Wave interaction with ‘⊥’-type breakwaters

Abstract The wave transmission, reflection and energy dissipation characteristics of ‘⊥’-type breakwaters were studied using physical models. Regular and random waves in a wide range of wave heights and periods and a constant water depth were used. Five different depths of immersion (two emerged, one surface flushing and two submerged conditions) of this breakwater were selected. The coefficient of transmission, K t , and coefficient of reflection, K r , were obtained from the measurements, and the coefficient of energy loss, K l was calculated using the law of balance of energy. It was found that the wave transmission is significantly reduced with increased relative water depth, d / L , whether the vertical barrier of the breakwater is surface piercing or submerged, where ‘ d ’ is the water depth and ‘ L ’ is the wave length. The wave reflection decreases and energy loss increases with increased wave steepness, especially when the top tip of the vertical barrier of this breakwater is kept at still water level (SWL). For any incident wave climate (moderate or storm waves), the wave transmission consistently decreases and the reflection increases with increased relative depth of immersion, Δ / d from −0.142 to 0.142. K t values less than 0.3 can be easily obtained for the case of Δ / d =+0.071 and 0.142, where Δ is the height of exposure (+ve) or depth of immersion (−ve) of the top tip of the vertical barrier. This breakwater is capable of dissipating wave energy to an extent of 50–80%. The overall performance of this breakwater was found to be better in the random wave fields than in the regular waves. A comparison of the hydrodynamic performance of ‘⊥’-type and ‘T’-type shows that ‘T’-type breakwater is better than ‘⊥’-type by about 20–30% under identical conditions.