Wave Transmission, Reflection and Energy Dissipation Characteristics of Floating and Fixed Pontoon Type Wave Barriers in Random Wave Fields

Abstract

Wave transmission, reflection and dissipation characteristics of floating and partially immersed fixed pontoon-type barriers dictate its optimal design. Model scaled experimental investigations were carried out on two different types of wave barriers; one, a floating slack moored pontoon barrier and another, a fixed partially immersed pontoon barrier. The hydrodynamic performances for a wide range of random wave conditions are assessed. The draft of both the pontoon models is kept constant with 12.66% of water depth. JONSWAP spectra was used for random wave generation. The investigations were carried out for incident wave steepness, His/Lp in the range of 0.007 to 0.097, relative water depth, d/Lp in the range of 0.093 to 0.452 and relative width of the breakwater, B/Lp in the range of 0.133 to 0.646. It was found from this study that increasing the relative width of the wave barrier could reduce the wave transmission coefficients and increase the wave reflection and energy dissipation coefficients for both the types of wave barriers. For floating barrier, for short period waves (B/Lp of 0.5 to 0.6), the transmission coefficients were in the range of 0.25 to 0.3 and for long period waves (B/Lp of 0.13 to 0.2), the transmission coefficients were in the range of 0.8 to 0.85. Fixing the barrier in space help to reduce the wave transmission to an extent of 10% to 15%, especially for the long waves. For d/Lp in the range of 0.093 to 0.452, for fixed barrier, the reflection coefficients were in the range of 0.6 to 0.8; whereas for floating barrier, it was only in the range of 0.3 to 0.6. It was found that the wave energy dissipation coefficient is better for floating barrier (0.4 to 0.8) compared to fixed barrier (0.2 to 0.5). For a design wave length of 40 m, in order to reduce 50%, 60%, and 70% of the incident wave height on the lee side, a floating pontoon barrier with width of 17.2 m, 21.6 m and 26.0 m respectively would be needed. For the same condition, barrier widths of 12.4 m, 18.8 m, and 26.0 m would be needed for a partially immersed fixed barrier with a draft of 12.66% of water depth. The data from this study will be useful for the design of floating, as well as partially immersed fixed pontoon type wave barriers.