Experiments on wave trapping by a submerged cylindrical island

A description is given of some experiments made to examine the possible trapping of surface waves by a submerged cylindrical sill in an otherwise uniform ocean. Theories by Longuet-Higgins (1967) and Renardy (1981) indicate that, at certain dangerous frequencies, nearly resonant conditions may obtain, leading to unusually large wavefields over the sill. These resonances have been sought experimentally by examining two functionals of the wavefield over the sill to see if they should become unusually large near certain of the dangerous frequencies. The results show no manifestation whatsoever of the resonances. Possible reasons for the discrepancies are considered. It has, however, been observed that, at certain locations over the sill, the waves were amplified by a factor of four or five times their magnitude in the ocean, irrespective of the frequency in a certain interval. This could be an important consideration for the siting of rigs on prominent topographic features of the ocean bed.