Experimental study of the short-term efficiency of different breakwater configurations on beach protection

A campaign of experimental tests on a 2D movable-bed physical model, reproducing an Italian beach on the Adriatic Sea, has been performed in the wave flume of the “Laboratorio di Idraulica e Costruzioni Marittime” of the Università Politecnica delle Marche (Ancona, Italy), with the aim to assess the fundamental features of various breakwater configurations to be used in a beach-defence system typical of sandy, low-coastline beaches. Three emerged and three submerged configurations of rubble-mound detached breakwaters, for beach protection, placed at different distances from the shore, were tested, as well as a free beach configuration. The short-term hydrodynamic performances of the different configurations were assessed using as forcing some typical real-life intense sea-storm conditions. Wave transmission and beach protection efficiency under various intense wave conditions were obtained and related to some dimensionless parameters, amongst which a recently introduced one, χ, that combines both wave and breakwater properties. Transmission coefficients were found to be about 0.4 for emerged breakwaters and in the range 0.5–0.8 for submerged breakwaters. A net damping coefficient, defined as the wave height decay solely due to the effect of the breakwater, was measured as 0.2 for submerged breakwaters and 0.4 for the emerged ones. Further, submerged breakwaters induce an inshore mean water superelevation that increases with χ, whilst it decreases in the case of emerged breakwaters. Wave transmission is well represented by existing literature relations for both emerged and submerged breakwaters. Emerged breakwaters are more protective than submerged ones, but, at the same time, are more sensitive to changes in structure dimensions or positions. This is confirmed by the analysis of the momentum flux within the nearshore region, which is much larger for the submerged breakwaters. Such structures induce large swash-zone motions and sediment transport, comparable to those occurring at an unprotected beach.