The Effect of Asymmetric Insular Shelf in Trapping Long Gravity Waves around an Island

The effect of asymmetric insular shelf in trapping long gravity waves around an island is investigated numerically by using a simple model. The model consists of a steep-sided, circular island which rises from the top of a circular sill of larger radius. The center of the island is placed off-center of the sill by Δr distance. The results are : (1) As the asymmetry becomes pronounced, the region of higher amplitude response shifts to the direction of the broader shelf. (2) The transfer function in the case of an asymmetric island can be estimated roughly by using the solution of a symmetric island model whose width of the round shelf coincides with the broadest width of the shelf in the asymmetric model. In water level observations at four stations around Oshima Island, a spectrum peak of period approximately 8 minutes could often be detected at the westernmost station. To understand this feature, numerical calculations were made both for the realistic Oshima-Island model (with actual bathymetry), and for the Island-Shelf model (which roughly approximated the topography of the Oshima Island area). And it was found that the oscillations of the observed peak period of 8 min. corresponded to a (1, 0) mode of edge wave. This mode is modified by the asymmetry of the insular shelf and forms a standing wave pattern with a node extending in the southwesterly direction vis-a-vis Oshima Island