SURF ZONE WAVE MODELING : IMPROVEMENT OF DISSIPATION BY DEPTH-INDUCED BREAKING IN SWAN AND APPLICATION IN MALACCA STRAITS

摘要

The focus of this study is to improve the prediction accuracy of SWAN in shallow waters. The current version of SWAN (40.20) uses the bore-based dissipation model by Battjes and Janssen (1978) to calculate dissipation due to depth-induced wave breaking. Inherent in this dissipation model are two simplifying assumptions that are addressed in this paper. Firstly the bore-based model assumes an underlying truncated Rayleigh probability density function to describe the distribution of random breaking waves. Secondly, headloss due to a hydraulic jump has been used to approximate energy loss in a broken wave. To overcome the shortcomings of the underlying truncated Rayleigh pdf and the hydraulic jump assumption, a new dissipation equation for breaking waves has been formulated based on the breaking wave height distribution by Thornton and Guza (1983) and the dissipation model by Rattanapitikon W. & Shibayama T. (1998). The model results, presented in terms of root mean square (rms) relative error, are compared against experimental results reported by Battjes and Janssen (1978). The proposed dissipation function gave the smallest rms relative error of 4.53% while the original model by Battjes and Janssen gave an error of 6.16%. Rattanapitikon and Shibayama's model gave an error of 17.9% and Thornton and Guza's model gave 4.9%. Hence the proposed dissipation function was able to improve the prediction accuracy and suggest that both energy dissipation and underlying breaking wave height distribution are important for accurate wave height prediction.