Exploring the Hydraulic Performance of Conical Pile Head Breakwater – An Experimental Investigation

Abstract

Conventional pile breakwater is a pervious structure built using prismatic circular piles and it has been proven to provide partial protection efficiently. Increasing the size of the pile breakwater in the vicinity of the free surface increases its hydraulic efficiency, as most of the wave energy is concentrated there. In the present study, the conventional pile breakwater model is modified by widening the cross-sectional area of the piles at the surface level in a conical shape termed as conical pile head breakwater (CPHB). The influence of the dimensionless structural parameters such as relative diameter (D/Hmax), relative height (Y/Hmax), relative clear spacing (b/D) and relative clear spacing between rows of CPHB (B/D) on the hydraulic performance is comprehensively explored through physical model studies. The hydraulic performance of the model includes wave transmission (Kt), wave reflection (Kr) and energy dissipation (Kd) coefficients. The study is carried out under monochromatic waves of varying wave height (0.06 m to 0.16 m) and wave period (1.4 s to 2 s) at different depths of water (0.35 m, 0.40 m and 0.45 m). For single-row non-perforated CPHB, the structural configuration of D/Hmax = 0.4, Y/Hmax = 1.5 and b/D = 0.1 have emerged as the best performing model for which a smaller value of Kt of 0.66 is obtained along with Kr of 0.22 and Kd of 0.72. Further, the investigation is carried out to determine the influence of the second row of similar piles arranged in a staggered manner. For two rows of CPHB, B/D of 0.4 is the optimum spacing, which provided a minimal Kt of 0.58 with Kr of 0.24 and Kd of 0.79. The addition of a second row of piles with a similar configuration reduces the Kt by a maximum of 12.34% compared to a single row of CPHB. However, from the construction point of view, driving two rows of piles at a closer spacing in the field may give rise to technical issues and practical difficulties such as disturbance of neighbouring piles, altering soil bearing capacity, equipment manoeuvring and restricted access for construction and maintenance personnel due to the limited space. iTo ward off such possibilities, an effort is made to enhance the functionality of single row of CPHB structure by introducing perforations to encourage energy dissipation. The influence of perforations on the performance of the perforated CPHB is comprehensively investigated through physical model studies. The effect of perforations and their distribution around the pile head (Pa), percentage of perforations (P) and size of perforations (S/D) on the wave attenuation characteristics are evaluated to arrive at an optimum configuration. A minimum Kt of 0.58 associated with Kr of 0.26 and Kd of 0.78 is obtained for an optimum configuration of Pa = 50%, P = 19.2% and S/D = 0.25 at a water depth of 0.45 m. This result compares exceptionally well with that of two rows of CPHB. Overall, providing the perforations is found to be effective in enhancing the wave attenuation capability by up to 12.4%. The Kt of the proposed CPHB is about 19 to 35% lesser than that of the perforated hollow pile breakwater under matching test conditions. To ascertain the suitability of an open-source software REEF3D in CPHB modelling, selected cases of CPHB are numerically simulated and the results are validated with the experimental data. For non-perforated CPHB, the numerical results are under predicted for Kt (less than 4%) and over predicted for Kr and Kd (less than 9%). For the perforated CPHB, the variation is slightly higher (up to 12%) compared to the non-perforated structure. Validation of the numerical results with the experimental data shows that REEF3D produces reliable results with acceptable RMSE values. In addition, a set of empirical equations is derived using the data fitting technique for quick prediction of Kt and Kr of CPHB. The empirical equations estimate the Kt and Kr values quite accurately with a high coefficient of determination (R2 ≥ 0.90). The overall performance of the CPHB is found to be promising and therefore, may be considered as one among the host of measures for the purpose of wave energy damping necessary for various shore/nearshore applications.