Parametric Studies on Stability of Reshaped Berm Breakwater with Concrete Cubes as Armor Unit

Abstract

The breakwater construction in deeper waters requires heavier armor units due to larger wave loads. Such large stones are uneconomical to quarry or transport or may not be available nearby. Another problem is uncertainty in the design conditions resulting in breakwater damage due to increased wave loads. The structural stability and economy in construction of breakwater are the need of hour. Under these circumstances, berm breakwaters can be a solution. For an economical solution, the quarry yield may be judiciously used and berm breakwater may be constructed with small size armor units. The present research work involves a detailed experimental study of influence of various sea states and structural parameters on the stability of statically stable reshaped berm breakwater made of concrete cubes as primary armor. Initially, a 0.70 m high of 1:30 scale model of conventional breakwater of 1V:1.5H slope and trapezoidal cross section is constructed on the flume bed with concrete cubes of weight 106 g as primary armor. This is designed for a non-breaking wave of height 0.10 m. This model is tested for armor stability with regular waves of heights 0.10 m to 0.16 m and periods 1.6 s to 2.6 s in water depths of 0.30 m, 0.35 m and 0.40 m. In the second phase, a 1V:1.5H sloped and 0.70 m high berm breakwater with varying size of concrete armor cubes, berm widths, thickness of primary layer is tested for stability with same test conditions. Based on the study of conventional and reshaped berm breakwater model the following conclusions are drawn. Damage level (S) was found increasing with the increase in stability number (Ns) in conventional breakwater. In conventional breakwater damages were in the range of 4.62 to 5.69 (intermediate), 9.75 to 11.46 (failure) and 9.46 to 10.22 (failure) in the depths of 0.3m, 0.35m and 0.4m respectively. Considering the complete iranges of Ho/gT2 and d/gT2, the maximum relative run-up Ru/Ho and relative run-down Rd/Ho were respectively 1.2 and 1.25. The stability of the berm breakwater is largely influenced by the storm duration. It was observed that relative berm position (hb/d) has a greater influence on berm recession than wave run-up and run-down. As relative berm position (hb/d) parameter increases from 1.00 to 1.50, the berm recession decreased by up to 77% while the wave run-up and run-down decreases by 7% and 14% respectively. The surface elevation of the water in front of the berm influences the recession and eroded area of the berm. Some of the available equations for berm recession, wave run-up over estimated the values for the considered conditions. The damage is reduced by about 47% in the present model when compared to stone armored berm breakwater. The wave runup and run-down are reduced by 34% and 49% compared to conventional cube armored breakwater respectively. The economic analysis showed that the cube armored berm breakwater is about 8% and 4% economical than the conventional cube armored breakwater and stone armored berm breakwater for the same design conditions. The design equations for berm recession, wave run-up and wave run-down are derived. Finally, it was found that 25% reduction in armor weight with 0.40 m berm width and 2 no. of primary armor layers is safe for the most of conditions considered during the study except for extreme waves of 0.16 m height and 1.6 s period. However, same breakwater with 3 armor layers was safe for the entire range of test conditions. In terms of safety as well as economy 25% reduction in armor weight with 0.40 m berm width and 2 no. of primary layer was cheaper compared to all other models studied.