Wave forces on an array of large-diameter cylinders have been ! measured in a three dimensional wave basin equipped a multi,: directional wave-maker. The array was constructed on a slope to study the effect of sloping bed on the wave forces and it was subjecte d to directional waves with various degrees of directional spreading. It has been lbund that the wave height between cylinders is significantly affected by the cylinder's spacing and it increases for small spacing between the cylinders. The interaction between waves diffracted from the cylinders, however, becomes less pronounced in case that the spacing is twice of the cylinder's diameter. The maximum wave pressure has been confirmed to occur in the zone between the cylinders at a location 60 degrees from the leading edge of the cylinder. The magnitude of the maximum pressure increases for smaller spacing between the cylinders, however, its location does not change neither changing the cylinder spacing nor the principal direction of incident waves. it has also been observed that the wave lbrces in the principal direction become less at smaller values of the directional spreading parameter for large values of the relative water depth, however, it increases when the relative water depth is small. This indicates that the design based on Unidirectional wave may not be always a conservative estimation and wave directionality should be properly considered.
Irregular waves have been commonly used in the design of offshore structures. In deep water, wind waves are essentially multidirectional and the effects of directional spreading on the wave lbrces acting on a structure were investigated (e.g., Ikegaya et al, 1994, Mizutani et al., 1998). On the contrary, in shallow water, waves have been commonly simplified by unidirectional waves because of wave refraction.