Analysis and Prediction of Wave Transformation from Offshore into the Surfzone under Storm Condition

Abstract

Surfzone wave transformation under storm conditions is investigated through field and laboratory measurements in this study. The observations have been used to examine currently available models of wave energy dissipation. Detailed field data has been collected by means of a novel method which was first introduced by Nielsen (1988). This method has been utilised through a common program between Griffith University and The University of Queensland at The Spit on the Gold Coast in Southeast Queensland. The facility primarily consists of a manometer tube array with 12 different manometer tube lengths varying from 60 m to 500 m offshore and a concrete manhole excavated into the dune system to house the monitoring station. Accordingly, this system has enabled the monitoring of a detailed wave height profile across the surfzone under any conditions from the safety of the “bunker” on land. The findings of new laboratory experiments on the frequency response of the semi-rigid manometer tubes are also presented which extend and improve upon the previous work of Nielsen et al. (1993). Testing was conducted over a range of frequencies (0.0067 Hz< f <2 Hz) and tube lengths (10 m< L <900 m). New frequency response factors are determined by fitting the semiempirical gain function of Nielsen et al. (1993) to the observed gain data. As a result, new predictive formulas for the empirical coefficients as a function of tube parameters are provided in this study. Wave induced pore pressure in the surfzone seabed is investigated based on the recorded field data. Two well-known models, i.e. Hsu and Jeng (1994) and Sleath (1970), are assessed against the field measurements. The findings validate the accuracy of the models and indicate that the extent of energy dissipation due to the overlying sand is less than 5% and depends on the incident wave length.