Comparative Study on Hydrodynamic Performance of Porous and Non-Porous Submerged Breakwater

Abstract

The paper offers a novel comparative case study on the performance of solid versus porous breakwater physically modeled under similar laboratory and wave conditions. Unidirectional waves were generated in a two dimensional laboratory flume having dimensions of 21.3 m length, 0.76 m width and 0.74 m height. Keeping still water depth of 50 cm, waves with T=1.6 sec, 1.7 sec, 1.8 sec and 2 sec were generated from a piston type wave generator. At 800 cm from the wave generator, horizontally slotted submerged breakwater of 100cm width,75cm length and 40 cm height were constructed with varying porosities of n=0.4, 0.5 and 0.6. Water level data were collected at six different locations for 12 unique run conditions. Using the measured data different hydrodynamic coefficients were calculated which include transmission co-efficient Kt, reflection co-efficient, Kr and wave energy loss co-efficient, KL. These co-efficient values were then, analyzed with respect to relative breakwater width (k.B), [where, k = wave number (2π/L), B = breakwater width] and porosity of breakwater. For the comparison, hydraulic performance of solid rectangular submerged breakwater of 40 cm height and 100 cm width was also reviewed which was modeled in the same laboratory under same wave condition of T=1.6,1.7 1.8 and 2.0 seconds and a still water depth of 50 cm. Experimental results from the porous breakwater and subsequent comparison of the results with solid breakwater indicate that, for transmitting less wave energy through the breakwater, maximum transmission co-efficient, Kt = 0.526 was obtained for porous breakwater with the lowest porosity (n=0.4) for T = 1.6 sec whereas for the solid breakwater maximum Kt= 0.5 was obtained for T=1.6 second. In terms of wave height, percentage wave height reduction was maximum 47% for porous breakwater whereas it was 68% for solid breakwater both at T=1.6 second. The hydrodynamic performance shows a significantly higher wave height reduction percentage for solid breakwater as anticipated by the physics of the interaction of the breakwater and the wave too. Besides, the position of wave breaking for different case scenario for both types of breakwater were recorded and analyzed which demonstrates that for porous breakwater waves tend to transmit through the slots and break at a certain distance beyond the breakwater whereas in case of solid breakwater, waves tend to break before or on the breakwater.