Experimental study on the performance of multilayer geotube breakwater with and without C.C. block under wave attack

Abstract

This study investigated the performance of a multilayer geotube breakwater with and without C.C block under wave attack as a shore protection structure. Experimental studies were carried out in a two-dimensional flume (21.3 m long, 0.76 m wide, and 0.74 m deep) of the Hydraulics and River Engineering Laboratory of Bangladesh University of Engineering and Technology. A set of experiments were carried out at d= 50 cm water depth with three different breakwater heights (h= 25 cm, 30 cm, and 35 cm) for three different breakwater conditions. A total of twenty-seven runs were conducted with monochromatic waves having three wave periods (T= 1.6 sec, 1.8 sec, and 2.0 sec) for 2-1 layer geotube with C.C. block breakwater, 2-1 layer geotube breakwater and 3-2 layer geotube breakwater. Wave heights were measured manually by providing a vertical scale on the flume side made of glass. Five different locations, three in the front, one over the breakwater, and one behind the breakwater were selected for data collection. At each measuring location, water surface data were recorded for 1 min, at 5 sec time intervals. Hydrodynamic co-efficient like transmission coefficient (Ct), were determined from the measuring water surface data for various run conditions. The transmission coefficient values were then, analyzed with respect to relative submergence (h/d) and relative breakwater width (B/L), [where, B= breakwater width, L= wavelength] to check the performance of three different breakwater conditions. The lowest average value of transmission coefficient for a 2-1 layer geotube with C.C. block breakwater is 0.53 (i.e. when h/d = 0.7 and T = 1.6 sec). From the experiments, it is observed that as the relative breakwater height increases the transmission coefficient decreases due to breaking for any particular wave period. The lowest transmission coefficient is obtained for low wave height with the highest breakwater height. The lowest transmission coefficient (Ct=0.46) is obtained for low wave height with the highest breakwater height in 2-1 layer geotube with C.C. block breakwater condition. Again, it is also found that for the same wave period higher breakwater can reduce incident wave height more than a shorter breakwater. For a 2-1 layer geotube with C.C. block breakwater, the wave height reduction (54%) is more compared to the other two types of the breakwater (i.e. when h/d = 0.7 and T = 1.6 sec). The performance of a 2-1 layer geotube with C.C. block breakwater is better than both a 2-1 layer geotube breakwater and a 3-2 layer geotube breakwater. It is hoped that this study can provide engineers with useful information for designing submerged geotube breakwaters.