Studies on the effects of an emerged impermeable and seaside perforated quarter circle breakwater on nearfield hydrodynamics

Abstract

Breakwaters are structures which are mainly used for the purpose of withstanding and dissipating the dynamic energy of ocean waves and thereby provide tranquility conditions on the lee side. Breakwaters are constructed either shore connected or detached to the coast. The main function of breakwaters is to create a tranquil medium on its leeside by reflecting the waves and also dissipating the wave energy arriving from seaside, resulting in ease of maneuverability to boats or ships to their berthing places. In modern times breakwaters are constructed for the purpose of protecting structures near to the coast and offshore, shoreline stabilization, forming an artificial harbour with a water area so protected from the ocean waves as to provide safe accommodation for ships and for preventing the siltation of river mouths. Different types of breakwaters have been developed in the past for the harbour development and protection of valuable coastal property, commercial activity and beach morphology. Among these, rubble mound breakwaters are the most common and provide good wave attenuation. In the beginning, primitive reefs and dykes of gentle slopes were built with natural stones. Later to save the material, steeper sloped structures with rubble mound, concrete block mound, rock fill over mound, caisson type etc. were tried. However, with time breakwaters with a variety of caisson designs have been proposed and developed. Later with development of technology various innovative types of breakwaters such as semicircular breakwater and quarter circle breakwater have been developed. Quarter circle breakwater (QBW) is a new-type breakwater first proposed by Xie et al. (2006) on the basis of semicircular breakwater. Quarter circle breakwater is usually placed on rubble mound foundation and its superstructure consists of a precast reinforced concrete quarter circular surface facing incident waves, a horizontal bottom slab and a rear vertical wall. A series of experiments are conducted in a two dimensional monochromatic wave flume on impermeable and seaside perforated quarter circle breakwater model. The present study investigates the wave reflection, loss characteristics, wave runup,ii wave rundown and sliding stability on an emerged seaside perforated quarter circle breakwater of three different radii 0.55 m, 0.575 m and 0.60 m with ratio of spacing to diameter of perforations (S/D) equal to 5, 4, 3, 2.5 and 2 for different water depths and wave conditions. A 1:30 scale model of quarter circle breakwater of 0.55 m radius is fabricated using Galvanized Iron (GI) sheet of 0.002 m thickness. The sheet is fixed to the slab with the help of stiffeners made up of flat plates of cross section 0.025 m x 0.005 m. The model is then placed over the rubble mound foundation of thickness 0.05m and stones weighing from 50 to 100 grams. Initially, impermeable quarter circle breakwater of different radius is tested for wave reflection and loss characteristics using regular waves of heights 0.03 m to 0.18 m and periods 1.2 s to 2.2 s in water depths of 0.30 m, 0.35 m and 0.40 m. Then runup and run down height on the curved QBW surface is noted and the vertical distance above and below the still water level is estimated. Later tests were conducted for determining minimum weight to be added to the QBW structure to prevent sliding. All the models were tested in the predetermined QBW dimensions as mentioned earlier. In the second phase, perforated QBW with different S/D ratios were tested to determine the reflection, loss characteristics, runup, rundown and stability with the same wave conditions and using the same structural parameters. Based on the experiments conducted, it was found that the reflection coefficient (Kr) increases but the loss coefficient (Kl) decreases with increase in incident wave steepness (Hi/gT2). The minimum Kr and the maximum Kl observed are 0.5054 and 0.8629 respectively for QBW of radius equal to 0.55 m at Hi/gT2 = 9.439 x10-4. The results shows that the value of K r decreases but Kl increases as the relative water depth (d/hs) increases for all values of Hi/gT2 and S/D ratio. The maximum percentage reduction in the value of Kr is observed for QBW of 0.55 m radius S/D= 2.5 and varies from 31.66% to 44.50% when the water depth increases from 0.35 m to 0.45 m. For seaside perforated QBW with d/hs= 0.732, percentage reduction iniii K r for S/D equal to 5, 4, 3, 2.5, 2 varies from 47% to 49%, 54% to 58%, 60% to 71%, 72% to 86% and 68% to 84% when compared to impermeable QBW. For all d/h s and Hi/gT2, the values for relative wave runup (Ru/Hi) and relative wave rundown (Rd/Hi) decreases with decrease in S/D ratio. But in the case of seaside perforated QBW with S/D = 2 the values of Ru/Hi and Rd/Hi are found to slightly more than that of S/D = 2.5 due to back propogation of waves from inside the chamber. Finally based on the studies on the sliding stability characteristics, it was observed that for all values of d/h s and S/D ratio, stability parameter (W/γHi2) decreases with increase in Hi/gT2. The minimum values for W/γHi2 for QBW of radius 0.55 m, 0.575 m and 0.60 m with S/D = 2.5 are 2.110, 1.998 and 1.967 respectively for Hi/gT2= 6.241 x10-3 and at 0.35 m water depth.