A double peaked spectrum is observed when wind waves and a swell from a remote source coexist. In the present paper, the statistical properties of double peaked spectral waves and wave forces on the upright section of composite breakwaters under the action of those waves are invesitgated by the experiments in the wave flume. The design wave periods for calculating the forces are also discussed on the basis of experimental results and simulation data.
　Laboratory test data demonstrate that period distributions of double peaked spectral waves are broader than those of single peaked spectral waves, however, relations between the representative wave periods (Tmax, T1/10 and T1/3) are same regardless of spectral shapes. As for wave height distributions, Rayleigh distribution is also appiocable for double peaked spectral waves.
　Random wave forces under the action of double peaked spectral waves have been investigated by hydraulic model tests with the aid of loadcells. Test results proved that wave forces on the upright section of composite breakwaters can be calculated by the application of Goda's formula with the significant wave period of double peaked spectral waves. The method for estimating the peirod is newly proposed and it can be calculated from the significant wave heights and periods of each wave train.