Effect of Short-Crestedness and Obliquity on Non-Breaking and Breaking Wave Forces Applied to Vertical Caisson Breakwaters

This paper addresses wave forces applied to vertical caisson breakwaters. Design diagrams are proposed to evaluate the reduction of the breaker wave force with increasing horizontal length of the units. A model in 1:100 scale of a typical Italian vertical breakwater was tested under multidirectional waves, causing mainly spilling and occasionally plunging breakers, in the CRF-LSF (Wallingford, UK) wave basin. Seven adjacent modules were instrumented with synchronized force transducers. Pulsating and breaking loads were distinguished on the basis of the frequency content and the spatial correlation of the force per unit length was fitted to a single or a double bell shaped distribution. The experimental autocorrelation function for pulsating loads agrees with theoretical solutions; for breaking waves it has a Taylor microscale which is approximately 7% of wavelength, only slightly dependent on spreading, and it goes to zero at great distances; it does not fall rapidly to zero possibly due to the residual correlation of wave troughs. The reduction factor of the load per unit span is hence derived following the argument developed by Taylor for random movements. The effect of wave obliquity is modeled describing the lack of correlation with a simple cosine function.