Experimental study on the effect of submerged breakwater configuration on long wave run-up reduction

Shing, Tony Kai Chung
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[Honolulu] : [University of Hawaii at Manoa], [December 2014]
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Experimental study was carried out to observe and quantify the effects that a submerged breakwater has on long wave run-up reduction. As shown in the past, long wave run-up, or tsunami can be very devastating. In 2011, the tsunami in Japan resulted in over 300 billion dollars of damages along with several thousand lives. A recent finding of coral and ocean sediments found in a sinkhole in Kauai suggest that this catastrophic event could happen in Hawaii. The focus of this thesis study is to find the optimal breakwater length for run-up reduction and examine the effects that a breakwater's geometry has on run-up reduction, aiming at seeking specific configurations and designs of submerged breakwater that may be effective in reducing long wave run-up. This included experimental test on waves of different amplitude (up to 10 different amplitude) propagating over breakwater models of different material (rigid/impermeable/smooth surface vs. flexible/porous/rough surface), length, spacing, and geometry, and running-up onto artificial beach of different slopes. In addition, the experimental results were compared with a previous numerical study done by Mohandie (2008) and Mohandie and Teng (2012). In their numerical study, it was found that when the length of a submerged rectangular breakwater is twice the wavelength of the generated long wave, the maximum run-up reduction rate was achieved. This numerical result had not been examined and validated in an experiment before this thesis study. In the present thesis study, it was found that foam/flexible breakwater material provided a better run-up reduction than plastic/rigid material. For the effect of the geometry of the breakwater, a triangular saw-tooth geometry provided better run-up reduction than a circular speed bump geometry. Both triangular and circular models provided better run-up reduction than a flat rectangular model. For the effect of breakwater length, the experimental results generally follow the same trend as the numerical simulations done by Mohandie and Teng (2012). The results showed a trend of increasing run-up reduction until a certain point before decreasing as the model length increases. The point at which the run-up reduction decreases varied in the experimental results and ranged from 1.25-1.9 wavelengths. In the 5 degree beach slope test, two rectangular breakwater models with a spacing in-between compared to a breakwater models of equal length without the spacing showed no significant difference in run-up reduction. However, in the 10 degree beach slope test, the results were mixed. The rigid rectangular model had a better run-up reduction for the breakwater model with spacing whereas the flexible rectangular model had a better run-up reduction for the breakwater without spacing. This issue may require further investigation. The results from this study may be useful in future design of submerged breakwaters for reducing long wave run-up. Specifically, we recommend breakwaters with a saw-tooth geometry, or of flexible/porous material with a rough surface. For the breakwater length, it should be comparable to the wavelength, if practically possible in near shore regions.
M.S. University of Hawaii at Manoa 2014.
Includes bibliographical references.
submerged breakwater configuration
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Theses for the degree of Master of Science (University of Hawaii at Manoa). Civil Engineering.
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