In nature coastal structures are exposed to oblique short-crested waves. The effect of wave incident angle on total wave force on a long caisson are twofold. The one is the force reduction due to the reduction of instantaneous point pressure on the caisson, named point-pressure force reduction. The other is the force reduction due to the fact that the peak pressures do not occur simultaneously along the caisson, named peak-delay force reduction. These two reduction effects can also be expected with short-crested waves, as the short-crestedness of waves means the spreading of wave energy over a range of incident angles. The peak-delay force reduction, i.e. no simultaneous peak along caisson, is of particular interest because the equipment improvement in construction enables the building of considerably long caissons. In Japan length of caissons exceeds often 100m. This paper will concentrate on the peak-delay force reduction of caissons exposed to non-breaking short-crested waves.
Battjes (1982) has investigated theoretically the peak-delay force reduction of shortcrested waves with only one frequency component. Such a force reduction factor cannot be applied because in nature waves are composed of many linear components with various frequencies. In this paper the peak-delay force reduction factor is defined on basis of zero-moment of the force spectrum. Based on linear wave theory, formulae for calculation of peakdelay force reduction factor for linear, long-crested and short-crested non-breaking waves are derived analytically. Based on the derived formulae the influence of wave obliquity and short-crestedness on the peak-delay force reduction is demonstrated.