Experimental study on the wave interaction with submerged geo- tube breakwater in front of cement concrete block revetment

Abstract

Engineering advancements on coastline protection have prompted more noteworthy counter-measures to protect the shorelines. Revetment and breakwater are the common counter-measures that are widely used to protect the shorelines from wave attack by damping the wave energy. Though individually, their effects on wave energy damping are well defined, their combined effects are poorly studied. To address this gap, the interaction between waves and submerged geo-tube breakwater in front of cement concrete block revetment has been investigated experimentally. To do this, a set of experiments have been carried out at still water depth, hw = 50 cm with the submerged geo-tube breakwater of four different breakwater heights, hb = 15 cm, 20 cm, 25 cm and 30 cm for four different wave periods, T = 1.7 sec, 1.8sec, 1.9 sec and 2.0 sec in a two-dimensional wave flume. A revetment consist of cement concrete blocks was constructed at the end of the flume with a slope of 4(H):1(V). Wave height (H) and wave period (T) were downscaled and Pilarczyk equation was used to determine the size of cement concrete block. The downscaled block size was 2.0 cm x 2.0 cm x 2.0 cm. The stability of the revetment against transmitted wave height was investigated. Though the incident wave heights, Hi = 11 cm, 12 cm, 14 cm, and 15 cm were larger than design wave height, Hs = 9.5 cm, only for three experimental runs the revetment failed partially among the sixteen experimental runs. For other experimental runs, the revetment was stable due to the reduction of incident wave height by the submerged breakwater. From the experimental investigations, it has been seen that for any particular wave period the relative breakwater height (hb/hw) and the relative breakwater width (breakwater width/wave length, B/L) are the important parameters for the reduction of incident wave height. As the relative breakwater height (hb/hw) increases, the transmitted wave reduces more due to breaking caused by the breakwater for any particular wave period. In this experiment, the highest reduction occurs for 60% submergence of the breakwater for any particular wave period. Also as the relative breakwater width (B/L) increases, the reduction of wave height also increases for any particular relative breakwater height. In this experiment, the highest reduction occurs, when breakwater width is 30% of the wave length for any particular relative breakwater height. A relationship among transmission coefficient (Kt), relative breakwater height (hb/hw), wave period (T) and significant wave height (Hs) has been established (〖 K〗_t=1.05-0.67(h_b/h_w )^0.83-2.4×〖10〗^(-4) ((gT^2)/H_s )) from the experimental results. This relationship will be helpful to design revetment at the shore and breakwater at shallow water for larger wave heights.