  The work highlights the importance of directional spreading effects on wave
overtopping estimation in shallow and mild sloping foreshores. Wave
short-crestedness leads, in general, to a reduction of mean overtopping
discharges on coastal structures. In the present work, the case of a sea dike
with gentle foreshore in very and extremely shallow water conditions is
analysed. Physical model tests have been carried out in order to investigate
the effect of directional spreading on overtopping and incident wave
characteristics. In the present experimental campaign, the effect of wave
spreading has only been investigated for perpendicular wave attack. Results
show that directional spreading is proved to cause a reduction of average
discharge of sea dikes with gentle and shallow foreshore. Expressions for the
reduction factor for directional spreading are derived, fitted on the tested
database. The use of this reduction factor leads to more accurate prediction
and avoids overtopping overestimation, however reduction-factor formulations
are overtopping-formula depending.
