The Conversion of Spectral Wave Heights to Design Wave Heights

The significant wave height can be either the zero-crossing wave height, H_1/3 or the spectral wave height, H_m0. The ratio of H_1/3/H_m0 in deeper water is about 0.95; in shallow water the ratio increases and may reach 1.1 for sea and about 1.5 for swell. Conversion of H_m0 to H_1/3 may be necessary when H_m0 is predicted by wave hindcast models or recorded by wave buoys while design formulae for coastal structures are based on H_1/3. Existing conversion methods by Thompson and Vincent (1985), Battjes and Groenendijk (2000) and Goda (2010) were reviewed and compared with field data. Non-linear wave transformation in shallow water and the resulting H_1/3/H_m0 ratio was investigated analytically with a line-model extending from deep to very shallow water. The effect of wave height, wave steepness, water depth and seabed slope were analysed. The findings were combined in a conversion formula of H_m0 and H_1/3 that is applicable inside and outside the surf zone. The new approach was validated against field data.