results from the field and laboratory were used to verify that the computer model was able to correctly simulate the interactions between waves and the wave barrier. This will allow future designs to be further refined using a computer before field testing. The design that resulted from this study will allow farmers to construct cost-effective wave barriers that can significantly increase the life span of earth levees by reducing wave induced erosion.
Technical Abstract: Floating breakwaters are commonly used to protect small marinas and for shoreline erosion control in coastal areas. They are efficient wave attenuation structures for relatively short waves and shallow water depths. The main objective of the current study is to investigate the hydrodynamic interaction of small scale waves with floating breakwaters experimentally and numerically. The breakwater models considered here are single or multiple cylindrical sections with different mooring configurations. For the model studies a wave flume with a flap type wave generator and a progressive wave absorber was designed, constructed, and used to investigate the wave transmission characteristics of multiple breakwater configurations for regular waves in deep and transitional water depths. The test results show that wave attenuation is achieved by different mechanisms depending on the wave characteristics and breakwater configuration. When the models were fully restrained primary mechanism to reduce transmission coefficient was reflection hence the efficiency strongly depend on the draft and the freeboard of the breakwater. For partially restrained models dissipation also becomes significant and therefore efficiency can be increased by increasing the damping characteristics of the breakwater. It was also observed that horizontal restraint is superior to vertical restraint in terms of breakwater efficiency. A floating breakwater made of polyethylene irrigation tubing was designed through a laboratory model study and subjected to a short-term prototype-scale field test. In the field test, wave amplitudes were reduced by an average of approximately 50%. It is concluded both from the model and the field studies that cylindrical pipes can be used effectively as breakwaters for small lakes and reservoirs. A two dimensional numerical wave tank was designed in Flow3D to further investigate the interaction of the waves with the cylindrical breakwaters. A new method is developed coded into Flow-3D to generate waves by using a mass source function. The source function allowed wave generation without blocking the reflected waves. The new method was coded into Flow-3D and tested for a wide range of linear monochromatic waves and irregular waves. The results are compared with analytical solutions and experimental data and it is shown that the new method can successfully be used within a wide range of deep and intermediate wave conditions. The numerical wave tank was used to examine the effect of overtopping and breakwater restraint for a wide range of wave parameters and breakwater configurations. The results are validated with the experimental outcomes using far field transmission and reflection properties. Prototype scale simulations were compared with the field data. Moreover, it is shown that the numerical model compared reasonably will with the experiments in case of overtopping.