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Wave transmission past a floating breakwater with a perforated fence Mulligan, Ryan Patrick


Floating breakwaters are used to attenuate wave action near shorelines and provide wave protection in marinas. Many different designs have been used. This thesis describes a field and numerical study of wave propagation past a particular floating breakwater design. The design corresponds to a pile restrained, concrete caisson, floating breakwater with a perforated fence. The floating breakwater has been installed at Ganges on Saltspring Island, British Columbia. Field tests were carried out to estimate the transmission characteristics of the breakwater by subjecting it to boat-generated waves with varying heights, periods and directions. Several phenomena that influence the wave transmission, including the boat-generated wave characteristics, reflected and evanescent waves and breakwater motions are addressed in the analysis. The numerical model is based on two-dimensional linear wave diffraction theory. The model is used to evaluate the breakwater's wave transmission performance for varying breakwater dimensions, wave periods, wave directions, breakwater motions and damping coefficients. The suitability and performance of the design predicted by the field study and numerical analysis are assessed. Of the parameters that were examined in the analysis of the floating breakwater the breakwater dimensions were determined to have the greatest effect on wave transmission, the degree of damping and breakwater motions both have a large influence on the results and the incident wave direction has a relatively small effect. Overall, the breakwater performs well for low wave-energy environments or in environments where the wave climate is dominated by boat-generated waves. The vertical perforated fence attached to the concrete caisson extends the effective draft of the floating breakwater and has been shown to improve wave reflection and cause increased damping. Therefore, the fence decreases wave transmission past the floating breakwater. Due to gaps in the pile wells, the breakwater may undergo some roll motions, and it tends to behave as freely floating for short period waves, in which case roll resonance can occur.

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