Experimental study on the efficiency of submerged breakwaters made of boulders and geotubes

An experimental study on submerged breakwaters made of boulders and geotubes in a three-dimensional (3D), mobile bottom physical model was carried out in the Coastal Engineering Laboratory of the Technical University of Bari - Italy. The objective of this study was to compare the efficiency of two types of coastal protection structures consisting of submerged boulders breakwater parallel to the coastal line, to which two different barriers were added on the offshore side; the first consisting of natural rocks with smaller dimensions than the breakwater boulders and the second one consisting of geotubes. The two barriers had the same average submergence, which was four times bigger than the submergence of the existing breakwater. In the model, irregular waves were generated with JONSWAP spectrums. The wave front was 29m long and had two different directions; one parallel to the coastline, whereas the other one inclined towards the coastline at an angle of 15°. For both structures, surveys of wavy and beach profile measurements were reported and analyzed. The evolution of the wave energy spectrum from the offshore zone to the coastline was studied and the wave set-up was defined inside the protected zone. A comparison between the behaviour of the two configurations was conducted analyzing the flux of energy incidence, reflex, dissipation and transmission beyond the structure. The results show that the two types of structures are equally efficient in terms of reduction of transmitted waves in the protected zone, although the causes that determine its decrease are substantially different. With the rocks barrier, a major dissipation of energy was revealed. With the geotubes barrier, significant reflecting behaviour was noted and was evident in the elevated values of reflected flux. The surveys of the beach profiles carried out before and after each wavy attack allow us to identify the erosion and deposit zones. The offshore submerged bed profiles were analysed accurately. An erosive scour hole at the structure toe occurs in this zone with both configurations. Just after the scour hole a bed undulation takes place. These results are in good agreement with other experimental studies (e.g., Sumer et al., 2005). The heights of the undulations appeared greater with the geotube barrier, which determines a larger reflection coefficient. The maximum scour depth which occurred at the toes of the two structures are comparable.