Estimating the JONSWAP Spectrum Parameter as a Function of Wave with Case in the Western Java Sea

The implementation of the JONSWAP spectrum model at a specific site requires adjustments to the shape of the spectral curve. These adjustments are made by modifying the key shape-defining parameters of the JONSWAP spectrum, namely the Phillips constant (α) and the peak enhancement factor (γ). In the absence of adequate wavefront profile data, marine structure designers often rely on recommended α and γ values derived from other locations. Several studies have proposed empirical formulas to estimate α and γ as functions of significant wave height (H_s) and peak frequency (f_p). This study has aimed to refine the estimation formulas for α and γ so they can be used as references for predicting the wave spectrum shape in the western Java Sea based on locally observed wave data. A curve-fitting optimization technique has been employed to align the model surface with the observed data distribution, which is assumed to vary as a function of H_s and f_p. The study has successfully proposed an improved empirical formula for estimating α, which has demonstrated higher accuracy than those that have been proposed by previous researchers, yielding a correlation coefficient (r) of 0.92 and a Root Mean Square Error (RMSE) of 0.00069. However, for the γ parameter, no clear correlation has been found between the estimated γ (H_s, f_p) and the observed γ values. The γ parameter exhibited high variability and appeared to be independent of H_s and f_p conditions.