PHYSICAL MODELLING OF INNOVATIVE SINGLE ROW ENLARGED PILE HEAD BREAKWATER

Abstract

Breakwaters are constructed to dissipate the wave energy and safeguard the coast, coastline infrastructures and communities from the destructive wave forces. Conventional type breakwaters are massive structures and suitable for the coastal sites where complete protection from the waves is essential. An environmentally friendly structure constructed for the protection of the coast without spoiling the aesthetics and advantages of a natural beach is always a better option. One such structure is the pile breakwater. Conventional pile breakwater is a non-gravity type breakwater consisting of closely placed single or multiple rows of circular or rectangular piles. The pile breakwater is generally constructed as an emergent structure. In pile breakwaters, the wave energy dissipation occurs due to wave structure interaction associated with turbulence, eddy formation and vortex shedding. For small recreational harbours or fisheries harbours and at locations where large littoral drift or onshore-offshore sediment movement exist, unconventional types of breakwaters like floating breakwater or piled breakwaters are highly preferred. Pile breakwaters are constructed and have been working successfully in many places like Auckland harbour in New Zealand, suspended breakwater in North-Western coast of Egypt, steel pipe breakwaters constructed in Asaka port at Japan, concrete pile breakwater at Pass Christian, Mississippi, USA and pile row breakwaters at Langkawi, Malaysia. An economical, eco-friendly and efficient breakwater system is vital for coastal protection and harbour tranquillity. In this regard, various researchers have been working to develop appropriate solutions to encounter site-specific challenges. With this viewpoint, the concept of enlarged pile head breakwater is developed. The wave energy is more concentrated at the surface and reduces gradually depthwise. On this basis, providing a larger area of the structure at the surface level may result in increased wave structure interaction inducing larger wave attenuation. Hence, the concept of structure enlargement at the surface and perforation is comprehended for the advancement of enlarged pile head breakwater. The enlarged portion is termed as pile head and the portion below is denoted as a trunk. ii In the present study, wave transmission, reflection and energy dissipation of the single row enlarged pile head breakwater are examined experimentally in a 1:30 scaled model. The experimental models are subjected to monochromatic wave heights ranging from 0.06 m to 0.16 m and wave period 1.4 s to 2 s, which in actual conditions corresponds to wave heights and wave periods of 1.8 m to 4.8 m and 7 s to 11 s, respectively. Initially, the experiments are conducted on non-perforated enlarged pile head breakwater to optimize the relative pile head spacing and depth of water. The effect of relative pile head diameter and height on wave transmission (Kt), reflection (Kr) and dissipation (Kd) characteristics are studied comprehensively. With the decrease in relative spacing between the piles from 0.9 to 0.2, a maximum of 19.75% reduction in Kt is obtained for the case of D/Hmax = 0.6 with Y/Hmax = 1.0 at 0.3 m water depth. It is observed that with an increase in the depth of water, Kt increases and Kr and Kd decreases. For 25% to 33.33% increase in water depth, Kt increases by an average of 4% to 6%, Kr decreases by 17% to 19% and Kd decreases by 7% to 8%. An increase in D/Hmax from 0.4 to 0.6 and Y/Hmax from 0.5 to 1.0 decreases the Kt and increases Kr and Kd. The enlarged pile head breakwater structure with the structural configuration of b/D = 0.2, D/Hmax = 0.6 and Y/Hmax = 1.0, has least value of Kt (0.62). Using the present experimental data, a hybrid theoretical solution is developed and validated with the available theoretical solutions. The proposed hybrid equation predicts encouragingly better transmission, reflection and dissipation coefficient than the existing solutions. Moreover, the results predicted by the proposed hybrid equation are in good agreement with the conventional pile breakwater model. In the second stage, on fixing the relative pile spacing and depth of water, investigations are continued on perforated enlarged pile head breakwater. The study focused on improving the hydraulic efficiency of enlarged pile head breakwater by incorporating perforations on the pile head. Effect of percentage distribution of perforations (pa), size of perforations (S) and percentage of perforations (P) on Kt, Kr, and Kd are investigated. Results indicate that the pore size highly dominates the wave attenuation than considering the increasing percentage of perforations with the small size of the pore. Perforations effectively reduce the Kt by about 10% to 18% iii compared to that of non-perforated pile head breakwater. Hydraulic efficiency of enlarged pile head breakwater is optimum when D/Hmax = 0.6, Y/Hmax = 1.0, b/D = 0.2, S = 0.25D, pa = 75% and P = 22.5% at 0.3 m water depth. For the quick estimate of hydraulic coefficients, a hybrid theoretical solution developed for non-perforated pile head breakwater is modified to suite for the perforated pile head breakwater. The proposed hybrid equation for the perforated pile breakwater predicts more reliable Kt, Kr and Kd values. The performance of the proposed breakwater is also compared with similar types of breakwaters. A samllest Kt of about 0.58 is obtained for the enlarged perforated pile breakwater structure with the structural configuration of b/D = 0.2, D/Hmax = 0.6, Y/Hmax = 1.0, S = 0.25D, pa =75% and P = 22.5 along with Kr = 0.36 and Kd = 0.73. The best performing configurations for non-perforated (D/Hmax = 0.6, Y/Hmax = 1.0 and b/D = 0.2) and perforated (D/Hmax = 0.6, Y/Hmax = 1.0, b/D=0.2, S = 0.25D, pa =75% and P = 22.5) structures as obtained from the present experimental work are numerically modelled using open source CFD software REEF3D. The results of Kt, Kr and Kd obtained from the REEF3D are in line with the experimental and theoretical data. REEF3D underpredicts Kt by about 1% to 3%, overpredicts Kr by 4% to 11% and variation of Kd is about 1% with reference to the experimental results. From the analysis, it is concluded that the REEF3D numerical model can be used for estimating the hydraulic performance of the enlarged pile head breakwater.