Performance of perforated piles as a wave attenuator

Abstract

Laboratory experiments on wave transmission through various types of piles with different perforations were conducted at the Coastal and Offshore Engineering Institute (COEI), Universiti Teknologi Malaysia City Campus, Kuala Lumpur. The study involved carrying out laboratory experiments on perforated pile models under unidirectional regular waves in a partially submerged set-up with water depth h = 0.19 m to 0.23 m; and under fully submerged set-ups at h = 0.27 m to 0.35 m. The experiments were conducted to study the performance of the one-row pile models as a wave attenuator using various pile types that are, Single Ring (SP), Double Ring (DP) and Single Small Ring Pile (SSP). Additional tests on two-row SP and DP models; and three-row DP models were also conducted to allow comparisons to be made with the one-row SP and DP models. The pile porosity used in this study was in the range of 0.0625 to 0.48. The effect of closely spaced pile of model spacing B = 0 was investigated and compared with piles arrangement with B = 0.5D and 0.75D. For the present study, the estimation of incident wave height Hi and reflected wave height Hr were obtained by using the Mansard and Funke (1980) equation. Two groups of wave gauges were used to measure the incident, reflected and transmitted waves. The first group, which consisted of three wave gauges positioned in front of the test model was to measure the incident and reflected waves, whilst the second group of two wave gauges positioned at the lee side of the model measured the transmitted waves. Analysis of transmission coefficient Kt and loss coefficient Kl were related to the non-dimensional structural geometric parameters that is, wave steepness (Hi/L), relative depth (h/L), porosity (e), relative spacing of model (B/L) and relative width of model (W/L). Comparison of experimental results of Kt and Kl between the three types of pile models, SP, DP and SSP are presented. The DP model was found to give better wave attenuation compared to the SP and SSP models. Subsequently, experimental results of wave transmission coefficient Kt for two-row DP model (e = 0.0625 and 0.48) are presented and compared to the results of previous studies by other researchers. Empirical equations for predicting the transmission coefficient have been developed for the onerow, two-row DP models with and without spacing, by using Multiple Regression Analysis. The experimental results showed that the two-row DP model under fully submerged condition with B = 0.5D and B = 0.75D were better able to attenuate waves up to 25% when T = 0.87 s to 1.03 s as compared to when T>1.03 s, where only 4% of wave is attenuated.